WorldWideScience
 
 
1

Chemistry of N-Heterocyclic Carbene Ligands  

Science.gov (United States)

N-Heterocyclic carbenes (NHCs) have developed into an important class of ligands in transition metal coordination chemistry. They have been employed successfully as spectator ligands in various catalytically active metal complexes and as organocatalysts. In this chapter we present some important synthetic methods for the preparation of various NHCs and their metal complexes.

Jahnke, Mareike C.; Hahn, F. Ekkehardt

2

Olefin Metathesis With Ruthenium-Arene Catalysts Bearing N-Heterocyclic Carbene Ligands  

Science.gov (United States)

In this chapter, we summarize the main results of our investigations on the ring-opening metathesis polymerization (ROMP) of cyclooctene catalyzed by various ruthenium (Ru)-arene complexes bearing imidazolin-2-ylidene, imidazolidin- 2-ylidene, or triazolin-5-ylidene ligands. Three major findings emerged from this study. First, we underscored the intervention of a photochemical activation step due to visible light illumination. Second, we established that the presence of an endocyclic double bond in the carbene ligand central heterocycle was not crucial to achieve high catalytic efficiencies. Third, we demonstrated that ortho-metallation of the N-heterocyclic carbene (NHC) ligand by the Ru center led to inactive catalysts.

Delaude, Lionel; Demonceau, Albert

3

Ligand-controlled regioselectivity in the hydrothiolation of alkynes by rhodium N-heterocyclic carbene catalysts.  

Science.gov (United States)

Rh-N-heterocyclic carbene compounds [Rh(?-Cl)(IPr)(?(2)-olefin)](2) and RhCl(IPr)(py)(?(2)-olefin) (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-carbene, py = pyridine, olefin = cyclooctene or ethylene) are highly active catalysts for alkyne hydrothiolation under mild conditions. A regioselectivity switch from linear to 1-substituted vinyl sulfides was observed when mononuclear RhCl(IPr)(py)(?(2)-olefin) catalysts were used instead of dinuclear precursors. A complex interplay between electronic and steric effects exerted by IPr, pyridine, and hydride ligands accounts for the observed regioselectivity. Both IPr and pyridine ligands stabilize formation of square-pyramidal thiolate-hydride active species in which the encumbered and powerful electron-donor IPr ligand directs coordination of pyridine trans to it, consequently blocking access of the incoming alkyne in this position. Simultaneously, the higher trans director hydride ligand paves the way to a cis thiolate-alkyne disposition, favoring formation of 2,2-disubstituted metal-alkenyl species and subsequently the Markovnikov vinyl sulfides via alkenyl-hydride reductive elimination. DFT calculations support a plausible reaction pathway where migratory insertion of the alkyne into the rhodium-thiolate bond is the rate-determining step. PMID:22536797

Di Giuseppe, Andrea; Castarlenas, Ricardo; Pérez-Torrente, Jesús J; Crucianelli, Marcello; Polo, Victor; Sancho, Rodrigo; Lahoz, Fernando J; Oro, Luis A

2012-05-16

4

Ligand-promoted carbene insertion into the aryl substituent of an N-heterocyclic carbene ligand in ruthenium-based metathesis catalysts.  

Science.gov (United States)

Addition of L = carbon monoxide or aryl isocyanides to the Grubbs second-generation carbene complexes Ru(H(2)IMes)(CHR)(PCy(3))Cl(2) (H(2)IMes = 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene; R = Ph, Me, H, CH=CMe(2)) triggers carbene insertion into an aromatic ring of the N-heterocyclic carbene supporting ligand, forming Ru{1-mesityl-3-(-7'-R-2',4',6'-trimethylcycloheptatrienyl)-4,5-dihydroimidazol-2-ylidene}L(2)(PCy(3))Cl(2). Insertions are also promoted for other PR(3) substituted complexes by carbon monoxide and aryl isocyanides, and for the phosphine-free Hoveyda-Blechert complex Ru(H(2)IMes)(CH(i-PrOC(6)H(4)))Cl(2) by aryl isocyanides and small phosphites but only after initial displacement of the coordinated ether. Heteroatom substituted carbenes do not undergo CO-promoted insertion unless poorer electron donor phosphine (PPh(3)) and carbene (CH(OC(6)H(4)-p-NO(2)) ligands are both present. Insertion depends on the added ligand, the carbene substituent, and to a lesser degree on the PR(3) ligand trans to the N-heterocyclic carbene. PMID:19397262

Galan, Brandon R; Pitak, Mateusz; Gembicky, Milan; Keister, Jerome B; Diver, Steven T

2009-05-20

5

Functionalised N-heterocyclic carbene complexes  

CERN Multimedia

N-heterocyclic carbenes (NHCs) have found increasing use as reagents for a range of organic transformations and in asymmetric organocatalysis. The performance of these molecules can be improved and tuned by functionalisation. Functionalised carbenes can anchor free carbenes to the metal site, introduce hemilability, provide a means to immobilise transition metal carbene catalysts, introduce chirality, provide a chelate ligand or bridge two metal centres. NHC can be attached to carbohydrates and campher, derived from amino acids and purines, they can be used as organocatalysts mimicking vitamin

Kühl, Olaf

2010-01-01

6

Electropositive metal N-heterocyclic carbene complexes  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The first chapter is an introduction to the f-elements, with a focus on the synthesis and chemistry of tetravalent cerium complexes. The synthesis, characterisation and reactivity of carbenes, particularly N-heterocyclic carbenes (NHCs), and anionic-functionalised NHC ligands is discussed. The synthesis and reactivity of s-block, Group three and fblock NHC complexes is reviewed. The synthesis of the alcohol-functionalised unsaturated imidazolium proligand, [H2L]I [H2L = HOCM...

Casely, Ian J.

2009-01-01

7

Cytotoxic Gold(I) N-heterocyclic Carbene Complexes with Phosphane Ligands as Potent Enzyme Inhibitors.  

Science.gov (United States)

Organometallic gold complexes with N-heterocyclic carbene (NHC) ligands have been demonstrating promising properties as novel anticancer agents. Gold(I) NHC complexes containing different phosphanes as secondary ligands were shown to trigger strong cytotoxic effects in cancer cells, and their effective uptake into the cells was quantified by atomic absorption spectroscopy. Moreover, the new compounds strongly inhibited the activity of the seleno-enzyme thioredoxin reductase (TrxR) and of the zinc-finger enzyme poly(ADP-ribose) polymerase?1 (PARP-1). In the case of TrxR inhibition, their activity depended clearly on the size of the alkyl/aryl residues of phosphorus atoms. Density functional theory (DFT) calculations showed that the Au?P bond of the triphenylphosphane complex [Au(I) (NHC)(PPh3 )]I had a lower bond dissociation energy compared to trialkylphosphane complexes [Au(I) (NHC)(PR3 )]I, indicating a higher kinetic reactivity of this particular compound. In fact, [Au(I) (NHC)(PPh3 )]I triggered an enhanced inhibitory activity against PARP-1. PMID:24677779

Rubbiani, Riccardo; Salassa, Luca; de Almeida, Andreia; Casini, Angela; Ott, Ingo

2014-06-01

8

Gold(I Complexes of N-Heterocyclic Carbene Ligands Containing Benzimidazole: Synthesis and Antimicrobial Activity  

Directory of Open Access Journals (Sweden)

Full Text Available Gold(I N-heterocyclic carbene (NHC complexes were obtained in good yields from the corresponding silver complexes by treatment with [AuCl(PPh3] following the commonly used silver carbene transfer route. The silver complexes were synthesized from the benzimidazolium halide salts by the in situ reactions with Ag2O in dichloromethane as a solvent at room temperature. All gold complexes have been characterized by 1H-NMR, 13C-NMR and IR spectroscopy and elemental analysis. Au-NHC complexes were evaluated for their in vitro antimicrobial activity against a variety of Gram-positive and Gram-negative bacteria and fungal species.

?lknur Özdemir

2010-03-01

9

[Ru(bpy)3]2+ analogues containing a N-heterocyclic carbene ligand  

Digital Repository Infrastructure Vision for European Research (DRIVER)

A synthetic procedure is described that provides access to [Ru(bpy)3]2+ analogues in which one bpy ligand is replaced by a C,N-bidentate coordinating carbene-benzimidazole ligand (bpy = 2,2’-bipyridine). These new complexes were prepared by first installing the chelating carbene ligand onto a Ru(cymene) platform and subsequent ligand substitution using bpy or terpy (terpy = 2:2’,6’:2’’-terpyridine). The carbene ligand significantly affects the optical properties of the complex and l...

2010-01-01

10

Accessible gold clusters using calix[4]arene N-heterocyclic carbene and phosphine ligands.  

Science.gov (United States)

We investigate the synthesis of accessible calix[4]arene-bound gold clusters consisting of open "coordinatively unsaturated" active sites, using a comparative approach that relies on calix[4]arene ligands with various upper- and lower-rim substituents. In contrast with a reported Au(I)-tert-butyl-calixarene phosphine complex, which exhibits a single cone conformer in solution, the H upper-rim analog exhibits multiple conformers in solution. This contrasts with observations of the tert-butyl upper-rim analog, which exhibits a single cone conformer in solution under similar conditions. In the solid state, as determined by single-crystal X-ray diffraction, both H and tert-butyl upper-rim analogs exhibit exclusively cone conformer. A detailed structural analysis of these two solid-state structures highlights a CH-? interaction involving a methoxy lower-rim substituent and phenyl substituent on P as the key feature that enforces a tight configuration of Au(I) atoms on the same side of the calix[4]arene lower-rim plane. We hypothesize that such a configuration promotes chelation of the ligand to a gold surface and facilitates the synthesis of small Au11-sized clusters after reduction of both complexes. The new cluster, like the one reported with the tert-butyl analog, has an extraordinary 25% of surface atoms that are open and accessible to a 2-NT (2-naphthalenethiol) probe in solution. We also investigated the effect of calix[4]arene lower-rim substituents that coordinate to the metal, by using N-heterocyclic carbene (NHC) functional groups rather than phosphines. Four small (<1.6 nm diameter) calix[4]arene NHC-bound gold clusters were synthesized, including three using novel calix[4]arene NHC ligands. The smallest calix[4]arene NHC-bound Au cluster consisted of a 1.2 nm gold core, and its number density of accessible and open surface sites was measured. This required development of a new titration method for open sites on gold clusters, using a SAMSA fluorescein dye molecule, which excites and emits at lower energy relative to the previously used 2-NT probe. The number density of open sites on the new calix[4]arene NHC-bound gold cluster measured by the SAMSA fluorescein probe strongly supports the generality of a mechanical model of accessibility, which does not depend on the functional group involved in binding to the gold surface and rather depends on the relative radii of curvature of bound ligands and the gold cluster core. PMID:23917776

Nigra, Michael M; Yeh, Alexander J; Okrut, Alexander; DiPasquale, Antonio G; Yeh, Sheila W; Solovyov, Andrew; Katz, Alexander

2013-09-21

11

Probing the limits of ligand steric bulk: backbone C-H activation in a saturated N-heterocyclic carbene.  

Science.gov (United States)

The consequences of extremely high steric loading have been probed for late transition metal complexes featuring the expanded ring N-heterocyclic carbene 6-Dipp. The reluctance of this ligand to form 2:1 complexes with d-block metals (rationalised on the basis of its percentage buried volume, %?Vbur , of 50.8%) leads to C-H and C-N bond activation processes driven by attack at the backbone ?-CH2 unit. In the presence of Ir(I) (or indeed H(+) ) the net result is the formation of an allyl formamidine fragment, while Au(I) brings about an additional ring (re-)closure step via nucleophilic attack at the coordinated alkene. The net transformation of 6-Dipp in the presence of [(6-Dipp)Au](+) represents to our knowledge the first example of backbone C-H activation of a saturated N-heterocyclic carbene, proceeding in this case via a mechanism which involves free carbene in addition to the Au(I) centre. PMID:24616039

Phillips, Nicholas; Tirfoin, Remi; Aldridge, Simon

2014-03-24

12

Non-classical N-Heterocyclic carbene complexes  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The expansion of the concept of N-heterocyclic carbenes as ligands for transition metals to mesoionic ligand systems has led to the discovery of a wide range of non-classical carbene-type ligands. These non-classical carbene-type ligands are characterised by a significantly lower heteroatom stabilisation of the (putative) free carbene, a situation that also affects the ligand donor properties and hence the reactivity of the coordinated metal centre. Based on the unique impact of non-classical...

Kru?ger, Anneke; Albrecht, Martin

2010-01-01

13

An N-Heterocyclic Carbene as a Bidentate Hemilabile Ligand: A Synchrotron X-ray Diffraction and Density Functional Theory Study  

International Nuclear Information System (INIS)

The N-heterocyclic carbene ligand IMes was shown by synchrotron crystallography and DFT computations to adopt a hemilabile bidentate coordination mode in CpM(CO)2(IMes)+B(C6F5)4- (M = Mo, W), with a C=C bond of one mesityl weakly coordinated to the metal.

2003-06-11

14

N-Heterocyclic Carbene Complexes: Decomposition Pathways  

Science.gov (United States)

N-heterocyclic carbenes, and their derivatives, are powerful donor ligands that form strong M-NHC bonds, particularly with late transition metals. Interest in these species has largely focused on their application as ligands in catalysis, in which they have shown some promise. However, they have not had the impact originally anticipated; NHCs undergo a number of processes that lead to catalyst deactivation and in the following chapter the range of deactivation reactions that have been reported will be discussed. By far the most detrimental deactivation process uncovered to date is reductive elimination, in which the NHC is lost as an imidazolium salt and the metal centre is reduced. This decomposition pathway is discussed in some detail and its impact on catalysis is put in context. Nevertheless, as apparent from some of the following discussion, the opportunity exists for an imaginative approach to the problem of catalyst decomposition, and it is possible to develop novel synthetic chemistry utilising these reaction pathways.

Cavell, Kingsley J.; Normand, Adrien T.

15

Optimized Syntheses of Cyclopentadienyl Nickel Chloride Compounds Containing "N"-Heterocyclic Carbene Ligands for Short Laboratory Periods  

Science.gov (United States)

Experiments are described for the preparation of imidazolium chloride precursors to "N"-heterocyclic carbenes and their cyclopentadienyl nickel chloride derivatives. The syntheses have been optimized for second- and third-year undergraduate laboratories that have a maximum programmed length of three hours per week. The experiments are flexible and…

Cooke, Jason; Lightbody, Owen C.

2011-01-01

16

Synthesis, photo-, and electrochemistry of ruthenium bis(bipyridine) complexes comprising a N-heterocyclic carbene ligand.  

Science.gov (United States)

Analogues of [Ru(bpy)3](2+) were prepared in which one pyridine ligand site is substituted by a N-heterocyclic carbene (NHC) ligand, that is, either by an imidazolylidene with a variable wingtip group R (R = Me, 3a; R = Et, 3b; R = iPr, 3c), or by a benzimidazolylidene (Me wingtip group, 3d), or by a 1,2,3-triazolylidene (Me wingtip group, 3e). All complexes were characterized spectroscopically, photophysically, and electrochemically. An increase of the size of the wingtip groups from Me to Et or iPr groups distorts the octahedral geometry (NMR spectroscopy) and curtails the reversibility of the ruthenium oxidation. NHC ligands with methyl wingtip groups display reversible ruthenium oxidation at a potential that reflects the donor properties of the NHC ligand (triazolylidene > imidazolylidene > benzimidazolylidene). The most attractive properties were measured for the triazolylidene ruthenium complex 3e, featuring the smallest gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in the series (2.41 eV), a slightly red-shifted absorption profile, and reasonable excited-state lifetime (188 ns) when compared to [Ru(bpy)3](2+). These features demonstrate the potential utility of triazolylidene ruthenium complexes as photosensitizers for solar energy conversion. PMID:23594091

Leigh, Vivienne; Ghattas, Wadih; Lalrempuia, Ralte; Müller-Bunz, Helge; Pryce, Mary T; Albrecht, Martin

2013-05-01

17

How does the addition of steric hindrance to a typical N-heterocyclic carbene ligand affect catalytic activity in olefin metathesis?  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Density functional theory (DFT) calculations were used to predict and rationalize the effect of the modification of the structure of the prototype 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) N-heterocyclic carbene (NHC) ligand. The modification consists in the substitution of the methyl groups of ortho isopropyl substituent with phenyl groups, and here we plan to describe how such significant changes affect the metal environment and therefore the related catalytic behaviour. Bearing in ...

Poater Teixidor, Albert; Falivene, Laura; Urbina-blanco, Cesar A.; Manzini, Simone; Nolan, Steven P.; Cavallo, Luigi

2013-01-01

18

N-Heterocyclic Carbene Complexes in Cross-Coupling Reactions  

Science.gov (United States)

N-heterocyclic carbenes (NHCs) are excellent ?-donor and poor ?-acceptor ligands. These properties make them not only behave as good phosphine mimics, but also result in more robust metal complexes. Recognising these advantages, the use of NHCs as spectator ligands in catalysis has become widespread. In this chapter we will focus on the advances in cross-coupling reactions using different metals and NHCs as ancillary ligands, a class of transformation that is a fundamental tool for synthetic chemists.

Jiménez-Núñez, Eloísa; Alcarazo, Manuel

19

Synthesis and Characterization of Ag(I) and Pd(II) Complexes with a Pyridine Substituted N-Heterocyclic Carbene Ligand  

International Nuclear Information System (INIS)

We have used our new tridentate pyridine substituted N-heterocyclic carbene to generate an interesting trinuclear [((MepyCH2)2-Im)3Ag3]3+ complex, displaying very short Ag-Ag separations. A Pd(II)-NHC complex was prepared from [((MepyCH2)2-Im)3Ag3]3+ via a facile transmetallation, leading to a dimeric [(MepyCH2)2-ImPdCl]22+ complex. Future plans are underway for the survey of the potential applications of these new NHC complexes as luminesent materials or homogeneous catalysts. Since Arduengo's discovery of the first isolable free carbene in 1991, N-heterocyclic carbenes (NHC) have been extensively utilized as ligands for transition metals. NHC are generally more stable than two extreme types of carbenes, the Fischer and the Schrock carbenes. They are good ? donors like most tertiary phosphins, PR3, but the ?-bonding with the metal is rather weak. The thriving studies of NHC-coordinated metal complexes produced a wide range of applications from homogeneous catalysts to materials science

2010-06-01

20

Synthesis and Characterization of Ag(I) and Pd(II) Complexes with a Pyridine Substituted N-Heterocyclic Carbene Ligand  

Energy Technology Data Exchange (ETDEWEB)

We have used our new tridentate pyridine substituted N-heterocyclic carbene to generate an interesting trinuclear [((MepyCH{sub 2}){sub 2}-Im){sub 3}Ag{sub 3}]{sup 3+} complex, displaying very short Ag-Ag separations. A Pd(II)-NHC complex was prepared from [((MepyCH{sub 2}){sub 2}-Im){sub 3}Ag{sub 3}]{sup 3+} via a facile transmetallation, leading to a dimeric [(MepyCH{sub 2}){sub 2}-ImPdCl]{sub 2}{sup 2+} complex. Future plans are underway for the survey of the potential applications of these new NHC complexes as luminesent materials or homogeneous catalysts. Since Arduengo's discovery of the first isolable free carbene in 1991, N-heterocyclic carbenes (NHC) have been extensively utilized as ligands for transition metals. NHC are generally more stable than two extreme types of carbenes, the Fischer and the Schrock carbenes. They are good ? donors like most tertiary phosphins, PR{sub 3}, but the ?-bonding with the metal is rather weak. The thriving studies of NHC-coordinated metal complexes produced a wide range of applications from homogeneous catalysts to materials science.

Kim, Ga Young; Jung, Hyun Jin; Lee, Dong Heon [Chonbuk National Univ., Jeonju (Korea, Republic of); Park, Gyung Se [Kunsan National Univ., Kunsan (Korea, Republic of)

2010-06-15

 
 
 
 
21

N-Heterocyclic Carbene Complexes in Arylation Reactions other than Cross-couplings  

Science.gov (United States)

This chapter highlights the use of N-heterocyclic carbenes as supporting ligands in arylation reactions different than the more common cross-coupling reactions, including C-F bond activation, catalytic arylation, homocoupling, direct arylation and oxidative Heck reactions.

Berini, Christophe; Navarro, Oscar

22

Theoretical study on the effect of annelation and carbonylation on the electronic and ligand properties of N-heterocyclic silylenes and germylenes: carbene comparisons begin to break down.  

Science.gov (United States)

Quantum chemical calculations have been carried out to investigate the effect of annelation and carbonylation on the electronic and ligand properties of N-heterocyclic silylenes and germylenes. The thermodynamic stability of these ligands has been found to increase with annelation, while the reverse is true for carbonylation. This is in sharp contrast to N-heterocyclic carbenes (NHCs) where annelation leads to a decrease in their thermodynamic stabilities. Compared to nonannelated derivatives, annelated and carbonylated ones are found to be weaker ? donors but better ? acceptors. The effect of carbonylation is more pronounced than annelation toward increasing the ? acidity of these ligands. Carbonylation at the ?-position with respect to the N atom attached to the Si/Ge center has been found to be the most effective way of enhancing the ? acidity of these ligands. The computed natural charges reveal that electrophilicity increases upon both annelation and carbonylation. The calculated values of (31)P NMR chemical shifts of corresponding phosphinidene adducts of these ligands have been found to correlate well with the ? acidity of these Si/Ge centers. PMID:24738711

Guha, Ankur Kanti; Phukan, Ashwini K

2014-05-01

23

Mechanisms and origins of switchable regioselectivity of palladium- and nickel-catalyzed allene hydrosilylation with N-heterocyclic carbene ligands: a theoretical study.  

Science.gov (United States)

The mechanisms and origins for the Pd- and Ni-catalyzed regioselective hydrosilylation of allene have been investigated by means of density functional theory (DFT) calculations. The free-energy profiles of Pd- and Ni-catalyzed reactions with small and bulky N-heterocyclic carbene (NHC) ligands are calculated to determine the mechanism for regioselectivities. The calculation results show that different metals (Ni vs Pd) lead to regiochemical reversals for the hydrosilylation of allene. The allylsilane is the major product via palladium catalysis with small NHC ligand, while the vinylsilane is the major product via nickel catalysis with bulky NHC ligand. Both electronic and steric factors play a key role in the regioselectivities for the hydrosilylation of allene via Pd and Ni catalysts. The calculation results are in good agreement with observed regioselectivities and could provide insights into the design of new catalysts for the regioselectivity of hydrosilylation reactions. PMID:24779730

Xie, Hujun; Zhao, Lijiang; Yang, Liu; Lei, Qunfang; Fang, Wenjun; Xiong, Chunhua

2014-05-16

24

N-heterocyclic carbene bonding to cobalt porphyrin complexes  

Digital Repository Infrastructure Vision for European Research (DRIVER)

N-heterocyclic carbene (NHC) coordination to a cobalt(III) center embedded in a porphyrin scaffold has been accomplished by decarboxylation from N,N’-dimethylimidazolium-2-carboxylate in the presence of Co(TPP)Cl (TPP = 5,10,15,20-tetraphenylporphyrin). The distal chloride ligand in the resulting complexes Co(NHC)(TPP)Cl was successfully substituted with imidazoles and alcohols. Single crystal X-ray diffraction of the latter complexes Co(NHC)(TPP)(ROH) (R = Me, Et) revealed a pronounced ruf...

Albrecht, Martin; Maji, Pathik; Ha?usl, Christina; Monney, Ange?le; Mu?ller-bunz, Helge

2012-01-01

25

N-Heterocyclic Carbene Complexes in Oxidation Reactions  

Science.gov (United States)

This chapter describes applications of N-heterocyclic carbenes (NHCs) in oxidation chemistry. The strong ?-donation capabilities of the NHCs allow an efficient stabilisation of metal centres in high oxidation states, while high metal-NHC bond dissociation energies suppress their oxidative decomposition. These properties make NHCs ideal ligands for oxidation processes. The first part of this chapter is dedicated to the reactivity of NHC-metal complexes towards molecular oxygen whilst the second half highlights all oxidation reactions catalysed by such complexes. These include oxidation of alcohols and olefins, oxidative cyclisations, hydrations of alkynes and nitriles, oxidative cleavage of alkenes and the oxidation of methane.

Jur?ík, Václav; Cazin, Catherine S. J.

26

Influence of pyrazolate vs N-heterocyclic carbene ligands on the slow magnetic relaxation of homoleptic trischelate lanthanide(III) and uranium(III) complexes.  

Science.gov (United States)

Two isostructural series of trigonal prismatic complexes, M(Bp(Me))3 and M(Bc(Me))3 (M = Y, Tb, Dy, Ho, Er, U; [Bp(Me)](-) = dihydrobis(methypyrazolyl)borate; [Bc(Me)](-) = dihydrobis(methylimidazolyl)borate) are synthesized and fully characterized to examine the influence of ligand donor strength on slow magnetic relaxation. Investigation of the dynamic magnetic properties reveals that the oblate electron density distributions of the Tb(3+), Dy(3+), and U(3+) metal ions within the axial ligand field lead to slow relaxation upon application of a small dc magnetic field. Significantly, the magnetization relaxation is orders of magnitude slower for the N-heterocyclic carbene complexes, M(Bc(Me))3, than for the isomeric pyrazolate complexes, M(Bp(Me))3. Further, investigation of magnetically dilute samples containing 11-14 mol % of Tb(3+), Dy(3+), or U(3+) within the corresponding Y(3+) complex matrix reveals thermally activated relaxation is favored for the M(Bc(Me))3 complexes, even when dipolar interactions are largely absent. Notably, the dilute species U(Bc(Me))3 exhibits Ueff ? 33 cm(-1), representing the highest barrier yet observed for a U(3+) molecule demonstrating slow relaxation. Additional analysis through lanthanide XANES, X-band EPR, and (1)H NMR spectroscopies provides evidence that the origin of the slower relaxation derives from the greater magnetic anisotropy enforced within the strongly donating N-heterocyclic carbene coordination sphere. These results show that, like molecular symmetry, ligand-donating ability is a variable that can be controlled to the advantage of the synthetic chemist in the design of single-molecule magnets with enhanced relaxation barriers. PMID:24650296

Meihaus, Katie R; Minasian, Stefan G; Lukens, Wayne W; Kozimor, Stosh A; Shuh, David K; Tyliszczak, Tolek; Long, Jeffrey R

2014-04-23

27

N-Heterocyclic carbene metal complexes: photoluminescence and applications.  

Science.gov (United States)

This review covers the advances made in the synthesis of luminescent transition metal complexes containing N-heterocyclic carbene (NHC) ligands. The presence of a high field strength ligand such as an NHC in the complexes gives rise to high energy emissions, and consequently, to the desired blue colour needed for OLED applications. Furthermore, the great versatility of NHC ligands for structural modifications, together with the use of other ancillary ligands in the complex, provides numerous possibilities for the synthesis of phosphorescent materials, with emission colours over the entire visible spectra and potential future applications in fields such as photochemical water-splitting, chemosensors, dye-sensitised solar cells, oxygen sensors, and medicine. PMID:24604135

Visbal, Renso; Gimeno, M Concepción

2014-05-21

28

Synthesis and Characterization of Divalent Manganese, Iron, and Cobalt Complexes in Tripodal Phenolate/N-Heterocyclic Carbene Ligand Environments  

DEFF Research Database (Denmark)

Two novel tripodal ligands, (BIMPNMes,Ad,Me)â?? and (MIMPNMes,Ad,Me)2â??, combining two types of donor atoms, namely, NHC and phenolate donors, were synthesized to complete the series of N-anchored ligands, ranging from chelating species with tris(carbene) to tris(phenolate) chelating arms. The complete ligand series offers a convenient way of tuning the electronic and steric environment around the metal center, thus, allowing for control of the complexâ??s reactivity. This series of divalent complexes of Mn, Fe, and Co was synthesized and characterized by 1H NMR, IR, and UV/vis spectroscopy as well as by single-crystal X-ray diffraction studies. Variable-temperature SQUID magnetization measurements in the range from 2 to 300 K confirmed high-spin ground states for all divalent complexes and revealed a trend of increasing zero-field splitting |D| from Mn(II), to Fe(II), to Co(II) complexes. Zero-field 57Fe MoÌ?ssbauer spectroscopy of the Fe(II) complexes 3, 4, 8, and 11 shows isomer shifts δ that increase gradually as carbenes are substituted for phenolates in the series of ligands. From the single-crystal structure determinations of the complexes, the different steric demand of the ligands is evident. Particularly, the molecular structure of 1î?¸in which a pyridine molecule is situated next to the Mnâ??Cl bondî?¸and those of azide complexes 2, 4, and 6 demonstrate the flexibility of these mixed-ligand derivatives, which, in contrast to the corresponding symmetrical TIMENR ligands, allow for side access of, e.g., organic substrates, to the reactive metal center.

Mossin, Susanne

2014-01-01

29

Dioxygen adducts of rhodium N-heterocyclic carbene complexes.  

Science.gov (United States)

Rhodium complexes functionalized by N-heterocyclic carbene ligands react with dioxygen to form adducts. Depending on the specifics of the ancillary ligands, oxygen binds to Rh either as a peroxide to form a fully oxidized Rh(III) complex, or as singlet dioxygen in a Rh(I) square planar complex. We have shown through analysis of a series of compounds, some previously published and some novel, that the presence of additional ligands that would support the formation of an octahedral geometry, as typically found with Rh(III) complexes, is critical for formation of the peroxide. In addition, we have demonstrated through DFT studies, that the potential energy surface with regard to the O-O bond length is relatively shallow, which provides a rationale for the distribution of bond lengths observed for apparently similar complexes analyzed by crystallography. PMID:23440044

Keske, Eric C; Zenkina, Olena V; Asadi, Ali; Sun, Hongsui; Praetorius, Jeremy M; Allen, Daryl P; Covelli, Danielle; Patrick, Brian O; Wang, Ruiyao; Kennepohl, Pierre; James, Brian R; Crudden, Cathleen M

2013-05-28

30

N-Heterocyclic Carbene Complexes in Cyclisation Reactions  

Science.gov (United States)

Cycloaddition reactions hold a prominent place in the arsenal of synthetic methods currently available to organic chemists. Although some cycloadditions can be promoted simply by heat, light, Lewis acids, high pressure or sonication, many cycloadditions require the use of a transition metal to catalyse the reaction. Transition metal catalysed cycloaddition is an attractive approach as it provides an opportunity to obtain regio- and/or enantio-pure cyclic products. The use of N-heterocyclic carbenes (NHCs) as ligands is a rapidly expanding field. As will be discussed in this chapter, these ligands have been used in conjunction with several metals in the synthesis of a wide range of carbocycles and heterocycles through cycloaddition reactions.

Louie, Janis

31

Alkyne hydroarylation with Au N-heterocyclic carbene catalysts  

Directory of Open Access Journals (Sweden)

Full Text Available Mono- and dinuclear gold complexes with N-heterocyclic carbene (NHC ligands have been employed as catalysts in the intermolecular hydroarylation of alkynes with simple unfunctionalised arenes. Both mono- and dinuclear gold(III complexes were able to catalyze the reaction; however, the best results were obtained with the mononuclear gold(I complex IPrAuCl. This complex, activated with one equivalent of silver tetrafluoroborate, exhibited under acidic conditions at room temperature much higher catalytic activity and selectivity compared to more commonly employed palladium(II catalysts. Moreover, the complex was active, albeit to a minor extent, even under neutral conditions, and exhibited lower activity but higher selectivity compared to the previously published complex AuCl(PPh3. Preliminary results on intramolecular hydroarylations using this catalytic system indicate, however, that alkyne hydration by traces of water may become a serious competing reaction.

Cristina Tubaro

2013-02-01

32

Tuning the luminescent properties of Pt(II) acetylide complexes through varying the electronic properties of N-heterocyclic carbene ligands.  

Science.gov (United States)

This Article reports the synthesis, structural characterization, electrochemistry, and photophysical investigations of five groups of luminescent Pt(II) alkynyl complexes bearing N-heterocyclic carbene (NHC) ligands with varying electronic properties. Complexes of the type [Pt(pmdb)(C?CR)2] 1a-c, [Pt(pm2tz)(C?CR)2] 2a-d, [Pt(pm3tz)(C?CR)2] 3a-c, [Pt(ppim)(C?CR)2] 4(a, b, e), and [Pt(ppbim)(C?CR)2] 5(a, b, e), where pmdb =1,1'-dipentyl-3,3'-methylene-dibenzimidazoline-2,2'-diylidene, pm2tz = 1,1'-dipentyl-3,3'-methylene-di-1,2,4-triazoline-5,5'-diylidene, pm3tz = 1,1'-dipentyl-3,3'-methylene-di-1,3,4-triazoline-5,5'-diylidene, ppim = 3-pentyl-1-picolylimidazoline-2-ylidene, and ppbim = 3-pentyl-1-picolylbenzimidazoline-2-ylidene, and R = 4-C6H4F, C6H5, 4-C6H4OMe, SiMe3, and 4-C6H4N(C6H5)2, were prepared, and the consequences of the electronic properties of the NHC ligands on the phosphorescent emission efficiencies were studied. Moreover, the emission quantum efficiencies of the previously reported complexes [Pt(pmim)(C?CR)2] where pmim = 1,1'-dipentyl-3,3'-methylene-diimidazoline-2,2'-diylidene and R = 4-C6H4F 6a, C6H5 6b, and 4-C6H4OMe 6c were also recorded in neat solid and in 10 wt % PMMA film. The square planar coordination geometry with the alkynyl ligands in cis configuration was corroborated for selected complexes by single crystal X-ray diffraction studies. The observed moderate difference in emission efficiencies of the bis-carbene complexes 6a-c, 1a-c, 2a-c, and 3a-c in conjunction with the decreasing electron-donating nature of the NHC ligands, pmim > pmdb > pm2tz ? pm3tz, can be attributed to the slight modification of the triplet emission parentage among the different complexes. The quantum efficiencies of complexes 4(a, b) and 5(a, b) bearing pyridyl-NHC ligand were significantly low in comparison to the bis-carbene complexes owing to the significant change in the charge transfer character of the triplet manifold. Complexes 4e and 5e bearing diarylamine phenylacetylenes display high ?em of 27% and 33% in 10 wt % PMMA film, respectively. PMID:24392802

Zhang, Yuzhen; Clavadetscher, Jessica; Bachmann, Michael; Blacque, Olivier; Venkatesan, Koushik

2014-01-21

33

An Electronic Rationale for Observed Initiation Rates in Ruthenium-Mediated Olefin Metathesis: Charge Donation in Phosphine And N-Heterocyclic Carbene Ligands  

Energy Technology Data Exchange (ETDEWEB)

Ru K-edge XAS data indicate that second generation ruthenium-based olefin metathesis precatalysts (L = N-heterocyclic carbene) possess a more electron-deficient metal center than in the corresponding first generation species (L = tricyclohexylphosphine). This surprising effect is also observed from DFT calculations and provides a simple rationale for the slow phosphine dissociation kinetics previously noted for second-generation metathesis precatalysts.

Getty, K.; Delgado-Jaime, M.U.; Kennepohl, P.

2009-06-01

34

N-Heterocyclic Carbene-Catalyzed Conjugate Additions of Alcohols  

Digital Repository Infrastructure Vision for European Research (DRIVER)

An efficient intermolecular conjugate addition of alcohols to activated alkenes catalyzed by N-heterocyclic carbenes has been developed. With 5 mol % of the free carbene derived from IMes•HCl, unsaturated ketones and esters are competent substrates and a variety of primary and secondary alcohols can be employed as the nucleophile. No oligomerization is observed under these mild conditions for effective hydroalkoxylation. In addition to reactions with activated alkenes, IMes catalyzes the fo...

Phillips, Eric M.; Riedrich, Matthias; Scheidt, Karl A.

2010-01-01

35

Beyond catalysis: N-heterocyclic carbene complexes as components for medicinal, luminescent, and functional materials applications  

Digital Repository Infrastructure Vision for European Research (DRIVER)

This tutorial review compiles the advances that have been achieved in using transition metal complexes containing N-heterocyclic carbene ligands as components for materials. Applications of metal carbene complexes in fields different from catalysis are remarkably scarce. During the last few years, promising results have been accomplished in particular by utilizing such complexes as antimicrobial and cytotoxic agents, as photoactive sites in luminescent materials, for self-assembly into liquid...

Mercs, Laszlo; Albrecht, Martin

2010-01-01

36

N-Heterocyclic Carbene Complexes in Industrial Processes  

Science.gov (United States)

N-Heterocyclic carbene complexes produced on industrial scale are presented in this chapter along with a discussion about their production. Details of processes employing NHC complexes on pilot to industrial scales are discussed. These are frequently oriented towards the synthesis of biologically active molecules, however, examples are given for rubber formation and for 1-octene synthesis, a comonomer for polyethylene synthesis.

Briel, Oliver; Cazin, Catherine S. J.

37

Synthesis and Properties of Chelating N-Heterocyclic Carbene Rhodium(I) Complexes: Synthetic Experiments in Current Organometallic Chemistry  

Science.gov (United States)

The preparation and characterization of two air-stable Rh(I) complexes bearing a chelating N-heterocyclic carbene (NHC) ligand is described. The synthesis involves the preparation of a Ag(I)-NHC complex and its use as carbene transfer agent to a Rh(I) precursor. The so obtained complex can be further reacted with carbon monoxide to give the…

Mata, Jose A.; Poyatos, Macarena; Mas-Marza, Elena

2011-01-01

38

N-Heterocyclic Carbene Complexes in Additions to Multiple Bonds  

Science.gov (United States)

The use of N-heterocyclic carbene (NHC) complexes as homogeneous catalysts in addition reactions across carbon-carbon double and triple bonds and carbon-heteroatom double bonds is described. The discussion is focused on the description of the catalytic systems, their current mechanistic understanding and occasionally the relevant organometallic chemistry. The reaction types covered include hydrogenation, transfer hydrogenation, hydrosilylation, hydroboration and diboration, hydroamination, hydrothiolation, hydration, hydroarylation, allylic substitution, addition, chloroesterification and chloroacylation.

Danopoulos, Andreas A.

39

SambVca: A Web Application for the Calculation of the Buried Volume of N-Heterocyclic Carbene Ligands  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We present a free web application for the calculation of the buried volume (% V(Bur)) of NHC ligands. The web application provides a graphic and user-friendly interface to the SambVca program, developed for the calculation of % V(Bur) values not only of NHC ligands but also of other classic organometallic ligands such as, for example, phosphanes and cyclopentadienyl-based ligands. To provide a reliable procedure for the calculation of % V(Bur) values we tested our approach in the interpretati...

2009-01-01

40

Control of emission colour with N-heterocyclic carbene (NHC) ligands in phosphorescent three-coordinate Cu(i) complexes.  

Science.gov (United States)

A series of three phosphorescent mononuclear (NHC)-Cu(i) complexes were prepared and characterized. Photophysical properties were found to be largely controlled by the NHC ligand chromophore. Variation of the NHC ligand leads to emission colour tuning over 200 nm range from blue to red, and emission efficiencies of 0.16-0.80 in the solid state. PMID:24853355

Krylova, Valentina A; Djurovich, Peter I; Conley, Brian L; Haiges, Ralf; Whited, Matthew T; Williams, Travis J; Thompson, Mark E

2014-06-10

 
 
 
 
41

Homobimetallic Ruthenium-N-Heterocyclic Carbene Complexes For Olefin Metathesis  

Science.gov (United States)

In this chapter, the synthesis and catalytic activity towards olefin metathesis of homobimetallic ruthenium (Ru)-alkylidene, -cyclodiene or -arene complexes bearing phosphine or N-heterocyclic carbene (NHC) ligands are reviewed. Emphasis is placed on the last category of bimetallic compounds. Three representatives of this new type of molecular scaffold were investigated. Thus, [(p-cymene)Ru(m-Cl)3RuCl (h2-C2H4)(L)] complexes with L = PCy3 (15a), IMes (16a), or IMesCl2 (16b) were prepared. They served as catalyst precursors for cross-metathesis (CM) of various styrene derivatives. These experiments revealed the outstanding aptitude of complex 16a (and to a lesser extent of 16b) to catalyze olefin metathesis reactions. Contrary to monometallic Ru-arene complexes of the [RuCl2(p-cymene)(L)] type, the new homobimetallic species did not require the addition of a diazo compound nor visible light illumination to initiate the ring-opening metathesis of norbornene or cyclooctene. When diethyl 2,2-diallylmalonate and N,N-diallyltosylamide were exposed to 16a,b, a mixture of cycloisomerization and ring-closing metathesis (RCM) products was obtained in a nonselective way. Addition of phenylacetylene enhanced the metathetical activity while completely repressing the cycloisomerization process.

Sauvage, Xavier; Demonceau, Albert; Delaude, Lionel

42

Cyclopentadienyl-functionalised N-heterocyclic carbenes: synthesis, coordination to Mo, Ru, Rh  

Digital Repository Infrastructure Vision for European Research (DRIVER)

This thesis deals with the synthesis of cyclopentadienyl-functionalised N-heterocyclic carbenes and its coordination to both middle and late transition metals. One of the goals was to gain chemical knowledge on the reactivity patterns of these complexes, and explore their potential applications in catalysis. The imidazolium salts synthesised in the course of this thesis represent a series containing changes in the electronic and steric parameters. The ligand precursors we...

Costa, Andre? Pontes Da

2011-01-01

43

N-donor functionalized N-heterocyclic carbene nickel(II) complexes in the Kumada coupling  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The synthesis and characterization of novel nickel(II) complexes bearing two bidentate N-heterocyclic carbene ligands functionalized with anionic N-donor moieties are described. Two different N-donor groups are employed, namely amido and benzimidazolato moieties. The solid-state structures of three of these complexes have been determined by X-ray crystallography. The amido-functionalized low-spin, square-planar Ni(II) complexes exhibit a cis geometry around the metal centre, while...

Berding, Joris; Dijkman, Thomas F.; Lutz, M.; Spek, A. L.; Bouwman, E.

2009-01-01

44

N-heterocyclic carbene catalyzed intramolecular acylation of allylic electrophiles.  

Science.gov (United States)

The N-heterocyclic carbene (NHC) catalyzed addition reaction has been well documented recently; however, the NHC-catalyzed substitution reaction especially the SN2' type reaction remains a challenge. As one of the most fundamental reaction types in organic chemistry, the SN2' reaction catalyzed by NHC would be a powerful tool in organic synthesis. Therefore, the first NHC-catalyzed intramolecular SN2' substitution reaction of aldehyde with allylic electrophiles has been developed. A variety of ?,?-unsaturated chromanones were obtained under a domino SN2' reaction and isomerization. Mechanistic experiments were conducted to confirm the nature of this SN2' reaction. PMID:24847954

Zhao, Ming; Yang, Hui; Li, Miao-Miao; Chen, Jie; Zhou, Ling

2014-06-01

45

A Tungsten Complex with a Bidentate, Hemilabile N-Heterocyclic Carbene Ligand, Facile Displacement of the Weakly Bound W-(C=C) Bond, and the Vulnerability of the NHC Ligand Toward Catalyst Deactivation During Ketone Hydrogenation  

Energy Technology Data Exchange (ETDEWEB)

The initial reaction observed between N-heterocyclic carbene IMes (IMes = 1,3-bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene) and molybdenum and tungsten hydride complexes CpM(CO)2(PPh3)H (M = Mo, W) is deprotonation of the metal hydride by IMes, giving [(IMes)H]+[CpM(CO)2(PPh3)]– . At longer reaction times and higher temperatures, the reaction of IMes with CpM(CO)2(PR3)H (M = Mo, W; R = Me, Ph) produces CpM(CO)2(IMes)H. Hydride transfer from CpW(CO)2(IMes)H to Ph3C+B(C6F5)4- gives CpW(CO)2(IMes)+B(C6F5)4- which was crystallographically characterized using x-ray radiation from a synchrotron. The IMes is bonded as a bidentate ligand, through the carbon of the carbene as well as forming a weak bond from the metal to a C=C bond of one mesityl ring. The weakly bound C=C ligand is hemilabile, being readily displaced by H2, THF, ketones or alcohols. Reaction of CpW(CO)2(IMes)+ with H2 gives the dihydride complex [CpW(CO)2(IMes)(H)2]+. Addition of Et2CH–OH to CpW(CO)2(IMes)+B(C6F5)4- gives the alcohol complex [CpM(CO)2(IMes)(Et2CH–OH)]+[B(C6F5)4]– which was characterized by crystallography and exhibits no evidence for hydrogen bonding of the bound OH group. Addition of H2 to the ketone complex [CpW(CO)2(IMes)(Et2C=O)]+[B(C6F5)4]– produces an equilibrium with the dihydride [CpW(CO)2(IMes)(H)2]+ (Keq = 1.1 ? 103 at 25 °C). The tungsten ketone complex [CpW(CO)2(IMes)(Et2C=O)]+[B(C6F5)4]– serves as a modest catalyst for hydrogenation of Et2C=O to Et2CH–OH in neat ketone solvent. Decomposition of the catalyst produces [H(IMes)]+B(C6F5)4-, indicating that these catalysts with N-heterocyclic carbenes ligands are vulnerable to decomposition by a reaction that produces a protonated imidazolium cation.

Wu, Fan; Dioumaev, Vladimir K.; Szalda, David J.; Hanson, Jonathan; Bullock, R. Morris

2007-09-24

46

Comproportionation of Cationic and Anionic Tungsten Complexes Having an N-Heterocyclic Carbene Ligand to Give the Isolable 17-Electron Tungsten Radical, CpW(CO)2(IMes)•  

Energy Technology Data Exchange (ETDEWEB)

A series consisting of a tungsten anion, radical and cation, supported by the N-heterocyclic carbene IMes (1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) and spanning formal oxidation states W(0), W(I) and W(II), has been synthesized, isolated, and characterized. Reaction of the hydride CpW(CO)2(IMes)H with KH and 18 crown 6 gives the tungsten anion [CpW(CO)2(IMes)]-[K(18 crown 6)]+. The crystal structure of this complex shows that the K+ interacts not only with the oxygen atoms in the crown ether, but also with the carbonyl oxygens. The electrochemical oxidation of [CpW(CO)2(IMes)]- in acetonitrile is fully reversible (E½ = ?1.65 V vs Cp2Fe+•/0) at all scan rates, indicating that CpW(CO)2(IMes)• is a persistent radical. Hydride transfer from CpW(CO)2(IMes)H to Ph3C+PF6 affords [cis-CpW(CO)2(IMes)(MeCN)]+PF6 . Comproportionation of [CpW(CO)2(IMes)]- with [CpW(CO)2(IMes)(MeCN)]+ gives the 17-electron tungsten radical CpW(CO)2(IMes)•. This complex shows paramagnetically shifted resonances in 1H NMR spectra and has been characterized by IR spectroscopy, low-temperature EPR spectroscopy, and X-ray diffraction. CpW(CO)2(IMes)• is very stable with respect to disproportionation and dimerization. NMR studies of degenerate electron transfer between CpW(CO)2(IMes)• and [CpW(CO)2(IMes)]- are reported. DFT calculations were carried out on CpW(CO)2(IMes)H, as well as on related complexes bearing NHC ligands with N,N´ substituents Me [CpW(CO)2(IMe)H] or H [CpW(CO)2(IH)H] to compare to the experimentally studied IMes complexes with mesityl substituents. These calculations reveal W H homolytic bond dissociation energies (BDEs) to decrease with increasing steric bulk of the NHC ligand, from 67 for CpW(CO)2(IH)H to 64 for CpW(CO)2(IMe)H to 63 kcal/mol for CpW(CO)2(IMes)H. The calculated spin density at W for CpW(CO)2(IMes)• is 0.63. The W radicals CpW(CO)2(IMe)• and CpW(CO)2(IH)• are calculated to form weak W W bonds. The weakly bonded complexes [CpW(CO)2(IMe)]2 and [CpW(CO)2(IH)]2, are predicted to have W-W BDEs of 6 and 18 kcal/mol, respectively, and to dissociate readily to the W-centered radicals CpW(CO)2(IMe)• and CpW(CO)2(IH)•. This work was supported by the US Department of Energy Basic Energy Sciences' Chemical Sciences, Geosciences & Biosciences Division. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.

Roberts, John A.; Franz, James A.; van der Eide, Edwin F.; Walter, Eric D.; Petersen, Jeffrey L.; DuBois, Daniel L.; Bullock, R. Morris

2011-09-21

47

Copolymerization of cyclohexene oxide with CO2 catalyzed by tridentate N-heterocyclic carbene titanium(IV) complexes.  

Science.gov (United States)

A new class of complexes based on titanium(IV) bearing a bisanionic mer-tridentate N-heterocyclic carbene ligand were investigated for the copolymerization of cyclohexene oxide with CO2. Upon addition of [PPN]X' salts, all complexes were found to be active and highly selective toward the formation of poly(cyclohexene oxide-alt-carbon dioxide). PMID:24292280

Quadri, Coralie C; Le Roux, Erwan

2014-03-21

48

N-Heterocyclic carbene functionalized goup 7-9 transition metal  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The N-heterocyclic carbene (NHC) functionalized phosphinidene complexes [(pCy)(IiPr2Me2)- RudPMes*] (4), [(pCy)(IiPr2Me2)OsdPMes*] (6), and [(Cp*)(IiPr2Me2)RhdPMes*] (7) were generated by a double-dehydrohalogenation-ligation sequence of the corresponding primary phosphine complexes with 3 equiv of NHC. The effect of the NHC ligand on the electronic properties of the phosphinidene complexes [(Ring)(NHC)MdPH] (8-16), bearing group 7-9 transition metals and cycloheptatrienyl (Cht...

Aktas, H.; Slootweg, J. C.; Ehlers, A. W.; Lutz, M.; Spek, A. L.; Lammertsma, K.

2009-01-01

49

N-heterocyclic carbene catalyzed nucleophilic substitution reaction for construction of benzopyrones and benzofuranones.  

Science.gov (United States)

N-Heterocyclic carbene as an efficient organic catalyst was employed to catalyze an intramolecular nucleophilic substitution reaction. When R(2) was a phenyl group, the cyclization process underwent isomerization, leading to generation of benzofuranone. PMID:16986969

He, Jinmei; Zheng, Jiyue; Liu, Jian; She, Xuegong; Pan, Xinfu

2006-09-28

50

Luminescence Studies of the Intracellular Distribution of a Dinuclear Gold(I) N-Heterocyclic Carbene Complex  

Energy Technology Data Exchange (ETDEWEB)

A bidentate cyclophane N-heterocyclic carbene ligand has been used to synthesize a new dinuclear Au{sup I} complex of the formula [Au{sub 2}L{sub 2}]{sup 2+}. The short Au {hor_ellipsis} Au distance imposed by the rigid cyclophane ligand leads to a red-shifted luminescence profile that enables the complex to be used as a luminescent probe for distribution studies in single living cancer cells.

Barnard, Peter J.; Wedlock, Louise E.; Baker, Murray V.; Berners-Price, Susan J.; Joyce, David A.; Skelton, Brian W.; Steer, James H. (UWA)

2008-10-03

51

Structural diversity of copper(I)-N-heterocyclic carbene complexes; ligand tuning facilitates isolation of the first structurally characterised copper(I)-NHC containing a copper(I)-alkene interaction.  

Science.gov (United States)

The preparation of a series of imidazolium salts bearing N-allyl substituents, and a range of substituents on the second nitrogen atom that have varying electronic and steric properties, is reported. The ligands have been coordinated to a copper(I) centre and the resulting copper(I)-NHC (NHC=N-heterocyclic carbene) complexes have been thoroughly examined, both in solution and in the solid-state. The solid-state structures are highly diverse and exhibit a range of unusual geometries and cuprophilic interactions. The first structurally characterised copper(I)-NHC complex containing a copper(I)-alkene interaction is reported. An N-pyridyl substituent, which forms a dative bond with the copper(I) centre, stabilises an interaction between the metal centre and the allyl substituent of a neighbouring ligand, to form a 1D coordination polymer. The stabilisation is attributed to the pyridyl substituent increasing the electron density at the copper(I) centre, and thus enhancing the metal(d)-to-alkene(?*) back-bonding. In addition, components other than charge transfer appear to have a role in copper(I)-alkene stabilisation because further increases in the Lewis basicity of the ligand disfavours copper(I)-alkene binding. PMID:24203461

Lake, Benjamin R M; Willans, Charlotte E

2013-12-01

52

A binuclear gold(i) complex with mixed bridging diphosphine and bis(n-heterocyclic carbene) ligands shows favorable thiol reactivity and inhibits tumor growth and angiogenesis in?vivo.  

Science.gov (United States)

In the design of anticancer gold(I) complexes with high in?vivo efficacy, tuning the thiol reactivity to achieve stability towards blood thiols yet maintaining the thiol reactivity to target cellular thioredoxin reductase (TrxR) is of pivotal importance. Herein we describe a dinuclear gold(I) complex (1-PF6 ) utilizing a bridging bis(N-heterocyclic carbene) ligand to attain thiol stability and a diphosphine ligand to keep appropriate thiol reactivity. Complex 1-PF6 displays a favorable stability that allows it to inhibit TrxR activity without being attacked by blood thiols. In?vivo studies reveal that 1-PF6 significantly inhibits tumor growth in mice bearing HeLa xenograft and mice bearing highly aggressive mouse B16-F10 melanoma. It inhibits angiogenesis in tumor models and inhibits sphere formation of cancer stem cells in?vitro. Toxicology studies indicate that 1-PF6 does not show systemic anaphylaxis on guinea pigs and localized irritation on rabbits. PMID:24729298

Zou, Taotao; Lum, Ching Tung; Lok, Chun-Nam; To, Wai-Pong; Low, Kam-Hung; Che, Chi-Ming

2014-06-01

53

Preparation of a N-Heterocyclic Carbene Nickel(II) Complex: Synthetic Experiments in Current Organic and Organometallic Chemistry  

Science.gov (United States)

A four-part experiment that leads to the synthesis of a cyclopentadienyl chloro-nickel(II) complex bearing a N-heterocyclic carbene (NHC) ligand is presented. In the first part, the preparation of 1,3-bis-(2,4,6-trimethylphenyl)imidazolium chloride (IMes[middle dot]HCl) in a one-pot procedure by reaction of 2,4,6-trimethylaniline with…

Ritleng, Vincent; Brenner, Eric; Chetcuti, Michael J.

2008-01-01

54

Antimicrobial Properties of Some Bis(Iminoacenaphthene (BIAN-Supported N-Heterocyclic Carbene Complexes of Silver and Gold  

Directory of Open Access Journals (Sweden)

Full Text Available The AgCl, AgOAc, AuCl, and AuOAc complexes of the new bis(iminoacenaphthene(BIAN-supported N-heterocyclic carbene ligand and the precursor imidazolium salt have been investigated with respect to their antimicrobial activities against Staphylococcus aureus, Bacillus subtilis, Escherichia coli and Psudomonas aeruginosa. The most active antimicrobial is the precursor imidazolium salt, which has a minimum inhibitory concentration (MIC value of

Rachel R. Butorac

2011-03-01

55

Isolation of a C5-Deprotonated Imidazolium, a Crystalline “Abnormal” N-Heterocyclic Carbene  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The discovery two decades ago of metal-free stable carbenes, especially imidazol-2-ylidenes [N-heterocyclic carbenes (NHCs)], has led to numerous breakthroughs in organic and organometallic catalysis. More recently, a small range of complexes has been prepared in which alternative NHC isomers, namely imidazol-5-ylidenes (also termed abnormal NHCs or aNHCs, because the carbene center is no longer located between the two nitrogens), coordinate to a transition metal. Here we report the synthesis...

Aldeco-perez, Eugenia; Rosenthal, Amos J.; Donnadieu, Bruno; Parameswaran, Pattiyil; Frenking, Gernot; Bertrand, Guy

2009-01-01

56

Synthesis and antimicrobial studies of silver N-heterocyclic carbene complexes bearing a methyl benzoate substituent  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Due to the properties of silver as an antimicrobial, our research group has synthesized many different silver carbene complexes. Two new silver N-heterocyclic carbene complexes derived from 4,5-dichloroimidazole and theobromine bearing methyl benzoate substituents were synthesized by in situ carbene formation using silver acetate as the base in the reaction. The new compounds were fully characterized by several methods including NMR spectroscopy and X-ray crystallography. Preliminary antimicr...

Knapp, Amanda R.; Panzner, Matthew J.; Medvetz, Doug A.; Wright, Brian D.; Tessier, Claire A.; Youngs, Wiley J.

2010-01-01

57

The Potential of N-Heterocyclic Carbene Complexes as Components for Electronically Active Materials  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The application of N-heterocyclic carbene complexes as active sites in materials other than catalysis has been remarkably scarce. Inspired by the — often misleading — ‘carbene’ label, which implies a substantial degree of M=C ? bonding, we have been interested in evaluating the potential of N-heterocylclic carbene complexes as building blocks for constructing electronically active materials. Electron mobility via the metal-carbon bond has been investigated in monometallic imidazol-2-...

Schuster, Oliver; Mercs, Laszlo; Albrecht, Martin

2010-01-01

58

Ultra stable self-assembled monolayers of N-heterocyclic carbenes on gold  

Science.gov (United States)

Since the first report of thiol-based self-assembled monolayers (SAMs) 30 years ago, these structures have been examined in a huge variety of applications. The oxidative and thermal instabilities of these systems are widely known, however, and are an impediment to their widespread commercial use. Here, we describe the generation of N-heterocyclic carbene (NHC)-based SAMs on gold that demonstrate considerably greater resistance to heat and chemical reagents than the thiol-based counterparts. This increased stability is related to the increased strength of the gold–carbon bond relative to that of a gold–sulfur bond, and to a different mode of bonding in the case of the carbene ligand. Once bound to gold, NHCs are not displaced by thiols or thioethers, and are stable to high temperatures, boiling water, organic solvents, pH extremes, electrochemical cycling above 0 V and 1% hydrogen peroxide. In particular, benzimidazole-derived carbenes provide films with the highest stabilities and evidence of short-range molecular ordering. Chemical derivatization can be employed to adjust the surface properties of NHC-based SAMs.

Crudden, Cathleen M.; Horton, J. Hugh; Ebralidze, Iraklii I.; Zenkina, Olena V.; McLean, Alastair B.; Drevniok, Benedict; She, Zhe; Kraatz, Heinz-Bernhard; Mosey, Nicholas J.; Seki, Tomohiro; Keske, Eric C.; Leake, Joanna D.; Rousina-Webb, Alexander; Wu, Gang

2014-05-01

59

N-Heterocyclic carbene/Brønsted acid cooperative catalysis as a powerful tool in organic synthesis  

Directory of Open Access Journals (Sweden)

Full Text Available The interplay between metals and N-heterocyclic carbenes (NHCs has provided a window of opportunities for the development of novel catalytic strategies within the past few years. The recent successful combination of Brønsted acids with NHCs has added a new dimension to the field of cooperative catalysis, enabling the stereoselective synthesis of functionalized pyrrolidin-2-ones as valuable scaffolds in heterocyclic chemistry. This Commentary will briefly highlight the concept of N-heterocyclic carbene/Brønsted acid cooperative catalysis as a new and powerful methodology in organic chemistry.

Rob De Vreese

2012-03-01

60

Mild and rational synthesis of palladium complexes comprising C(4)-bound N-heterocyclic carbenes  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Oxidative addition of pyridyl-functionalised 4-iodoimidazolium salts to palladium(0) gives catalytically active complexes in which the N-heterocyclic carbene is bound to the palladium(II) centre in a non-classical bonding mode via C(4).

Kluser, Evelyne; Neels, Antonia; Albrecht, Martin

2007-01-01

 
 
 
 
61

Bifunctional N-Heterocyclic Carbene-Catalyzed Highly Enantioselective Synthesis of Spirocyclic Oxindolo-?-lactams.  

Science.gov (United States)

The N-heterocyclic carbene-catalyzed Staudinger reaction of ketenes with isatin-derived ketimines was investigated. The bifunctional NHCs with a free hydroxyl group were demonstrated as efficient catalysts for the reaction, giving the corresponding spirocyclic oxindolo-?-lactams in high yields with excellent diastereo- and enantioselectivities. PMID:24856000

Zhang, Han-Ming; Gao, Zhong-Hua; Ye, Song

2014-06-01

62

Formal Diels-Alder reactions of chalcones and formylcyclopropanes catalyzed by chiral N-heterocyclic carbenes.  

Science.gov (United States)

Highly enantioselective (formal) hetero-Diels-Alder reactions between chalcones and formylcyclopropanes are disclosed. The challenging N-heterocyclic carbene (NHC)-bounded enolate intermediates from formylcyclopropanes were captured for new C-C bond forming reactions. The reaction products were obtained with high diastereo- and enantioselectivities and could be easily transformed to optically pure multisubstituted cyclohexane derivatives. PMID:21877756

Lv, Hui; Mo, Junming; Fang, Xinqiang; Chi, Yonggui Robin

2011-10-01

63

N-heterocyclic carbene-catalyzed enantioselective annulation of indolin-3-ones with bromoenals.  

Science.gov (United States)

N-Heterocyclic carbene-catalyzed reactions of indolin-3-ones with 2-bromoenals opened an asymmetric access to 3,4-dihydropyrano[3,2-b]indol-2(5?H)-ones in good yields and with good to excellent enantioselectivities. This protocol tolerates a broad substrate scope. In addition, a possible mechanism for the annulation reaction is presented. PMID:24729588

Ni, Qijian; Song, Xiaoxiao; Raabe, Gerhard; Enders, Dieter

2014-06-01

64

Backbone tuning in indenylidene–ruthenium complexes bearing an unsaturated N-heterocyclic carbene  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The steric and electronic influence of backbone substitution in IMes-based (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) N-heterocyclic carbenes (NHC) was probed by synthesizing the [RhCl(CO)2(NHC)] series of complexes to quantify experimentally the Tolman electronic parameter (electronic) and the percent buried volume (%V

2010-01-01

65

Catalytically active palladium pyridylidene complexes: pyridinium ionic liquids as N-heterocyclic carbene precursors  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Pyridinium salts similar to those used as ionic liquid solvents readily undergo palladation in the presence of a base, thus giving palladium-pyridylidene N-heterocyclic carbene complexes that are active catalysts for Suzuki-type cross-coupling reactions.

Albrecht, Martin; Stoeckli-evans, Helen

2006-01-01

66

The photochemistry of rhenium(I) tricarbonyl N-heterocyclic carbene complexes.  

Science.gov (United States)

The photophysical and photochemical properties of the new tricarbonyl rhenium(I) complexes bound to N-heterocyclic carbene ligands (NHC), fac-[Re(CO)3(N^C)X] (N^C = 1-phenyl-3-(2-pyridyl)imidazole or 1-quinolinyl-3-(2-pyridyl)imidazole; X = Cl or Br), are reported. The photophysics of these complexes highlight phosphorescent emission from triplet metal-to-ligand ((3)MLCT) excited states, typical of tricarbonyl rhenium(I) complexes, with the pyridyl-bound species displaying a ten-fold shorter excited state lifetime. On the other hand, these pyridyl-bound species display solvent-dependent photochemical CO dissociation following what appear to be two different mechanisms, with a key step being the formation of cationic tricarbonyl solvato-complexes, being themselves photochemically active. The photochemical mechanisms are illustrated with a combination of NMR, IR, UV-Vis, emission and X-ray structural characterization techniques, clearly demonstrating that the presence of the NHC ligand is responsible for the previously unobserved photochemical behavior in other photoactive tricarbonyl rhenium(I) species. The complexes bound to the quinolinyl-NHC ligand (which possess a lower-energy (3)MLCT) are photostable, suggesting that the photoreactive excited state is not any longer thermally accessible. The photochemistry of the pyridyl complexes was investigated in acetonitrile solutions and also in the presence of triethylphosphite, showing a competing and bifurcated photoreactivity promoted by the trans effect of both the NHC and phosphite ligands. PMID:23939232

Vaughan, Jamila G; Reid, Brodie L; Ramchandani, Sushil; Wright, Phillip J; Muzzioli, Sara; Skelton, Brian W; Raiteri, Paolo; Brown, David H; Stagni, Stefano; Massi, Massimiliano

2013-10-21

67

Zincocene and dizincocene N-heterocyclic carbene complexes and catalytic hydrogenation of imines and ketones.  

Science.gov (United States)

The N-heterocyclic carbene (NHC) adducts Zn(Cp(R) )2 (NHC)] (Cp(R) =C5 HMe4 , C5 H4 SiMe3 ; NHC=ItBu, IDipp (Dipp=2,6-diisopropylphenyl), IMes (Mes=mesityl), SIMes) were prepared and shown to be active catalysts for the hydrogenation of imines, whereas decamethylzincocene [ZnCp*2 ] is highly active for the hydrogenation of ketones in the presence of noncoordinating NHCs. The abnormal carbene complex [Zn(OCHPh2 )2 (aItBu)]2 was formed from spontaneous rearrangement of the ItBu ligand during incomplete hydrogenation of benzophenone. Two isolated Zn(I) adducts [Zn2 Cp*2 (NHC)] (NHC=ItBu, SIMes) are presented and characterized as weak adducts on the basis of (13) C?NMR spectroscopic and X-ray diffraction experiments. A mechanistic proposal for the reduction of [ZnCp*2 ] with H2 to give [Zn2 Cp*2 ] is discussed. PMID:24861200

Jochmann, Phillip; Stephan, Douglas W

2014-07-01

68

Dehydrogenative Synthesis of Imines from Alcohols and Amines Catalyzed by a Ruthenium N-Heterocyclic Carbene Complex  

Digital Repository Infrastructure Vision for European Research (DRIVER)

A new method for the direct synthesis of imines from alcohols and amines is described where hydrogen gas is liberated. The reaction is catalyzed by the ruthenium N-heterocyclic carbene complex [RuCl2(IiPr)(p-cymene)] in the presence of the ligand DABCO and molecular sieves. The imination can be applied to a variety of primary alcohols and amines and can be combined with a subsequent addition reaction. A deuterium labeling experiment indicates that the catalytically active species is a rutheni...

Maggi, Agnese; Madsen, Robert

2011-01-01

69

Room-temperature hydrohydrazination of terminal alkynes catalyzed by saturated abnormal N-heterocyclic carbene-gold(i) complexes.  

Science.gov (United States)

A number of saturated abnormal N-heterocyclic carbene (NHC) complexes of gold, in combination with KBAr(F) 4 as activator, were successfully applied in the chemoselective addition of hydrazine to alkynes. The reaction proceeds even at room temperature, which was not possible to date with gold catalysts. The reaction can be applied to a number of substituted arylalkynes. With alkylalkynes the yields are low. The saturated abnormal NHC ligands are resistant to isomerization to the saturated normal NHC coordination mode under basic reaction conditions. Under acidic conditions, a simple protonation at the nitrogen atom not neighboring the carbene center was observed and unambiguously characterized by an X-ray crystal-structure analysis. Computational studies confirm that such an isomerization would be highly exothermic, the observed kinetic stability probably results from the need to shift two protons in such a process. PMID:24771293

Manzano, Rubén; Wurm, Thomas; Rominger, Frank; Hashmi, A Stephen K

2014-06-01

70

The hydrosilylation of alkenes and alkynes catalyzed by N-heterocyclic carbene platinum(0) complexes  

Digital Repository Infrastructure Vision for European Research (DRIVER)

This PhD deals with the fundamental understanding of the hydrosilylation of alkene and alkynes catalyzed by N-heterocyclic carbene platinum(0) complexes ((NHC)Pt(dvtms)). These catalysts have been found to be uniquely selective for the hydrosilylation of alkenes and yield low amounts of isomerized alkene byproducts (< 3%). The detailed mechanistic study of the mode of action of (NHC)Pt(dvtms) complexes has revealed that the high selectivity observed is due to their capability to act as stable...

Berthon-gelloz, Guillaume

2007-01-01

71

Polyaminoborane main chain scission using N-heterocyclic carbenes; formation of donor-stabilised monomeric aminoboranes.  

Science.gov (United States)

The reaction of N-heterocyclic carbenes with polyaminoboranes [MeNH-BH2]n or [NH2-BH2]n at 20 °C led to depolymerisation and the formation of labile, monomeric aminoborane-NHC adducts, RNH-BH2-NHC (R = Me or H); a similar NHC adduct of Ph2N=BCl2 was characterized by single crystal X-ray diffraction. PMID:23982163

Stubbs, Naomi E; Jurca, Titel; Leitao, Erin M; Woodall, Christopher H; Manners, Ian

2013-10-14

72

On the Mechanism of N-Heterocyclic Carbene-Catalyzed Reactions Involving Acyl Azoliums  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Catalytic reactions promoted by N-heterocyclic carbenes (NHCs) have exploded in popularity since 2004 when several reports described new fundamental reactions that extended beyond the long-studied generation of aryl anion equivalents. These new NHC-catalyzed reactions allow chemists to generate unique reactive species from otherwise inert starting materials, all under simple, mild reaction conditions and with exceptional selectivities. In analogy to transition metal catalysis, the use of NHCs...

2014-01-01

73

Alkyne-Azide Cycloaddition Catalyzed by Silver Chloride and "Abnormal" Silver N-Heterocyclic Carbene Complex  

Science.gov (United States)

A library of 1,2,3-triazoles was synthesized from diverse alkynes and azides using catalytic amounts of silver chloride instead of copper compounds. In addition, a novel “abnormal” silver N-heterocyclic carbene complex was tested as catalyst in this process. The results suggest that the reaction requires only 0.5% of silver complex, affording 1,2,3-triazoles in good yields.

Ortega-Arizmendi, Aldo I.; Aldeco-Perez, Eugenia; Cuevas-Yanez, Erick

2013-01-01

74

Cascade olefin isomerization/intramolecular diels-alder reaction catalyzed by N-heterocyclic carbenes.  

Science.gov (United States)

The addition of an N-heterocyclic carbene to the carbonyl group of an ?,?,?,?-unsaturated enol ester affords a hemiacetal azolium intermediate that enables a cascade olefin isomerization/Diels-Alder reaction, for which mechanistic studies implicate Lewis base catalysis. Preliminary studies into the utility of the products have been undertaken with reductive and oxidative cleavage, giving materials for potential use in complex-target synthesis. PMID:24706584

Kowalczyk, Marcin; Lupton, David W

2014-05-19

75

Zwitterionic borane adducts of N-heterocyclic carbenes from mesomeric betaines of uracil.  

Science.gov (United States)

We prepared a series of imidazolium-substituted uracil-anions which are members of the class of cross-conjugated heterocyclic mesomeric betaines. They are in tautomeric equilibrium with their N-heterocyclic carbenes, uracil-6-yl-imidazol-2-ylidenes. These carbenes can be trapped by reaction with sulfur, selenium, as well as by triethylborane and triphenylborane, respectively. The latter trapping reaction yielded the first representatives of a new heterocyclic zwitterionic ring system, imidazo[2',1':3,4][1,4,2]diazaborolo[1,5-c]pyrimidinium-10-ide. Results of two single crystal X-ray structure analyses are presented. PMID:24658502

Zhang, Jiaxi; Pidlypnyi, Nazar; Nieger, Martin; Namyslo, Jan C; Schmidt, Andreas

2014-05-01

76

N-Heterocyclic carbenes (NHCs) as organocatalysts and structural components in metal-free polymer synthesis.  

Science.gov (United States)

The chemistry of N-heterocyclic carbenes (NHCs) has witnessed tremendous development in the past two decades: NHCs have not only become versatile ligands for transition metals, but have also emerged as powerful organic catalysts in molecular chemistry and, more recently, in metal-free polymer synthesis. To understand the success of NHCs, this review first presents the electronic properties of NHCs, their main synthetic methods, their handling, and their reactivity. Their ability to activate key functional groups (e.g. aldehydes, esters, heterocycles, silyl ketene acetals, alcohols) is then discussed in the context of molecular chemistry. Focus has been placed on the activation of substrates finding analogies with monomers (e.g. bis-aldehydes, multi-isocyanates, cyclic esters, epoxides, N-carboxyanhydrides, etc.) and/or initiators (e.g. hydroxy- or trimethylsilyl-containing reagents) employed in such "organopolymerisation" reactions utilizing NHCs. A variety of metal-free polymers, including aliphatic polyesters and polyethers, poly(?-peptoid)s, poly(meth)acrylates, polyurethanes, or polysiloxanes can be obtained in this way. The last section covers the use of NHCs as structural components of the polymer chain. Indeed, NHC-based photoinitiators, chain transfer agents or functionalizing agents, as well as bifunctional NHC monomer substrates, can also serve for metal-free polymer synthesis. PMID:23288304

Fèvre, Maréva; Pinaud, Julien; Gnanou, Yves; Vignolle, Joan; Taton, Daniel

2013-03-01

77

Two Fe3(?3-S)2(CO)8 clusters with terminal N-heterocyclic carbenes.  

Science.gov (United States)

The title compounds with terminal N-heterocyclic carbenes, namely octacarbonyl(imidazolidinylidene-?C(2))di-?3-sulfido-triiron(II)(2 Fe-Fe), [Fe3(C3H6N2)(?3-S)2(CO)8], (I), and octacarbonyl(1-methylimidazo[1,5-a]pyridin-3-ylidene-?C(3))di-?3-sulfido-triiron(II)(2 Fe-Fe), [Fe3(C8H8N2)(?3-S)2(CO)8], (II), have been synthesized. Each compound contains two Fe-Fe bonds and two S atoms above and below a triiron triangle. One of the eight carbonyl ligands deviates significantly from linearity. In (I), dimers generated by an N-H...S hydrogen bond are linked into [001] double chains by a second N-H...S hydrogen bond. These chains are packed by a C-H...O hydrogen bond to yield [101] sheets. In (II), dimers generated by an N-H...S hydrogen bond are linked by C-H...O hydrogen bonds to form [111] double chains. PMID:24898951

Yang, Wei; Fu, Qiang; Zhao, Jing; Cheng, Huan Ren; Shi, Yao Cheng

2014-06-15

78

Photophysical and photochemical trends in tricarbonyl rhenium(I) N-heterocyclic carbene complexes.  

Science.gov (United States)

A family of tricarbonyl Re(I) complexes of the formulation fac-[Re(CO)3(NHC)L] has been synthesized and characterized, both spectroscopically and structurally. The NHC ligand represents a bidentate N-heterocyclic carbene species where the central imidazole ring is substituted at the N3 atom by a butyl, a phenyl, or a mesityl group and substituted at the N1 atom by a pyridyl, a pyrimidyl, or a quinoxyl group. On the other hand, the ancillary L ligand alternates between chloro and bromo. For the majority of the complexes, the photophysical properties suggest emission from the lowest triplet metal-to-ligand charge transfer states, which are found partially mixed with triplet ligand-to-ligand charge transfer character. The nature and relative energy of the emitting states appear to be mainly influenced by the identity of the substituent on the N3 atom of the imidazole ring; thus, the pyridyl complexes have blue-shifted emission in comparison to the more electron deficient pyrimidyl complexes. The quinoxyl complexes show an unexpected blue-shifted emission, possibly occurring from ligand-centered excited states. No significant variations were found upon changing the substituent on the imidazole N3 atom and/or the ancillary ligand. The photochemical properties of the complexes have also been investigated, with only the complexes bound to the pyridyl-substituted NHC ligands showing photoinduced CO dissociation upon excitation at 370 nm, as demonstrated by the change in the IR and NMR spectra as well as a red shift in the emission profile after photolysis. Temperature-dependent photochemical experiments show that CO dissociation occurs at temperatures as low as 233 K, suggesting that the Re-C bond cleaves from excited states of metal-to-ligand charge transfer nature rather than thermally activated ligand field excited states. A photochemical mechanism that takes into account the reactivity of the complexes bound to the pyridyl-NHC ligand as well as the stability of those bound to the pyrimidyl- and quinoxyl-NHC ligands is proposed. PMID:24665819

Vaughan, Jamila G; Reid, Brodie L; Wright, Phillip J; Ramchandani, Sushil; Skelton, Brian W; Raiteri, Paolo; Muzzioli, Sara; Brown, David H; Stagni, Stefano; Massi, Massimiliano

2014-04-01

79

Covalency in Ce(IV) and U(IV) halide and N-heterocyclic carbene bonds.  

Science.gov (United States)

Oxidative halogenation with trityl chloride provides convenient access to Ce(IV) and U(IV) chloroamides [M(N{SiMe(3)}(2))(3)Cl] and their N-heterocyclic carbene derivatives, [M(L)(N{SiMe(3)}(2))(2)Cl] (L = OCMe(2)CH(2)(CNCH(2)CH(2)NDipp) Dipp = 2,6-iPr(2)C(6)H(3)). Computational analysis of the bonding in these and a fluoro analogue, [U(L)(N{SiMe(3)}(2))(2)F], provides new information on the covalency in this relative rare oxidation state for molecular cerium complexes. Computational studies reveal increased Mayer bond orders in the actinide carbene bond compared with the lanthanide carbene bond, and natural and atoms-in-molecules analyses suggest greater overall ionicity in the cerium complexes than in the uranium analogues. PMID:20658507

Arnold, Polly L; Turner, Zoë R; Kaltsoyannis, Nikolas; Pelekanaki, Panagiota; Bellabarba, Ronan M; Tooze, Robert P

2010-08-16

80

CH-? and CF-? interactions lead to structural changes of N-heterocyclic carbene palladium complexes.  

Science.gov (United States)

The role of CH-? and CF-? interactions in determining the structure of N-heterocyclic carbene (NHC) palladium complexes were studied using (1) H?NMR spectroscopy, X-ray crystallography, and DFT calculations. The CH-? interactions led to the formation of the cis-anti isomers in 1-aryl-3-isopropylimidazol-2-ylidene-based [(NHC)2 PdX2 ] complexes, while CF-? interactions led to the exclusive formation of the cis-syn isomer of diiodobis(3-isopropyl-1-pentafluorophenylimidazol-2-ylidene) palladium(II). PMID:24382838

Xu, Xiangya; Pooi, Benjamin; Hirao, Hajime; Hong, Soon Hyeok

2014-01-27

 
 
 
 
81

Backbone tuning in indenylidene–ruthenium complexes bearing an unsaturated N-heterocyclic carbene  

Directory of Open Access Journals (Sweden)

Full Text Available The steric and electronic influence of backbone substitution in IMes-based (IMes = 1,3-bis(2,4,6-trimethylphenylimidazol-2-ylidene N-heterocyclic carbenes (NHC was probed by synthesizing the [RhCl(CO2(NHC] series of complexes to quantify experimentally the Tolman electronic parameter (electronic and the percent buried volume (%Vbur, steric parameters. The corresponding ruthenium–indenylidene complexes were also synthesized and tested in benchmark metathesis transformations to establish possible correlations between reactivity and NHC electronic and steric parameters.

César A. Urbina-Blanco

2010-11-01

82

N-Heterocyclic Carbene Complexes of Rh: Reaction With Dioxygen Without Oxidation  

Energy Technology Data Exchange (ETDEWEB)

The reaction of oxygen with rhodium complexes containing N-heterocyclic carbenes was found to give dioxygen complexes with rare square planar geometries and unusually short O-O bond lengths. Analysis of the bonding in these complexes by Rh L-edge X-ray absorption spectroscopy (XAS), Raman spectroscopy, and DFT calculations provides evidence for a bonding model in which singlet oxygen is bound to a Rh(I) d{sup 8} metal complex, rather than the more common Rh(III) d{sup 6} peroxo species with octahedral geometry and O-O bond lengths in the 1.4-1.5 {angstrom} range.

Praetorius, J.M.; Allen, D.P.; Wang, R.; Webb, J.D.; Grein, F.; Kennepohl, P.; Crudden, C.M.

2009-05-21

83

N-Heterocyclic Carbene Complexes in Olefin Metathesis  

Science.gov (United States)

Olefin metathesis is now a synthetic tool found ubiquitously in various fields involving synthesis. Of its many variations, three are prominently used: (1) catalytic ring closing metathesis (RCM) is an extremely powerful method for the construction of carbon-carbon double bonds in organic chemistry; (2) ring opening metathesis polymerisation (ROMP) where polymers are formed by use of the energy released from cyclic strain; and (3) cross metathesis (CM) where non-cyclic partners are coupled through C-C double bond formation. These important transformations and variations on these themes mediated by second generation ruthenium complexes bearing a NHC ligand will be presented in the following sections.

Luan, Xinjun; Dorta, Reto; Leitgeb, Anita; Slugovc, Christian; Tiede, Sascha; Blechert, Siegfried

84

N-heterocyclic carbene and phosphine complexes of osmium and ruthenium carbonyls  

Digital Repository Infrastructure Vision for European Research (DRIVER)

1,3-bis-(2,4,6-trimethylphenyl)imidazol-2-ylidene-silver(I)chloride ([IMes]AgCl), a carbene transfer agent, was employed in the reaction with various osmium clusters. Reaction of ([IMes]AgCl) with [Os3(µ-H)2(CO)10] gave [Os3(µ-H)(µ-Cl)(CO)9(IMes)] (1), an osmium N-heterocyclic carbene complex. The reaction of ([IMes]AgCl) with the cluster [Os3(CO)10(CH3CN)2] yielded two products, [Os3(µ-Cl)(CO)10(µ-Ag(IMes)] (2) and [(IMes-H)][Os3(µ-Cl)(CO)10](µ4-Ag)[Os3(µ-Cl)(CO)10] (3). Compound (2)...

Cooke, Craig E.

2008-01-01

85

High-throughput screening of metal-N-heterocyclic carbene complexes against biofilm formation by pathogenic bacteria.  

Science.gov (United States)

A set of molecules including a majority of metal-N-heterocyclic carbene (NHC) complexes (metal=Ag, Cu, and Au) and azolium salts were evaluated by high-throughput screening of their activity against biofilm formation associated with pathogenic bacteria. The anti-planktonic effects were compared in parallel. Representative biofilm-forming strains of various genera were selected (Listeria, Pseudomonas, Staphylococcus, and Escherichia). All the compounds were tested at 1 mg L(-1) by using the BioFilm Ring Test. An information score (IS, sum of the activities) and an activity score (AS, difference between anti-biofilm and anti-planktonic activity) were determined from normalized experimental values to classify the most active molecules against the panel of bacterial strains. With this method we identified lipophilic Ag(I) and Cu(I) complexes possessing aromatic groups on the NHC ligand as the most efficient at inhibiting biofilm formation. PMID:24729552

Bernardi, Thierry; Badel, Stéphanie; Mayer, Pascal; Groelly, Jérome; de Frémont, Pierre; Jacques, Béatrice; Braunstein, Pierre; Teyssot, Marie-Laure; Gaulier, Christelle; Cisnetti, Federico; Gautier, Arnaud; Roland, Sylvain

2014-06-01

86

Oxyanion steering and CH-? interactions as key elements in an N-heterocyclic carbene-catalyzed [4 + 2] cycloaddition.  

Science.gov (United States)

The N-heterocyclic carbene catalyzed [4 + 2] cycloaddition has been shown to give ?,?-unsaturated ?-lactones in excellent enantio- and diastereoselectivity. However, preliminary computational studies of the geometry of the intermediate enolate rendered ambiguous both the origins of selectivity and the reaction pathway. Here, we show that a concerted, but highly asynchronous, Diels-Alder reaction occurs rather than the stepwise Michael-type or Claisen-type pathways. In addition, two crucial interactions are identified that enable high selectivity: an oxyanion-steering mechanism and a CH-? interaction. The calculations accurately predict the enantioselectivity of a number of N-heterocyclic carbene catalysts in the hetero-Diels-Alder reaction. PMID:22765294

Allen, Scott E; Mahatthananchai, Jessada; Bode, Jeffrey W; Kozlowski, Marisa C

2012-07-25

87

Efficient electronic communication of two ruthenium centers through a rigid ditopic N-heterocyclic carbene linker  

Digital Repository Infrastructure Vision for European Research (DRIVER)

A ditopic benzobis(carbene) ligand precursor was prepared that contained a chelating pyridyl moiety to ensure co-planarity of the carbene ligand and the coordination plane of a bound octahedral metal center. Bimetallic ruthenium complexes comprising this ditopic ligand [L?Ru-C,N-bbi-C,N-RuL?] were obtained by a transmetalation methodology (C,N-bbi-C,N=benzobis(N-pyridyl-N?-methyl-imidazolylidene). ...

Nussbaum, Michael; Schuster, Oliver; Albrecht, Martin

2014-01-01

88

Synthesis of Well-Defined Copper "N"-Heterocyclic Carbene Complexes and Their Use as Catalysts for a "Click Reaction": A Multistep Experiment that Emphasizes the Role of Catalysis in Green Chemistry  

Science.gov (United States)

A multistep experiment for an advanced synthesis lab course that incorporates topics in organic-inorganic synthesis and catalysis and highlights green chemistry principles was developed. Students synthesized two "N"-heterocyclic carbene ligands, used them to prepare two well-defined copper(I) complexes and subsequently utilized the complexes as…

Ison, Elon A.; Ison, Ana

2012-01-01

89

One-pot synthesis and immobilisation of sulfonate-tethered N-heterocyclic carbene complexes on polycationic dendrimers  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Easy does it! Anion-tethered N-heterocyclic carbene metal complexes (metal=AuI, RhI) were efficiently synthesised and immobilised on a polycationic dendrimer by an in situ transmetallation-immobilisation reaction, leading to discrete non-covalent metallodendritic assemblies

Virboul, M. A. N.; Lutz, M.; Siegler, M. A. M.; Spek, A. L.; Koten, G.; Klein Gebbink, R. J. M.

2009-01-01

90

Efficient synthesis of benzofuranones: N-heterocyclic carbene (NHC)/base-catalyzed hydroacylation-Stetter-rearrangement cascade.  

Science.gov (United States)

A N-heterocyclic carbene/base-catalyzed cascade reaction leading to the formation of functionalized benzofuranones is reported. The reaction proceeds via an intramolecular hydroacylation of unactivated alkynes followed by an intermolecular Stetter reaction and a base-catalyzed chromanone to benzofuranone rearrangement. PMID:21919536

Padmanaban, Mohan; Biju, Akkattu T; Glorius, Frank

2011-10-21

91

A short and modular synthesis of bulky and electron-rich N-phosphinomethyl-functionalised N-heterocyclic carbene complexes.  

Science.gov (United States)

Bulky and electron-rich N-phosphinomethyl-substituted N-heterocyclic carbene transition metal complexes have been prepared in a short and efficient protocol. The modular synthesis allows one to convert borane-protected phosphino-functionalized imidazolium salts into their corresponding metal chelate complexes in a one-pot procedure. PMID:23903289

Brill, Marcel; Kühnel, Erik; Scriban, Corina; Rominger, Frank; Hofmann, Peter

2013-09-28

92

Functionalization of benzylic C(sp(3))-H bonds of heteroaryl aldehydes through N-heterocyclic carbene organocatalysis.  

Science.gov (United States)

Aryl aldehyde activation: Oxidative activation of 2-methylindole-3-carboxaldehyde (I) through N-heterocyclic carbene (NHC) organocatalysis generates heterocyclic ortho-quinodimethane (II) as a key intermediate. This intermediate then undergoes formal [4+2] cycloaddition with trifluoromethyl ketones or isatins to form polycyclic lactones containing a quaternary carbon center. PMID:24038663

Chen, Xingkuan; Yang, Song; Song, Bao-An; Chi, Yonggui Robin

2013-10-11

93

Pd-N-Heterocyclic Carbene (NHC) Organic Silica: Synthesis and Application in Carbon-Carbon Coupling Reactions  

Science.gov (United States)

The first Pd-N-heterocyclic carbene (NHC) complex in the form of organic silica was prepared using sol-gel method and its application in Heck and Suzuki reaction were demonstrated. These C-C coupling reactions proceeded efficiently under the influence of microwave irradiation, wi...

94

Stereoelectronic effects in C-h bond oxidation reactions of ni(i) N-heterocyclic carbene complexes.  

Science.gov (United States)

Activation of O2 by the three-coordinate Ni(I) ring-expanded N-heterocyclic carbene complexes Ni(RE-NHC)(PPh3)Br (RE-NHC = 6-Mes, 1; 7-Mes, 2) produced the structurally characterized dimeric Ni(II) complexes Ni(6-Mes)(Br)(?-OH)(?-O-6-Mes')NiBr (3) and Ni(7-Mes)(Br)(?-OH)(?-O-7-Mes')NiBr (4) containing oxidized ortho-mesityl groups from one of the carbene ligands. NMR and mass spectrometry provided evidence for further oxidation in solution to afford bis-?-aryloxy compounds; the 6-Mes derivative was isolated, and its structure was verified. Low-temperature UV-visible spectroscopy showed that the reaction between 1 and O2 was too fast even at ca. -80 °C to yield any observable intermediates and also supported the formation of more than one oxidation product. Addition of O2 to Ni(I) precursors containing a less electron-donating diamidocarbene (6-MesDAC, 7) or less bulky 6- or 7-membered ring diaminocarbene ligands (6- or 7-o-Tol; 8 and 9) proceeded quite differently, affording phosphine and carbene oxidation products (Ni(O?PPh3)2Br2 and (6-MesDAC)?O) and the mononuclear Ni(II) dibromide complexes (Ni(6-o-Tol)(PPh3)Br2 (10) and (Ni(7-o-Tol)(PPh3)Br2 (11)) respectively. Electrochemical measurements on the five Ni(I) precursors show significantly higher redox potentials for 1 and 2, the complexes that undergo oxygen atom transfer from O2. PMID:24972307

Poulten, Rebecca C; López, Isidoro; Llobet, Antoni; Mahon, Mary F; Whittlesey, Michael K

2014-07-21

95

Dimerisation, rhodium complex formation and rearrangements of N-heterocyclic carbenes of indazoles  

Science.gov (United States)

Summary Deprotonation of indazolium salts at low temperatures gives N-heterocyclic carbenes of indazoles (indazol-3-ylidenes) which can be trapped as rhodium complexes (X-ray analysis). In the absence of Rh, the indazol-3-ylidenes spontaneously dimerize under ring cleavage of one of the N,N-bonds and ring closure to an indazole–indole spiro compound which possesses an exocyclic imine group. The E/Z isomers of the imines can be separated by column chromatography when methanol is used as eluent. We present results of a single crystal X-ray analysis of one of the E-isomers, which equilibrate in solution as well as in the solid state. Heating of the indazole–indole spiro compounds results in the formation of quinazolines by a ring-cleavage/ring-closure sequence (X-ray analysis). Results of DFT calculations are presented.

Guan, Zong; Namyslo, Jan C; Drafz, Martin H H; Nieger, Martin

2014-01-01

96

The Crystallographic and Quantum Mechanical Analysis of some Pd(II) N-heterocyclic carbene Complexes  

International Nuclear Information System (INIS)

Because of their extraordinary properties, N-heterocyclic carbenes (NHC) have found access to a great variety of catalytic processes which include C-C coupling reactions, formation of furans, cyclopropanation, olefin metathesis, hydroformylation, polymerization and hydrosilylation reactions. In this study, molecular and crystal structures of Pd(II) NHC complexes have been determined by single crystal x-ray diffraction technique. In addition, molecular geometries of all complexes under study were optimized at the B3LYP level of density functional theory (DFT) and the effective core potentials of Hay and Wadt with LanL2DZ basis set were used. In order to investigate binding orbitals of metal and charge transfer mechanism occurred in NHC ring, natural bond orbital (NBO) analyses were performed at the B3LYP/LanL2DZ level on the basis of the optimized ground state for complexes

2008-08-25

97

Structure and bonding of three-coordinate N-heterocyclic carbene nickel nitrosyl complexes: Theoretical study  

Science.gov (United States)

The structure and bonding of the for C3N3H2X2Ni(Cp)NO (X = H, F, Cl, Br) and their linkage isomers C3N3H2X2Ni(Cp)ON has been studied by carrying out density functional theory. The bonding nature of NiC bonds has been further explored by means of AIM method and natural bond orbital (NBO) analysis. Nucleus-independent chemical shift ( NICS) values calculated at several points above ring center indicate aromaticity of heterocyclic cycle. Also, the effect of substitution (X = F, Cl, Br, CN) in N-heterocyclic carbene on the properties of complex has been shown.

Ghlasi, R.; Mokarram, E. E.

2011-07-01

98

?-Carbon activation of saturated carboxylic esters through N-heterocyclic carbene organocatalysis  

Science.gov (United States)

The activation of the ?-carbons of carboxylic esters and related carbonyl compounds to generate enolate equivalents as nucleophiles is one of the most powerful strategies in organic synthesis. We reasoned that the horizons of chemical synthesis could be greatly expanded if the typically inert ?-carbons of saturated esters could be used as nucleophiles. However, despite the rather significant fundamental and practical values, direct use of the ?-carbons of saturated carbonyl compounds as nucleophiles remains elusive. Here we report the catalytic activation of simple saturated ester ?-carbons as nucleophiles (?-carbon activation) using N-heterocyclic carbene organocatalysts. The catalytically generated nucleophilic ?-carbons undergo enantioselective reactions with electrophiles such as enones and imines. Given the proven rich chemistry of ester ?-carbons, we expect this catalytic activation mode for saturated ester ?-carbons to open a valuable new arena for new and useful reactions and synthetic strategies.

Fu, Zhenqian; Xu, Jianfeng; Zhu, Tingshun; Leong, Wendy Wen Yi; Chi, Yonggui Robin

2013-10-01

99

Nanofiber composites containing N-heterocyclic carbene complexes with antimicrobial activity  

Directory of Open Access Journals (Sweden)

Full Text Available Ahmed A Elzatahry1,4, Abdullah M Al-Enizi1, Elsayed Ahmed Elsayed2,5, Rachel R Butorac3, Salem S Al-Deyab1, Mohammad AM Wadaan2, Alan H Cowley31Petrochemical Research Chair, Department of Chemistry, 2Chair of Advanced Proteomics & Cytomics Research, Faculty of Science, King Saud University, Riyadh, Kingdom of Saudi Arabia; 3Department of Chemistry and Biochemistry, University of Texas at Austin, Austin, TX, USA; 4Institute of Advanced Technology and New Materials, City for Scientific Research and Technology Applications, New Borg Alrab, Alexandria, Egypt; 5Natural & Microbial Products Department, National Research Centre, Dokki, Cairo, EgyptAbstract: This report concerns nanofiber composites that incorporate N-heterocyclic carbenes and the use of such composites for testing antimicrobial and antifungal activities. The nanofiber composites were produced by electrospinning mixtures of the gold chloride or gold acetate complexes of a bis(iminoacenaphthene (BIAN-supported NHC with aqueous solutions of polyvinyl alcohol (PVA. The products were characterized by scanning-electron microscopy, which revealed that nanofibers in the range of 250–300 nm had been produced. The biological activities of the nanofiber composites were tested against two Gram-positive bacteria, six Gram-negative bacteria, and two fungal strains. No activity was evident against the fungal strains. However, the gold chloride complex was found to be active against all the Gram-positive pathogens and one of the Gram-negative pathogens. It was also found that the activity of the produced nanofibers was localized and that no release of the bioactive compound from the nanofibers was evident. The demonstrated antimicrobial activities of these novel nanofiber composites render them potentially useful as wound dressings.Keywords: nanofiber, electrospinning, N-Heterocyclic carbene, biopolymer, antimicrobial

Elzatahry AA

2012-06-01

100

Catalytic hydrogenation using abnormal N-heterocyclic carbene palladium complexes: catalytic scope and mechanistic insights  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Palladium complexes containing abnormally bound C4-bound dicarbene ligands have been exploited for catalytic alkene hydrogenation. Comparison to normally C2-bound homologues indicates that the carbene bonding mode critically influences the catalytic activity. Good catalytic performance in the hydrogenation of cis-disubstituted olefins and non-isomerizable terminal olefins under mild conditions (RT, 0.1?MPa H?) only occurs when the carbene is abnormally bound to the palladium cent...

Heckenroth, Marion; Khlebnikov, Vsevolod; Neels, Antonia; Schurtenberger, Peter; Albrecht, Martin

2011-01-01

 
 
 
 
101

Gold (I) N-heterocyclic carbene complex inhibits mouse melanoma growth by p53 upregulation  

Science.gov (United States)

Background Cancer treatment using gold (I) complexes is becoming popular. In this study, a gold (I) N-heterocyclic complex designated as complex 3 was synthesized, its cytotoxicity was examined, and its anti-melanoma activity was evaluated in vitro and in vivo. Methods Viability of cancer cells was determined by MTT assay upon treatment with various concentrations of a gold (I) N-heterocyclic carbene complex (complex 3) in a dose and time dependent manner. Mouse melanoma cells B16F10 were selected for further apoptotic studies, including flowcytometric analysis of annexin binding, cell cycle arrest, intracellular ROS generation and loss in the mitochondrial membrane potential. ELISA based assays were done for caspase activities and western blots for determining the expression of various survival and apoptotic proteins. Immunocytology was performed to visualize the translocation of p53 to the nucleus. B16F10 cells were inoculated into mice and post tumor formation, complex 3 was administered. Immunohistology was performed to determine the expressions of p53, p21, NF-?B (p65 and p50), MMP-9 and VEGF. Student’s t test was used for determining statistical significance. The survival rate data were analyzed by Kaplan-Meier plots. Results Complex 3 markedly inhibited the growth of HCT 116, HepG2, and A549, and induced apoptosis in B16F10 cells with nuclear condensation, DNA fragmentation, externalization of phosphatidylserine, activation of caspase 3 and caspase 9, PARP cleavage, downregulation of Bcl-2, upregulation of Bax, cytosolic cytochrome c elevation, ROS generation, and mitochondrial membrane potential loss indicating the involvement of an intrinsic mitochondrial death pathway. Further, upregulation of p53, p-p53 (ser 15) and p21 indicated the role of p53 in complex 3 mediated apoptosis. The complex reduced tumor size, and caused upregulation of p53 and p21 along with downregulation of NF-?B (p65 and p50), VEGF and MMP-9. These results suggest that it induced anti-melanoma effect in vitro and in vivo by modulating p53 and other apoptotic factors. Conclusions The gold (I) N-heterocyclic carbene complex (C22H26N6AuO2PF6) designated as complex 3 induced ROS and p53 dependent apoptosis in B16F10 cells involving the mitochondrial death pathway along with suppression of melanoma tumor growth by regulating the levels of pro and anti apoptotic factors (p53, p21, NF-?B, VEGF and MMP-9).

2014-01-01

102

Expedient syntheses of the N-heterocyclic carbene precursor imidazolium salts IPr·HCl, IMes·HCl and IXy·HCl  

Directory of Open Access Journals (Sweden)

Full Text Available Abstract The 1,3-diaryl-imidazolium chlorides IPr·HCl (aryl = 2,6-diisopropylphenyl, IMes·HCl (aryl = 2,4,6-trimethylphenyl and IXy·HCl (aryl = 2,6-dimethylphenyl, precursors to widely used N-heterocyclic carbene (NHC ligands and catalysts, were prepared in high yields (81%, 69% and 89%, respectively by the reaction of 1,4-diaryl-1, 4-diazabutadienes, paraformaldehyde and chlorotrimethylsilane in dilute ethyl acetate solution. A reaction mechanism involving a 1,5-dipolar electrocyclization is proposed.

Hintermann Lukas

2007-08-01

103

Expedient syntheses of the N-heterocyclic carbene precursor imidazolium salts IPr·HCl, IMes·HCl and IXy·HCl  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Abstract The 1,3-diaryl-imidazolium chlorides IPr·HCl (aryl = 2,6-diisopropylphenyl), IMes·HCl (aryl = 2,4,6-trimethylphenyl) and IXy·HCl (aryl = 2,6-dimethylphenyl), precursors to widely used N-heterocyclic carbene (NHC) ligands and catalysts, were prepared in high yields (81%, 69% and 89%, respectively) by the reaction of 1,4-diaryl-1, 4-diazabutadienes, paraformaldehyde and chlorotrimethylsilane in dilute ethyl acetate solution. A reaction mechanism involving a 1,5-dipolar ele...

Hintermann Lukas

2007-01-01

104

Zwitterionic dithiocarboxylates derived from N-heterocyclic carbenes: coordination to gold surfaces.  

Science.gov (United States)

The zwitterionic dithiocarboxylates 1(+)-CS(2)(-)-4(+)-CS(2)(-) were prepared by reacting the corresponding N-heterocyclic carbenes 1,3-bis(2,6-diisoproylphenyl)imidazol-2-ylidene (1), 1,3-diisopropylimidazol-2-ylidene (2), 1,3-dibenzylimidazol-2-ylidene (3) and 1,3-diethylbenzimidazol-2-ylidene (4) with CS(2). In the latter two cases, the corresponding N-heterocylic carbene was generated in situ. Compounds 2(+)-CS(2)(-)-4(+)-CS(2)(-) were structurally characterised by single-crystal X-ray diffraction studies. The chemisorption of these zwitterionic dithiocarboxylates on solid gold substrates was investigated in situ and in real time by optical second harmonic generation (SHG). The resulting thin films were exemplarily characterised by near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS) in the case of 1(+)-CS(2)(-) and 2(+)-CS(2)(-), revealing the formation of almost contamination-free self-assembled monolayers, which exhibit a remarkable degree of orientational order. PMID:22274745

Siemeling, Ulrich; Memczak, Henry; Bruhn, Clemens; Vogel, Florian; Träger, Frank; Baio, Joe E; Weidner, Tobias

2012-03-14

105

Nickel-Catalyzed Cycloisomerization of Enynes: Catalyst Generation via C-H Activation of Carbene Ligands  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The combination of Ni(0) and an N-heterocyclic carbene act as a precatalyst for the cycloisomerization of enynes to afford 1,3-dienes. During the course of the reaction, a nickel hydride is formed from oxidative addition of the ortho C-H on the carbene ligand. Deuteriumn labeling studies are presented.

Tekavec, Thomas N.; Louie, Janis

2008-01-01

106

N-Heterocyclic carbene–palladium(II)-1-methylimidazole complex catalyzed Mizoroki–Heck reaction of aryl chlorides with styrenes  

Digital Repository Infrastructure Vision for European Research (DRIVER)

A well-defined N-heterocyclic carbene–palladium(II)-1-methylimidazole [NHC-Pd(II)-Im] complex 1 was found to be an effective catalyst for the Mizoroki–Heck reaction of a variety of aryl chlorides with styrenes. Both activated and deactivated aryl chlorides work well to give the corresponding coupling products in good to excellent yields by using tetrabutylammonium bromide (TBAB) as the ionic liquid.

Gao, Ting-ting; Jin, Ai-ping; Shao, Li-xiong

2012-01-01

107

Investigation of the Role of Electrogenerated N-Heterocyclic Carbene in the Staudinger Synthesis in Ionic Liquid  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Electrogenerated N-heterocyclic carbene (NHC), obtained by cathodic reduction of Bmim-BF4, behaves as organocatalyst and base in the Staudinger synthesis from an acyl chloride and a deactivated imine in ionic liquid to yield ?-lactams. The effect of many parameters (temperature, amount of electricity, substituents, presence of an external base) has been evaluated and a tentative mechanism for the Staudinger synthesis in a very polar medium like the ionic liquid reported. The yields of isolate...

Marta Feroci

2011-01-01

108

Suzuki-Miyaura Reactions Catalyzed by C2-Symmetric Pd-Multi-Dentate N-Heterocyclic Carbene Complexes  

Directory of Open Access Journals (Sweden)

Full Text Available Suzuki-Miyaura coupling reactions are promoted by Pd complexes ligated with C2-symmetric multi-dentate N-heterocyclic carbenes derived in situ from Pd(OAc2 and imidazolium salts. Good to excellent yields were obtained for aryl bromides as substrates. Turnover numbers of up to 105 could be achieved with 5 × 10?4 mol% of Pd(OAc2/1 × 10?3 mol% NHC precatalyst in 24 h.

Zhengning Li

2012-10-01

109

N-Heterocyclic carbene-catalyzed direct cross-aza-benzoin reaction: Efficient synthesis of ?-amino-?-keto esters  

Directory of Open Access Journals (Sweden)

Full Text Available An efficient catalytic synthesis of ?-amino-?-keto esters has been newly developed. Cross-coupling of various aldehydes with ?-imino ester, catalyzed by N-heterocyclic carbene, leads chemoselectively to ?-amino-?-keto esters in moderate to good yields with high atom efficiency. The reaction mechanism is discussed, and it is proposed that the ?-amino-?-keto esters are formed under thermodynamic control.

Takuya Uno

2012-09-01

110

Enantioselective sulfonation of enones with sulfonyl imines by cooperative N-heterocyclic-carbene/thiourea/tertiary-amine multicatalysis.  

Science.gov (United States)

Many hands make light work: In an organocatalytic asymmetric sulfonation of enones, the activation of a sulfonyl imine by an N-heterocyclic carbene (NHC) catalyst led to the release of a sulfinic anion, which underwent nucleophilic addition to the enone. The enantioselectivity of the process was controlled by a chiral thiourea/amine co-catalyst through anion recognition and hydrogen-bonding interactions. Tol=p-tolyl. PMID:24115649

Jin, Zhichao; Xu, Jianfeng; Yang, Song; Song, Bao-An; Chi, Yonggui Robin

2013-11-18

111

Synthesis and reactivity of new bis(N-heterocyclic carbene) iridium(I) complexes.  

Science.gov (United States)

New complexes of the type trans-[IrCl(?(2)-COE)(NHC)2] (COE = cis-cyclooctene; NHC = N-heterocyclic carbene) have been prepared in one step from the reaction of ca. 4 equiv of NHC or [AgCl(NHC)] with [IrCl(?(2)-COE)2]2 in benzene. These new complexes have been characterized by techniques including NMR and IR spectroscopy, X-ray crystallography, and elemental analysis. Exposing trans-[IrCl(COE)(I(i)Pr(Me))2] to CO yielded trans-[IrCl(CO)(I(i)Pr(Me))2], which is the only bis(NHC) analogue of Vaska's complex trans-[IrCl(CO)(PPh3)2] known to date. The synthesis of trans-[Ir(CO)(I(i)Pr(Me))2(R)] (R = MeO, PhCC, OSiPh3, O2CPh) complexes has been achieved via deprotonation reactions involving the new hydroxide species trans-[Ir(OH)(CO)(I(i)Pr(Me))2]. PMID:24147789

Nelson, David J; Truscott, Byron J; Slawin, Alexandra M Z; Nolan, Steven P

2013-11-01

112

Mechanistic Investigation of the Rutheniumâ??N-Heterocyclic-Carbene-Catalyzed Amidation of Alcohols and Amines  

DEFF Research Database (Denmark)

The mechanism of the rutheniumâ??N-heterocyclic-carbene-catalyzed formation of amides from alcohols and amines was investigated by experimental techniques (Hammett studies, kinetic isotope effects) and by a computational study by using dispersion-corrected density functional theory (DFT/ M06). The Hammett study indicated that a small positive charge builds-up at the benzylic position in the transition state of the turnover-limiting step. The kinetic isotope effect was determined to be 2.29ACHTUNGTRENUNG(!0.15), which suggests that the breakage of the C"H bond is not the rate-limiting step, but that it is one of several slow steps in the catalytic cycle. Rapid scrambling of hydrogen and deuterium at the a position of the alcohol was observed with deuterium-labeled substrates, which implies that the catalytically active species is a ruthenium dihydride. The experimental results were supported by the characterization of a plausible catalytic cycle by using DFT/M06. Both cisdihydride and trans-dihydride intermediates were considered, but when the theoretical turnover frequencies (TOFs) were derived directly from the calculated DFT/M06 energies, we found that only the trans-dihydride pathway was in agreement with the experimentally determined TOFs.

Makarov, Ilya; Fristrup, Peter

2012-01-01

113

Synthesis, structural characterization and properties of new N-heterocyclic carbene Ag(I) complexes  

Science.gov (United States)

The syntheses and properties of new N-heterocyclic carbene complexes [{1-(2,3,5,6-tetra methyl benzyl)-3-benzylbimy}AgCl] (2a), [{1-(benzyl)-3-benzylbimy}2Ag][BF4] (2b), [{1-(benzyl)-3-butylbimy}AgBr]2 (2c), [{1-(benzyl)-3-(4-nitro-benzyl)bimy}AgBr] (2d), [{1-(benzyl)-3-(4-bromo-benzyl)bimy}AgBr] (2e), [{1-(benzyl)-3-(4-nitro-benzyl)bimy}2Ag][BF4] (2f), [{1-(benzyl)-3-(4-bromo-benzyl)bimy}2Ag][BF4] (2g) (bimy = benzimidazol-2-ylidene), and [{1-(10-bromo-9-anthracenylmethyl)-3-butylimy}AgBr] (2h) (imy = imidazol-2-ylidene) have been described. Compounds 2a-2h have been characterized by 1H NMR-, UV-vis- LC/MS-spectral studies and elemental analysis. Compounds 2a-2c and 2h are unambiguously characterized by single crystal X-ray crystallography. Compound 2h shows solution emission at room temperature.

Kishore, Ravada; Das, Samar K.

2013-12-01

114

On the electronic impact of abnormal C4-bonding in N-heterocyclic carbene complexes  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Sterically similar palladium dicarbene complexes have been synthesized that comprise permethylated dicarbene ligands which bind the metal center either in a normal coordination mode via C2 or abnormally via C4. Due to the strong structural analogy of the complexes, differences in reactivity patterns may be attributed to the distinct electronic impact of normal versus abnormal carbene bonding, while stereoelectronic effects are negligible. Unique reactivity patterns have been identified for th...

2009-01-01

115

Investigation of the Role of Electrogenerated N-Heterocyclic Carbene in the Staudinger Synthesis in Ionic Liquid  

Directory of Open Access Journals (Sweden)

Full Text Available Electrogenerated N-heterocyclic carbene (NHC, obtained by cathodic reduction of Bmim-BF4, behaves as organocatalyst and base in the Staudinger synthesis from an acyl chloride and a deactivated imine in ionic liquid to yield ?-lactams. The effect of many parameters (temperature, amount of electricity, substituents, presence of an external base has been evaluated and a tentative mechanism for the Staudinger synthesis in a very polar medium like the ionic liquid reported. The yields of isolated ?-lactams are good, starting from non-electrophilic imines, and predominantly trans lactams are obtained with a good diastereomeric ratio.

Marta Feroci

2011-12-01

116

Stereospecific asymmetric N-heterocyclic carbene (NHC)-catalyzed redox synthesis of trifluoromethyl dihydropyranones and mechanistic insights.  

Science.gov (United States)

N-heterocyclic carbene (NHC)-catalyzed redox asymmetric hetero-Diels-Alder reactions of ?-aroyloxyaldehydes with ?-trifluoromethyl enones generates synthetically useful dihydropyranones containing a stereogenic trifluoromethyl substituent in good yields (up to 81%) and excellent diastereoselectivity and enantioselectivity (up to >95:5 dr and >99% ee). The process is stereospecific, with use of either (E)- or (Z)-?-trifluoromethyl enones forming syn- or anti-dihydropyranone products, respectively. Mechanistic studies through in situ kinetic analysis of the reaction reveal key differences in reactivity between chiral NHC precursor 1 and an achiral NHC precursor. PMID:23957790

Davies, Alyn T; Taylor, James E; Douglas, James; Collett, Christopher J; Morrill, Louis C; Fallan, Charlene; Slawin, Alexandra M Z; Churchill, Gwydion; Smith, Andrew D

2013-09-20

117

Spiro-fused six-membered N-heterocyclic carbene: a new scaffold toward unique properties and activities.  

Science.gov (United States)

A six-membered N-heterocyclic carbene fused with a spiro-scaffold is designed. The new NHC shows stronger ?-donation ability than typical 5-membered NHCs. This property leads to interesting reactivities of this spiro-fused six-membered NHC. For example, the NHC-BF3 Lewis pair complex can be readily prepared by using LiBF4 as the BF3 source, or through a direct bond-reconstruction of the tetrafluoroborate salt NHC·HBF4. PMID:24851922

Yang, Bin-Miao; Xiang, Kai; Tu, Yong-Qiang; Zhang, Shi-Heng; Yang, Deng-Tao; Wang, Shao-Hua; Zhang, Fu-Ming

2014-06-10

118

N-Heterocyclic Carbene Catalyzed Umpolung of Styrenes: Mechanistic Elucidation and Selective Tail-to-Tail Dimerization.  

Science.gov (United States)

The reaction between N-heterocyclic carbenes (NHCs) and styrenes yields alkyl-substituted azolium salts, which are able to form nucleophilic deoxy Breslow intermediates by simple deprotonation. This hitherto unknown reaction of NHCs represents a new way to generate deoxy Breslow intermediates and paves the way for the selective NHC-catalyzed tail-to-tail homodimerization of styrenes. This reaction significantly broadens the scope of the Michael umpolung and provides a new method to generate 1,4-diaryl compounds. PMID:24824735

Schedler, Michael; Wurz, Nathalie E; Daniliuc, Constantin G; Glorius, Frank

2014-06-01

119

A cationic rhodium(I N-heterocyclic carbene complex isolated as an aqua adduct  

Directory of Open Access Journals (Sweden)

Full Text Available The title complex, aqua[1,3-bis(2,6-diisopropylphenylimidazol-2-ylidene](?4-cycloocta-1,5-dienerhodium(I tetrafluoridoborate, [Rh(C8H12(C27H36N2(H2O]BF4, exihibits a square-planar geometry around the Rh(I atom, formed by a bidentate cycloocta-1,5-diene (cod ligand, an N-heterocylcic carbene and an aqua ligand. The complex is cationic and a BF4? anion balances the charge. The structure exists as a hydrogen-bonded dimer in the solid state, formed via interactions between the aqua ligand H atoms and the BF4? F atoms.

Ashley L. Huttenstine

2011-09-01

120

Quinobis(imidazolylidene): synthesis and study of an electron-configurable bis(N-heterocyclic carbene) and its bimetallic complexes.  

Science.gov (United States)

Reaction of bromanil with N,N'-dimesitylformamidine followed by deprotonation with NaN(SiMe(3))(2) afforded 1,1',3,3'-tetramesitylquinobis(imidazolylidene) (1), a bis(N-heterocyclic carbene) (NHC) with two NHC moieties connected by a redox active p-quinone residue, in 72 % yield of isolated compound. Bimetallic complexes of 1 were prepared by coupling to FcN(3) (2) or FcNCS (3; Fc=ferrocenyl) or coordination to [M(cod)Cl] (4 a or 4 b, where M=Rh or Ir, respectively; cod=1,5-cyclooctadiene). Treatment of 4 a and 4 b with excess CO(g) afforded the corresponding [M(CO)(2)Cl] complexes 5 a and 5 b, respectively. Analysis of 2-5 by NMR spectroscopy and X-ray diffraction indicated that the electron-deficient quinone did not significantly affect the inherent spectral properties or coordination chemistry of the flanking imidazolylidene units, as compared to analogous NHCs. Infrared spectroscopy and cyclic voltammetry revealed that decreasing the electron density at ML(n) afforded an increase in the stretching energy and a decrease in the reduction potential of the quinone, indicative of metal-quinone electronic interaction. Differential pulse voltammetry and chronoamperometry of the metal-centered oxidations in 2-4 revealed two single, one-electron peaks. Thus, the metal atoms bound to 1 are oxidized at indistinguishable potentials and do not appear electronically coupled. However, the metal-quinone interaction was used to increase the electron density at coordinated metal atoms. Infrared spectroelectrochemistry revealed that the average nu(CO) values for 5 a and 5 b decreased by 14 and 15 cm(-1), respectively, upon reduction of the quinone embedded within 1. These shifts correspond to 10 and 12 cm(-1) decreases in the Tolman electronic parameter of this ditopic ligand. PMID:19946903

Tennyson, Andrew G; Ono, Robert J; Hudnall, Todd W; Khramov, Dimitri M; Er, Joyce A V; Kamplain, Justin W; Lynch, Vincent M; Sessler, Jonathan L; Bielawski, Christopher W

2010-01-01

 
 
 
 
121

Iridium(III) Hydrido N-Heterocyclic Carbene-Phosphine Complexes as Catalysts in Magnetization Transfer Reactions  

Science.gov (United States)

The hyperpolarization (HP) method signal amplification by reversible exchange (SABRE) uses para-hydrogen to sensitize substrate detection by NMR. The catalyst systems [Ir(H)2(IMes)(MeCN)2(R)]BF4 and [Ir(H)2(IMes)(py)2(R)]BF4 [py = pyridine; R = PCy3 or PPh3; IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene], which contain both an electron-donating N-heterocyclic carbene and a phosphine, are used here to catalyze SABRE. They react with acetonitrile and pyridine to produce [Ir(H)2(NCMe)(py)(IMes)(PPh3)]BF4 and [Ir(H)2(NCMe)(py)(IMes)(PCy3)]BF4, complexes that undergo ligand exchange on a time scale commensurate with observation of the SABRE effect, which is illustrated here by the observation of both pyridine and acetonitrile HP. In this study, the required symmetry breaking that underpins SABRE is provided for by the use of chemical inequivalence rather than the previously reported magnetic inequivalence. As a consequence, we show that the ligand sphere of the polarization transfer catalyst itself becomes hyperpolarized and hence that the high-sensitivity detection of a number of reaction intermediates is possible. These species include [Ir(H)2(NCMe)(py)(IMes)(PPh3)]BF4, [Ir(H)2(MeOH)(py)(IMes)(PPh3)]BF4, and [Ir(H)2(NCMe)(py)2(PPh3)]BF4. Studies are also described that employ the deuterium-labeled substrates CD3CN and C5D5N, and the labeled ligands P(C6D5)3 and IMes-d22, to demonstrate that dramatically improved levels of HP can be achieved as a consequence of reducing proton dilution and hence polarization wastage. By a combination of these studies with experiments in which the magnetic field experienced by the sample at the point of polarization transfer is varied, confirmation of the resonance assignments is achieved. Furthermore, when [Ir(H)2(pyridine-h5)(pyridine-d5)(IMes)(PPh3)]BF4 is examined, its hydride ligand signals are shown to become visible through para-hydrogen-induced polarization rather than SABRE.

2013-01-01

122

On the mechanism of N-heterocyclic carbene-catalyzed reactions involving acyl azoliums.  

Science.gov (United States)

Catalytic reactions promoted by N-heterocyclic carbenes (NHCs) have exploded in popularity since 2004 when several reports described new fundamental reactions that extended beyond the long-studied generation of acyl anion equivalents. These new NHC-catalyzed reactions allow chemists to generate unique reactive species from otherwise inert starting materials, all under simple, mild reaction conditions and with exceptional selectivities. In analogy to transition metal catalysis, the use of NHCs has introduced a new set of elementary steps that operate via discrete reactive species, including acyl anion, homoenolate, and enolate equivalents, usually generated by oxidation state reorganization ("redox neutral" reactions). Nearly all NHC-catalyzed reactions offer operationally simple reactions, proceed at room temperature without the need for stringent exclusion of air, and do not generate reaction byproducts. Variation of the catalyst or reaction conditions can profoundly influence reaction outcomes, and researchers can tune the desired selectivities through careful choice of NHC precursor and base. The catalytically generated homoenolate and enolate equivalents are nucleophilic species. In contrast, the catalytically generated acyl azolium and ?,?-unsaturated acyl azoliums are electrophilic cationic species with unique and unprecedented chemistry. For example, when generated catalytically, these species transformed an ?-functionalized aldehyde to an ester under redox neutral conditions without coupling reagents or waste. In addition to providing new approaches to catalytic esterifications, acyl azoliums offer unique reactivities that chemists can exploit for selective reactions. This Account focuses on the discovery and mechanistic investigation of the catalytic generation of acyl azoliums and ?,?-unsaturated acyl azoliums. These chemical species are fascinating, and their catalytic generation is an important development. Studies of their unusual chemistry, however, date back to the intense investigation of thiamine-dependent enzymatic processes in the 1960s. Acyl azoliums are remarkably reactive in acylation chemistry and are unusually chemoselective. These two properties have led to a new wave of reactions such as redox esterification reaction (1) and the catalytic kinetic resolution of challenging substrates (i.e., 3). Our group and others have also developed methods to generate and exploit ?,?-unsaturated acyl azoliums, which have facilitated new C-C bond-forming annulations, including a catalytic, enantioselective variant of the Claisen rearrangement (2). From essentially one class of catalysts, the N-mesityl derived triazolium salts, researchers can easily prepare highly enantioenriched dihydropyranones and dihydropyridinones. Although this field is now one of the most explored areas of enantioselective C-C bond forming reactions, many mechanistic details remained unsolved and in dispute. In this Account, we address the mechanistic inquiries about the characterization of the unsaturated acyl triazolium species and its kinetic profile under catalytically relevant conditions. We also provide explanations for the requirement and effect of the N-mesityl group in NHC catalysis based on detailed experimental data within given specific reactions or conditions. We hope that our studies provide a roadmap for catalyst design/selection and new reaction discovery based on a fundamental understanding of the mechanistic course of NHC reactions. PMID:24410291

Mahatthananchai, Jessada; Bode, Jeffrey W

2014-02-18

123

Polyoxometalate-based N-heterocyclic carbene (nhc) complexes for palladium-mediated C-C coupling and chloroaryl dehalogenation catalysis  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Better together: A novel hybrid N-heterocyclic carbene (NHC) palladium complex, integrating a totally inorganic and polyanionic decatungstate unit, has been synthesized following a convergent strategy. The interplay of the Pd binding domains with the inorganic scaffold is instrumental in accessing multi-turnover catalysis in C-C cross-coupling and aromatic dehalogenation reactions under MW-assisted protocols

2010-01-01

124

Synthesis of Pd-N-heterocyclic carbene Pd-catalyst and its application in MW-assisted Heck and Suzuki reaction  

Science.gov (United States)

The first Pd-N-heterocyclic carbene (NHC) complex in the form of organic silica is prepared using sol-gel method and its application in Heck and Suzuki reactions are demonstrated. These C-C coupling reactions proceeded efficiently under the influence of microwave irradiation, wit...

125

Activation of carbon dioxide and carbon disulfide by a scandium N-heterocyclic carbene complex.  

Science.gov (United States)

A Sc NHC complex readily activates three equivalents of CO2 showing 'Frustrated Lewis Pair' type reactivity with each metal-carbene bond, but whilst CS2 is also activated by the labile carbenes, no metal involvement is observed. PMID:24162511

Arnold, Polly L; Marr, Isobel A; Zlatogorsky, Sergey; Bellabarba, Ronan; Tooze, Robert P

2014-01-01

126

Main-chain organometallic microporous polymers bearing triphenylene-tris(N-heterocyclic carbene)-gold species: catalytic properties.  

Science.gov (United States)

Two triphenylene-based tris(N-heterocyclic carbene)-gold-acetylide main-chain organometallic microporous polymers (MOMPs) were obtained and fully characterized. Both materials show spherical shapes, and their size is highly dependent on the type of acetylene used in the synthetic protocol. The new solids were tested in the catalytic reduction of nitroarenes with NaBH4 and in the three-component Strecker reaction for the synthesis of ?-aminonitriles, and showed high activity in both processes. Whereas the activity of the solids in the reduction of nitroarenes may be attributed to the formation of Au nanoparticles due to the use of NaBH4 as reducing agent, the activity in the Strecker reaction may originate from the Lewis acidic activation of the ketone or imine on coordination to Au. PMID:24668881

Gonell, Sergio; Poyatos, Macarena; Peris, Eduardo

2014-05-01

127

A highly active water-soluble cross-coupling catalyst based on dendritic polyglycerol N-heterocyclic carbene palladium complexes.  

Science.gov (United States)

A new water-soluble polyglycerol derivative functionalized with N-heterocyclic carbene palladium complexes was prepared and applied as catalyst for Suzuki cross-coupling reactions in water. The complex displays a metal loading of around 65 metal centers per dendrimeric molecule, which is estimated to contain 130 chelating groups and thus corresponds approximately to the formation of 2:1 NHC/metal complexes. Monomeric analogues were also synthesized to validate the reactivity of the dendritic catalyst. Both types of catalysts were tested with various aryl bromides and arylboronic acids. Turnover frequencies of up to 2586 h(-1) at 80 degrees C were observed with the dendritic catalyst along with turnover numbers of up to 59 000, which are among the highest turnover numbers reported for polymer-supported catalysts in neat water. The dendritic catalyst could be used (reused) in five consecutive reactions without loss in activity. PMID:18702166

Meise, Markus; Haag, Rainer

2008-01-01

128

Oxyanion-steering and CH-? Interactions as Key Elements in a N-Heterocyclic Carbene-Catalyzed [4+2] Cycloaddition  

Science.gov (United States)

The N-heterocyclic carbene catalyzed [4+2] cycloaddition has been shown to give ?,?-unsaturated ?-lactones in excellent enantio- and diastereoselectivity. However, preliminary computational studies of the geometry of the intermediate enolate rendered ambiguous both the origins of selectivity and the reaction pathway. In this communication, we show that a concerted, but highly asynchronous, Diels-Alder reaction occurs rather than the stepwise Michael-type or Claisen-type pathways. In addition, two crucial interactions are identified that enable high selectivity: an oxyanion-steering mechanism and a CH-? interaction. The calculations accurately predict the enantioselectivity of a number of N-heterocyclic carbene catalysts in the hetero-Diels-Alder reaction.

Allen, Scott E.; Mahatthananchai, Jessada; Bode, Jeffrey W.; Kozlowski, Marisa C.

2012-01-01

129

Asymmetric N-heterocyclic carbene catalyzed addition of enals to nitroalkenes: controlling stereochemistry via the homoenolate reactivity pathway to access ?-lactams.  

Science.gov (United States)

An asymmetric intermolecular reaction between enals and nitroalkenes to yield ?-nitroesters has been developed, catalyzed by a novel chiral N-heterocyclic carbene. Key to this work was the development of a catalyst that favors the ?-nitroester pathway over the established Stetter pathway. The reaction proceeds in high stereoselectivity and affords the previously unreported syn diastereomer. We also report an operationally facile two-step, one-pot procedure for the synthesis of ?-lactams. PMID:23713683

White, Nicholas A; DiRocco, Daniel A; Rovis, Tomislav

2013-06-12

130

Trans [O=Re(V)-OH] core stabilised by chelating N-heterocyclic dicarbene ligands.  

Science.gov (United States)

Three Re(v) N-heterocyclic carbene complexes [ReO(OH)(L(Me))(2)][PF(6)](1.4)[ReO(4)](0.6), [ReO(OH)(L(iPr))(2)][PF(6)](1.4)[ReO(4)](0.6) and [ReO(OH)(L(Bn))(2)][PF(6)](1.3)[ReO(4)](0.7) (L(Me) = 1,1'-methylene-bis(3-methylimidazole-2-ylidene); L(iPr) = 1,1'-methylene-bis(3-isopropylimidazole-2-ylidene) and L(Bn) = 1,1'-methylene-bis(3-benzylimidazole-2-ylidene)) with trans oxo and hydroxo at axial positions have been synthesized. PMID:23172573

Lum, Regina; Zhang, Hu; Zhang, Wenhua; Bai, Shi-Qiang; Zhao, Jin; Hor, T S Andy

2013-01-28

131

Cooperative N-Heterocyclic Carbene/Brønsted Acid Catalysis for the Tail-to-Tail (Co)dimerization of Methacrylonitrile.  

Science.gov (United States)

The first tail-to-tail dimerization of methacrylonitrile (MAN) has been realized by the cooperative use of N-heterocyclic carbene (NHC) and Brønsted acid catalysts, producing 2,5-dimethylhex-2-enedinitrile with the E/Z ratio of 24:76. Although the NHC alone was not effective for the catalysis, the addition of alcohols resulted in the significant increase of the dimer yield up to 82% in the presence of 5 mol % NHC. Detailed experimental studies including the ESI-MS analysis of the intermediates, stoichiometric (co)dimerizations, and deuterium-labeling experiments revealed the mechanistic aspects of the proton transfer, isomerization, umpolung, and rate-limiting steps, allowing us to observe several mechanistic differences between the dimerization of MAN and that of methyl methacrylate. The stoichiometric reactions in the presence and absence of an alcohol suggest that the alcohol additives play a role in promoting the intermolecular proton transfers from the deoxy-Breslow intermediate to the regenerated NHC in the second half of the catalytic cycle. In addition, the codimerizations of MAN with n-butyl methacrylate (n-BuMA) have been studied. While the dimerization of n-BuMA was sluggish in the presence of an alcohol, the catalytic activity for the codimerization was enhanced by the cooperative systems. PMID:24773333

Kato, Terumasa; Matsuoka, Shin-Ichi; Suzuki, Masato

2014-05-16

132

Electrogenerated N-heterocyclic carbene: N-acylation of chiral oxazolidin-2-ones in ionic liquids  

International Nuclear Information System (INIS)

An electrochemical procedure for the N-acylation of chiral oxazolidin-2-ones, in the absence of volatile molecular organic solvents, has been set up via electrolyses of ionic liquid [bmim]BF4 containing oxazolidin-2-ones followed by addition of saturated or unsaturated anhydrides. N-acyloxazolidin-2-ones were isolated in good to elevated yields. The electrochemically induced N-acylation of chiral oxazolidin-2-ones occurs with total retention of the absolute configuration of all the chiral atoms. The electrogenerated carbene (1-butyl-3-methyl-1H-imidazol-2-ylidene) has been indicated as the base involved in the deprotonation of chiral oxazolidin-2-ones

2009-02-01

133

Electrogenerated N-heterocyclic carbene: N-acylation of chiral oxazolidin-2-ones in ionic liquids  

Energy Technology Data Exchange (ETDEWEB)

An electrochemical procedure for the N-acylation of chiral oxazolidin-2-ones, in the absence of volatile molecular organic solvents, has been set up via electrolyses of ionic liquid [bmim]BF{sub 4} containing oxazolidin-2-ones followed by addition of saturated or unsaturated anhydrides. N-acyloxazolidin-2-ones were isolated in good to elevated yields. The electrochemically induced N-acylation of chiral oxazolidin-2-ones occurs with total retention of the absolute configuration of all the chiral atoms. The electrogenerated carbene (1-butyl-3-methyl-1H-imidazol-2-ylidene) has been indicated as the base involved in the deprotonation of chiral oxazolidin-2-ones.

Chiarotto, Isabella [Dipartimento Ingegneria Chimica Materiali Ambiente, Universita ' La Sapienza' , via Castro Laurenziano, 7, 00161 Roma (Italy)], E-mail: isabella.chiarotto@uniroma1.it; Feeney, Michelle M.M.; Feroci, Marta [Dipartimento Ingegneria Chimica Materiali Ambiente, Universita ' La Sapienza' , via Castro Laurenziano, 7, 00161 Roma (Italy); Inesi, Achille [Dipartimento Ingegneria Chimica Materiali Ambiente, Universita ' La Sapienza' , via Castro Laurenziano, 7, 00161 Roma (Italy)], E-mail: achille.inesi@uniroma1.it

2009-02-01

134

Non-NHCs Stable Singlet Carbene Ligands  

Science.gov (United States)

Carbene chemistry has been tremendously developed during the last 20 years. NHC carbenes have found many applications as ligands in organometallic chemistry or as catalysts in organocatalysis. On the other hand, the so-called "non-NHC stable carbenes" have had a restrained development to date in spite of the high potential of this family of carbenes which offers many more possibilities in structure modifications, resulting in original reactivities. This chapter gives an overview over non-NHC stable singlet carbenes and their properties as ligands.

Kato, Tsuyoshi; Maerten, Eddy; Baceiredo, Antoine

135

Electronic structure trends in N-heterocyclic carbenes (NHCs) with varying number of nitrogen atoms and NHC-transition-metal bond properties.  

Science.gov (United States)

Carbenes derived from five-membered heterocycles with different numbers of nitrogen atoms ranging from two to four lead formally either to normal N-heterocyclic or mesoionic carbenes with, in some cases, the same skeletal structure. The electronic structures of fourteen of these compounds were examined by means of DFT calculations at the B3LYP/aug-cc-pVTZ level. The examined parameters include the energies of the ?-lone pair at Ccarbene and the ?-HOMO of the protonated form, which are correlated to the first and second proton affinities. The singlet-triplet energy gap was used as a measure of the stability of the N-heterocyclic carbene (NHC) towards dimerisation. Natural population analysis provided insight into the variation of the p? population and the natural charge at Ccarbene with NHC structure. Additionally, the transition metal?NHC bond in L-AuCl and L-TiCl4 and the nature of the orbital interactions between the NHC and the transition-metal fragment were analysed in detail by the extended transition state-natural orbitals for chemical valence (ETS-NOCV) approach at the BP86/TZ2P level. Similarities and differences between the NHC?gold and the NHC?titanium bond are discussed, and trends in key bonding properties can be traced back to the variation of the electronic parameters of the NHC. PMID:23955586

Bernhammer, Jan C; Frison, Gilles; Huynh, Han Vinh

2013-09-16

136

Unveiling the stereoelectronic properties of a triphenylene-based tris N-heterocyclic carbene.  

Science.gov (United States)

Two rhodium complexes with a triphenylene-based tris-NHC have been fully characterized. DFT and electrochemical studies suggest that the electronic communication between the metals is very weak, and that the electron donating properties of the ligand are very similar to those shown by its benzimidazolylidene analogue. PMID:23828447

Gonell, Sergio; Alabau, Roberto G; Poyatos, Macarena; Peris, Eduardo

2013-08-18

137

Ruthenium complexes of an abnormally bound, anionic N-heterocyclic carbene.  

Science.gov (United States)

The abnormally bound, anionic NHC-borane complex [Ru(IDipp-BF3 )(p-cymene)Cl]2 (4; IDipp-BF3 =1,3-(2,6-iPr2 C6 H3 )2 -2-BF3 (C3 HN2 )-4-yl) was synthesized by transmetalation from Li[(IDipp-BF3 )2 Ag]. Addition of donors gave species of the form [Ru(IDipp-BF3 )(p-cymene)(L)Cl], whereas halide abstraction with Ag(Et2 O)[B(C6 F5 )4 ] gave C?H activation of the methine position of the IDipp?BF3 ligand. PMID:24782270

Pranckevicius, Conor; Stephan, Douglas W

2014-05-26

138

Air-stable, convenient to handle Pd based PEPPSI (pyridine enhanced precatalyst preparation, stabilization and initiation) themed precatalysts of N/O-functionalized N-heterocyclic carbenes and its utility in Suzuki-Miyaura cross-coupling reaction.  

Science.gov (United States)

Several new air-stable, convenient to handle and easily synthesized Pd based PEPPSI (Pyridine Enhanced Precatalyst Preparation, Stabilization and Initiation) type precatalysts supported over N/O-functionalized N-heterocyclic carbenes (NHC) namely, trans-[1-(benzyl)-3-(N-t-butylacetamido)imidazol-2-ylidene]Pd(pyridine)Cl2 (), trans-[1-(2-hydroxy-cyclohexyl)-3-(benzyl)imidazol-2-ylidene]Pd(pyridine)Cl2 () and trans-[1-(o-methoxybenzyl)-3-(t-butyl)imidazol-2-ylidene]Pd(pyridine)Br2 (), have been designed. Specifically, the Pd-NHC complexes, , and , were conveniently synthesized from their respective imidazolium halide salts by the reaction with PdCl2 in pyridine in presence of K2CO3 as a base. A new imidazolium chloride salt, 1-(benzyl)-3-(N-t-butylacetamido)imidazolium chloride () was synthesized by the alkylation reaction of benzyl imidazole with N-t-butyl-2-chloroacetamide. The molecular structures of the imidazolium chloride salt, , and the Pd-NHC complexes, , and , have been determined by X-ray diffraction studies. The density functional theory studies of the , and complexes were carried out to in order to gain insight about their structure, bonding and the electronic properties. The nature of the NHC-metal bond in these complexes was examined using Charge Decomposition Analysis (CDA), which revealed that the N-heterocyclic carbene ligands are effective sigma-donors. In addition, the catalysis studies revealed that the Pd-NHC complexes, , and , are effective catalysts for the Suzuki-Miyaura type C-C cross-coupling reactions. PMID:17928912

Ray, Lipika; Shaikh, Mobin M; Ghosh, Prasenjit

2007-10-28

139

Synthesis, spectroscopic studies and reactivity of triphenylphosphine ruthenium (II) complexes with N-heterocyclic ligands  

International Nuclear Information System (INIS)

Reported is the chemistry of triphenylphosphine ruthenium (II) complexes of general formula RuCl_2(PPh_3)_2L_2 and RuCl_2(PPh_3)_2A, obtained from the reaction of RuCl_2(PPh_3)_3 with N-heterocyclic ligands L, or A (of ambidentate nature). The electronic spectra exhibit two strong metal-to-ligand charge-transfer bands, ascribed to the b_1(dxz)->b_1(pi) and a_2(dxy)->a_2(pi) transitions, and a third, weak band ascribed to the b_2(dyz)->a_2(pi) transition. The electronic states and the vibrational modes of the complexes were characterized by means of their resonance Raman and infrared absorption spectra. Thermogravimetric and thermodifferential analysis indicated that the melting process is succeeded by an exothermic reaction, and that the weigh loss starts to occur only after this step. The complexes dissociated in CHCl_3 solution, showing preferential labilization of the phosphine ligands, as in the case of the hydrogenation catalyst Ru(PPh_3)_3Cl_2. In the presence of CO, RuCl_2(CO)_2L_2 complexes were gennerated. Several derivatives were isolated and characterized. (author)

1989-01-01

140

Supramolecular NHC ligands: on the influence of ZnII-templates on the activity of RhI(cod) complexes in carbene polymerization  

Digital Repository Infrastructure Vision for European Research (DRIVER)

New RhI(cod) complexes of N-heterocyclic carbene ligands containing a pending pyridyl moiety for subsequent binding of ZnII-templates were designed for supramolecular catalyst control in carbene polymerization reactions. ZnII-templates indeed have an influence on the yields and the polymer molecular weights (and weight distributions) produced by these supramolecular assemblies. However, control experiments reveal that the effect of the ZnII-template on the polymerization results is general an...

Rubio, Miguel; Jellema, Erica; Siegler, M. A. M.; Spek, A. L.; Reek, J. N. H.; Bruin, Bas

2009-01-01

 
 
 
 
141

Quantum mechanical study and vibrational spectra of indazolium-3-carboxylate and its decarboxylation product, the N-heterocyclic carbene indazol-3-ylidene.  

Science.gov (United States)

Indazolium-3-carboxylate is a molecule that can be found as the nucleus of several pseudo-cross-conjugated mesomeric betaines, such as the alkaloid nigellicine. From a chemical point of view, one of the more interesting properties of this class of molecules is the possibility of forming an N-heterocyclic carbene by thermal decarboxylation. In this paper we have studied the carbene generation by decarboxylation of 1,2-dimethyl indazolium-3-carboxylate, using vibrational (infrared and Raman) spectroscopy and quantum chemistry calculations. Normal mode analysis allowed us to analyse the changes in the stretching force constants upon decarboxylation and to establish spectroscopic-structure relationships. We also investigate the effect of 5-halogen (fluoro, chloro) substitution on the carbene generation. Decarboxylation energy profiles of the three derivatives were calculated. Crossing of the energy paths when going from the transition state to the final product were obtained. The theoretical tendency found for the activation energies agrees with that observed for the decarboxylation temperatures and for the calculated NICS values of the benzene moieties. PMID:19088990

Schmidt, Andreas; Snovydovych, Bohdan; Casado, Juan; Quirante, José Joaquín; Navarrete, Juan Teodomiro López; Ramírez, Francisco Javier

2009-01-14

142

Expanded-ring N-heterocyclic carbenes efficiently stabilize gold(I) cations, leading to high activity in ?-acid-catalyzed cyclizations.  

Science.gov (United States)

A series of six- and seven-membered expanded-ring N-heterocyclic carbene (er-NHC) gold(I) complexes has been synthesized using different synthetic approaches. Complexes with weakly coordinating anions [(er-NHC)AuX] (X(-) = BF4(-), NTf2(-), OTf(-)) were generated in solution. According to their (13)C?NMR spectra, the ionic character of the complexes increases in the order X(-) = Cl(-) carbene complexes [(NHC)AuX], [(Ph3 P)AuX], [(Me2S)AuX], and inorganic compounds of gold in model reactions of indole and benzofuran synthesis. It was found that increased ionic character of the complexes was correlated with increased catalytic activity in the cyclization reactions. As a result, we developed an unprecedentedly active monoligand cationic [(THD-Dipp)Au]BF4 (1,3-bis(2,6-diisopropylphenyl)-3,4,5,6-tetrahydrodiazepin-2-ylidene gold(I) tetrafluoroborate) catalyst bearing seven-membered-ring carbene and bulky Dipp substituents. Quantitative yields of cyclized products were attained in several minutes at room temperature at 1?mol?% catalyst loadings. The experimental observations were rationalized and fully supported by DFT calculations. PMID:24682976

Morozov, Oleg S; Lunchev, Andrey V; Bush, Alexander A; Tukov, Aleksandr A; Asachenko, Andrey F; Khrustalev, Victor N; Zalesskiy, Sergey S; Ananikov, Valentine P; Nechaev, Mikhail S

2014-05-12

143

Anionic and zwitterionic copper(I) complexes incorporating an anionic N-heterocyclic carbene decorated with a malonate backbone: synthesis, structure and catalytic applications.  

Science.gov (United States)

The anionic malonate-derived N-heterocyclic carbenes (maloNHCs) react cleanly and rapidly with copper chloride to generate the anionic complexes of type [(maloNHC)CuCl]·Li, which crystallize in the solid state either in an oligomeric trimer arrangement or in polymeric helixes depending on the substitution pattern and the solvent. Ten zwitterionic heteroleptic Cu(I) complexes combining the anionic maloNHC and a neutral imidazol-2-ylidene are also obtained in a very selective manner and fully characterized. Whereas the anionic complexes are relatively active catalysts for the hydrosilylation of carbonyl compounds, the zwitterionic complexes reveal to be efficient and extremely robust pre-catalysts for the intramolecular cyclopropanation reaction of a diazo ester and outperform the corresponding cationic Cu(i) complexes with classical imidazol-2-ylidenes. PMID:23361332

César, Vincent; Barthes, Cécile; Farré, Yoann C; Cuisiat, Stéphane V; Vacher, Bernard Y; Brousses, Rémy; Lugan, Noël; Lavigne, Guy

2013-05-28

144

The ambivalent chemistry of a free anionic N-heterocyclic carbene decorated with a malonate backbone: the plus of a negative charge.  

Science.gov (United States)

The anionic heterocycle "[maloNHC](-)", ([1](-)), is the archetype of a growing family of N-heterocyclic carbenes incorporating an anionic backbone; here, a malonate group. A comprehensive experimental exploration of its chemistry as a free entity (in the form of its lithium salt [1]·Li) is presented, and rationalized using DFT calculations at the B3LYP/6-31+G** level of theory. For the sake of comparison, similar computations were performed on other representative carbene types. Reactions of [1]·Li with a broad series of electrophilic reagents were used to ascertain its intrinsic nature as a nucleophilic carbene. Unexpectedly, [1]·Li was also seen to react with the nucleophilic tert-butylisocyanide, to give an anionic ketenimine, which could be subsequently derivatized, either into an imine by protonation of the ketenimine moiety, or into a neutral ketenimine by alkylation of the intracyclic malonate moiety. Further experiments on the electrophilic behavior of [1]·Li revealed its unexpected reactivity toward p-chlorobenzaldehyde, resulting in a formal C-H activation and the first structurally characterized keto-tautomer of the Breslow intermediate. Finally, [1]·Li remarkably activates polar E-H bonds, including N-H bonds from ammonia and amines, Si-H bonds, and B-H bonds. Importantly, DFT calculations indicate the importance of counterion effects. In particular, the key to the observed reactivity appears to be a modulation of energy levels associated with a dynamic variability of the Li-O distance between the remote malonate group and the counterion. PMID:24307368

César, Vincent; Labat, Stéphane; Miqueu, Karinne; Sotiropoulos, Jean-Marc; Brousses, Rémy; Lugan, Noël; Lavigne, Guy

2013-12-01

145

A new, mild one-pot synthesis of iodinated heterocycles as suitable precursors for N-heterocyclic carbene complexes  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The use of I2/AgOAc in dichloromethane constitutes a cheap, mild, and efficient method for the selective iodination of a variety of heterocycles. In a number of cases, this method provides superior yields than other literature methods and affords iodo-functionalized heterocycles that are suitable precursors for carbene complexes.

Iglesias, Manuel; Schuster, Oliver; Albrecht, Martin

2010-01-01

146

DFT investigation on mechanisms and stereoselectivities of [2 + 2 + 2] multimolecular cycloaddition of ketenes and carbon disulfide catalyzed by N-heterocyclic carbenes.  

Science.gov (United States)

The first theoretical investigation using density functional theory (DFT) methods to study the detailed reaction mechanisms of stereoselective [2 + 2 + 2] multimolecular cycloaddition of ketene (two molecules) and carbon disulfide (CS2, one molecule) which is catalyzed by N-heterocyclic carbene (NHC) is presented in this paper. The calculated results indicate that this reaction occurs through four steps: the complexation of NHC with ketene (channel 1a) rather than with CS2 (channel 1b), addition of CS2 (channel 2b) but not dimerization of ketene (channel 2a), formal [4 + 2] cycloaddition with a second molecule of ketene (channel 3a) rather than intramolecular [2 + 2] cycloaddition (channel 3b), and finally regeneration of NHC. The second step is revealed to be the rate-determining step. Moreover, the stereoselectivities associated with the chiral carbon center and the carbon double bond are predicted to be respectively determined in the second and third steps and respectively R and E configurations dominated, which are in good agreement with the experimental results. Furthermore, the possible mechanisms of the identical [2 + 2 + 2] cycloaddition catalyzed by N,N-dimethylpyridin-4-amine (DMAP) have also been investigated to help understand the ring closure mechanism proceeding in the third step. PMID:24188481

Zhang, Wen-Jing; Wei, Dong-Hui; Tang, Ming-Sheng

2013-12-01

147

The catalytic role of N-heterocyclic carbene in a metal-free conversion of carbon dioxide into methanol: a computational mechanism study.  

Science.gov (United States)

A density functional theory study at the M05-2X(IEFPCM, THF)/6-311+G**//M05-2X/6-31G* level has been conducted to gain insight into the catalytic mechanism of the first metal-free N-heterocyclic carbene (NHC)-catalyzed conversion of carbon dioxide into methanol. Among the various examined reaction pathways, we found that the most favorable leads to the experimentally detected intermediates, including formoxysilane (FOS), bis(silyl)acetal (BSA), silylmethoxide (SMO), and disiloxane (DSO). However, our study also revealed that formaldehyde (CH(2)O), generated from the dissociation of BSA into DSO and CH(2)O via a mechanism somewhat similar to the Brook rearrangement, should be an inevitable intermediate, although it was not reported by the experimentalists. When NHC catalyzes the reactions of CO(2)/FOS/CH(2)O with silane, there are two activation modes. It was found that NHC prefers to activate Si-H bonds of silane and push electron density to the H atoms of the Si-H bonds in favor of transferring a hydridic atom of silane to the electrophilic C center of CO(2)/FOS/CH(2)O. This holds true in particular for the NHC-catalyzed reactions of silane with FOS/CH(2)O to produce BSA/SMO. The preferred activation mode can operate by first passing an energetically unfavorable NHC-silane local minimum via pi-pi interactions or by directly crossing a transition state involving three components simultaneously. The activation mode involving initial coordination of NHC with the electrophilic C atom of CO(2)/FOS/CH(2)O is less favorable or inoperable. The predicted catalytic mechanism provides a successful interpretation of the experimental observation that phenylsilane is more efficient than diphenylsilane in performing the conversion. PMID:20707349

Huang, Fang; Lu, Gang; Zhao, Lili; Li, Haixia; Wang, Zhi-Xiang

2010-09-01

148

Three-coordinate gold(I) N-heterocyclic carbene complexes: a new class of strongly luminescent derivatives.  

Science.gov (United States)

A selected group of cationic three-coordinate Au(I)-NHC complexes of the form [Au(NHC)(dppbz)]OTf have been prepared from a commercially available bidentate phosphine. All complexes have been fully characterised by NMR and mass spectroscopy. The [Au(NHC)](+) fragment shows a pronounced tendency to form linear complexes which is confirmed by the molecular structure of [Au(IPr)(dppbz)]OTf in the solid state. The complexes are brightly luminescent and present very high quantum yield values in the solid state. The assignments of the electronic transitions involved in the emissions are of a phosphorescent nature and it is proposed that the origin of the emissions is derived from the ligand (dppbz) to metal-ligand (Au-NHC) charge-transfer (LML'CT) transition. PMID:24105255

Visbal, Renso; López-de-Luzuriaga, José M; Laguna, Antonio; Gimeno, M Concepción

2014-01-01

149

Organocatalytic conjugate-addition polymerization of linear and cyclic acrylic monomers by N-heterocyclic carbenes: mechanisms of chain initiation, propagation, and termination.  

Science.gov (United States)

This contribution presents a full account of experimental and theoretical/computational investigations into the mechanisms of chain initiation, propagation, and termination of the recently discovered N-heterocyclic carbene (NHC)-mediated organocatalytic conjugate-addition polymerization of acrylic monomers. The current study specifically focuses on three commonly used NHCs of vastly different nucleophilicity, 1,3-di-tert-butylimidazolin-2-ylidene (I(t)Bu), 1,3-dimesitylimidazolin-2-ylidene (IMes), and 1,3,4-triphenyl-4,5-dihydro-1H-1,2,4-triazol-5-ylidene (TPT), and two representative acrylic monomers, the linear methyl methacrylate (MMA) and its cyclic analog, biomass-derived renewable ?-methyl-?-methylene-?-butyrolactone (MMBL). For MMA, there exhibits an exquisite selectivity of the NHC structure for the three types of reactions it promotes: enamine formation (single-monomer addition) by IMes, dimerization (tail-to-tail) by TPT, and polymerization by I(t)Bu. For MMBL, all three NHCs promote no dimerization but polymerization, with the polymerization activity being highly sensitive to the NHC structure and the solvent polarity. Thus, I(t)Bu is the most active catalyst of the series and converts quantitatively 1000-3000 equiv of MMBL in 1 min or 10,000 equiv in 5 min at room temperature to MMBL-based bioplastics with a narrow range of molecular weights of M(n) = 70-85 kg/mol, regardless of the [MMBL]/[I(t)Bu] ratio employed. The I(t)Bu-catalyzed MMBL polymerization reaches an exceptionally high turnover frequency up to 122 s(-1) and a high initiator efficiency value up to 1600%. Unique chain-termination mechanisms have been revealed, accounting for the production of relative high-molecular-weight linear polymers and the catalytic nature of this NHC-mediated conjugate-addition polymerization. Computational studies have provided mechanistic insights into reactivity and selectivity between two competing pathways for each NHC-monomer zwitterionic adduct, namely enamine formation/dimerization through proton transfer vs polymerization through conjugate addition, and mapped out extensive energy profiles for chain initiation, propagation, and termination steps, thereby satisfactorily explaining the experimental observations. PMID:24245532

Zhang, Yuetao; Schmitt, Meghan; Falivene, Laura; Caporaso, Lucia; Cavallo, Luigi; Chen, Eugene Y-X

2013-11-27

150

Luminescent pillared LnIII–ZnII heterometallic coordination frameworks with two kinds of N-heterocyclic carboxylate ligands  

Science.gov (United States)

In our efforts toward rational design and systematic synthesis of 'pillar-layer' structure coordination frameworks, four new LnIII–ZnII heterometallic coordination polymers (CPs) based on two kinds of N-heterocyclic carboxylic ligands with formula {[LnZn(L1)2(L2)(H2O)m]·nH2O}? (Ln=La (1), Eu (2), Gd (3) and Dy (4), m=3 (for 1) and 2 (for 2–4), n=8 (for 1) and 7 (for 2–4), H2L1=pyridine-2,3-dicarboxylate acid, HL2=isonicotinic acid), have been synthesized under hydrothermal reaction of Ln2O3, ZnO, H2L1 and HL2. CP 1 has a three-dimensional (3D) structure with a (3,6)-connected sit topology network, while CPs 2–4 are isostructural with 3D single-node pcu alpha-Po topology network. Also, luminescent properties of these CPs have also been investigated. The emission of 1 and 3 should be attributed to the coordination-perturbed ligand-centered luminescence and the emission spectra of 2 and 4 show the characteristic bands of the corresponding LnIII ions.

Liu, Sui-Jun; Jia, Ji-Min; Cui, Yu; Han, Song-De; Chang, Ze

2014-04-01

151

Metal and carbene organocatalytic relay activation of alkynes for stereoselective reactions.  

Science.gov (United States)

Transition metal and organic catalysts have established their own domains of excellence. It has been expected that merging the two unique domains should provide complimentary or unprecedented opportunities in converting simple raw materials to functional products. N-heterocyclic carbenes alone are excellent organocatalysts. When used with transition metals such as copper, N-heterocyclic carbenes are routinely practiced as strong-coordinating ligands. Combination of an N-heterocyclic carbene and copper therefore typically leads to deactivation of either or both of the two catalysts. Here we disclose the direct merge of copper as a metal catalyst and N-heterocyclic carbenes as an organocatalyst for relay activation of alkynes. The reaction involves copper-catalysed activation of alkynes to generate ketenimine intermediates that are subsequently activated by an N-heterocyclic carbene organocatalyst for stereoselective reactions. Each of the two catalysts (copper metal catalyst and N-heterocyclic carbene organocatalyst) accomplishes its own missions in the activation steps without quenching each other. PMID:24865392

Namitharan, Kayambu; Zhu, Tingshun; Cheng, Jiajia; Zheng, Pengcheng; Li, Xiangyang; Yang, Song; Song, Bao-An; Chi, Yonggui Robin

2014-01-01

152

Labile coordination approach for the modulation of the electronic properties of ruthenium(ii) and iridium(iii) complexes within an "N-heterocyclic carbene (NHC)-pyridyl" dynamic platform.  

Science.gov (United States)

Modulating the functionality of a synthetic transition metal complex by external stimuli is highly important for designing switchable systems. One prerequisite for achieving such dynamic activity is to generate molecular systems with in situ controllable electronic properties. To achieve dynamic control of the electronic properties, here we report the synthesis of two new N-heterocyclic carbene (NHC)-pyridyl Ir(III)/Ir(III) () and Ru(II)/Ru(II) () bimetallic complexes. These complexes include a latent stimuli-responsive labile site. This is utilized successfully for the on-demand, real-time modulation of the electronic properties of the systems in a reversible manner using external agents, as probed by spectroscopic and electrochemical techniques. These results display promising scope in the domain of transition metal-NHC chemistry, which can guide us in developing future smart organometallic systems. PMID:24828753

Mondal, Moumita; Ranjeesh, T K; Gupta, Suraj K; Choudhury, Joyanta

2014-05-28

153

Chiral N-heterocyclic carbene-catalyzed generation of ester enolate equivalents from ?,?-unsaturated aldehydes for enantioselective Diels–Alder reactions  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The catalytic generation of chiral ester enolate equivalents from ?,?-unsaturated aldehydes with chiral N-hetereocyclic carbene catalysts makes possible highly enantioselective hetero-Diels–Alder reactions. The reactions proceed under simple, mild conditions with both aliphatic and aromatic substituted enals as substrates. Previous attempts to employ these starting materials as enolate precursors gave structurally different products via catalytically generated homoenolate equivalents. Cri...

2010-01-01

154

Gold(I)-phosphine-N-heterocycles: biological activity and specific (ligand) interactions on the C-terminal HIVNCp7 zinc finger.  

Science.gov (United States)

The syntheses and the characterization by chemical analysis, (1)H and (31)P NMR spectroscopy, and mass spectrometry of a series of linear triphenylphosphine gold(I) complexes with substituted N-heterocycle ligands (L), [(PPh3)Au(I)(L)](+), is reported. The reaction of [(PPh3)Au(L)](+) (L = Cl(-) or substituted N- heterocyclic pyridine) with the C-terminal (Cys3His) finger of HIVNCp7 shows evidence by mass spectrometry (ESI-MS) and (31)P NMR spectroscopy of a long-lived {(PPh3)Au}-S-peptide species resulting from displacement of the chloride or pyridine ligand by zinc-bound cysteine with concomitant displacement of Zn(2+). In contrast, reactions with the Cys2His2 finger-3 of the Sp1 transcription factor shows significantly reduced intensities of {(PPh3)Au} adducts. The results suggest the possibility of systematic (electronic, steric) variations of "carrier" group PR3 and "leaving" group L as well as the nature of the zinc finger in modulation of biological activity. The cytotoxicity, cell cycle signaling effects, and cellular accumulation of the series are also reported. All compounds display cytotoxicity in the micromolar range upon 96 h continuous exposure to human tumor cells. The results may have relevance for the reported inhibition of viral load in simian virus by the gold(I) drug auranofin. PMID:24063530

Abbehausen, Camilla; Peterson, Erica J; de Paiva, Raphael E F; Corbi, Pedro P; Formiga, André L B; Qu, Yun; Farrell, Nicholas P

2013-10-01

155

Facile Thermal W-W Bond Homolysis in the N-Heterocyclic Carbene-Containing Tungsten Dimer [CpW(CO)2(IMe)]2  

Energy Technology Data Exchange (ETDEWEB)

The thermal W-W bond homolysis in [CpW(CO)2(IMe)]2 (IMe = 1,3-dimethylimidazol-2-ylidene) was investigated and was found to occur to a large extent compared to other tungsten dimers such as [CpW(CO)3]2. CpW(CO)2(IMe)H was prepared by heating a solution of [IMeH]+[CpW(CO)2(PMe3)]?, and exists in solution as a mixture of interconverting cis and trans isomers. The carbene rotation in CpW(CO)2(IMe)H was explored by DFT calculations, and low enthalpic barriers (< 3.5 kcal mol?1) are predicted. CpW(CO)2(IMe)H has pKaMeCN = 31.5(3) and deprotonation with KH gives K+[CpW(CO)2(IMe)]? (• MeCN). Hydride abstraction from CpW(CO)2(IMe)H with Ph3C+PF6? in the presence of a coordinating ligand L (MeCN or THF) gives [CpW(CO)2(IMe)(L)]+PF6?. Electrochemical measurements on the anion [CpW(CO)2(IMe)]? in MeCN, together with digital simulations, give an E1/2 of ?1.54(2) V vs Cp2Fe+/0 for the [CpW(CO)2(IMe)]•/? couple. A thermochemical cycle provides the solution bond dissociation free energy of the W-H bond of CpW(CO)2(IMe)H as 61.3(6) kcal mol?1. In the electrochemical oxidation of [CpW(CO)2(IMe)]?, reversible dimerization of the electrogenerated radical CpW(CO)2(IMe)• occurs, and digital simulation provides kinetic and thermodynamic parameters for the monomer-dimer equilibrium: kdimerization ~ 2.5 ? 104 M?1 s?1, khomolysis ~ 0.5 s?1 (i.e., Kdim ~ 5 ? 104 M?1). Reduction of [CpW(CO)2(IMe)(MeCN)]+PF6? with cobaltocene gives the dimer [CpW(CO)2(IMe)]2, which in solution exists as a mixture of anti and gauche rotomers. As expected from the electrochemical experiments, the dimer is in equilibrium with detectable amounts of CpW(CO)2(IMe)•. This species was observed by IR spectroscopy, and its presence in solution is also in accordance with the observed reactivity toward 2,6-di-tert-butyl-1,4-benzoquinone, chloroform and dihydrogen. This work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Biosciences and Geosciences. Pacific Northwest National Laboratory is a multiprogram national laboratory operated for DOE by Battelle. The EPR studies were performed at EMSL, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research located at Pacific Northwest National Laboratory.

van der Eide, Edwin F.; Liu, Tianbiao L.; Camaioni, Donald M.; Walter, Eric D.; Bullock, R. Morris

2012-03-05

156

Palladium carbene complexes for selective alkene di- and oligomerization  

Digital Repository Infrastructure Vision for European Research (DRIVER)

A series of palladium complexes were synthesized that comprise three sterically different C,N-bidentate coordinating NHC-pyridine ligands (NHC = N-heterocyclic carbene). In one set, the pyridine and the carbene are linked by a flexible CH2 group (a), in the other two sets, the two ligand units are directly linked and feature a shielding mesityl substituent on the carbene and either an unsubstituted pyridine (b) or a xylyl-substituted pyridine unit (c). Investigation of the reactivity of catio...

Khlebnikov, Vsevolod; Meduri, Angelo; Mu?ller-bunz, Helge; Montini, Tiziano; Fornasiero, Paolo; Zangrando, Ennio; Milani, Barbara; Albrecht, Martin

2012-01-01

157

Unexpected ring expansion of an enantiopure imidazoline carbene ligand.  

Science.gov (United States)

[reaction: see text] We report an unexpected ring expansion reaction of an enantiopure fenchone-derived imidazolinium salt during attempts to form copper complexes of the corresponding imidazoline carbene ligand. A N,N'-difenchyl piperazinone was formed in low yield together with the difenchyl-substituted five-membered urea. PMID:16805549

Pelegrí, Adela Sanchez; Elsegood, Mark R J; McKee, Vickie; Weaver, George W

2006-07-01

158

Thiophene decorated with Fischer carbene ligands  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The activation of a section, or of all the carbons of thiophene by bromine in lithium–halogen exchange reactions, was implemented in a step-wise manner to facilitate the preparation of novel multiple Fischer carbene-bearing thiophene compounds.

Jaarsveld, Nina A.; Liles, David C.; Lotz, Simon

2010-01-01

159

Imidazol(in)ium hydrogen carbonates as a genuine source of N-heterocyclic carbenes (NHCs): applications to the facile preparation of NHC metal complexes and to NHC-organocatalyzed molecular and macromolecular syntheses.  

Science.gov (United States)

Anion metathesis of imidazol(in)ium chlorides with KHCO(3) afforded an easy one step access to air stable imidazol(in)ium hydrogen carbonates, denoted as [NHC(H)][HCO(3)]. In solution, these compounds were found to be in equilibrium with their corresponding imidazol(in)ium carboxylates, referred to as N-heterocyclic carbene (NHC)-CO(2) adducts. The [NHC(H)][HCO(3)] salts were next shown to behave as masked NHCs, allowing for the NHC moiety to be readily transferred to both organic and organometallic substrates, without the need for dry and oxygen-free conditions. In addition, such [NHC(H)][HCO(3)] precursors were successfully investigated as precatalysts in two selected organocatalyzed reactions of molecular chemistry and polymer synthesis, namely, the benzoin condensation reaction and the ring-opening polymerization of d,l-lactide, respectively. The generation of NHCs from [NHC(H)][HCO(3)] precursors occurred via the formal loss of H(2)CO(3)via a concerted low energy pathway, as substantiated by Density Functional Theory (DFT) calculations. PMID:22455795

Fèvre, Maréva; Pinaud, Julien; Leteneur, Alexandre; Gnanou, Yves; Vignolle, Joan; Taton, Daniel; Miqueu, Karinne; Sotiropoulos, Jean-Marc

2012-04-18

160

Water-soluble tetrapodal N, O ligands incorporating soft N-heterocycles for the selective complexation of Am(III) over Ln(III)  

International Nuclear Information System (INIS)

A series of four water-soluble N, O-tetrapodal ligands derived from ethylenediamine, bearing hard acetate groups and soft N-heterocycles, either pyridine or pyrazine, was developed to study the impact of the softness of N-donors on the complexation properties with trivalent f ions. Two novel ligands of enhanced soft character, bearing three pyridines (L3py) or three pyrazines (L3pz), were synthesized and the related lanthanide complexes were studied in solution. The ligand containing three pyridyl-methyl moieties L3py gives complexes with a coordination similar to EDTA, i.e. a hexa-dentate coordination mode as indicated by NMR and luminescence decays (q = 3) and stability constants in the range log ?110 = 6.99-9.3 (La-Lu). On the other hand, the softest molecule L3pz forms much less stable complexes with log ?110 = 4.0-4.4 (La-Eu). The selective back-extraction of Am(III) from organic solutions containing 4f and 5f elements was tested with the four water-soluble complexing agents. The ligand L3pz demonstrates poor stripping ability and selectivity. In contrast, the three ligands Lpy, Lpz and L3py give interesting back-extraction results with Eu/Am separation factors ranging from 36 to 46, which are significantly higher than with HEDTA. This exemplifies the role of the N-hetero-cycle softness in enhancing the separation between Am(III) and Eu(III). Interestingly, the pyrazine-based ligand, Lpz, demonstrates the best stripping properties, with a distribution factor that approaches that of HEDTA in the same conditions (DAm?0.3). This molecule is a good compromise between softness and hardness and forms complexes still stable at pH 3 due to its low basicity. (authors)

2010-01-01

 
 
 
 
161

Hydride-Rhodium(III)-N-Heterocyclic Carbene Catalysts for Vinyl-Selective H/D Exchange: A Structure-Activity Study.  

Science.gov (United States)

A series of neutral and cationic Rh(III) -hydride and Rh(III) -ethyl complexes bearing a NHC ligand has been synthesized and evaluated as catalyst precursors for H/D exchange of styrene using CD3 OD as a deuterium source. Various ligands have been examined in order to understand how the stereoelectronic properties can modulate the catalytic activity. Most of these complexes proved to be very active and selective in the vinylic H/D exchange, without deuteration at the aromatic positions, displaying very high selectivity toward the ?-positions. In particular, the cationic complex [RhClH(CH3 CN)3 (IPr)]CF3 SO3 showed excellent catalytic activity, reaching the maximum attainable degree of ?-vinylic deuteration in only 20?min. By modulation of the catalyst structure, we obtained improved ?/? selectivity. Thus, the catalyst [RhClH(?(2) -O,N-C9 H6 NO)(SIPr)], bearing an 8-quinolinolate ligand and a bulky and strongly electron-donating SIPr as the NHC, showed total selectivity for the ?-vinylic positions. This systematic study has shown that increased electron density and steric demand at the metal center can improve both the catalytic activity and selectivity. Complexes bearing ligands with very high steric hindrance, however, proved to be inactive. PMID:24895153

Di Giuseppe, Andrea; Castarlenas, Ricardo; Pérez-Torrente, Jesús J; Lahoz, Fernando J; Oro, Luis A

2014-07-01

162

A C(3v)-symmetrical tribenzotriquinacene-based threefold N-heterocyclic carbene. Coordination to rhodium(I) and stereoelectronic properties.  

Science.gov (United States)

A novel tribenzotriquinacene-based tris-NHC has been obtained and coordinated to rhodium. The new ligand displays a unique rigid C3v symmetry. The electrochemical analysis of the tri-rhodium complex reveals that the three metals are essentially disconnected. PMID:24092258

Segarra, Candela; Linke, Jens; Mas-Marzá, Elena; Kuck, Dietmar; Peris, Eduardo

2013-11-21

163

Azobenzene-functionalized N-heterocyclic carbenes as photochromic ligands in silver(i) and gold(i) complexes.  

Science.gov (United States)

The reaction of meta- and para-bromomethylene-azobenzenes with 1-methyl-imidazole yields the respective meta-/para-functionalized azobenzenes tagged with an imidazolium group. Similar reactions of ortho- and para-bromo-azobenzene with imidazole and successive quaternation with benzylbromide give the analogues, with an imidazolium group in ortho/para substituted azobenzenes. With the exception of the ortho derivative, all imidazolium salts could be transformed into their respective silver(i) complexes by reaction with Ag2O. Transmetallation of these silver(i) complexes with (Me2S)AuCl gives the azobenzene-containing complexes (NHC)AuCl. Two of these formed crystals suitable for X-ray diffraction, which revealed the typical linear coordination geometry of the NHC-Au-Cl moiety. All gold complexes feature E?Z photo-isomerisation upon irradiation with UV light. The thermal back reaction to the E-isomers is relatively slow and comparable to that of other azobenzene compounds. PMID:23922028

Kaiser, Manuel; Leitner, Sebastian P; Hirtenlehner, Christa; List, Manuela; Gerisch, Alexander; Monkowius, Uwe

2013-10-01

164

Modulation of the stacking interaction of MN4 (M=Pt, Pd, Au) complexes with tryptophan through N-heterocyclic ligands.  

Science.gov (United States)

A survey of selected N-heterocycle ligands showed that platination of 4-N-dimethylaminopyridine (DMAP) in [Pt(dien)L](2+) (dien=diethylenetriamine) gave especially strong ?-? stacking interactions with tryptophan and the tryptophan-containing C-terminal zinc finger (ZF) of the HIV (human immunodeficiency virus) nucleocapsid protein NCp7. The association constants (all at 10(3)M(-1)) were significantly stronger (25.0 and 28.1 for tryptophan and ZF respectively) than those previously measured for the purine nucleobase 9-ethylguanine (9EtG) in [Pt(dien)(9EtG)](2+) (6.88 and 7.55 for tryptophan and ZF respectively). Extension to Pd and Au complexes also confirmed the utility of DMAP in assisting stacking interactions. The results confirm the utility of a "bioinorganic" approach to targeting and inactivation of medicinal chemistry targets using the dual approach of target recognition (non-covalent) followed by target fixation (covalent). PMID:24206773

Tsotsoros, Samantha D; Bate, Aaron B; Dows, Martina G; Spell, Sarah R; Bayse, Craig A; Farrell, Nicholas P

2014-03-01

165

Mono- and dinuclear Ag(I), Au(I), and Au(III) metallamacrocycles containing N-heterocyclic dicarbene ligands.  

Science.gov (United States)

Ag(I) dicarbene complexes [Ag(m)(L(n))m]X(m) (L(n) = Im(Me)(CH2)(n)Im(Me), Im(Me) = N-methylimidazol-N-yl-2-ylidene; n = 3, X = PF6, m = 2; n = 6-8, 10, X = AgBr2, m = 1, 2) were prepared by reacting Ag2O with 1 equiv of the corresponding bisimidazolium salt [H2L(n)]A2 (A = PF6, Br). The dibromoargentates react with 1 equiv of AgTfO to afford [Ag(m)(L(n))m](TfO)m (m = 1, 2). The room temperature transmetalation reaction of [Ag(m)(L(n))m][AgBr2]m (n = 3, 5, 6-8, 10) with [AuCl(SMe2)] and AgTfO (L(n):Au:TfO = 1:1:1) affords [Au2(?-L(n))2](TfO)2 (n = 3, 5, 10), or mixtures of [Au(?(2)-L(n))]TfO (main product for n = 7) and [Au2(?-L(n))2](TfO)2 (main product for n = 6, 8). At room temperature, the equilibrium between [M2(?-L(n))2](TfO)2 and [M(?(2)-L(n))]TfO is fast for M = Ag, but slow for M = Au, in the NMR time scale. When n ? 7 and M = Ag or Au, the equilibrium is shifted toward the mononuclear complexes in the order 8 > 10 > 7, which proves that the (CH2)8 linker has the optimal length for trans chelation. Correspondingly, the high-temperature metalation of [H2L(n)]Br2 (n = 8, 10) with 1 equiv of [AuCl(SMe2)] and excess of NaAcO, affords [Au(?(2)-L(n))]Br with a small amount of [Au2(?-L(n))2]Br2. If AgTfO is added to the reaction mixture, [Au(?(2)-L(8))]2[AgBr3] is isolated instead of the desired triflate, which can be obtained by reacting the mixture of [Au(?(2)-L(8))]Br and [Au2(?-L(8))2]Br2 with AgTfO. [Au(?(2)-L(10))]TfO was isolated after thermal conversion of [Au2(?-L(10))2](TfO)2. [Au(?(2)-L(8))]TfO reacts with I2 to give trans-[AuI2(?(2)-L(8))]TfO, which is the first Au(III) complex containing a trans-spanning bidentate ligand. We have determined the crystal structures of complexes [Ag2(?-L(3))2](PF6)2, [Ag(?(2)-L(7))]TfO, [Au2(?-L(3))2](TfO)2, [Au(?(2)-L(8))]Br, [Au(?(2)-L(8))]2[AgBr3], and trans-[AuI2(?(2)-L(8))]TfO. PMID:23488928

Gil-Rubio, Juan; Cámara, Verónica; Bautista, Delia; Vicente, José

2013-04-01

166

C4-bound imidazolylidenes: from curiosities to high-impact carbene ligands  

Digital Repository Infrastructure Vision for European Research (DRIVER)

This feature article summarizes the progress achieved thus far in using C4-bound imidazolylidenes as a new class of ligands for transition metals. Since the discovery of this unusual carbene bonding mode in 2001, various rational routes towards complexes containing C4-bound carbenes have evolved. These advances allowed for studying the impact of this new type of ligand on the transition metal center, both from a fundamental point of view as well as from a more applied perspective, in particul...

Albrecht, Martin

2008-01-01

167

Cyclic (Amino)[bis(ylide)]carbene as an Anionic Bidentate Ligand for Transition Metal Complexes  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The 3-triphenylphosphonio-N-(2,6-diisopropylphenyl)-pyrole reacts with two equivalents of methyl lithium to afford a lithium adduct in which a cyclic (amino)[bis(ylide)]carbene, a novel type of NHC, acts as a 1,4-bidentate ligand via the carbene center and the exocyclic ylidic carbon. This species readily undergoes transmetallation reactions, which allows for the synthesis of a variety of transition metal complexes.

Asay, Matthew; Donnadieu, Bruno; Baceiredo, Antoine; Soleilhavoup, Michele; Bertrand, Guy

2008-01-01

168

1,2,3-triazolylidenes as versatile abnormal carbene ligands for late transition metals  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The [3 + 2] cycloaddition of azides and acetylenes followed by nitrogen quaternization was applied for the generation of novel and highly modular triazolium salts. The selective substitution of the 1,3,4-substitution pattern presets such salts as precursors for a new class of abnormal carbene ligands, thus expanding the family of these high-impact ligands. Metalation of the triazolium salts is highly versatile and is illustrated by direct C?H bond activation as well as by applying a transme...

Mathew, Paulson; Neels, Antonia; Albrecht, Martin

2009-01-01

169

High oxidation state carbene complexes for C-H bond activation catalysis  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Chapter one is an introduction to the less common coordination and oxidation chemistry of palladium; complexes containing Pd-OR, Pd-NR2 and those in the oxidation states of +IV. An outline of PdII/IV catalysed ligand-directed oxidative functionalisation is also included. Chapter two covers the design and synthesis of a range of tethered N-heterocyclic carbene (NHC) complexes of Pd. In addition, the syntheses of a number of new tethered NHC ligands are described. The use of Dens...

Pearson, Stephen

2010-01-01

170

A new stable C(NHC)--CH--C(NHC)N-heterocyclic dicarbene ligand: its mono- and dinuclear Ir(I) and Ir(I)-Rh(I) complexes.  

Science.gov (United States)

The ligand [(1,3-phenylene)bis(methylene)]bis(1-ethyl-imidazol-2-ylidene) ( 2) (abbreviated as EtCNHC--CH--CNHC with CH = central aromatic ring, -- = CH2 spacer) has been synthesised, and isolated, by deprotonation of [(1,3-phenylene)bis(methylene)]bis(1-ethyl-imidazolium) dichloride, Et(CHimid.--CH--CHimid.)Cl2 ( 1) with LiN(SiMe3)2. This new, remarkably stable NHC dicarbene ligand is quantitatively protonated with HBr to form [(1,3-phenylene)bis(methylene)]bis(1-ethyl-imidazolium) dibromide Et(CHimid.--CH--CHimid.)Br2 ( 3) and reprotonated slowly enough in MeOH to allow observation of the monoprotonated intermediate 4. Dicarbene 2 reacts with S8 to give the dithione compound [(1,3-phenylene)bis(methylene)]bis(1-ethyl-imidazol-2-thione) Et[(SC)--CH--(CS)] ( 6). Reaction of 2 with [IrCl(CO)(PPh3)2] and [Ir(mu-Cl)(cod)]2 yielded the mononuclear [Ir(CO)(PPh3)2Et(CHimid.--CH--CNHC)(PF6)](PF6) ( 8) and [IrCl(cod)Et(CHimid.--CH--CNHC)](PF6) ( 10) complexes, respectively, and the dinuclear complexes [{Ir(CO)(PPh3)2}2Et(mu-CNHC--CH--CNHC)](PF6)2 ( 7) and [{IrCl(cod)}2Et(mu-CNHC--CH--CNHC)] ( 9), respectively, in which 2 acts as a bridging, non-pincer type ligand. Only one other Ir(I) complex has been reported before containing a CNHC--CH--CNHC ligand. The structures of 6 and 9 were determined by X-ray diffraction and depict different conformations of the (1,3-phenylene)bis(methylene) (or xylylene) moiety. The mononuclear Ir complex 9 reacted smoothly with [Rh(mu-Cl)(cod)]2 and Cs2CO3 to generate a rare example of a heteronuclear NHC complex, [IrRhCl2(cod)2Et(mu-CNHC--CH--CNHC)] ( 11). PMID:19417949

Raynal, Matthieu; Cazin, Catherine S J; Vallée, Christophe; Olivier-Bourbigou, Hélène; Braunstein, Pierre

2009-05-21

171

Carbene catalysis: An internal affair  

Science.gov (United States)

Preparing powerful reactive intermediates such as enolates and homoenolates for C-C bond formation used to require strong bases and stoichiometric reagents. They can now be catalytically generated from ?-functionalized aldehydes or even from saturated esters under mild conditions using N-heterocyclic carbene catalysts.

Bode, Jeffrey W.

2013-10-01

172

Synthesis of saturated N-heterocycles.  

Science.gov (United States)

Saturated N-heterocycles are prevalent in biologically active molecules and are increasingly attractive scaffolds in the development of new pharmaceuticals. Unlike their aromatic counterparts, there are limited strategies for facile construction of substituted saturated N-heterocycles by convergent, predictable methods. In this Synopsis, we discuss recent advances in the synthesis of these compounds, focusing on approaches that offer generality and convenience from widely available building blocks. PMID:24617516

Vo, Cam-Van T; Bode, Jeffrey W

2014-04-01

173

Nitrenium ions as ligands for transition metals  

Science.gov (United States)

Unlike N-heterocyclic carbenes (NHCs), which are now used ubiquitously in metal-based chemistry, the nitrogen-derived analogue (in which a carbon is replaced with the isoelectronic nitrogen cation, a nitrenium ion) has remained elusive as a ligand for metals. This is especially intriguing, because several other main-group analogues of NHCs have been prepared, and have been shown to coordinate with transition-metal complexes. Here, we describe the preparation of several N-heterocyclic nitrenium ions that are isoelectronic and isostructural to NHCs, and study their ligand properties. The formation of relatively strong nitrenium-metal bonds is unambiguously confirmed, in solution by selective 15N-labelling experiments, and in the solid state by X-ray crystallography. Experimental and computational studies of the electronic properties of this novel type of ligand suggest that they are poor ?-donors and good ?-acceptors.

Tulchinsky, Yuri; Iron, Mark A.; Botoshansky, Mark; Gandelman, Mark

2011-07-01

174

N-Heterocyclic Carbene Complexes in Dehalogenation Reactions  

Science.gov (United States)

Catalytic dehalogenation represents an underdeveloped transformation in M-NHC chemistry with a small number of reports detailing the reactivity of Co, Ru, Ni and Pd catalysts. In situ generated nickel and palladium NHC complexes catalyse the hydrodechlorination of aryl chlorides. Lower coordinate Ni complexes are proposed to operate in the hydrodefluorination of mono- and poly-fluorinated substrates. The single example of Ru-NHC catalysed hydrodefluorination of fully and partially fluorinated aromatic substrates is characterised by an unusual regioselectivity. The highly regioselective dehydrohalogenation of relatively unreactive alkyl halide substrates is achieved with a cobalt NHC catalyst.

Mas-Marzá, Elena; Page, Michael J.; Whittlesey, Michael K.

175

Synthesis and Antimicrobial Activity of Novel Ag-N-Hetero-cyclic Carbene Complexes  

Directory of Open Access Journals (Sweden)

Full Text Available A series of imidazolidinium ligand precursors are metallated with Ag2O to give silver(I N-heterocyclic carbene complexes. All compounds were fully characterized by elemental analyses, 1H-NMR, 13C-NMR and IR spectroscopy techniques. All compounds studied in this work were screened for their in vitro antimicrobial activities against the standard strains: Enterococcus faecalis (ATCC 29212, Staphylococcus aureus (ATCC 29213, Escherichia coli (ATCC 25922, Pseudomonas aeruginosa (ATCC 27853 and the fungi Candida albicans and Candida tropicalis. The new imidazolidin-2-ylidene silver complexes have been found to display effective antimicrobial activity against a series of bacteria and fungi.

?lknur Özdemir

2010-04-01

176

Molecular versus ionic structures in adducts of GaX3 with monodentate carbon-based ligands.  

Science.gov (United States)

A molecular donor-acceptor adduct has been isolated by the reaction of the N-heterocyclic carbene 1,3-dimethyl imidazol-2-ylidene (diMe-IMD) with GaCl(3). In contrast, the structurally related, yet much more nucleophilic, 1,3-dimethyl-2-methylene-2,3-dihydro-1H-imidazole (diMe-MDI) gave rise to ion pairs of type [L(2)GaX(2)][GaX(4)], where X = Cl, Br, or I. With IBioxMe(4), a N-heterocyclic carbene that is more nucleophilic than diMe-IMD, the outcome of the reaction was dependent on the nature of the halide. Ionic 1:1 adducts between monodentate ligands and GaX(3) salts have only one precedent in the literature. The peculiar behavior of carbon-based ligands was explained on the basis of their electronic properties and reaction kinetics. PMID:23256783

El-Hellani, Ahmad; Monot, Julien; Guillot, Régis; Bour, Christophe; Gandon, Vincent

2013-01-01

177

Impact of terminal dimethylation on the resistance profile of ?-N-heterocyclic thiosemicarbazones  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Triapine is an ?-N-heterocyclic thiosemicarbazone with promising anticancer activity against hematologic malignancies but widely ineffective against solid tumor types in clinical trials. The anticancer activity of thiosemicarbazones can be dramatically increased by terminal dimethylation. KP1089 is a gallium compound containing two terminal dimethylated thiosemicarbazone ligands. To gain insights on the vulnerability of this highly active terminal dimethylated thiosemicarbazone to drug resis...

Heffeter, Petra; Pirker, Christine; Kowol, Christian R.; Herrman, Gerrit; Dornetshuber, Rita; Miklos, Walter; Jungwirth, Ute; Koellensperger, Gunda; Keppler, Bernhard K.; Berger, Walter

2012-01-01

178

Role of wingtip substituents on benzene-platform-based tetrapodal ligands toward the formation of a self-assembled silver carbene cage.  

Science.gov (United States)

Tetrapodal imidazolium ligands L(1)-L(3) as their PF6(-) salts are synthesized in good yields by reacting 1,2,4,5-tetrakis(bromomethyl)benzene with N-methylimidazole, N-benzylimidazole, and N-ethylimidazole, respectively. Single-crystal X-ray diffraction studies of L(1)·4PF6, L(2)·4PF6, and L(3)·4PF6 show the chair conformation of the tetrapodal imidazoliums (L(1)-L(3)), where 1,5- and 2,4-imidazolium moieties are oriented in opposite directions of the benzene plane. The PF6(-) salts of L(1)-L(3) are reacted with Ag2O to synthesize different silver complexes of N-heterocyclic carbene (NHC), 1-3, respectively, in good yields. Crystals of all three complexes suitable for single-crystal X-ray diffraction study are also isolated. Structural analysis of 1, i.e., the complex of L(1)·4PF6 containing methyl as a wingtip substituent, and Ag2O shows the formation of a bimetallic silver NHC (NHC-Ag) complex, [(L(1)-4H)·2Ag]·2PF6, which is rotationally disordered over an inversion of symmetry of the space group P2(1)/c. Elemental analysis and solution-state (1)H and (13)C NMR studies confirm the above molecular formula of complex 1. When L(2)·4PF6 functionalized with the benzyl wingtip moiety is explored for similar complexation with Ag2O, the isolated complex 2 shows the formation of a simple NHC-Ag complex with molecular formula [(L(2)-4H)·2Ag]·2PF6, as observed in the case of 1. Interestingly, the reaction of L(3)·4PF6 containing ethyl as the wingtip substituent and Ag2O shows the formation of a silver-ion-assisted tetranuclear molecular box of [Ag4(L(3)-4H)2](4+) (3). PMID:23521517

Ahamed, B Nisar; Dutta, Ranjan; Ghosh, Pradyut

2013-04-15

179

Chemistry and biology of two novel gold(I) carbene complexes as prospective anticancer agents.  

Science.gov (United States)

Two novel gold carbene compounds, namely, chlorido (1-butyl-3-methyl-imidazole-2-ylidene) gold(I) (1) and bis(1-butyl-3-methyl-imidazole-2-ylidene) gold(I) (2), were prepared and characterized as prospective anticancer drug candidates. These compounds consist of a gold(I) center linearly coordinated either to one N-heterocyclic carbene (NHC) and one chloride ligand (1) or to two identical NHC ligands (2). Crystal structures were solved for both compounds, the resulting structural data being in good agreement with expectations. We wondered whether the presence of two tight carbene ligands in 2 might lead to biological properties distinct from those of the monocarbene complex 1. Notably, in spite of their appreciable structural differences, these two compounds manifested similarly potent cytotoxic actions in vitro when challenged against A2780 human ovarian carcinoma cells. In addition, both were able to overcome resistance to cisplatin in the A2780R line. Solution studies revealed that these gold carbene complexes are highly stable in aqueous buffers at physiological pH. Their reactivity with proteins was explored: no adduct formation was detected even upon a long incubation with the model proteins cytochrome c and lysozyme; in contrast, both compounds were able to metalate, to a large extent, the copper chaperone Atox-1, bearing a characteristic CXXC motif. The precise nature of the resulting gold-Atox-1 adducts was elucidated through ESI-MS analysis. On the basis of these findings, it is proposed that the investigated gold(I) carbene compounds are promising antiproliferative agents warranting a wider pharmacological evaluation. Most likely these gold compounds produce their potent biological effects through selective metalation and impairment of a few crucial cellular proteins. PMID:24547701

Messori, Luigi; Marchetti, Lorella; Massai, Lara; Scaletti, Federica; Guerri, Annalisa; Landini, Ida; Nobili, Stefania; Perrone, Gabriele; Mini, Enrico; Leoni, Piero; Pasquali, Marco; Gabbiani, Chiara

2014-03-01

180

Synthesis of axially chiral oxazoline–carbene ligands with an N-naphthyl framework and a study of their coordination with AuCl·SMe2  

Directory of Open Access Journals (Sweden)

Full Text Available Axially chiral oxazoline–carbene ligands with an N-naphthyl framework were successfully prepared, and their coordination behavior with AuCl·SMe2 was also investigated, affording the corresponding Au(I complexes in moderate to high yields.

Feijun Wang

2012-05-01

 
 
 
 
181

Stereoselectively fluorinated N-heterocycles: a brief survey.  

Science.gov (United States)

The stereoselective incorporation of fluorine atoms into N-heterocycles can lead to dramatic changes in the molecules' physical and chemical properties. These changes can be rationally exploited for the benefit of diverse fields such as medicinal chemistry and organocatalysis. This brief review will examine some of the effects that fluorine substitution can have in N-heterocycles, including changes to the molecules' stability, their conformational behaviour, their hydrogen bonding ability, and their basicity. Finally, some methods for the synthesis of stereoselectively fluorinated N-heterocycles will also be reviewed. PMID:24367435

Hu, Xiang-Guo; Hunter, Luke

2013-01-01

182

Stereoselectively fluorinated N-heterocycles: a brief survey  

Science.gov (United States)

Summary The stereoselective incorporation of fluorine atoms into N-heterocycles can lead to dramatic changes in the molecules’ physical and chemical properties. These changes can be rationally exploited for the benefit of diverse fields such as medicinal chemistry and organocatalysis. This brief review will examine some of the effects that fluorine substitution can have in N-heterocycles, including changes to the molecules’ stability, their conformational behaviour, their hydrogen bonding ability, and their basicity. Finally, some methods for the synthesis of stereoselectively fluorinated N-heterocycles will also be reviewed.

Hu, Xiang-Guo

2013-01-01

183

Cycloalkyl-based unsymmetrical unsaturated (U?)-NHC ligands: flexibility and dissymmetry in ruthenium-catalysed olefin metathesis.  

Science.gov (United States)

Air-stable Ru-indenylidene and Hoveyda-type complexes bearing new unsymmetrical unsaturated N-heterocyclic carbene (U2-NHC) ligands combining a mesityl unit and a flexible cycloalkyl moiety as N-substituents were synthesised. Structural features, chemical stabilities and catalytic profiles in olefin metathesis of this new library of cycloalkyl-based U2-NHC Ru complexes were studied and compared with their unsymmetrical saturated NHC-Ru homologues as well as a set of commercially available Ru-catalysts bearing either symmetrical SIMes or IMes NHC ligands. PMID:24647372

Rouen, Mathieu; Borré, Etienne; Falivene, Laura; Toupet, Loic; Berthod, Mikaël; Cavallo, Luigi; Olivier-Bourbigou, Hélène; Mauduit, Marc

2014-05-21

184

Cyclodehydrations leading to indene products having N-heterocyclic substituents.  

Science.gov (United States)

In this Note, we describe superacid-promoted cyclodehydrations leading to functionalized indenes. The product indenes are synthesized having N-heterocyclic substituents, including pyridyl, imidazolyl, pyrimdyl, and other groups. A mechanism is proposed involving dicationic, superelectrophilic intermediates. The protonated N-heterocyclic rings are shown to dramatically lower the LUMO energy level of the carboxonium electrophile (compared to that of a similar uncharged system). PMID:24841486

Boblak, Kenneth N; Klumpp, Douglas A

2014-06-20

185

Carbenic nitrile imines: properties and reactivity.  

Science.gov (United States)

Structures and properties of nitrile imines were investigated computationally at B3LYP and CCSD(T) levels. Whereas NBO analysis at the B3LYP DFT level invariably predicts a propargylic electronic structure, CCSD(T) calculations permit a clear distinction between propargylic, allenic, and carbenic structures. Nitrile imines with strong IR absorptions above ca. 2150 cm(–1) have propargylic structures with a CN triple bond (RCNNSiMe3 and R2BCNNBR2), and those with IR absorptions below ca. 2150 cm(–1) are allenic (HCNNH, PhCNNH, and HCNNPh). Nitrile imines lacking significant cumulenic IR absorptions at 1900–2200 cm(–1) are carbenic (R–(C:)–N?N–R?). Electronegative but lone pair-donating groups NR2, OR, and F stabilize the carbenic form of nitrile imines in the same way they stabilize “normal” singlet carbenes, including N-heterocyclic carbenes. NBO analyses at the CCSD(T) level confirm the classification into propargylic, allenic, and carbenic reactivity types. Carbenic nitrile imines are predicted to form azoketenes 21 with CO, to form [2+2] and [2+4] cycloadducts and borane adducts, and to cyclize to 1H-diazirenes of the type 24 in mildly exothermic reactions with activation energies in the range 29–38 kcal/mol. Such reactions will be readily accessible photochemically and thermally, e.g., under the conditions of matrix photolysis and flash vacuum thermolysis. PMID:24400935

Bégué, Didier; Wentrup, Curt

2014-02-01

186

A molecular iron catalyst for the acceptorless dehydrogenation and hydrogenation of N-heterocycles.  

Science.gov (United States)

A well-defined iron complex (3) supported by a bis(phosphino)amine pincer ligand efficiently catalyzes both acceptorless dehydrogenation and hydrogenation of N-heterocycles. The products from these reactions are isolated in good yields. Complex 3, the active catalytic species in the dehydrogenation reaction, is independently synthesized and characterized, and its structure is confirmed by X-ray crystallography. A trans-dihydride intermediate (4) is proposed to be involved in the hydrogenation reaction, and its existence is verified by NMR and trapping experiments. PMID:24877556

Chakraborty, Sumit; Brennessel, William W; Jones, William D

2014-06-18

187

A dimer of bis(N-heterocyclic carbenerhodium(I centres spanned by a dibenzo-18-crown-6 bridge from synchrotron radiation  

Directory of Open Access Journals (Sweden)

Full Text Available The compound (?-3,3?,3??,3???-{[2,5,8,15,18,21-hexaoxatricyclo[20.4.0.09,14]hexacosa-1(22,9,11,13,23,25-hexaene-11,12,24,25-tetrayl]tetrakis(methylene}tetrakis(1-methyl-1H-imidazol-2-ylbis[(?4-cycloocta-1,4-dienerhodium(I] bis(hexafluoridophosphate acetonitrile sesquisolvate dihydrate, [Rh2(C8H122(C40H42N8O6](PF62·1.5CH3CN·2H2O, crystallized from acetonitrile under an atmosphere of diethyl ether. In the crystal structure, the complex cation exhibits two square-planar RhI centres, each bound by a cyclooctadiene (COD ligand and by two adjacent imidazolylidene N-heterocyclic carbene (NHC donors from the same phenoxy ring of the {[dibenzo-18-crown-6-11,12,24,25-tetrayl]tetrakis(methylene}tetrakis(1-methyl-1H-imidazol-2-yl (L ligand. The dibenzo-crown ether bridge of L spans the Rh centres and forms hydrogen bonds with water molecules. One water molecule with half occupancy bridges adjacent macrocycles in the lattice. Another water with full occupancy forms weak hydrogen bonds to the crown ether O atoms and is, in turn, part hydrogen bonded by a lattice water with half occupancy. The latter water is within hydrogen-bonding distance of a fourth water also with partial occupancy. The result of these interactions is the formation of a layer in the ab plane. Two PF6? ions, one of which is twofold disordered, and one ordered and one twofold disordered (with 0.5 occupancy lattice acetonitrile molecules complete the crystal structure.

Stephen B. Colbran

2013-01-01

188

A Stable (Amino)(Phosphino)Carbene as bidentate ligand for Palladium and Nickel Complexes: Synthesis, Structure, and Catalytic Activity  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Two original complexes featuring an (amino)(phosphino)carbene ?2-bonded to the metal have been obtained in 60 and 80 % yields, by addition of the corresponding stable carbene to PdCl2(cod) and NiCl2(PPh3)2, respectively. Both complexes have been fully characterized including X-ray diffraction studies. The catalytic activity of the palladium complex has been evaluated for aryl amination reactions.

Teuma, Emmanuelle; Lyon-saunier, Ce?line; Gornitzka, Heinz; Mignani, Ge?rard; Baceiredo, Antoine; Bertrand, Guy

2005-01-01

189

The Synthesis of Some Perhydrobenzimidazolinium Salts and Their Application in Pd-Carbene Catalyzed Heck and Suzuki Reactions  

Directory of Open Access Journals (Sweden)

Full Text Available Novel 1,3-dialkylperhydrobenzimidazolinium chloride salts were prepared as precursors of N-heterocyclic carbenes 3a-e by reacting N,N’-dialkylcyclohexandiamine, triethyl orthoformate and ammonium chloride. The salts were characterized spectroscopically and the complexes formed in situ from Pd(OAc2 and 3 have been tested as catalysts in homogenous Heck and Suzuki reactions.

Murat Yi?it

2009-06-01

190

Reactivity of Phosphine- and Thioether-Tethered N-Heterocyclic Carbenes with Ruthenium Carbonyl  

Digital Repository Infrastructure Vision for European Research (DRIVER)

{The reactions of the NHC-functionalized phosphines 1-[{{2-(diphenylphosphino)phenyl}methyl]-3-methylimidazol-2-ylidene (Ph(2)PC(6)H(4)CH(2)ImMe) and 1-[{{2-(dicyclohexylphosphino)phenyl}methyl}-3-methylimidazol-2-ylidene (Cy(2)PC(6)H(4)CH(2)ImMe) and the NHC-functionalized thioether 1-{{(2-methylsulfide)ethyl}-3-methylimidazol-2-ylidene (MeSCH(2)CH(2)ImMe) with [Ru(3)(CO)(12)] have been studied. Both NHC-phosphines react with equimolar amounts of [Ru(3)(CO)(12)], in THF at room temperature, ...

Cabeza, Javier A.; Damonte, Marina; Garcia-alvarez, Pablo; Guadalupe Hernandez-cruz, M.; Kennedy, Alan R.

2012-01-01

191

Binuclear luminescent silver(I) - N-heterocyclic carbene complex derived from 1-picolyl-3-pyrimidylbenzimidazoliumhexaflurophosphate  

Science.gov (United States)

Starting from the novel proligand 1-picolyl-3-pyrimidylbenzimidazoliumhexaflurophosphate (1), the binuclear silver (I) complex (2) has been synthesized and both the compounds (1-2) have been characterized by different spectroscopic techniques. Finally, the solid state structure of 2 has been determined by single crystal X-ray diffraction studies. The role of non-covalent ?-? stacking interactions in stabilizing the self-assembly process appears to be of great importance in the title complex. Additional weak forces like C-H⋯? and anion⋯? interactions are also found to be operating in 2. A rare combination of these interactions in 2, of the type C-H⋯?/?⋯?/?⋯H-C and anion⋯?/?⋯?/?⋯anion, is observed, and this unusual supramolecular network is fully described here. Moreover, the proligand 1 is non-luminescent at room temperature in acetonitrile whereas complex 2 is luminescent. Closed shell d10-d10 interactions of Ag(I) is expected to be the sources of luminescence.

Adhikary, Sirsendu Das; Seth, Saikat Kumar; Senapati, Manas Ranjan; Dinda, Joydev

2013-06-01

192

N-Heterocyclic Carbene Complexes in other Transition Metal Mediated Reactions  

Science.gov (United States)

This chapter describes reactions involving NHC-transition metal complexes that have not been considered in previous chapters. The reactions are treated in six sections, presenting borrowing hydrogen reactions where C-C and C-N bonds are formed from alcohols in the first section. Then dehydration reactions either with or without coupling are discussed. The dynamic kinetic resolution of alcohols using a combination of NHC-TM catalysed racemisation coupled with enzyme-catalysed resolution is described. The next section considers the emerging area of dehydrogenation reactions, followed by a section discussing the isomerisation of alkene-containing substrates. The final section details hydrogen/deuterium exchange reactions within aryl and alkyl substrates.

Nixon, Tracy D.; Williams, Jonathan M. J.

193

N-Heterocyclic Carbene Complexes in Polymerisation, Oligomerisation and Telomerisation Reactions  

Science.gov (United States)

Over the past decade significant advances have been made in the fields of polymerisation, oligomerisation and telomerisation with metal-NHC catalysts. Complexes from across the transition series, as well as lanthanide examples, have been employed as catalysts for these reactions. Recent developments in the use of metal-NHC complexes in ?-olefin polymerisation and oligomerisation, CO/olefin copolymerisation, atom-transfer radical polymerisation (ATRP) and diene telomerisation are discussed in subsequent sections.

McGuinness, David S.; Cavell, Kingsley J.

194

Stereoelectronic basis for the kinetic resolution of N-heterocycles with chiral acylating reagents.  

Science.gov (United States)

The kinetic resolution of N-heterocycles with chiral acylating agents reveals a previously unrecognized stereoelectronic effect in amine acylation. Combined with a new achiral hydroxamate, this effect makes possible the resolution of various N-heterocycles by using easily prepared reagents. A transition-state model to rationalize the stereochemical outcome of this kinetic resolution is also proposed. PMID:24839065

Hsieh, Sheng-Ying; Wanner, Benedikt; Wheeler, Philip; Beauchemin, André M; Rovis, Tomislav; Bode, Jeffrey W

2014-06-10

195

Synthesis, characterization, and antimicrobial activity of silver carbene complexes derived from 4,5,6,7-tetrachlorobenzimidazole against antibiotic resistant bacteria†  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Silver N-heterocyclic carbene complexes have been shown to have great potential as antimicrobial agents, affecting a wide spectrum of both Gram-positive and Gram-negative bacteria. A new series of three silver carbene complexes (SCCs) based on 4,5,6,7-tetrachlorobenzimidazole has been synthesized, characterized, and tested against a panel of clinical strains of bacteria. The imidazolium salts and their precursors were characterized by elemental analysis, mass spectrometry, 1H and 13C NMR spec...

Wright, Brian D.; Shah, Parth N.; Mcdonald, Lucas J.; Shaeffer, Michael L.; Wagers, Patrick O.; Panzner, Matthew J.; Smolen, Justin; Tagaev, Jasur; Tessier, Claire A.; Cannon, Carolyn L.; Youngs, Wiley J.

2012-01-01

196

Electronic structure of transition metal carbene complexes  

International Nuclear Information System (INIS)

The analysis of the electronic structure of transition metal (V, Nb, Ta) carbene complexes is given. The properties and peculaiarities of the electronic structure of Fischer and Schrock type complexes are compared. The results of the most accurate nonempirical calculations of the simplest systems simulating the transition metal-carbene ligand and some less strict semiempirical and nonempirical calculations which permit to find out the regularities of the electronic structure of carbene complexes, depending on position of the metal in the Periodic system and the nature of ligand surrounding, are discussed. Main factors causing the stabilization of covalently bound or donor-acceptor systems are singled out

1984-01-01

197

Backbone tuning in indenylidene–ruthenium complexes bearing an unsaturated N-heterocyclic carbene  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The steric and electronic influence of backbone substitution in IMes-based (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) N-heterocyclic carbenes (NHC) was probed by synthesizing the [RhCl(CO)2(NHC)] series of complexes to quantify experimentally the Tolman electronic parameter (electronic) and the percent buried volume (%Vbur, steric) parameters. The corresponding ruthenium–indenylidene complexes were also synthesized and tested in benchma...

Urbina-blanco, C. Sar A.; Xavier Bantreil; Hervé Clavier; Slawin, Alexandra M. Z.; Nolan, Steven P.

2010-01-01

198

Gold complexes as antimicrobial agents: an overview of different biological activities in relation to the oxidation state of the gold ion and the ligand structure.  

Science.gov (United States)

Interest in antimicrobial gold complexes originated from the work of Robert Koch at the end of 19th century, who demonstrated that potassium dicyanidoaurate(I), K[Au(CN)2], showed activity against Mycobacterium tuberculosis, a causative agent of tuberculosis. Subsequently, a large number of gold(I) and gold(III) complexes have been evaluated as possible antimicrobial agents against a broad spectrum of bacteria, fungi and parasites. The first part of the present review article summarizes the results achieved in the field of antibacterial and antifungal activity of gold(I) and gold(III) complexes. The represented gold(I) complexes have been divided into three distinct classes based on the type of coordinated ligand: (i) complexes with phosphine-type ligands, (ii) complexes with N-heterocyclic carbene ligands and (iii) various other gold(I) complexes, while the results related to the antibacterial and antifungal gold(III) complexes have been mainly focused on the organometallic-type of complexes. The second section of this article represents findings obtained from the evaluation of antimalarial activity of gold complexes against chloroquine-sensitive and chloroquine-resistant strains of Plasmodium falciparum parasite. Antimalarial gold(I) and gold(III) complexes have been divided into the following classes, based on the nature of the coordinated ligand: (i) complexes with chloroquine and its derivatives, (ii) complexes with N-heterocyclic carbene ligands, (iii) complexes containing functionalised alkynes and (iv) thiosemicarbazonato ligands, as well as (v) other gold(I) and gold(III) complexes. In the last section of the review, gold(I) and gold(III) complexes have been reported to be potential agents against parasites that cause amoebiasis, leishmaniasis and trypanosomiasis. A systematic summary of these results could contribute to the future design of new gold(I) and gold(III) complexes as potential antimicrobial agents. PMID:24598838

Gliši?, Biljana ?; Djuran, Miloš I

2014-04-28

199

(C^C*) Cyclometalated binuclear N-heterocyclic biscarbene platinum(II) complexes--highly emissive phosphorescent emitters.  

Science.gov (United States)

A series of bimetallic N-heterocyclic carbene (NHC) platinum(II) complexes with the general formula [Pt(NHC)(L)]2Ph were synthesized, which are composed of two [Pt(NHC)(L)] (L = acetylacetone, dipivaloylmethane or dimesitoylmethane; NHC = 3-methylimidazole or 3-benzylimidazole) fragments. Both fragments are cyclometalated to the same central phenyl ring in the para- or meta-position. All complexes have been fully characterized by standard techniques, two of them additionally by solid-state structures. Photoluminescence properties were investigated and very high quantum yields of 76-93% at room temperature have been observed. For a single-matrix device with an emitter concentration of 30%, a current efficiency of 25 cd A(-1), an external quantum efficiency of 8.5%, and a luminance efficiency of 10 lm W(-1) were achieved at 300 cd m(-2). Density functional theory (DFT) calculations were performed to support experimental data and gain further insight into the photoluminescence behaviour. PMID:23689316

Tronnier, Alexander; Strassner, Thomas

2013-07-21

200

Triruthenium and triosmium carbonyl clusters containing chiral bidentate NHC-thiolate ligands derived from levamisole.  

Science.gov (United States)

The trinuclear complexes [M3(mu-Cl)(mu-S approximately CH)(CO)9] (M=Ru, Os; S approximately CH=1-ethylenethiolate-3-H-4-(S)-phenylimidazolin-2-ylidene) and [M3(mu-H)(mu-S approximately CMe)(CO)9] (M=Ru, Os; S approximately CMe=1-ethylenethiolate-3-methyl-4-(S)-phenylimidazolin-2-ylidene) have been prepared by treating [Ru3(CO)12] and [Os3(CO)10(MeCN)2] with levamisolium chloride or [M3(mu-H)(CO)11]- with methyl levamisolium triflate, respectively. The chiral N-heterocyclic carbene-thiolate ligands S approximately CH and S approximately CMe arise from the oxidative addition of the C-S bond of levamisolium or methyl levamisolium cations to anionic trinuclear clusters. PMID:17028704

Cabeza, Javier A; da Silva, Iván; del Río, Ignacio; Sánchez-Vega, M Gabriela

2006-09-01

 
 
 
 
201

Catalytic Enantioselective Total Syntheses of Bakkenolides I, J and S: Application of a Carbene-Catalyzed Desymmetrization  

Digital Repository Infrastructure Vision for European Research (DRIVER)

A general strategy for the catalytic asymmetric syntheses of the bakkenolides is reported. The key bond-forming step involves an N-heterocyclic carbene catalyzed desymmetrization of a 1,3-diketone to form three new bonds in one step with excellent enantio– and diastereoselectivity. This intramolecular reaction allows direct access to the hydrindane core of the bakkenolides family and enables a facile synthesis of these natural products.

Phillips, Eric M.; Roberts, John M.; Scheidt, Karl A.

2010-01-01

202

Water oxidation catalyzed by mononuclear ruthenium complexes with a 2,2'-bipyridine-6,6'-dicarboxylate (bda) ligand: how ligand environment influences the catalytic behavior.  

Science.gov (United States)

A new water oxidation catalyst [Ru(III)(bda)(mmi)(OH2)](CF3SO3) (2, H2bda = 2,2'-bipyridine-6,6'-dicarboxylic acid; mmi = 1,3-dimethylimidazolium-2-ylidene) containing an axial N-heterocyclic carbene ligand and one aqua ligand was synthesized and fully characterized. The kinetics of catalytic water oxidation by 2 were measured using stopped-flow technique, and key intermediates in the catalytic cycle were probed by density functional theory calculations. While analogous Ru-bda water oxidation catalysts [Ru(bda)L2] (L = pyridyl ligands) are supposed to catalyze water oxidation through a bimolecular coupling pathway, our study points out that 2, surprisingly, undergoes a single-site water nucleophilic attack (acid-base) pathway. The diversion of catalytic mechanisms is mainly ascribed to the different ligand environments, from nonaqua ligands to an aqua ligand. Findings in this work provide some critical proof for our previous hypothesis about how alternation of ancillary ligands of water oxidation catalysts influences their catalytic efficiency. PMID:24422472

Staehle, Robert; Tong, Lianpeng; Wang, Lei; Duan, Lele; Fischer, Andreas; Ahlquist, Mårten S G; Sun, Licheng; Rau, Sven

2014-02-01

203

Copper(I)-catalyzed nucleophilic addition of ynamides to acyl chlorides and activated N-heterocycles.  

Science.gov (United States)

The addition of ynamides to acyl chlorides and N-heterocycles activated in situ with ethyl chloroformate has been accomplished at room temperature using copper iodide as catalyst. This economical and practical carbon-carbon bond formation provides convenient access to a variety of 3-aminoynones from aliphatic and aromatic acyl chlorides in up to 99% yield. The addition to pyridines and quinolines occurs under almost identical conditions and proceeds with good to high regioselectivity, producing the corresponding 1,2-dihydro-N-heterocycles in up to 95% yield. PMID:24724797

Zhang, Peng; Cook, Andrea M; Liu, Yang; Wolf, Christian

2014-05-01

204

Correlations between electrochemical and spectrochemical parameters of ruthenium sulfoxides series with N-heterocyclic  

International Nuclear Information System (INIS)

A systematic study of Ru Cl sub(2) (DMSO) sub(2) L sub(2) derivates, where L = N-heterocyclic base is described, contributing for a best understanding of chemical behaviour and electronic structure of the ruthenium sulfoxides. The correlations between the electrochemical and the spectroscopical parameters of the serie are presented with more emphasis. (author)

1990-04-08

205

Formation of azaborines by photoelimination of B,N-heterocyclic compounds.  

Science.gov (United States)

Highly fluorescent ?-conjugated polycyclic azaborines can be prepared from B,N-heterocyclic compounds with a BR2 -CH2 unit through the elimination of an R-H molecule (see scheme). These clean photoelimination reactions occur both in solution and in polymers doped with the precursors. PMID:23564599

Lu, Jia-Sheng; Ko, Soo-Byung; Walters, Nicholas R; Kang, Youngjin; Sauriol, Francoise; Wang, Suning

2013-04-22

206

On the reactivity and stability of electrogenerated N-heterocyclic carbene in parent 1-butyl-3-methyl-1H-imidazolium tetrafluoroborate: Formation and use of N-heterocyclic carbene-CO2 adduct as latent catalyst  

International Nuclear Information System (INIS)

A simple electrolysis (under galvanostatic conditions) of the room temperature ionic liquid 1-butyl-3-methyl-1H-imidazolium tetrafluoroborate, BMIm-BF4, yields, after bubbling CO2 into the catholyte, the adduct NHC-CO2. The considerable stability of this NHC-CO2 adduct, at room temperature, in the parent ionic liquid as solvent, has been compared with the one of free NHC in the same BMIm-BF4. The BMIm-BF4 solution containing NHC-CO2 adduct, suitably triggered (US irradiation or 120 °C), is able to release free NHC. The NHC-CO2 adduct usefulness has been demonstrated using it as efficient latent catalyst, in BMIm-BF4 as solvent, in the benzoin condensation and in the oxidative esterification of cinnamaldehyde with benzyl alcohol

2013-10-30

207

Synthesis and structure of ruthenium-silylene complexes: activation of Si-Cl bonds in N-heterocyclic silanes.  

Science.gov (United States)

Ru(0) complexes of bis(imino)pyridine ligands, [eta2-N3]Ru(eta6-Ar) and {[N3]Ru}2(mu-N2), where Ar = C6H6 or C6H5Me and [N3] = 2,6-(MesN=CMe)2C5H3N, react with N-heterocyclic silicon(IV) compounds to yield Ru(II) silylene complexes of the type [N3]Ru(X)(Cl){Si(NN)} (X = H, Cl, and Si(NN) = N,N'-bis(neopentyl)-1,2-phenylenedi(amino)silylene). The activation of two groups on the silane occurs in a stepwise fashion: initial oxidative addition of a Si-X bond, followed by 1,2-migration (alpha-elimination) of the Si-Cl group to the metal. Reversible dissociation from the Ru(II) center leads to free silylene, which can be preferentially trapped with Ru(0) complexes to generate a zero-valent silylene complex, [N3]Ru(N2){Si(NN)}, which also contains a terminal dinitrogen ligand. PMID:16669664

Yoo, Hyojong; Carroll, Patrick J; Berry, Donald H

2006-05-10

208

Chemoselective N-heterocyclic carbene-catalyzed cross-benzoin reactions: importance of the fused ring in triazolium salts.  

Science.gov (United States)

Morpholinone- and piperidinone-derived triazolium salts are shown to catalyze highly chemoselective cross-benzoin reactions between aliphatic and aromatic aldehydes. The reaction scope includes ortho-, meta-, and para-substituted benzaldehyde derivatives with a range of electron-donating and -withdrawing groups as well as branched and unbranched aliphatic aldehydes. Catalytic loadings as low as 5 mol % give excellent yields in these reactions (up to 99%). PMID:24809936

Langdon, Steven M; Wilde, Myron M D; Thai, Karen; Gravel, Michel

2014-05-28

209

Enantioselective Conjugate Silyl Additions to Cyclic and Acyclic Unsaturated Carbonyls Catalyzed by Cu Complexes of Chiral N-Heterocyclic Carbenes  

Digital Repository Infrastructure Vision for European Research (DRIVER)

An efficient Cu-catalyzed protocol for enantioselective addition of a dimethylphenylsilanyl group to a wide range of cyclic and acyclic unsaturated ketones, esters, acrylonitriles and ?,?,?,?-dienones is disclosed. Reactions are performed in the presence of 1–2 mol % of commercially available and inexpensive CuCl, a readily accessible monodentate imidazolinium salt as well as commercially available (dimethylphenylsilyl)pinacolatoboron. Cu-catalyzed enantioselective conjugate additions p...

2010-01-01

210

N-Heterocyclic carbene–palladium catalysts for the direct arylation of pyrrole derivatives with aryl chlorides  

Digital Repository Infrastructure Vision for European Research (DRIVER)

New Pd–NHC complexes have been synthesized and employed for palladium-catalyzed direct arylation of pyrrole derivatives by using electron-deficient aryl chlorides as coupling partners. The desired coupling products were obtained in moderate to good yields by using 1 mol % of these air-stable palladium complexes. This is an advantage compared to the procedures employing air-sensitive phosphines, which have been previously shown to promote the coupling of aryl chlorides with heteroarenes.

O?zdemir, Ismail; Gu?rbu?z, Nevin; Kalog?lu, Nazan; Dog?an, O?znur; Kalog?lu, Murat; Bruneau, Christian; Doucet, Henri

2013-01-01

211

Dehydrogenative Coupling of Primary Alcohols To Form Esters Catalyzed by a Ruthenium N-Heterocyclic Carbene Complex  

DEFF Research Database (Denmark)

The ruthenium complex [RuCl2(IiPr)(p-cymene)] catalyzes the direct condensation of primary alcohols into esters and lactones with the release of hydrogen gas. The reaction is most effective with linear aliphatic alcohols and 1,4-diols and is believed to proceed with a ruthenium dihydride as the catalytically active species.

Sølvhøj, Amanda Birgitte; Madsen, Robert

2011-01-01

212

Concise formal synthesis of (-)-7-deoxyloganin via N-heterocyclic carbene catalysed rearrangement of ?,?-unsaturated enol esters.  

Science.gov (United States)

NHC catalysed rearrangement of ?,?-unsaturated enol esters derived from formyl acetates and cyclopentyl annulated ?,?-unsaturated acids provides the cyclopentapyranone core of (-)-7-deoxyloganin (1) with diastereo- and chemoselectivity in 6 steps starting from (-)-citronellal. The elaboration to the natural product has been investigated using two new approaches. The most successful intercepts our previous work on (-)-7-deoxyloganin (1) allowing completion of a formal total synthesis in 10-steps. PMID:22024770

Candish, Lisa; Lupton, David W

2011-12-01

213

N-Heterocyclic carbene (NHC)-modulated Pd/Cu cocatalyzed three-component synthesis of 2,6-diarylquinolines.  

Science.gov (United States)

Two new NHC adducts of cyclopalladated ferrocenylpyrazine complexes 1-2 have been prepared and characterized. An efficient NHC-modulated Pd/Cu cocatalyzed three-component coupling reaction for the synthesis of 2,6-diarylquinolines from aminobenzyl alcohols, aryl ketones, and arylboronic acids in air is described. The reaction involves oxidation, cyclization and Suzuki reactions. The luminescence of the resulting arylquinolines 3-30 was also investigated. PMID:24710846

Xu, Chen; Li, Hong-Mei; Yuan, Xiao-Er; Xiao, Zhi-Qiang; Wang, Zhi-Qiang; Fu, Wei-Jun; Ji, Bao-Ming; Hao, Xin-Qi; Song, Mao-Ping

2014-05-21

214

Neutral sulfur nucleophiles: synthesis of thioethers and thioesters by substitution reactions of N-heterocyclic carbene boryl sulfides and thioamides.  

Science.gov (United States)

Newly discovered boryl sulfides and N-borylthioamides are shown to serve as neutral sources of sulfur nucleophiles in substitutions reactions. For example, heating of diMe-Imd-BH(SPh)2 with benzyl bromides, primary bromides, or acid chlorides provides the corresponding thioethers or thioesters in high yields. Likewise, N-phenyltetrazole thioethers/esters are made from a readily available N-borylthionotetrazole. The formation of the boryl sulfide and its onward nucleophilic substitution can be telescoped down to a one-pot reaction whose components are an NHC-borane (NHC-BH3), a disulfide, and an electrophile. PMID:24802672

Pan, Xiangcheng; Curran, Dennis P

2014-05-16

215

Transition-metal phosphors with cyclometalating ligands: fundamentals and applications.  

Science.gov (United States)

One goal of this critical review is to provide advanced methodologies for systematic preparation of transition-metal based phosphors that show latent applications in the field of organic light emitting diodes (OLEDs). We are therefore reviewing various types of cyclometalating chelates for which the favorable metal-chelate bonding interaction, on the one hand, makes the resulting phosphorescent complexes highly emissive in both fluid and solid states at room temperature. On the other hand, fine adjustment of ligand-centered pi-pi* electronic transitions allows tuning of emission wavelength across the whole visible spectrum. The cyclometalating chelates are then classified according to types of cyclometalating groups, i.e. either aromatic C-H or azolic N-H fragment, and the adjacent donor fragment involved in the formation of metallacycles; the latter is an N-containing heterocycle, N-heterocyclic (NHC) carbene fragment or even diphenylphosphino group. These cyclometalating ligands are capable to react with heavy transition-metal elements, namely: Ru(II), Os(II), Ir(III) and Pt(II), to afford a variety of highly emissive phosphors, for which the photophysical properties as a function of chelate or metal characteristics are systematically discussed. Using Ir(III) complexes as examples, the C--N chelates possessing both C-H site and N-heterocyclic donor group are essential for obtaining phosphors with emission ranging from sky-blue to saturated red, while the N--N chelates such as 2-pyridyl-C-linked azolates are found useful for serving as true-blue chromophores due to their increased ligand-centered pi-pi* energy gap. Lastly, the remaining NHC carbene and benzyl phosphine chelates are highly desirable to serve as ancillary chelates in localizing the electronic transition between the metal and remaining lower energy chromophoric chelates. As for the potential opto-electronic applications, many of them exhibit remarkable performance data, which are convincing to pave a broad avenue for further development of all types of phosphorescent displays and illumination devices (94 references). PMID:20111785

Chi, Yun; Chou, Pi-Tai

2010-02-01

216

Structure and properties of N-heterocycle-containing benzotriazoles as UV absorbers  

Science.gov (United States)

Two N-heterocycle-containing benzotriazole compounds (CBHDCP, CBMPP) were synthesized and their structures were confirmed by FT-IR, 1H NMR, mass spectroscopy and elemental analysis. Their spectral properties compared with that of a common commercial benzotriazole UV absorber Tinuvin 326 in solvents were measured and analyzed using molecular orbital calculations. It was found that the experimentally obtained optical properties were in close agreement with the theoretically obtained results. Besides, CBMPP is much superior to CBHDCP acting as an effective UV absorber on polyethylene terephthalate fabric due to its combinative higher exhaustion and higher molar extinction coefficient. The exhaustion difference on polyethylene terephthalate fabric among N-heterocycle-containing benzotriazoles (CBHDCP, CBMPP) and Tinuvin 326 were then measured and explained by the solubility parameter theory. The results agreed with the rule that compounds with higher exhaustion on fiber have solubility parameter much closer to that of fiber polymer.

Cui, Zhihua; Li, Xin; Wang, Xidong; Pei, Kemei; Chen, Weiguo

2013-12-01

217

Formation and ecotoxicity of N-heterocyclic compounds on ammoxidation of mono- and polysaccharides.  

Science.gov (United States)

Ammoxidation of technical lignins under mild conditions is a suitable approach to artificial humic substances. However, carbohydrates as common minor constituents of technical lignins have been demonstrated to be a potential source of N-heterocyclic ecotoxic compounds. Ethyl acetate extracts of ammoxidation mixtures of the monosaccharides glucose and xylose exhibited considerable growth inhibiting activity in the OECD 201 test, with 4-methyl-1H-imidazole, 4-(hydroxymethyl)-1H-imidazole, and 3-hydroxypyridine being the most active compounds. The amount of N-heterocyclic compounds formed at moderate ammoxidation conditions (70 °C, 0.2 MPa O2, 3 h) was significantly lower for the polysaccharides cellulose and xylan (16-30 ?g/g of educt) compared to glucose (15.4 mg). Ammoxidation at higher temperature is not recommendable for carbohydrate-rich materials as much higher amounts of N-heterocyclic compounds were formed from both monosaccharides (100 °C: 122.4-160.5 mg/g of educt) and polysaccharides (140 °C: 5.52-16.03 mg/g of educt). PMID:23967874

Klinger, Karl Michael; Liebner, Falk; Fritz, Ines; Potthast, Antje; Rosenau, Thomas

2013-09-25

218

Supramolecular hydrogen-bonded structures between melamine and N-heterocycles  

Science.gov (United States)

Melamine (M) forms hydrogen bonded adducts with the N-heterocycles tris(2-pyridyl)-triazine (TPTZ) and tetrakis(4-N-methylpyridinumyl)porphyrin tetratosylate (TMPyP-Tos). The 4:3 adduct 4M·3TPTZ was obtained under solvent-free conditions and the 3:1 adduct 3M·TMPyP-Tos·9H2O in aqueous solution. In both adduct phases N-heterocycles are arranged in ?-? stacks and melamine molecules are arranged perpendicular to these stacks. Whereas cohesion between the extended ?-systems of N-heterocycles is mainly achieved by dispersive interactions no stacking interactions between melamine molecules occur. Melamine units are orientated in such a way to be located most suitable as donors and acceptors, respectively, for a dense hydrogen bonding network. In 4M·3TPTZ all molecules are involved in hydrogen bonding, whereas in 3M·TMPyP-Tos·9H2O hydrogen bridges are primarily formed between melamine, tosylate and water units and the macrocycle TMPyP is mainly bound by electrostatic interactions.

Makowski, Sophia J.; Lacher, Monika; Lermer, Claudia; Schnick, Wolfgang

2012-04-01

219

Direct, catalytic, and regioselective synthesis of 2-alkyl-, aryl-, and alkenyl-substituted N-heterocycles from N-oxides.  

Science.gov (United States)

A one-step transformation of heterocyclic N-oxides to 2-alkyl-, aryl-, and alkenyl-substituted N-heterocycles is described. The success of this broad-scope methodology hinges on the combination of copper catalysis and activation by lithium fluoride or magnesium chloride. The utility of this method for the late-stage modification of complex N-heterocycles is exemplified by facile syntheses of new structural analogues of several antimalarial, antimicrobial, and fungicidal agents. PMID:24410049

Larionov, Oleg V; Stephens, David; Mfuh, Adelphe; Chavez, Gabriel

2014-02-01

220

SnAP reagents for the one-step synthesis of medium-ring saturated N-heterocycles from aldehydes  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Interest in saturated N-heterocycles as scaffolds for the synthesis of bioactive molecules is increasing. Reliable and predictable synthetic methods for the preparation of these compounds, especially medium-sized rings, are limited. We describe the development of SnAP (Sn amino protocol) reagents for the transformation of aldehydes into seven-, eight- and nine-membered saturated N-heterocycles. This process occurs under mild, room-temperature conditions and offers exceptional substrate scope ...

2014-01-01

 
 
 
 
221

Synthesis and Structural Investigations of manganese carbene complexes  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The study involves the synthesis and structural characterization of manganese carbene complexes. The synthesis of dimanganese monocarbene complexes [Mn2(CO)9{C(OEt)(heteroaryl)}] was done via the classical Fischer method, and a range of complexes containing heteroaromatic substituents, e.g. 2,2'-bithiophene, thiophene, furan and N-methyl pyrrole, was isolated. These complexes displayed a novel configuration with the carbene ligand in the axial position, in contrast to the equatorial position ...

Bezuidenhout, Daniela Ina

2006-01-01

222

Recent advances in the field of multicarbene and multimetal carbene complexes of the Fischer-type  

Digital Repository Infrastructure Vision for European Research (DRIVER)

This review article covers the development of Fischer carbene complexes since the year 2000, with specific focus on carbene complexes bearing metal-containing fragments as substituents, as well as multicarbene systems. The role of the metal-containing substituents on the character and reactivity of such complexes are discussed. In addition, larger systems containing more than one carbene ligand are also covered (rodlike biscarbenes, chelates, macrosystems, etc.) in terms of the sy...

Bezuidenhout, Daniela Ina; Lotz, Simon; Liles, David C.; Westhuizen, Belinda

2012-01-01

223

Synthesis of P,N-Heterocycles from ?-Amino-H-Phosphinates: Conformationally Restricted ?-Amino Acid Analogs  

Digital Repository Infrastructure Vision for European Research (DRIVER)

P,N-Heterocycles (3-hydroxy-1,3-azaphospholane and 3-hydroxy-1,3-azaphosphorinane-3-oxide) are synthesized in moderate yield from readily available ?-amino-H-phosphinates and aldehydes or ketones via an intramolecular Kabachnik-Fields reaction. The products are conformationally restricted phosphinic analogs of ?-amino acids. The multi-gram scale syntheses of the H2N(CH2)nPO2H2 phosphinic precursors (n = 1, 2, 3) and some derivatives are also described.

Queffelec, Cle?mence; Ribie?re, Patrice; Montchamp, Jean-luc

2008-01-01

224

Titanium-based molecular squares and rectangles: syntheses by self-assembly reactions of titanocene fragments and aromatic N-heterocycles.  

Science.gov (United States)

This paper reports on the potential of titanium compounds as building blocks for supramolecular polygons. Self-assembly reactions of low-valent titanocene units and N-heterocyclic bridging ligands lead to novel titanium-based supramolecular squares. Pyrazine (3), 4,4'-bipyridine (4), and tetrazine (5) were used as bridging ligands, and the acetylene complexes [Cp2Ti{eta2-C2(SiMe3)2}] (1) and [(tBuCp)2Ti{eta2-C2(SiMe3)2}] (2) as sources of titanocene fragments. Molecular rectangles can be synthesized by stepwise reduction of the titanocene dichlorides [Cp(2)TiCl2] and [(tBuCp)2TiCl2] and consecutive coordination of two different bridging ligands. The resulting complexes are the first examples of molecular rectangles containing bent metallocene corner units. Single-crystal X-ray analyses of the tetranuclear compounds revealed the geometric properties of the molecular polygons in the solid state. Comparison of bond lengths and angles in coordinated and free ligands reveals the reduced state of the bridging ligand in the low-valent titanium compounds. The syntheses and properties of these novel, highly air- and moisture-sensitive compounds are discussed. PMID:15612051

Kraft, Susanne; Hanuschek, Edith; Beckhaus, Rüdiger; Haase, Detlev; Saak, Wolfgang

2005-01-21

225

Metallocene carbene chemistry  

International Nuclear Information System (INIS)

A review of chemical properties of group IV element chamfered metallocenes Cp2M (M=Ti, Zr, Hf) is presented. It is shown that specific features of their stereoelectronic structure and clearly pronounced oxophily of M metals permit using the complexes as components of reaction systems involving carbenes. The use of ruthenium carbene complexes for realizing the catalytic metathesis of lateral olefin substituents in the chamfered metallocenes is described by way of example

2004-05-01

226

SnAP reagents for the one-step synthesis of medium-ring saturated N-heterocycles from aldehydes  

Science.gov (United States)

Interest in saturated N-heterocycles as scaffolds for the synthesis of bioactive molecules is increasing. Reliable and predictable synthetic methods for the preparation of these compounds, especially medium-sized rings, are limited. We describe the development of SnAP (Sn amino protocol) reagents for the transformation of aldehydes into seven-, eight- and nine-membered saturated N-heterocycles. This process occurs under mild, room-temperature conditions and offers exceptional substrate scope and functional-group tolerance. Air- and moisture-stable SnAP reagents are prepared on a multigram scale from inexpensive starting materials by simple reaction sequences. These new reagents and processes allow widely available aryl, heteroaryl and aliphatic aldehydes to be converted into diverse N-heterocycles, including diazepanes, oxazepanes, diazocanes, oxazocanes and hexahydrobenzoxazonines, by a single synthetic operation.

Vo, Cam-Van T.; Luescher, Michael U.; Bode, Jeffrey W.

2014-04-01

227

Fast CO2 sequestration, activation, and catalytic transformation using N-heterocyclic olefins.  

Science.gov (United States)

N-Heterocyclic Olefin (NHO) with high electronegativity at the terminal carbon atom was found to show a strong tendency for CO2 sequestration, affording a CO2 adduct (NHO-CO2). X-ray single crystal analysis revealed the bent geometry of the binding CO2 in the NHO-CO2 adducts with an O-C-O angle of 127.7-129.9°, dependent on the substitute groups of N-heterocyclic ring. The length of the C(carboxylate)-C(NHO) bond is in the range of 1.55-1.57 Å, significantly longer than that of the C(carboxylate)-C(NHC) bond (1.52-1.53 Å) of the previously reported NHC-CO2 adducts. The FTIR study by monitoring the ?(CO2) region of transmittance change demonstrated that the decarboxylation of NHO-CO2 adducts is easier than that of the corresponding NHC-CO2 adducts. Notably, the NHO-CO2 adducts were found to be highly active in catalyzing the carboxylative cyclization of CO2 and propargylic alcohols at mild conditions (even at ambient temperature and 0.1 MPa CO2 pressure), selectively giving ?-alkylidene cyclic carbonates in good yields. The catalytic activity is about 10-200 times that of the corresponding NHC-CO2 adducts at the same conditions. Two reaction paths regarding the hydrogen at the alkenyl position of cyclic carbonates coming from substrate (path A) or both substrate and catalyst (path B) were proposed on the basis of deuterium labeling experiments. The high activity of NHO-CO2 adduct was tentatively ascribed to its low stability for easily releasing the CO2 moiety and/or the desired product, a possible rate-limiting step in the catalytic cycle. PMID:23865980

Wang, Yan-Bo; Wang, Yi-Ming; Zhang, Wen-Zhen; Lu, Xiao-Bing

2013-08-14

228

Fused Tetrazoles as Azide Surrogates in Click Reaction: Efficient Synthesis of N-Heterocycle-substituted 1,2,3-Triazoles  

Science.gov (United States)

It has been shown that various pyrido-, quinolino-, pyrazino- and qunoxalinotetrazoles can efficienly be used as azide components in the Cu-catalyzed click reaction with alkynes. This method allows for efficient synthesis of a wide variety of N-heterocyclic derivatives of 1,2,3-triazoles.

Chattopadhyay, Buddhadeb; Rivera Vera, Claudia I.; Chuprakov, Stepan; Gevorgyan, Vladimir

2010-01-01

229

Fused tetrazoles as azide surrogates in click reaction: efficient synthesis of N-heterocycle-substituted 1,2,3-triazoles.  

Science.gov (United States)

It has been shown that various pyrido-, quinolino-, pyrazino-, and quinoxalinotetrazoles can be used efficiently as azide components in Cu-catalyzed click reaction with alkynes. This method allows for efficient synthesis of a wide variety of N-heterocyclic derivatives of 1,2,3-triazoles. PMID:20380424

Chattopadhyay, Buddhadeb; Vera, Claudia I Rivera; Chuprakov, Stepan; Gevorgyan, Vladimir

2010-05-01

230

Formation of quinones, indanones and furans by the reaction of molybdenum carbene complexes with alkynes  

International Nuclear Information System (INIS)

(Alkoxy)carbene complexes of molybdenum react with terminal alkynes to give carbene annulation of cycloaddition products, the skeleton of which depends on the carbene substitution pattern and the alkyne used. (CO)5Mo=C(OMe)-p-tol undergoes carbene annulation upon reaction with trimethylsilylacetylene leading to naphthoquinone after oxidative work-up. Similar products are obtained from (CO)5Mo=C(OMe)2-furyl and hex-1-yne or oct-1-yne. The reaction of these alkynes results in the formation of indanones as five-membered annulation products. In the presence of 3.3-dimethylbut-1-yne the (phenyl) carbene ligands act as a C1-synthon, which is incorporated into the furan cycloaddition products

1992-01-01

231

Synthesis, Characterization, and Reactivity of a Uranium(VI) Carbene Imido Oxo Complex.  

Science.gov (United States)

We report the uranium(VI) carbene imido oxo complex [U(BIPM(TMS) )(NMes)(O)(DMAP)2 ] (5, BIPM(TMS) =C(PPh2 NSiMe3 )2 ; Mes=2,4,6-Me3 C6 H2 ; DMAP=4-(dimethylamino)pyridine) which exhibits the unprecedented arrangement of three formal multiply bonded ligands to one metal center where the coordinated heteroatoms derive from different element groups. This complex was prepared by incorporation of carbene, imido, and then oxo groups at the uranium center by salt elimination, protonolysis, and two-electron oxidation, respectively. The oxo and imido groups adopt axial positions in a T-shaped motif with respect to the carbene, which is consistent with an inverse trans-influence. Complex 5 reacts with tert-butylisocyanate at the imido rather than carbene group to afford the uranyl(VI) carbene complex [U(BIPM(TMS) )(O)2 (DMAP)2 ] (6). PMID:24842784

Lu, Erli; Cooper, Oliver J; McMaster, Jonathan; Tuna, Floriana; McInnes, Eric J L; Lewis, William; Blake, Alexander J; Liddle, Stephen T

2014-06-23

232

NHC-coordinated silagermenylidene functionalized in allylic position and its behaviour as a ligand.  

Science.gov (United States)

Vinylidenes are common in transition metal chemistry with catalytic applications in alkene and alkyne metathesis. We report here the isolation of a heavier analogue of vinylidene, an ?-chlorosilyl functionalized silagermenylidene stabilized by an N-heterocyclic carbene (NHC). Silagermenylidene (Tip2Cl)Si-(Tip)Si=Ge·NHC(iPr2Me2) (4-E/Z; Tip = 2,4,6-(i)Pr3C6H2; NHC(iPr2Me2) = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) is available as an E/Z-equilibrium mixture from Tip2Si=Si(Tip)Li and NHC(iPr2Me2)·GeCl2. Reaction of 4-E/Z with Fe2(CO)9 affords a silagermenylidene Fe(CO)4 complex, which slowly isomerizes to its E-isomer at 25 °C. A rearranged Fe(CO)3 complex with an allylic SiGeSi ligand is obtained as a side product at 65 °C. PMID:24503610

Jana, Anukul; Majumdar, Moumita; Huch, Volker; Zimmer, Michael; Scheschkewitz, David

2014-04-01

233

Mineralization of polycyclic and n-heterocyclic aromatic compounds in hydrocarbon-contaminated soils  

International Nuclear Information System (INIS)

The comparative mineralization of eight polycyclic aromatic compounds in five soils collected from an abandoned coal tar refinery in eastern Ohio was determined. The soils showed differences only in total extractable hydrocarbon content of the soil chemical characteristics measured. The compounds studied included five polycyclic aromatic hydrocarbons (phenanthrene, anthracene, pyrene, and carcinogenic benz[a]anthracene and benzo[a]pyrene) and three N-heterocyclic aromatics (9H-carbazole, and carcinogenic 7H-dibenzo[c,g]carbazole and dibenz[a,j]acridine). Mineralization was measured by serum bottle radiorespirometry. Only phenanthrene, anthracene, pyrene, benz[a]anthracene, and carbazole were mineralized in the soils after 64 d. Two of the soils with eight to 15 times the hexane -extractable hydrocarbon content consistently showed more rapid initial rates and higher overall extents of mineralization compared to the other three soils. Overall extents of mineralization ranged from 38 to 55% for phenanthrene, 10 to 60% for anthracene, 25 to 70% for pyrene, background to 40% for benz[a]anthracene, and 25 to 50% for carbazole after 64 d. Extents of mineralization by indigenous soil microbiota appear to be more dependent on the chemical characteristics of the soil and not soil total biomass and activity. Cultures capable of degrading phenanthrene, anthracene, and pyrene were obtained following enrichment techniques. A Mycobacterium sp. capable of degrading these three compounds was isolated and reintroduced into two of the soils, resulting in mineralization enhanced above that of the indigenous soil microbial population. These data indicate that the future success of bioremediation methods relies on the characterization of environmental parameters affecting microbial degradation as well as the isolation of microbial populations that can reduce toxicity in the environment

1995-03-01

234

Donor Substituted Sulfonyl Carbenes, 2: Organothio Sulfonyl Carbenes  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Organothio sulfonyl carbenes 3 have been generated via ylid thermolysis or via a-elimination starting from a-chloro a-organothio sulfones and their derivatives. They have been captured by suitable nucleophilic trapping reagents (diazomethane, enol ethers, and others). Their nucleophilic carbenoid precursors could be trapped by an electrophilic olefin (ketene dithioacetal S,S-dioxides as Michael acceptors). Stable carbene Z-dimers could be obtained under various conditions. Bromine catalyzed i...

Abdel Wahab, Aboel Magd A.; Bu?gler, Stephan; Eigen, Peter; Jager, Ju?rgen; Jost, Klaus; Schank, Kurt

1994-01-01

235

Rh-Catalyzed rearrangement of vinylcyclopropane to 1,3-diene units attached to N-heterocycles  

Directory of Open Access Journals (Sweden)

Full Text Available Dienes embedded in quinolizidine and indolizidine structures can be prepared in four steps from cyclic nitrones and bicyclopropylidene. The key intermediates ?-spirocyclopropanated N-heterocyclic ketones, generated via a domino 1,3-dipolar cycloaddition/thermal rearrangement sequence, were converted by Wittig methylenation to the corresponding vinylcyclopropanes (VCPs, which underwent rearrangement to 1,3-dienes in the presence of the Wilkinson Rh(I complex under microwave heating. The previously unexplored Rh(I-catalyzed opening of the VCP moiety embedded in an azapolycyclic system occurs at high temperature (110–130 °C to afford the corresponding 1,3-dienes in moderate yield (34–53%.

Alberto Brandi

2011-03-01

236

Direct C-H bond arylation of (benzo)oxazoles with aryl chlorides catalyzed by N-heterocyclic carbene-palladium(II)-1-methylimidazole complex.  

Science.gov (United States)

The direct C-H bond arylation of (benzo)oxazoles with aryl chlorides was achieved catalyzed by a well-defined NHC-Pd(II)-Im complex. Under the optimal conditions, various aryl chlorides were successfully applied as the arylating reagents to achieve the 2-aryl (benzo)oxazoles in acceptable to high yields, providing a convenient and alternative method for the direct C-H bond arylation of (benzo)oxazoles and enriching the chemistry of the NHC-Pd(II) complex in organic synthesis. PMID:24670076

Shen, Xiao-Bao; Zhang, Yun; Chen, Wen-Xin; Xiao, Zheng-Kang; Hu, Ting-Ting; Shao, Li-Xiong

2014-04-01

237

[CNN]-pincer nickel(ii) complexes of N-heterocyclic carbene (NHC): synthesis and catalysis of the Kumada reaction of unactivated C-Cl bonds.  

Science.gov (United States)

Three novel [CNN]-pincer nickel(ii) complexes with NHC-amine arms were synthesized in three steps. Complex was proven to be an efficient catalyst for the Kumada coupling of aryl chlorides or aryl dichlorides under mild conditions. PMID:24828414

Sun, Yunqiang; Li, Xiaoyan; Sun, Hongjian

2014-06-01

238

Synthesis of chiral mono(N-heterocyclic carbene) palladium and gold complexes with a 1,1'-biphenyl scaffold and their applications in catalysis  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Axially chiral mono(NHC)–Pd(II) and mono(NHC)–Au(I) complexes with one side shaped 1,1'-biphenyl backbone have been prepared from chiral 6,6'-dimethoxybiphenyl-2,2'-diamine. The complexes were characterized by X-ray crystal structure diffraction. The Pd(II) complex showed good catalytic activities in the Suzuki–Miyaura and Heck–Mizoroki coupling reactions, and the (S)-Au(I) complexes also showed good catalytic activities in the asymmetric intramolecular hydroami...

Lian-jun Liu; Feijun Wang; Wenfeng Wang; Mei-xin Zhao; Min Shi

2011-01-01

239

Cooperative N-heterocyclic carbene (NHC)-Lewis acid-mediated regioselective umpolung formal [3 + 2] annulations of alkynyl aldehydes with isatins.  

Science.gov (United States)

A novel and regioselective umpolung synthesis of spirooxindoles has been developed by cooperative NHC-Lewis acid-mediated formal [3 + 2] annulations of alkynyl aldehydes with isatins. In most cases, the reactions proceeded via a(3)-d(3) umpolung of alkynyl aldehydes resulting in spirooxindole butenolides. In a few cases, spirooxindole furan-3(2H)-ones were formed as the major products via an a(1)-d(1) umpolung process by controlling the reaction temperature. These newly formed spirooxindoles could provide promising candidates for chemical biology and drug lead discovery. PMID:24718717

Zhang, Yu; Lu, Yingyan; Tang, Weifang; Lu, Tao; Du, Ding

2014-05-21

240

Suzuki–Miyaura cross-coupling reaction of 1-aryltriazenes with arylboronic acids catalyzed by a recyclable polymer-supported N-heterocyclic carbene–palladium complex catalyst  

Directory of Open Access Journals (Sweden)

Full Text Available The Suzuki–Miyaura cross-coupling reaction of 1-aryltriazenes with arylboronic acids catalyzed by a recyclable polymer-supported Pd–NHC complex catalyst has been realized for the first time. The polymer-supported catalyst can be re-used several times still retaining high activity for this transformation. Various aryltriazenes were investigated as electrophilic substrates at room temperature to give biaryls in good to excellent yields and showed good chemoselectivity over aryl halides in the reactions.

Guangming Nan

2010-06-01

 
 
 
 
241

Suzuki–Miyaura cross-coupling reaction of 1-aryltriazenes with arylboronic acids catalyzed by a recyclable polymer-supported N-heterocyclic carbene–palladium complex catalyst  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The Suzuki–Miyaura cross-coupling reaction of 1-aryltriazenes with arylboronic acids catalyzed by a recyclable polymer-supported Pd–NHC complex catalyst has been realized for the first time. The polymer-supported catalyst can be re-used several times still retaining high activity for this transformation. Various aryltriazenes were investigated as electrophilic substrates at room temperature to give biaryls in good to excellent yields and showed good chemoselectivity over aryl hali...

Guangming Nan; Fang Ren; Meiming Luo

2010-01-01

242

Mechanistic investigations on e-N bond-breaking and ring expansion for N-heterocyclic carbene analogues containing the group 14 elements (e).  

Science.gov (United States)

The potential-energy surfaces for the ring-expansion reactions (i)Pr N(CH)2N((i)Pr)E :(Rea-E) + SiH2Ph2 ? six-membered ring heterocyclcic product (E = C, Si, Ge, Sn, and Pb) and (i)Pr N(CH)2N((i)Pr)C :(Rea-C) + EH2Ph2 ? six-membered ring heterocyclcic product are studied at the M06-2X/Def2-TZVP level of theory. These theoretical investigations suggest that for a given SiH2Ph2, the relative reactivity of Rea-E toward the ring-ring expansion reaction decreases as the atomic weight of the central atom E increases, that is, in the order Rea-C ? Rea-Si > Rea-Ge > Rea-Sn > Rea-Pb. However, for a given Rea-C, these theoretical observations demonstrate that the reactivity of the EH2Ph2 molecule that undergoes the ring-expansion reaction decreases in the order SiH2Ph2 ? GeH2Ph2 ? SnH2Ph2 > PbH2Ph2 ? CH2Ph2. This theoretical study indicates that both the electronic structure and steric effects play a crucial role in determining their reactivities. The model conclusions are consistent with available experimental findings. Furthermore, a valence bond state correlation diagram model can be used to rationalize the computational results. These results allow a number of predictions to be made. PMID:24786838

Su, Ming-Der

2014-05-19

243

Synthesis of chiral mono(N-heterocyclic carbene palladium and gold complexes with a 1,1'-biphenyl scaffold and their applications in catalysis  

Directory of Open Access Journals (Sweden)

Full Text Available Axially chiral mono(NHC–Pd(II and mono(NHC–Au(I complexes with one side shaped 1,1'-biphenyl backbone have been prepared from chiral 6,6'-dimethoxybiphenyl-2,2'-diamine. The complexes were characterized by X-ray crystal structure diffraction. The Pd(II complex showed good catalytic activities in the Suzuki–Miyaura and Heck–Mizoroki coupling reactions, and the (S-Au(I complexes also showed good catalytic activities in the asymmetric intramolecular hydroamination reaction to give the corresponding product in moderate ee.

Min Shi

2011-05-01

244

N-Heterocyclic Carbene-Palladium(II)-1-Methylimidazole Complex-Catalyzed Direct C-H Bond Arylation of (Benz)imidazoles with Aryl Chlorides.  

Science.gov (United States)

(Benz)imidazoles can be efficiently functionalized by (hetero)aryl chlorides via direct C-H bond arylation in the presence of a well-defined NHC-Pd(II)-Im complex. Under the optimal conditions, various activated, unactivated, and deactivated (hetero)aryl chlorides were successfully applied as the arylating reagents to achieve the 2-(hetero)aryl (benz)imidazoles in acceptable to high yields, giving a facile and alternative methodology for the direct C-H bond arylation of (benz)imidazoles. PMID:24869774

Gu, Zheng-Song; Chen, Wen-Xin; Shao, Li-Xiong

2014-06-20

245

Magnetic Properties of 1:2 Mixed Cobalt(II) Salicylaldehyde Schiff-Base Complexes with Pyridine Ligands Carrying High-Spin Carbenes (Scar = 2/2, 4/2, 6/2, and 8/2) in Dilute Frozen Solutions: Role of Organic Spin in Heterospin Single-Molecule Magnets.  

Science.gov (United States)

The 1:2 mixtures of Co(p-tolsal)2, p-tolsal = N-p-tolylsalicylideniminato, and diazo-pyridine ligands, DXpy; X = 1, 2, 3l, 3b, and 4, in MTHF solutions were irradiated at cryogenic temperature to form the corresponding 1:2 cobalt-carbene complexes Co(p-tolsal)2(CXpy)2, with Stotal = 5/2, 9/2, 13/2, 13/2, and 17/2, respectively. The resulting Co(p-tolsal)2(CXpy)2, X = 1, 2, 3l, 3b, and 4, showed magnetic behaviors characteristic of heterospin single-molecule magnets with effective activation barriers, Ueff/kB, of 40, 65, 73, 72, and 74 K, for reorientation of the magnetic moment and temperature-dependent hysteresis loops with a coercive force, Hc, of ?0, 6.2, 10, 6.5, and 9.0 kOe at 1.9 K, respectively. The relaxation times, ?Q, due to a quantum tunneling of magnetization (QTM) were estimated to be 1.6 s for Co(p-tolsal)2(C1py)2, ?2.0 × 10(3) s for Co(p-tolsal)2(C2py)2, and >10(5) s for Co(p-tolsal)2(CXpy)2; X = 3b, 3l, and 4. In heterospin complexes, organic spins, carbenes interacted with the cobalt ion to suppress the QTM pathway, and the ?Q value increased with increasing the Stotal values. PMID:24816331

Karasawa, Satoru; Nakano, Kimihiro; Yoshihara, Daisuke; Yamamoto, Noriko; Tanokashira, Jun-Ichi; Yoshizaki, Takahito; Inagaki, Yuji; Koga, Noboru

2014-06-01

246

Formations of mixed-valence oxovanadiumV,IV citrates and homocitrate with N-heterocycle chelated ligand.  

Science.gov (United States)

Dimeric mixed-valence oxovanadium citrate [V 2O 3(phen) 3(Hcit)].5H 2O ( 1) (H 4cit = citric acid, phen = 1,10-phenanthroline) was isolated from a weak acidic medium. It could be converted quantitatively into a tetrameric oxovanadium citrate adduct of 1,10-phenanthroline [V 2O 3(phen) 3(Hcit) 2(phen) 3O 3V 2].12H 2O ( 2). This was supported by the trace of infrared spectra and X-ray diffraction patterns. The two compounds feature a bidentate citrate group that chelates only to one vanadium center through their negatively charged alpha-alkoxy and alpha-carboxy oxygen atoms, while the other beta-carboxy and beta-carboxylic acid groups are free to participate in strong intramolecular and intermolecular hydrogen bonding [2.45(1) in 1 and 2.487(2) A in 2], respectively. This is also the case of homocitrato vanadate(V/IV) [V 2O 3(phen) 3( R, S-H 2homocit)]Cl.6H 2O ( 3) (H 4homocit = homocitric acid), which features a binding mode similar to that found in the R-homocitrato iron molybdenum cofactor of Mo-nitrogenase. Moreover, the homocitrato vanadate(V) [VO 2(phen) 2] 2[V 2O 4( R,S-H 2homocit) 2].4H 2O.2C 2H 5OH ( 4) is isolated as a molecular precursor for the formation of mixed-valence complex 3. The V-O alpha-alkoxy and V-O alpha-carboxy bond distances of homocitrate complexes 3 and 4 are 1.858(4) and 1.968(6) av and 2.085(4) and 1.937(5) A, respectively. They are shorter than those of homocitrate to FeVco (2.15 A). The gamma-carboxy groups of coordinated homocitrato complexes 3 and 4, and the free homocitrate salt Na 3(Hhomocit).H 2O ( 5), form strong hydrogen bonds with the chloride ion and the water molecule [2.982(5) in 3, 2.562(9) in 4, and 2.763(1) A in 5], respectively. PMID:18722422

Chen, Can-Yu; Zhou, Zhao-Hui; Chen, Hong-Bin; Huang, Pei-Qiang; Tsai, Khi-Rui; Chow, Yuan L

2008-10-01

247

Simple building-block route to (Phosphanyl-carbene)palladium complexes via intermolecular addition of functionalised phosphanes to isocyanides  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We present a straightforward protocol for making (phosphanyl- carbene)PdII complexes. These complexes have bidentate ligands containing an acyclic diamino- or aminooxycarbene and a phosphane. The synthesis gives good yields (typically 70–90%) for a variety of complexes (22 compounds). Moreover, it does not require the synthesis of imidazolium salts nor the a priori generation of free carbenes. Three of the new complexes were tested as catalysts for Sonogashira and Hay c...

Eberhard, M. R.; Vliet, Bart; Dura?n Pa?chon, L.; Rothenberg, G.; Eastham, G.; Kooijman, H.; Spek, A. L.; Elsevier, C. J.

2009-01-01

248

Metal-ligand cooperation on a diruthenium platform: selective imine formation through acceptorless dehydrogenative coupling of alcohols with amines.  

Science.gov (United States)

Metal-metal singly-bonded diruthenium complexes, bridged by naphthyridine-functionalized N-heterocyclic carbene (NHC) ligands featuring a hydroxy appendage on the naphthyridine unit, are obtained in a single-pot reaction of [Ru2(CH3COO)2(CO)4] with 1-benzyl-3-(5,7-dimethyl-1,8-naphthyrid-2-yl)imidazolium bromide (BIN?HBr) or 1-isopropyl-3-(5,7-dimethyl-1,8-naphthyrid-2-yl)imidazolium bromide (PIN?HBr), TlBF4, and substituted benzaldehyde containing an electron-withdrawing group. The modified NHC-naphthyridine-hydroxy ligand spans the diruthenium unit in which the NHC carbon and hydroxy oxygen occupy the axial sites. All the synthesized compounds catalyze acceptorless dehydrogenation of alcohols to the corresponding aldehydes in the presence of a catalytic amount of weak base 1,4-diazabicyclo[2.2.2]octane (DABCO). Further, acceptorless dehydrogenative coupling (ADHC) of the alcohol with amines affords the corresponding imine as the sole product. The substrate scope is examined with 1 (BIN, p-nitrobenzaldehyde). A similar complex [Ru2(CO)4(CH3COO)(3-PhBIN)][Br], that is devoid of a hydroxy arm, is significantly less effective for the same reaction. Neutral complex 1?a, obtained by deprotonation of the hydroxy arm in 1, is found to be active for the ADHC of alcohols and amines under base-free conditions. A combination of control experiments, deuterium labeling, kinetic Hammett studies, and DFT calculations support metal-hydroxyl/hydroxide and metal-metal cooperation for alcohol activation and dehydrogenation. The bridging acetate plays a crucial role in allowing ?-hydride elimination to occur. The ligand architecture on the diruthenium core causes rapid aldehyde extrusion from the metal coordination sphere, which is responsible for exclusive imine formation. PMID:24715433

Saha, Biswajit; Rahaman, S M Wahidur; Daw, Prosenjit; Sengupta, Gargi; Bera, Jitendra K

2014-05-19

249

Dynamics of carbene cycloadditions.  

Science.gov (United States)

Quasiclassical trajectory calculations using quantum mechanical energies and forces generated by the Venus and Gaussian programs provide for the first time a detailed dynamical picture of singlet carbene, CCl(2) and CF(2), cycloadditions to alkenes on the B3LYP/6-31G* surface. For CF(2), B3LYP/6-31G* with exact exchange reduced to 12% HF was also employed to better mimic the high accuracy surface. The range of geometries sampled in reactive trajectories and the timing of bond formation were explored. All trajectories follow the nonlinear approach proposed by Moore and Hoffmann. The reaction of CCl(2) with ethylene is a dynamically concerted reaction, with an average time gap between formation of the two bonds of 50 fs. The reaction of CF(2) with ethylene is dynamically complex with biexponential decay of the diradical species formed from the first bond formation. A general quantitative dynamical classification of cycloaddition mechanisms is proposed, based on the timing of bond formation. PMID:21961828

Xu, Lai; Doubleday, Charles E; Houk, K N

2011-11-01

250

Highly enantioselective synthesis of chiral 7-ring O- and N-heterocycles by a one-pot nitro-Michael-cyclization tandem reaction.  

Science.gov (United States)

A concise enantioselective approach to synthesise medium-sized 7-ring O- and N-heterocycles has been developed. The synthetic strategy relies on an organocatalytic nitro-Michael-nitrile oxide cycloaddition tandem reaction, leading to the corresponding chiral benzoxe- and benzazepine derivatives containing an additional fused dihydroisoxazoline ring in good yields and excellent enantioselectivities (up to 97% ee). PMID:24190160

Rohlmann, Renate; Daniliuc, Constantin-Gabriel; Mancheño, Olga García

2013-12-25

251

A Three-Step Laboratory Sequence to Prepare a Carbene Complex of Silver(I) Chloride  

Science.gov (United States)

We have developed a multistep inorganic synthesis experiment for our second-year undergraduate teaching laboratory that introduces students to modern organometallic chemistry. The ligands are prepared in two simple steps and the preparation of an air-stable silver carbene complex is accomplished in the third step. The students are introduced to…

Canal, John P.; Ramnial, Taramatee; Langlois, Lisa D.; Abernethy, Colin D.; Clyburne, Jason A. C.

2008-01-01

252

Synthesis, characterization, and antimicrobial activity of silver carbene complexes derived from 4,5,6,7-tetrachlorobenzimidazole against antibiotic resistant bacteria†  

Science.gov (United States)

Silver N-heterocyclic carbene complexes have been shown to have great potential as antimicrobial agents, affecting a wide spectrum of both Gram-positive and Gram-negative bacteria. A new series of three silver carbene complexes (SCCs) based on 4,5,6,7-tetrachlorobenzimidazole has been synthesized, characterized, and tested against a panel of clinical strains of bacteria. The imidazolium salts and their precursors were characterized by elemental analysis, mass spectrometry, 1H and 13C NMR spectroscopy, and single crystal X-ray diffraction. The silver carbene complexes, SCC32, SCC33, and SCC34 were characterized by elemental analysis, 1H and 13C NMR spectroscopy, and single crystal X-ray diffraction. These complexes proved highly efficacious with minimum inhibitory concentrations (MICs) ranging from 0.25 to 6 ?g mL?1. Overall, the complexes were effective against highly resistant bacteria strains, such as methicillin-resistant Staphylococcus aureus (MRSA), weaponizable bacteria, such as Yersinia pestis, and pathogens found within the lungs of cystic fibrosis patients, such as Pseudomonas aeruginosa, Alcaligenes xylosoxidans, and Burkholderia gladioli. SCC33 and SCC34 also showed clinically relevant activity against a silver-resistant strain of Escherichia coli based on MIC testing.

Wright, Brian D.; Shah, Parth N.; McDonald, Lucas J.; Shaeffer, Michael L.; Wagers, Patrick O.; Panzner, Matthew J.; Smolen, Justin; Tagaev, Jasur; Tessier, Claire A.

2013-01-01

253

Synthesis, characterization, and antimicrobial activity of silver carbene complexes derived from 4,5,6,7-tetrachlorobenzimidazole against antibiotic resistant bacteria.  

Science.gov (United States)

Silver N-heterocyclic carbene complexes have been shown to have great potential as antimicrobial agents, affecting a wide spectrum of both Gram-positive and Gram-negative bacteria. A new series of three silver carbene complexes (SCCs) based on 4,5,6,7-tetrachlorobenzimidazole has been synthesized, characterized, and tested against a panel of clinical strains of bacteria. The imidazolium salts and their precursors were characterized by elemental analysis, mass spectrometry, (1)H and (13)C NMR spectroscopy, and single crystal X-ray diffraction. The silver carbene complexes, SCC32, SCC33, and SCC34 were characterized by elemental analysis, (1)H and (13)C NMR spectroscopy, and single crystal X-ray diffraction. These complexes proved highly efficacious with minimum inhibitory concentrations (MICs) ranging from 0.25 to 6 ?g mL(-1). Overall, the complexes were effective against highly resistant bacteria strains, such as methicillin-resistant Staphylococcus aureus (MRSA), weaponizable bacteria, such as Yersinia pestis, and pathogens found within the lungs of cystic fibrosis patients, such as Pseudomonas aeruginosa, Alcaligenes xylosoxidans, and Burkholderia gladioli. SCC33 and SCC34 also showed clinically relevant activity against a silver-resistant strain of Escherichia coli based on MIC testing. PMID:22402409

Wright, Brian D; Shah, Parth N; McDonald, Lucas J; Shaeffer, Michael L; Wagers, Patrick O; Panzner, Matthew J; Smolen, Justin; Tagaev, Jasur; Tessier, Claire A; Cannon, Carolyn L; Youngs, Wiley J

2012-06-01

254

Quest for even higher stabilized foiled carbenes.  

Science.gov (United States)

Foiled carbene structures comprising strong stabilizing interactions between the divalent carbon and the intramolecular double bond have been located by DFT calculations. These tetracyclic species bearing fused five-membered rings impeding intramolecular rearrangements are theoretically predicted to lie in a deep potential energy well. A suitable dibromocyclopropane precursor for this type of foiled carbene has been prepared in 12 steps. Its treatment with methyllithium led to the isolation of a product of a formal carbene dimerization, a bicyclopropylidene. PMID:21809818

Su, Kuan-Jen; Pa?ar, Mirjana; Mieusset, Jean-Luc; Arion, Vladimir B; Brinker, Udo H

2011-09-16

255

Phosphorus carbene and olefine analogues  

Digital Repository Infrastructure Vision for European Research (DRIVER)

A representative of the yet unknown parent iminophosphane is ButP=NBut 1 which has been recognized as an carbenic analogue for some time. The unusual properties of 1 prompted our investigations of the synthesis of further phosphorus(III)-(p-p)?-bonds systems with an orbital sequence (HOMO= n(P), LUMO= T (P=X)) as well as the reaction behaviour of this class of compounds.

Niecke, Edgar; Gudat, Dietrich; Leuer, Martina; Lysek, Manfred; Symalla, Erhard

1987-01-01

256

H-bonded N-heterocyclic base-pair phototautomerizational potential barrier and mechanism: The 7-azaindole dimer.  

Science.gov (United States)

A theoretical analysis of the double proton transfer (PT) in a hydrogen-bonded N-heterocyclic base pair is presented. The calculated (time-dependent density functional theory) double PT barrier calculated for the concerted process of the 7-azaindole C(2h) dimer in the first excited singlet electronic state S(1) conforms well to the kinetic data and the photophysical evidence reported in this article. The calculated PT energy barrier of 4.8 kcal/mol height, and the corresponding zero point energy value, yield for the S(1) state an activation energy barrier of 0.3 kcal/mol. This finding implies that the double PT concerted process is almost barrierless, confirming previous experiments. Upon N-H deuteration of the 7-azaindole dimer, the theoretical excited-state activation energy for the double deuterium transfer is determined to be 1.4 kcal/mol, in agreement with experiment, which in low-temperature spectroscopy is shown to negate excited-state double-deuteron transfer. PMID:14701906

Catalán, J; Pérez, P; del Valle, J C; de Paz, J L G; Kasha, M

2004-01-13

257

Microwave-assisted synthesis of N-heterocycles and their evaluation using an acetylcholinesterase immobilized capillary reactor  

Scientific Electronic Library Online (English)

Full Text Available SciELO Brazil | Language: English Abstract in portuguese Os etenoditioacetais polarizados são conhecidos como importantes intermediários em muitos processos de síntese. Neste trabalho, reporta-se a anelação livre de metal de transição de 1,1-bis(tiometil)-2-nitroetileno com hidroxilalquilaminas e alquildiaminas. Esta metodologia permite a síntese direta d [...] e N-heterociclos tais como imidazolidinas, oxazolidinas e benzoxazóis sob irradiação de micro-ondas. Estes compostos foram avaliados como inibidores da acetilcolinesterase usando um reator capilar contendo a enzima imobilizada acoplado a um espectrômetro de massas. Abstract in english Polarized ketene dithioketals have been recognized as useful building blocks in many synthetic operations. In this work, a transition-metal-free annulations of 1,1-bis(thiomethyl)-2-nitroethylene with hydroxylalkylamines or alkyldiamines have been reported. This methodology provides a directed appro [...] ach to N-heterocycles, e.g., imidazolidines, oxazolidines and benzoxazoles under microwave conditions. These compounds were evaluated as acetylcholinesterase inhibitors by using an enzyme immobilized capillary reactor-tandem mass spectrometry.

Diego P., Sangi; Julia L., Monteiro; Kenia L., Vanzolini; Quezia B., Cass; Marcio W., Paixão; Arlene G., Corrêa.

258

Computational Chemistry Studies on the Carbene Hydroxymethylene  

Science.gov (United States)

A density functional theory computational chemistry exercise on the structure and vibrational spectrum of the carbene hydroxymethylene is presented. The potential energy curve for the decomposition reaction of the carbene to formaldehyde and the geometry of the transition state are explored. The results are in good agreement with recent…

Marzzacco, Charles J.; Baum, J. Clayton

2011-01-01

259

Steriese en elektroniese effek van metaalbevattende substituente op Fischer-karbeenmetaalbondelverbindings The steric and electronic effects of metal-containing substituents on Fischer carbene metal clusters  

Directory of Open Access Journals (Sweden)

Full Text Available Die insluiting van verskillende oorgangsmetaalfragmente by komplekse word reeds die afgelope aantal jare wyd bestudeer om die effek van die verskillende metaalfragmente op die reaktiwiteit van die ligande en die chemie van die komplekse te ondersoek.Novel Group VI transition metal Fischer carbene complexes containing three different transition metal substituents, all in electronic contact with the carbene carbon atom, were synthesised.

Editorial Office

2009-09-01

260

Assessing the ligand properties of 1,3-dimesitylbenzimidazol-2-ylidene in ruthenium-catalyzed olefin metathesis.  

Science.gov (United States)

The deprotonation of 1,3-dimesitylbenzimidazolium tetrafluoroborate with a strong base afforded 1,3-dimesitylbenzimidazol-2-ylidene (BMes), which was further reacted in situ with rhodium or ruthenium complexes to afford three new organometallic products. The compounds [RhCl(COD)(BMes)] (COD is 1,5-cyclooctadiene) and cis-[RhCl(CO)2(BMes)] were used to probe the steric and electronic parameters of BMes. Comparison of the percentage of buried volume (%V(Bur)) and of the Tolman electronic parameter (TEP) of BMes with those determined previously for 1,3-dimesitylimidazol-2-ylidene (IMes) and 1,3-dimesitylimidazolin-2-ylidene (SIMes) revealed that the three N-heterocyclic carbenes (NHCs) had very similar profiles. Nonetheless, changes in the hydrocarbon backbone subtly affected the stereoelectronic properties of these ligands. Accordingly, the corresponding [RuCl2(PCy3)(NHC)(=CHPh)] complexes displayed different catalytic behaviors in the ring-closing metathesis (RCM) of ?,?-dienes. In the benchmark cyclization of diethyl 2,2-diallylmalonate, the new [RuCl2(PCy3)(BMes)(=CHPh)] compound (1d) performed slightly better than the Grubbs second-generation catalyst (1a), which was in turn significantly more active than the related [RuCl2(PCy3)(IMes)(=CHPh)] initiator (1b). For the formation of a model trisubstituted cycloolefin, complex 1d ranked in-between catalyst precursors 1a and 1b, whereas in the RCM of tetrasubstituted cycloalkenes it lost its catalytic efficiency much more rapidly. PMID:22990296

Borguet, Yannick; Zaragoza, Guillermo; Demonceau, Albert; Delaude, Lionel

2013-05-28

 
 
 
 
261

Unsaturated carbenes from primary vinyl triflates. IX. Intramolecular rearrangement via free carbenes  

International Nuclear Information System (INIS)

The mechanism and exact nature of the intermediates in the base-initiated rearrangement of arylvinyl triflates were examined to determine if the reaction proceeds via a carbenoid or the unencumbered carbene. A clear-cut difference was noted in the intramolecular rearrangement of aryl-substituted unsaturated carbenes, with the reaction proceeding via a carbenoid in the case of arylvinyl halides and via the free carbene in the case of arylvinyl triflates

1978-01-20

262

Nonempirical investigation of electronic structure of simple model alkyl and carbene complexes of metals of the first transition period beginning  

International Nuclear Information System (INIS)

The nonempirical LCAO MO SCF method in small bases of groupped Gauss functions is used to calculate electronic structure of model complexes of the Hsub(n)MCHsub(3) and Hsub(m)MCHsub(2) composition, where M=Ti, V, Cr. The character of the metal-carbon bond depending on the nature of metal and hydrocarbon ligand is analyzed. The energetics of some processes accompanied by the breaking of ?- or ?-component of the metal-carbon bond is considered. The growth of relative ability to the formation of carbene complex from initial alkyl in the process od ?-elimination in the Ti< V< Cr series is established, which is explained by relative ?-component strengthening of metal-carbon bond in carbene complex as compared with the initial alkyl one. Carbene complexes with increased coordination of the transition metal are characterized by resuced populations of the 2psub(?)-AO of C atom of carbene ligand. On the basis of obtained results some general regularities of the M=C bond nature and the problems of stabilization of carbene complexes are discussed

1985-01-01

263

Photochemistry of group 6 Fischer carbene complexes: beyond the photocarbonylation reaction.  

Science.gov (United States)

The seminal report by Hegedus in 1982, showing that alkoxychromium(0) carbenes reacted with imines under bright Colorado sunlight to yield ?-lactams, marked the beginning of a key reaction in organometallic chemistry. Very little was known about the mechanism of this reaction. In fact, Hegedus proposed the reversible generation of a chromium-coordinated ketene, which would react with nucleophiles. This coordinated species would show all the advantages of ketenes without their shortcomings, namely, dimerization, formation of undesired adducts, and so forth. The quest for the detection of these species and the pursuit of the mechanism of the photocarbonylation (a reaction exclusive to Cr(0) and Mo(0) carbene complexes, not W(0) carbene complexes) remained unabated over the next 15 years. In fact, all attempts to experimentally determine the mechanism of this useful reaction have been fruitless. At the same time, the photocarbonylation of Cr(0) carbenes matured into a valuable synthetic reaction, allowing access to several families of organic compounds. Unfortunately, reactions other than photocarbonylation remained elusive. We used a combination of experimental and computational methodologies to study the photocarbonylation of Cr(0) carbene complexes and the subsequent reaction of the photogenerated ketenes with nucleophiles. In parallel, we discovered new photochemical processes and succeeded in making photoreactive the so-called "unreactive" W(0) carbene complexes. In this Account, we discuss the disentangling of the mechanisms of these transformations, thereby shedding some light onto the photochemistry of group 6 metal (Fischer) carbene complexes. The original designation of the electronic transitions of group 6 carbene complexes was reassigned, and the photocarbonylation step was analyzed again, resulting in the sequence S(0)-T(1)-S(0), which is far removed from conventional organic photochemistry. The T(1) species is a chromacyclopropanone; its unpaired electrons are primarily localized in the metal fragment and in the former carbene carbon atom. The T(1)-S(0) intersystem crossing occurs with the participation of the solvent through an unusual loose-bolt radiationless mechanism. The photogenerated S(0) species reacts with imines to form the final ?-lactams in a mechanism that resembles the organic Staudinger reaction, but here the metal is present during the entire reaction coordinate. The selectivity of these reactions is defined by the nucleophilic attack on the O-bonded metallaketene instead of the subsequent conrotatory ring closure, a distinct departure from the organic reaction. Appropriate modification of the substituents of the carbene ligand or in the coordination sphere of the complex results in new photoprocesses; these include 1,2-metalladyotropic rearrangements as well as ?-fragmentations in which W(0) carbene complexes become photoreactive. Moreover, the inclusion of additional metal centers usually results in new reactions, such as the formation of fulvenes by ?(5)? ?(3) photoslippage, or in the complete inhibition of the photoreactivity. The photochemistry of group 6 metal-carbene complexes thus offers unexplored territory for pursuing new reactions and reaction mechanisms. PMID:21517052

Fernández, Israel; Cossío, Fernando P; Sierra, Miguel A

2011-07-19

264

Synthesis and bonding in carbene complexes of an unsymmetrical dilithio methandiide: a combined experimental and theoretical study.  

Science.gov (United States)

Herein, we report the preparation of a new unsymmetrical, bis(thiophosphinoyl)-substituted dilithio methandiide and its application for the synthesis of zirconium- and palladium-carbene complexes. These complexes were found to exhibit remarkably shielded (13)C NMR shifts, which are much more highfield-shifted than those of "normal" carbene complexes. DFT calculations were performed to determine the origin of these observations and to distinguish the electronic structure of these and related carbene complexes compared with the classical Fischer and Schrock-type complexes. Various methods show that these systems are best described as highly polarized Schrock-type complexes, in which the metal-carbon bond possesses more electrostatic contributions than in the prototype Schrock systems, or even as "masked" methandiides. As such, geminal dianions represent a kind of "extreme" Schrock-type ligands favoring the ionic resonance structure M(+)-CR2(-) as often used in textbooks to explain the nucleophilic nature of Schrock complexes. PMID:24150833

Gessner, Viktoria H; Meier, Florian; Uhrich, Diana; Kaupp, Martin

2013-12-01

265

A fullerene-carbene adduct as a crystalline molecular rotor: remarkable behavior of a spherically-shaped rotator.  

Science.gov (United States)

A new fullerene structure was recently obtained from the reaction of a Lewis basic N-heterocyclic carbene (NHC) and the Lewis acidic C60. The molecular features of the zwitterionic adduct can be described as a molecular rotor with the fullerene cage acting as the rotator that spins about one distinct axis given by its C-C single bond linkage with the imidazolium heterocycle stator. A detailed structural analysis of the compound by means of single-crystal X-ray diffraction (XRD) revealed significant differences in the packing motifs of solvent-free and solvent-containing crystals. Variable temperature single-crystal XRD experiments (80 K ? T ? 480 K) carried out to investigate the rotational dynamics of the fullerene group in the higher quality solvent-free structure revealed atomic displacement parameters consistent with fast rotation of the highly symmetric fullerene in the solid state, whereas the imidazolium unit remains in a fixed position and therefore represents the stator. DFT and semiempirical calculations were applied to get insight into the profile of the rotational potential of the fullerene unit, particularly considering interactions with the neighboring molecules in the crystal lattice. The results indicate that the crystal environment leads to the presence of one lowest energy minimum that is connected to seven others that are slightly higher in energy through rotational barriers of approximately 1.5-2.5 kcal mol(-1). PMID:24852314

Lorbach, Andreas; Maverick, Emily; Carreras, Abel; Alemany, Pere; Wu, Guang; Garcia-Garibay, Miguel A; Bazan, Guillermo C

2014-06-01

266

[3,3?-Dimesityl-1,1?-(4,5:16,17-dibenzo-3,6,9,12,15,18-hexaoxaeicosane-1,20-diyldiimidazolin-2-ylidene]dithiocyanatopalladium(II  

Directory of Open Access Journals (Sweden)

Full Text Available The coordination geometry of the Pd atom in the title compound, [Pd(SCN2(C46H54N4O6], is approximately square-planar. The N-heterocyclic carbene (NHC metallacrown ether ligand binds to the Pd atom in a trans orientation through the carbene C atoms of the two imidazole rings and generates a 25-membered chelate ring. Two mutually trans S-bound thiocyanate ligands complete the coordination.

Mei-Ming Luo

2009-05-01

267

Copper-catalyzed trifluoromethylation of internal olefinic C-H bonds: efficient routes to trifluoromethylated tetrasubstituted olefins and N-heterocycles.  

Science.gov (United States)

The functionalization of internal olefins has been a challenging task in organic synthesis. Efficient CuII-catalyzed trifluoromethylation of internal olefins, that is, ?-oxoketene dithioacetals, has been achieved by using Cu(OH)2 as a catalyst and TMSCF3 as a trifluoromethylating reagent. The push-pull effect from the polarized olefin substrates facilitates the internal olefinic C-H trifluoromethylation. Cyclic and acyclic dithioalkyl ?-oxoketene acetals were used as the substrates and various substituents were tolerated. The internal olefinic C-H bond cleavage was not involved in the rate-determining step, and a mechanism that involves radicals is proposed based on a TEMPO-quenching experiment of the trifluoromethylation reaction. Further derivatization of the resultant CF3 olefins led to multifunctionalized tetrasubstituted CF3 olefins and trifluoromethylated N-heterocycles. PMID:24677229

Mao, Zhifeng; Huang, Fei; Yu, Haifeng; Chen, Jiping; Yu, Zhengkun; Xu, Zhaoqing

2014-03-17

268

Copper(I) complexes based on five-membered P^N heterocycles: structural diversity linked to exciting luminescence properties.  

Science.gov (United States)

Bridging P(^)N ligands bearing five-membered heterocyclic moieties such as tetrazoles, 1,2,4-triazoles, oxadiazoles, thiadiazoles, and oxazoles have been investigated regarding their complexation behavior with copper(I) iodide as metal salts. Different complex structures were found, depending either on the ligand itself or on the ligand-to-metal ratios used in the complexation reaction. Two different kinds of luminescent dinuclear complex structures and a kind of tetranuclear complex structure were revealed by X-ray single-crystal analyses and were further investigated for their photophysical properties. The emission maxima of these complexes are in the blue to yellow region of the visible spectrum for the dinuclear complexes and in the yellow to orange region for the tetranuclear complexes. Further investigations using density functional theory (DFT) show that the highest occupied molecular orbital (HOMO) is located mainly on the metal halide cores, while the lowest unoccupied molecular orbital (LUMO) resides mostly in the ligand sphere of the complexes. The emission properties were further examined in different environments such as neat powders, neat films, PMMA matrices, or dichloromethane solutions, revealing the high potential of these complexes for their application in organic light-emitting diodes. Especially complexes with 1,2,4-triazole moieties feature emission maxima in the blue region of the visible spectrum and quantum yields up to 95% together with short decay times of about 1-4 ?s and are therefore promising candidates for blue-emitting materials in OLEDs. PMID:24205840

Zink, Daniel M; Baumann, Thomas; Friedrichs, Jana; Nieger, Martin; Bräse, Stefan

2013-12-01

269

Laser-induced carbene-carbene rearrangement in solution: the diphenylcarbene-fluorene rearrangement.  

Science.gov (United States)

Diphenylcarbene (DPC) generated by high-intensity laser photolysis of diphenyldiazomethane rearranges to fluorene (FL) by two distinct mechanisms as revealed by methyl-group labeling. Thus, excimer laser irradiation of p,p'-dimethyldiphenyldiazomethane generates 3,6-dimethylfluorene (3,6-DMF) and 2,7-dimethylfluorene (2,7-DMF), which were identified by fluorescence measurements as well as GC-MS and comparison with authentic materials. 3,6-DMF corresponds to direct bond formation between ortho positions in DPC, referred to as ortho,ortho' coupling. 2,7-DMF corresponds to a carbene-carbene rearrangement, whereby DPC undergoes ring expansion to phenylcycloheptatetraene (PhCHT) followed by ring contraction to o-biphenylylcarbene (o-BPC), which then cyclizes to FL. The carbene-carbene rearrangement dominates over the ortho,ortho' coupling under all conditions employed. The ortho,ortho' coupling must take place in a higher excited state (most likely S2 or T1) of DPC, because it is not observed at all under thermolysis conditions, where only S1 and T0 are populated. The carbene-carbene rearrangement may take place either in a hot S1 state or more likely in a higher excited state (S2 or T1). PMID:23941300

Régimbald-Krnel, Michèle J; Wentrup, Curt

2013-09-01

270

Enyne Metathesis Catalyzed by Ruthenium Carbene Complexes  

DEFF Research Database (Denmark)

Enyne metathesis combines an alkene and an alkyne into a 1,3-diene. The first enyne metathesis reaction catalyzed by a ruthenium carbene complex was reported in 1994. This review covers the advances in this transformation during the last eight years with particular emphasis on methodology development and synthetic applications.

Poulsen, Carina Storm; Madsen, Robert

2003-01-01

271

Chemistry of Transition Metal Carbene Complexes.  

Science.gov (United States)

Significant contributions to the understanding of the synthesis, structure and reactivity of d4 and d6 metal carbene complexes of the type Cp(CO)(L)Fe=CHR+ (R=aryl, alkyl; L=P(C6H5)3, CO) and Cp(CO2)LM=CHR+ (M=Mo,W; R=H, aryl) are described. The increasin...

G. R. Husk M. S. Brookhart

1983-01-01

272

Easy access to uranium nucleophilic carbene complexes  

International Nuclear Information System (INIS)

Metathesis reactions of UCl4 with Li2C(Ph2PS)2 in Et2O only afforded the tris-carbene complex [{Li(OEt2)}2-U{=C(Ph2PS)2}3](1), while the bis- and mono-carbene compounds [U{=C(Ph2PS)2}2(THF)2] (2) and [{Li(THF)2}2U{=C(Ph2PS)2}Cl4] (3) were obtained by treatment of UCl4 with Li2C(Ph2PS)2 in a mixture of THF and toluene. The bis- carbene complex 2 was also obtained either from the comproportionation reaction of 1 and UCl4 or protonolysis reaction of U(NEt2)4 with H2C(Ph2PS)2 and was transformed into the mono- carbene complex [U{=C(Ph2PS)2}Cl2(THF)2] (4) by further reaction with UCl4. The utility of these complexes as precursors is illustrated by the synthesis of the bis-cyclopentadienyl derivative [Cp2U{=C(Ph2PS)2}] (5) by treatment of 3 with TlCp. The crystal structures of [U{=C(Ph2PS)2}2(py)2]center dot 1.5py.0.5THF and 5.toluene are reported. (authors)

2010-01-01

273

Mutagenic activity of some platinum and ruthenium complexes with N-heterocyclic ligands in salmonella typhimurium Ta 1530 and Ta 98  

International Nuclear Information System (INIS)

The mutagenic activity of some platinum and ruthenium complexes with 2,2'-biquinoline (b iq) and 2-(2' -pyridyl) quinoline (p q) was examined in strains of salmonella typhimurium Ta 1530 and Ta 98. The complexes cis-[Pt(Nn)X2)] (Nn=b iq, X=CI; Nn=p q, X=CI, Br), Pt3(b iq)2I6 and me r-[Ru(b iq)2CI3].2H2O exhibit significant mutagenic activity while me r-[Ru(p q)CI3(C2H5OH)], [Ru(p q)CI4]. 3a-2a and [Ru(p q) (DMSO)2CI2].0.5H2O show much weaker mutagenic activity. the platinum complexes appear to be more active via induction of frameshift than base substitution mutation while me r-[Ru(b iq)2CI3].2H2O is highly mutagenic via base substitution. (authors). 16 refs., 2 tabs

1997-04-01

274

White Phosphorus Degradation with a NacNac Aluminum Carbene Analogue: The Biradical Reaction Mechanism.  

Science.gov (United States)

The title compound Al-NacNac is isolobal to the imidazol-2-ylidene (NHC); the latter is considered as a nucleophilic carbene. However, the title compound is different from a typical carbene, as aluminum is a heavier group 13 element with a predominant inert s orbital. Its singlet ground state is a poor Lewis donor (acceptor) toward white phosphorus, but its corresponding lowest energy triplet state forms a strong Al-P bond with (opened) white phosphorus. The reaction of Al-NacNac with white phosphorus proceeds in two steps: after the addition of a first carbene analogue, a second one is added, resulting in a transient biradicaloid species. This undergoes facile subsequent rearrangement, and a final ring closure reaction leads to the observed product with a bicyclobutane moiety. It is determined by intramolecular bond formation of two phosphorus centered radicals. Finally, a structure with a large singlet-triplet energy separation is formed. An analogy to the noninnocent ligand character as well as the exciplex view of the monoadduct of white phosphorus with the Al-NacNac system is drawn. PMID:24800980

Schoeller, Wolfgang W; Frey, Guido D

2014-05-19

275

Sacrificial spacer and non-covalent routes toward the molecular imprinting of 'poorly-functionalized' N-heterocycles  

Energy Technology Data Exchange (ETDEWEB)

A comparison of three different methods for the imprinting of small aromatic heterocycles containing only a single nitrogen atom, for the preparation of specific analytical phases, was carried out. A conventional non-covalent approach to the imprinting of pyridine using methacrylic acid as the functional monomer was compared with two sacrificial spacer methods, in which heterocycles were imprinted as covalent template analogues. The results of binding experiments showed that discrimination based on ligand size was possible when polymers were prepared using a silyl ester-based template. The most selective polymer was able to bind pyridine in preference to quinoline or acridine which is opposite to the trend predicted by the pK{sub HB} values for the three ligands. Curve fitting of the isotherm for pyridine binding to this polymer to the Langmuir model gave an approximate K{sub d} of 1.1{+-}0.1 mM and a binding site concentration of 57{+-}2 mmol g{sup -1}. Acridine binding did not show saturation behaviour and was non-specific and cooperative in nature.

Kirsch, N.; Alexander, C.; Davies, S.; Whitcombe, M.J

2004-02-16

276

Sacrificial spacer and non-covalent routes toward the molecular imprinting of 'poorly-functionalized' N-heterocycles  

International Nuclear Information System (INIS)

A comparison of three different methods for the imprinting of small aromatic heterocycles containing only a single nitrogen atom, for the preparation of specific analytical phases, was carried out. A conventional non-covalent approach to the imprinting of pyridine using methacrylic acid as the functional monomer was compared with two sacrificial spacer methods, in which heterocycles were imprinted as covalent template analogues. The results of binding experiments showed that discrimination based on ligand size was possible when polymers were prepared using a silyl ester-based template. The most selective polymer was able to bind pyridine in preference to quinoline or acridine which is opposite to the trend predicted by the pKHB values for the three ligands. Curve fitting of the isotherm for pyridine binding to this polymer to the Langmuir model gave an approximate Kd of 1.1±0.1 mM and a binding site concentration of 57±2 mmol g-1. Acridine binding did not show saturation behaviour and was non-specific and cooperative in nature

2004-02-16

277

Insights on the mechanism of thioredoxin reductase inhibition by Gold N-heterocyclic carbene compounds using the synthetic linear Selenocysteine containing C-terminal peptide hTrxR(488-499): An ESI-MS investigation.  

Science.gov (United States)

Gold-based drugs typically behave as strong inhibitors of the enzyme thioredoxin reductase (hTrxR), possibly as the consequence of direct Gold(I) coordination to its active site selenocysteine. To gain a deeper insight into the molecular basis of enzyme inhibition and prove gold-selenocysteine coordination, the reactions of three parent Gold(I) NHC compounds with the synthetic C-terminal dodecapeptide of hTrxR containing Selenocysteine at position 498, were investigated by electrospray ionization mass spectrometry (ESI-MS). Formation of 1:1 Gold-peptide adducts, though in highly different amounts, was demonstrated in all cases. In these adducts the same [Au-NHC](+) moiety is always associated to the intact peptide. Afterward, tandem MS experiments, conducted on a specific Gold-peptide complex, pointed out that Gold is coordinated to the selenolate group. The relatively large strength of the Gold-selenolate coordinative bond well accounts for potent enzyme inhibition typically afforded by these Gold(I) compounds. In a selected case, the time course of enzyme inhibition was explored. Interestingly, enzyme inhibition turned out to show up very quickly and reached its maximum just few minutes after mixing. Overall, the present results offer some clear insight into the process of thioredoxin reductase inhibition by Gold-based compounds. PMID:24524917

Pratesi, Alessandro; Gabbiani, Chiara; Michelucci, Elena; Ginanneschi, Mauro; Papini, Anna Maria; Rubbiani, Riccardo; Ott, Ingo; Messori, Luigi

2014-07-01

278

Pd-carbene catalyzed carbonylation reactions of aryl iodides.  

Science.gov (United States)

A series of carbene complexes [PdBr(2)((i)Pr(2)-bimy)L] (C2-C13) with different types of co-ligands (L) have been tested for their catalytic activities in the carbonylative annulation of 2-iodophenol with phenylacetylene in DMF to afford the respective flavone 2a. Complex C12 with an N-phenylimidazole co-ligand showed the best activity and also afforded high yields when the substrate scope was extended to other aryl or pyridyl acetylenes. In addition, catalyst C12 was also efficient in the carbonylative annulation of 2-iodoaniline with acid chlorides giving the desirable 2-substituted 4H-3,1-benzoxazin-4-ones (4) in good yields. Additionally, this Pd-NHC complex also proved to be a very efficient catalyst for the hydroxycarbonylation of iodobenzene derivatives at low catalyst loading and under low CO pressure. These results demonstrate the versatility and efficiency of this phosphine-free Pd(II)-NHC complex in different types of carbonylations of aryl iodides under mild conditions. PMID:21677940

Xue, Liqin; Shi, Lijun; Han, Yuan; Xia, Chungu; Huynh, Han Vinh; Li, Fuwei

2011-08-01

279

Cu(i) and Ag(i) complexes of 7,10-bis-N-heterocycle-diazafluoranthenes: programmed molecular grids?  

Science.gov (United States)

Reactions of 7,10-disubstituted diazafluoranthene derivatives with three different silver(i) salts AgX (X = [PF6](-), [SbF6](-), [CB11HCl11](-)) and [Cu(CH3CN)4]PF6 afforded complexes exhibiting five different motifs. The crystal structures of the free ligands and nine new complexes from this series of reactions are reported. The use of 2,5-di-tert-butyl-7,10-di(pyridin-2-yl)-8,9-diazafluoranthene as a ligand leads to the formation of the tetranuclear compounds [Ag4(C32H30N4)4][PF6]4·3C6H6·4MeCN, [Ag4(C32H30N4)4][SbF6]4·4C5H12 and [Cu4(C32H30N4)4][PF6]4·8C3H6O, which exhibit "propeller" and saddle-type geometry, respectively, as well as a dinuclear complex [Ag2(C32H30N4)2][CHB11Cl11]2·4C6H4Cl2·CH2Cl2. The reactions involving the less sterically hindered 2,5-di-tert-butyl-7,10-di-(pyrimidin-2-yl)-8,9-diazafluoranthene and 2,5-di-tert-butyl-7,10-di(thiazol-2-yl)-8,9-diazafluoranthene afforded crystals of the dinuclear complexes [Ag2(C30H28N6)2][PF6]2·0.5CH2Cl2·0.5C6H5Cl·0.5C6H12, [Ag2(C30H28N6)2][SbF6]2·C3H6O·0.5C6H14·0.5C6H6, the polymeric species [Ag2(C28H26N4S2)2]n·2n[PF6]n·nC3H6O and the tetranuclear compounds [Cu4(C26H25N4S2)4][PF6]4·2CHCl3·2C3H6O and [Cu4(C30H28N6)4][PF6]4·2.17H2O, which possess saddle and grid-like architectures, respectively. Conformational analysis of the free ligands showed that they exhibit N-C-C-N torsion angles ranging from syn clinal (58°) to fully anti-periplanar conformations; the syn clinal conformation dominates in the complexes. The relative energies of the possible structural conformations of the synthesized ligands, as well as of oxazole disubstituted diazafluoranthenes, were calculated using density functional theory at the B97D/Def2-TZVPP level of theory. PMID:24915516

Rahanyan, Nelli; Duttwyler, Simon; Linden, Anthony; Baldridge, Kim K; Siegel, Jay S

2014-06-24

280

Reactions of Fischer carbene complexes with Electron-deficient olefins: Scope and limitations of this route to donor-acceptor-substituted cyclopropanes  

Energy Technology Data Exchange (ETDEWEB)

The Fischer carbene complex ((CO){sub 5}Cr{double bond}C(OMe)Ph) (1) is able to transfer its carbene ligand to a variety of electron-deficient olefins and provides donor-acceptor-substituted cyclopropanes in good yields. Apt activating groups with respect to the alkene are ester, amide, nitrile, sulfone, and dialkyl phosphonate functions. Methyl vinyl ketone (19) affords products in low yield that may arise from an intermediate cyclopropane derivative. Phenyl vinyl sulfoxide (24) mainly acts as an oxidizing agent, transforming 1 into methyl benzoate. for olefin 24 and {alpha}-(N-methylanilino)acrylonitrile the authors found products that should be formed on an olefin metathesis pathway. The methyl-substituted carbene complex 48 also affords the expected donor-acceptor-substituted cyclopropanes; however, acyclic isomers are formed in higher amounts. The molybdenum and tungsten complexes 55 and 56, respectively, also furnish cyclopropane derivatives, but the yields are lower than with the chromium compound 1. Disubstituted olefins and complex 1 still give the cyclopropanes in moderate yields, while all trisubstituted and most of the difunctionalized alkenes do not react with this Fischer carbene complex. The cyclopropanes synthesized can be deprotonated and alkylated or transformed into ring-opened products. These model reactions demonstrate the synthetic potentials of donor-acceptor-substituted cyclopropanes prepared via Fischer carbene complexes.

Wienand, A.; Reissig, H.U. (Inst. fuer Organische Chemie der Technischen Hochschule Darmstadt (West Germany))

1990-12-01

 
 
 
 
281

(Bis{2-[3-(2,4,6-trimethylbenzylimidazolin-2-yliden-1-yl-?C2]-4-methylphenyl}amido-?Nchloridopalladium(II  

Directory of Open Access Journals (Sweden)

Full Text Available The coordination geometry about the Pd centre in the title compound, [Pd(C40H42N5Cl], is approximately square-planar. The CNC pincer-type N-heterocyclic carbene ligand binds to the Pd atom in a tridentate fashion by the amido N atom and the two carbene atoms and generates two six-membered chelate rings, completing the coordination.

Guan-Jun Cheng

2010-02-01

282

Synthesis and Reactivity of Iron Carbene Complexes.  

Science.gov (United States)

In an effort to more fully understand the chemistry of carbene complexes, two series of complexes, Cp(CO) sub 2 Fe- (C(XR)YR) exp + and Cp(CO) (L) Fe(C(SCH sub 3 ) sub 2 ) exp + where X and Y are O, S, Se, and/or NR and L is CH sub 3 NC, P(OPh) sub 3 , P(...

F. B. McCormick

1980-01-01

283

Easy access to uranium nucleophilic carbene complexes  

Energy Technology Data Exchange (ETDEWEB)

Metathesis reactions of UCl{sub 4} with Li{sub 2}C(Ph{sub 2}PS){sub 2} in Et{sub 2}O only afforded the tris-carbene complex [{l_brace}Li(OEt{sub 2}){r_brace}{sub 2}-U{l_brace}=C(Ph{sub 2}PS){sub 2{r_brace}3}](1), while the bis- and mono-carbene compounds [U{l_brace}=C(Ph{sub 2}PS){sub 2{r_brace}2}(THF){sub 2}] (2) and [{l_brace}Li(THF){sub 2{r_brace}2}U{l_brace}=C(Ph{sub 2}PS){sub 2}{r_brace}Cl{sub 4}] (3) were obtained by treatment of UCl{sub 4} with Li{sub 2}C(Ph{sub 2}PS){sub 2} in a mixture of THF and toluene. The bis- carbene complex 2 was also obtained either from the comproportionation reaction of 1 and UCl{sub 4} or protonolysis reaction of U(NEt{sub 2}){sub 4} with H{sub 2}C(Ph{sub 2}PS){sub 2} and was transformed into the mono- carbene complex [U{l_brace}=C(Ph{sub 2}PS){sub 2}{r_brace}Cl{sub 2}(THF){sub 2}] (4) by further reaction with UCl{sub 4}. The utility of these complexes as precursors is illustrated by the synthesis of the bis-cyclopentadienyl derivative [Cp{sub 2}U{l_brace}=C(Ph{sub 2}PS){sub 2}{r_brace}] (5) by treatment of 3 with TlCp. The crystal structures of [U{l_brace}=C(Ph{sub 2}PS){sub 2}{r_brace}2(py){sub 2}]center dot 1.5py.0.5THF and 5.toluene are reported. (authors)

Tourneux, Jean-Christophe; Berthet, Jean-Claude; Thuery, Pierre; Ephritikhine, Michel [CEA, IRAMIS, SIS2M, CNRSUMR3299, 91191, Gif-sur-Yvette (France); Mezailles, Nicolas; Le Floch, Pascal [Laboratoire ' Heteroelements et Coordination' , Ecole Polytechnique, CNRS, 91128, Palaiseau Cedex (France)

2010-07-01

284

Uranium(IV) nucleophilic carbene complexes  

International Nuclear Information System (INIS)

The mono-, bis-, and tris-carbene uranium complexes [Li(THF)2U(SCS)Cl3(THF)] (2a), [U(SCS)2(THF)2] (3a), and [{Li(OEt2)}2U(SCS)3] (1) were synthesized in good yields by reactions of UCl4 and the stoichiometric amount of Li2(SCS) [(SCS)2 = [Ph2P(dS)]2C2]. Complex 3a was also obtained by comproportionation reaction of 1 and 0.5 molar equiv of UCl4 and further reacted with 1 molar equiv of UCl4 to give the neutral mono-carbene [U(SCS)Cl2(THF)2] (5a). Treatment of U(NEt2)4 with H2C(Ph2PS)2 in THF led to a mixture of 3a and [U(SCS)(NEt2)2] (6), while the same reaction in Et2O gave the mixed alkyl-carbene compound [U(SCS)(SCHS)(NEt2)] (7) in 85% yield. The cationic uranium carbene complex [U(SCS)(NEt2)(THF)3][BPh4] (9) was isolated in almost quantitative yield from reaction of [U(NEt2)3][BPh4] and H2C(Ph2PS)2. Mono-carbenes 2a, 5a, and 9 were used as precursors for the synthesis of Cp and COT derivatives (Cp = C5H5, COT=C8H8). Treatment of 2a with 1 or 2 molar equiv of TlCp gave [Tl{U(Cp)(SCS)}2(?-Cl)3] (10) and [U(Cp)2(SCS)] (11) in 90% and 79% yield, respectively, whereas [U(Cp*)2(SCS)] (12) (Cp* = C5Me5) was obtained only by reaction of [U(Cp*)2Cl2] and Li2(SCS). Reactions of 5a or 9 with K2COT gave [U(COT)(SCS)(THF)] (13) in 78% and 99% yield, respectively. 2a, [Li(THF)(Et2O)U(SCS)(?-Cl)3]2 (2b), [U(SCS)2(py)2] 3 1.5py 3 0.5THF (3b 3 1.5py 3 0.5THF), [U(SCS){CS(Ph2PS)2}(py)] (4), 7.toluene, [Li(THF)2U(SCS)(NEt2)(?-O)]2 (8), 10.2 toluene, 11, 12, and 13.0.5 pentane were characterized by X-ray diffraction. The crystal data revealed that, in contrast to transition metal complexes, changes in the coordination sphere of the U(IV) center have little influence on the U(double bond)C bond. This feature was explained by DFT analysis of analogous U(IV) and Zr(IV) compounds [M(SCS1)Cl2(py)2] and [M(SCS1)(Cp)2] [M = U, Zr; SCS1 = C(H2PS)2]. Although the 5f orbitals are more radially contracted than the 6d atomic orbitals, the 5f AOs are lower in energy in uranium and can lead to greater angular overlaps in symmetry-constrained systems. As a result, the seven 5f orbitals play a 'buffer' role by engaging in covalent interactions with the carbon center to stabilize the nucleophilic carbene lone pairs. (authors)

2011-01-01

285

Heterogeneous catalysts for carbene insertion reactions  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Copper catalysts supported on silica or silica-alumina are used to promote the insertion of carbenes, coming from methyl phenyldiazoacetate and ethyl diazoacetate, into one C-H bond of THF, constituting the first example of this reaction promoted by a purely inorganic catalyst. Cu/SiO2-Al 2O3 leads to better results, regarding yield and catalyst recovery, than with Cu/SiO2. With the former, yields are similar or even slightly better than those reached in solution using Cu (OTf)2 as the cataly...

2011-01-01

286

Chemistry of transition metal carbene complexes: nucleophilic substitution reactions of cyanamide anion to Fischer carbene complexes.  

Science.gov (United States)

Rate constants for the reactions of cyanamide anion N?C-NH(-) with Fischer carbene complexes of the type (CO)(5)M=C(XR)C(6)H(4)Z with M = Cr and W, XR = SMe and OMe, Z = NMe(2), OMe, Me, H, F, Cl and CF(3) in 50% MeCN-50% water (v/v) at 25 °C are reported. N?C-NH(-) shows a much higher reactivity towards these carbene complexes than OH(-), primary aliphatic amines (e.g. n-butylamine) or secondary alicyclic amines (e.g. piperidine) but is slightly less reactive than thiolate ions (e.g. HOCH(2)CH(2)S(-)). The alkoxy carbene complexes were found to react faster than the thiomethyl derivatives, consistent with previous findings for alkoxide ion, CH(CN)(2)(-), OH(-), amine and thiolate ion nucleophiles. Hammett ? values are 3.00 ± 0.08 (k(1)) and 2.98 ± 0.08 (k(2)) for Cr-OMe-Z-N?C-NH(-) reactions and 0.94 ± 0.05 (k(1)) for Cr-SMe-Z-N?C-NH(-) reactions. The ? values for the reaction of Cr-OMe-Z and Cr-SMe-Z with CH(CN)(2)(-) and DABCO (1,4-diazabicyclo[2.2.2]octane) (in 50% MeCN-50% H(2)O (v/v) are comparable to the present reactions. The much higher reactivity and hence much higher ? value for methoxy carbene complexes over the corresponding thiomethyl derivatives is best explained by considering the substituent effects not only on the transition state (TS) but also on the reactant, and consistent with the previously observed pattern. Higher k(1)(W)/k(1)(Cr) ratios for (thiomethyl)carbene complexes as well as methoxy and ethoxycarbene complexes are related to the intrinsic rate constant which is higher for tungsten-carbene complexes than the corresponding Cr ones resulting in an enhanced ratio. This can also be explained by considering the electronegativity of Cr and W, which is higher for the latter; as a result the negative charge on the central metal atom is more localized in case of W causing destabilization of the TS and hence higher reactivity. PMID:23076623

Gangopadhyay, Sumana; Mistri, Tarun; Dolai, Malay; Alam, Rabiul; Ali, Mahammad

2013-01-14

287

Synthesis of Endohedral Metallofullerene Glycoconjugates by Carbene Addition  

Directory of Open Access Journals (Sweden)

Full Text Available Endohedral metallofullerene glycoconjugates were synthesized under mild conditions by carbene addition using appropriate glycosylidene-derived diazirine with La2@Ih-C80. NMR spectroscopic studies revealed that the glycoconjugate consists of two diastereomers of [6,6]-open mono-adducts. The electronic properties were characterized using Vis/NIR absorption spectroscopy and electrochemical measurements. This study demonstrates that glycosylidene carbene is useful to incorporate carbohydrate moieties onto endohedral metallofullerene surfaces.

Takeshi Akasaka

2011-11-01

288

Synthesis of Endohedral Metallofullerene Glycoconjugates by Carbene Addition  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Endohedral metallofullerene glycoconjugates were synthesized under mild conditions by carbene addition using appropriate glycosylidene-derived diazirine with La2@Ih-C80. NMR spectroscopic studies revealed that the glycoconjugate consists of two diastereomers of [6,6]-open mono-adducts. The electronic properties were characterized using Vis/NIR absorption spectroscopy and electrochemical measurements. This study demonstrates that glycosylidene carbene is useful to incorporate carbohydrate moie...

2011-01-01

289

An unusual norcaradiene/tropylium rearrangement from a persistent amino-phosphonio-carbene  

Digital Repository Infrastructure Vision for European Research (DRIVER)

An amino-phosphonio-carbene featuring a bromobiphenyl backbone was prepared and spectroscopically characterized at low temperature. This carbene was found to readily rearrange upon warm up, affording an original tricyclic phospholium derivative, presumably via a norcaradiene/tropylium isomerization.

Vignolle, Joan; Donnadieu, Bruno; Bourissou, Didier; Bertrand, Guy

2007-01-01

290

Rhodium-catalyzed carbene transfer to alkynes via 2-furylcarbenes generated from enynones.  

Science.gov (United States)

A rhodium-catalyzed carbene transfer to alkynes is reported. Easily available enynones served as the carbene source replacing diazocompounds. The overall process involved a cyclization-cyclopropenation-ring enlargement sequence. PMID:24175321

González, María J; López, Enol; Vicente, Rubén

2014-05-25

291

Tuning the photophysical properties of BODIPY molecules by ?-conjugation with Fischer carbene complexes.  

Science.gov (United States)

The synthesis, structure, and photophysical properties of novel BODIPY-Fischer alkoxy-, thio-, and aminocarbene dyads are reported. The BODIPY chromophore is directly attached to the carbene ligand by an ethylenic spacer, thus forming donor-bridge-acceptor ?-extended systems. The extension of the ?-conjugation is decisive in the equilibrium geometries of the dyads and is clearly reflected in the corresponding absorption and emission spectra. Whereas the BODIPY fragment is mainly isolated in aminocarbene complexes, it is fully conjugated in alkoxycarbene derivatives. The former thus exhibit the characteristic photophysical properties of BODIPY units, whereas complete suppression of the BODIPY fluorescence emission is observed in the latter, as a direct consequence of the strong electron-accepting character of the (CO)5M=C moiety. As the ?-acceptor character of the metal-carbene group can be modified, the electronic properties of the conjugated BODIPY can be tuned. Density functional calculations have been carried out to gain insight into the photophysical properties. PMID:24382813

Chu, Gong Ming; Guerrero-Martínez, Andrés; Fernández, Israel; Sierra, Miguel Ángel

2014-01-27

292

Incorporation of hexafluorobutyne into furans or phenols via reaction with iron(0) carbene or vinylketene complexes  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Reaction of iron(0) carbene complexes with hexafluorobut-2-yne produces 3,4-bis(trifluoromethylated)furans in a process that is favored for electron rich carbenes. No traces of furannulation or benzannulation are observed with Group 6 Fischer carbenes. Reaction of hexafluorobut-2-yne with a vinylketene iron(0) complex gives a 2,3-bis-trifluoromethylated phenol.

Akhani, Ravish K.; Atiq-ur-Rehman; Schnatter, Wayne F. K.

2009-01-01

293

Intramolecular Cyclohexadienone Annulations of Fischer Carbene Complexes: Model Studies for the Synthesis of Phomactins  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The intramolecular cyclohexadienone annulation of chromium carbene complexes is examined as a method to provide general access to the Phomactin family of natural products. The importance of the stereochemistry of the carbene complex and the number of carbons in the tether connecting the carbene complex and the alkyne are probed. Additionally, the degree of the 1,4-asymmetric induction is examined.

Huang, Jie; Wang, Huan; Wu, Chunrui; Wulff, William D.

2007-01-01

294

Design and Synthesis of Ruthenium based Olefin Metathesis Catalysts  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The present Master thesis seeks to develop new unsymmetrical ruthenium-based olefin metathesis catalysts and therein a better understanding of olefin metathesis catalysis with unsymmetrical active complexes. Such catalysts have a potential for chemoselectivity and in best case, stereoselectivity. Two different classes of catalysts, coordinated by a hemilabile amine ligand and by a novel N-heterocyclic carbene (NHC) ligand respectively, have been investigated. Two new amine-based olefin metath...

Singstad, A?smund

2010-01-01

295

Carbene formation from excited C6-C10 cycloalkane molecules  

International Nuclear Information System (INIS)

During radiolysis and vacuum-ultraviolet photolysis of liquid n-alkanes and larger cycloalkanes, part of the main product, hydrogen is formed by unimolecular elimination. Unimolecular H2 elimination from a single carbon atom is demonstrated to be the basic decomposition process occurring from the low energy excited states of C6-C10 cycloalkanes. The carbene produced in this reaction is unimolecularly stabilized by transannular insertion or hydrogen atom migration. The carbene lifetime is probably longer than 10-100 ps. (author)

1982-09-19

296

A highly conjugated benzimidazole carbene-based ruthenium sensitizer for dye-sensitized solar cells.  

Science.gov (United States)

A new type of carbene-based ruthenium sensitizer, CB104, with a highly conjugated ancillary ligand, diphenylvinylthiophene-substituted benzimidazolepyridine, was designed and developed for dye-sensitized solar cell applications. The influence of the thiophene antenna on the performance of the cell anchored with CB104 was investigated. Compared with the dye CBTR, the conjugated thiophene in the ancillary ligand of CB104 enhanced the molar extinction coefficient of the intraligand ?-?* transition and the intensity of the lower energy metal-to-ligand charge-transfer band. However, the incident photon-to-current conversion efficiency spectrum of the cell anchored with CB104 (0.15?mM) showed a maximum of 63?% at 420?nm. The cell sensitized with the dye CB104 attained a power conversion efficiency of 7.30?%, which was lower than that of the cell with nonconjugated sensitizer CBTR (8.92?%) under the same fabrication conditions. The variation in the performance of these two dyes demonstrated that elongating the conjugated light-harvesting antenna resulted in the reduction of short-circuit photocurrent density, which might have been due to the aggregation of dye molecules. In the presence of a coabsorbate, chenodeoxycholic acid, the CB104-sensitized cell exhibited an enhanced photocurrent density and achieved a photovoltaic efficiency of 8.36?%. PMID:23832840

Ho, Shih-Yu; Su, Chaochin; Li, Chung-Yen; Prabakaran, Kumaresan; Shen, Ming-Tai; Chen, Ying-Fan; Chang, Wei-Chun; Tingare, Yogesh S; Akula, Suribabu; Tsai, Sheng-Han; Li, Wen-Ren

2013-09-01

297

Reactivities of Metal-Carbenes Toward Alkenes and Alkynes.  

Science.gov (United States)

Stabilized metal-carbenes like the P's in eqs 2 and 3 are very much more reactive toward phenylacetylene than toward cyclopentene (by a factor of 119-170) or cyclooctene (by a factor > 10 to the 4th power). The result is strange but accords with 1 (also a...

T. J. Katz E. B. Savage S. J. Lee M. Nair

1980-01-01

298

Fundamental spectroscopic studies of carbenes and hydrocarbon radicals  

Energy Technology Data Exchange (ETDEWEB)

Highly reactive carbenes and carbon-chain radicals are studied at millimeter wavelengths by observing their rotational spectra. The purpose is to provide definitive spectroscopic identification, accurate spectroscopic constants in the lowest vibrational states, and reliable structures of the key intermediates in reactions leading to aromatic hydrocarbons and soot particles in combustion.

Gottlieb, C.A.; Thaddeus, P. [Harvard Univ., Cambridge, MA (United States)

1993-12-01

299

Synthesis and electrochemical study of iron, chromium and tungsten aminocarbenes: Role of ligand structure and central metal nature  

Energy Technology Data Exchange (ETDEWEB)

Several series of Fischer-type aminocarbene complexes with central Fe, Cr or W atoms and with various carbene substitution were synthesized and electrochemically investigated by dc-polarography and cyclic voltammetry. The shifts and changes of reduction and oxidation potentials were evaluated using the linear free energy relationship (LFER) approach with respect to (a) the type of coordination, (b) the substitution on the carbene ligand and (c) the nature of the central metal atom. The analysis of measured data confirms that the reduction center is localized on the carbene moiety and is strongly influenced by both electronic and sterical properties of its substituents. The oxidation proceeds on the metal and depends mainly on its nature and on the {pi}-acidity of the ligands. Electrochemistry thus represents an important experimental approach to the description and understanding of the molecular electronic structure and redox properties. Experimental results are supported by DFT calculation of HOMO and LUMO orbitals shape and composition.

Hoskovcova, Irena [Department of Inorganic Chemistry, Institute of Chemical Technology, Technicka 5, 166 28 Prague 6 (Czech Republic); Rohacova, Jana; Dvorak, Dalimil; Tobrman, Tomas [Department of Organic Chemistry, Institute of Chemical Technology, Technicka 5, 166 28 Prague 6 (Czech Republic); Zalis, Stanislav [J. Heyrovsky Institute of Physical Chemistry, Academy of Science of the Czech Republic, Dolejskova 3, 182 23 Prague 8 (Czech Republic); Zverinova, Radka [Department of Inorganic Chemistry, Institute of Chemical Technology, Technicka 5, 166 28 Prague 6 (Czech Republic); J. Heyrovsky Institute of Physical Chemistry, Academy of Science of the Czech Republic, Dolejskova 3, 182 23 Prague 8 (Czech Republic); Ludvik, Jiri, E-mail: jiri.ludvik@jh-inst.cas.c [J. Heyrovsky Institute of Physical Chemistry, Academy of Science of the Czech Republic, Dolejskova 3, 182 23 Prague 8 (Czech Republic)

2010-11-30

300

Synthesis and electrochemical study of iron, chromium and tungsten aminocarbenes: Role of ligand structure and central metal nature  

International Nuclear Information System (INIS)

Several series of Fischer-type aminocarbene complexes with central Fe, Cr or W atoms and with various carbene substitution were synthesized and electrochemically investigated by dc-polarography and cyclic voltammetry. The shifts and changes of reduction and oxidation potentials were evaluated using the linear free energy relationship (LFER) approach with respect to (a) the type of coordination, (b) the substitution on the carbene ligand and (c) the nature of the central metal atom. The analysis of measured data confirms that the reduction center is localized on the carbene moiety and is strongly influenced by both electronic and sterical properties of its substituents. The oxidation proceeds on the metal and depends mainly on its nature and on the ?-acidity of the ligands. Electrochemistry thus represents an important experimental approach to the description and understanding of the molecular electronic structure and redox properties. Experimental results are supported by DFT calculation of HOMO and LUMO orbitals shape and composition.

2010-11-30

 
 
 
 
301

Bis(1,3-dimesitylimidazolylgold(I 2,4,8,10-tetraphenyl-1,3,5,7,9,11-hexaoxa-2,4,8,10-tetrabora-6-borataspiro[5.5]undecane  

Directory of Open Access Journals (Sweden)

Full Text Available The AuI atom in the title compound, [Au(C21H24N22](C24H20B5O6, adopts a slightly distorted linear AuC2 coordination geometry arising from its coordination by two mesitylenic N-heterocyclic carbene ligands, forming an overall cationic complex. The dihedral angle between the imidazole rings is 57.3?(6°. In the crystal, the components are linked by weak C—H...O hydrogen bonds.

Lennart Möhlmann

2011-06-01

302

Synthesis and reactivity of some tungsten carbene complexes  

International Nuclear Information System (INIS)

Dithiocarbene tungsten complexes, W(CO)5[C(SR)(SR')], were prepared in good yield. The substituted tetracarbonyl dimethyldithiocarbene tungsten complexes are prepared by an analogous route. Cyclic dithiocarbene tungsten complexes may be prepared in a similar manner. Isocyanide complexes are formed when W(CO)5[C(SCH3)2] reacts with primary amines. Net effect of the reaction of tungsten dithiocarbene complexes with tertiary phosphine is to transfer the carbene group from the tungsten to the phosphorus atom

1981-01-01

303

Synthesis and characterization of distonic radical, biradical and carbene ions  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The synthesis and characterization of distonic radical, biradical and carbene anions were studied with the use of a flowing afterglow-triple quadrupole instrument. A new synthetic strategy for gas-phase synthesis of negative ions with multiple open valence sites was developed. This method, which is based on the reaction of organic anions with molecular fluorine, allowed effective synthesis of a series of negative ions of biradicals and triradicals that are of great theoretical importance. The...

1997-01-01

304

First Astronomical Detection of the Carbene Chain H2C6  

Science.gov (United States)

The cumulene carbenes are important components of hydrocarbon chemistry in low mass star forming cores. Here we report the first astronomical detection of the long chain cumulene carbene H(sub 2)C(sub 6), in the insterstellar cloud TMC1, from observations of two of its rotational transitions:...

Langer, W. D.; Velusamy, T.; Kuiper, T. B. H.; McCarthy, M. C.; Travers, M. J.; Kovacs, A.; Gottlieb, C. A.; Thaddeus, P.

1996-01-01

305

?3-Chlorido-tris­(bis­{1-[2-(dimethyl­amino)­eth­yl]-3-methyl­imidazol-2-yl­idene}silver(I)) dichloride  

Digital Repository Infrastructure Vision for European Research (DRIVER)

In the crystal structure of the title compound, [Ag3Cl(C8H15N3)6]Cl2, the AgI ion, which is located on a twofold rotation axis, exists in a T-shape coordination environment. Two carbene C atoms of the N-heterocyclic carbene (NHC) ligands are bonded tightly forming a slightly bent [Ag(NHC)2]+ cation [C—Ag—C angle = 162.80?(18)°]. Three of these complex cations are further aggregated by one bridging chloride anion, which is lying on a threefold rotoinversion axis and is only loosely bind...

Topf, Christoph; Leitner, Sebastian; Monkowius, Uwe

2012-01-01

306

Actinide metals with multiple bonds to carbon: synthesis, characterization, and reactivity of U(IV) and Th(IV) bis(iminophosphorano)methandiide pincer carbene complexes.  

Science.gov (United States)

Treatment of ThCl(4)(DME)(2) or UCl(4) with 1 equiv of dilithiumbis(iminophosphorano) methandiide, [Li(2)C(Ph(2)P?NSiMe(3))(2)] (1), afforded the chloro actinide carbene complexes [Cl(2)M(C(Ph(2)P?NSiMe(3))(2))] (2 (M = Th) and 3 (M = U)) in situ. Stable PCP metal-carbene complexes [Cp(2)Th(C(Ph(2)P?NSiMe(3))(2))] (4), [Cp(2)U(C(Ph(2)P?NSiMe(3))(2))] (5), [TpTh(C(Ph(2)P?NSiMe(3))(2))Cl] (6), and [TpU(C(Ph(2)P?NSiMe(3))(2))Cl] (7) were generated from 2 or 3 by further reaction with 2 equiv of thallium(I) cyclopentadienide (CpTl) in THF to yield 4 or 5 or with 1 equiv of potassium hydrotris(pyrazol-1-yl) borate (TpK) also in THF to give 6 or 7, respectively. The derivative complexes were isolated, and their crystal structures were determined by X-ray diffraction. All of these U (or Th)-carbene complexes (4-7) possess a very short M (Th or U)?carbene bond with evidence for multiple bond character. Gaussian 03 DFT calculations indicate that the M?C double bond is constructed by interaction of the 5f and 6d orbitals of the actinide metal with carbene 2p orbitals of both ? and ? character. Complex 3 reacted with acetonitrile or benzonitrile to cyclo-add C?N to the U?carbon double bond, thereby forming a new C-C bond in a new chelated quadridentate ligand in the bridged dimetallic complexes (9 and 10). A single carbon-U bond is retained. The newly coordinated uranium complex dimerizes with one equivalent of unconverted 3 using two chlorides and the newly formed imine derived from the nitrile as three connecting bridges. In addition, a new crystal structure of [CpUCl(3)(THF)(2)] (8) was determined by X-ray diffraction. PMID:21667978

Ma, Guibin; Ferguson, Michael J; McDonald, Robert; Cavell, Ronald G

2011-07-18

307

Conformation analysis of triphenylphosphine in trans and cis triphenylphosphine-substituted Fischer carbene complexes  

Science.gov (United States)

The synthesis and selected crystal structures of novel (M = Cr or Mo) and known (M = W) [(CO)4(PPh3)M = C(OEt)R] complexes, M = Cr, R = 2-thienyl (1), 2-furyl (2); M = Mo, R = 2-thienyl (3), 2-furyl (4); M = W, R = 2-thienyl (5), 2-furyl (6), are presented. Experimental crystal structures and DFT calculations of selected trans- and cis-triphenylphosphine-substituted Fischer carbene complexes, illustrate that the minimum energy conformation of triphenylphosphine (PPh3) in octahedral [(CO)4(PPh3)ML]-complexes generally have distinct features that can be described in terms of the "plane of nadir energy", a plane linking all points of minimum steric compression between the ligands. The generally observed orientation of PPh3 involves a correlated feathering of the phenyl groups with the PCipso bond of one phenyl group orientated near parallel to the nadir plane, and a meta carbon (Cm) of the other two phenyl groups orientated as near as possible to the nadir plane, orthogonal to the first. Although the orientation of PPh3 in 6-trans, [(CO)4(PPh3)W = C(OEt)2-furyl], deviates from this, DFT correctly calculated the unexpected and not generally observed PPh3 orientation.

Landman, Marilé; Levell, Tamzyn; van Rooyen, Petrus H.; Conradie, Jeanet

2014-05-01

308

Probing Protein Surface with a Solvent Mimetic Carbene Coupled to Detection by Mass Spectrometry  

Science.gov (United States)

Much knowledge into protein folding, ligand binding, and complex formation can be derived from the examination of the nature and size of the accessible surface area (SASA) of the polypeptide chain, a key parameter in protein science not directly measurable in an experimental fashion. To this end, an ideal chemical approach should aim at exerting solvent mimicry and achieving minimal selectivity to probe the protein surface regardless of its chemical nature. The choice of the photoreagent diazirine to fulfill these goals arises from its size comparable to water and from being a convenient source of the extremely reactive methylene carbene (:CH2). The ensuing methylation depends primarily on the solvent accessibility of the polypeptide chain, turning it into a valuable signal to address experimentally the measurement of SASA in proteins. The superb sensitivity and high resolution of modern mass spectrometry techniques allows us to derive a quantitative signal proportional to the extent of modification (EM) of the sample. Thus, diazirine labeling coupled to electrospray mass spectrometry (ESI-MS) detection can shed light on conformational features of the native as well as non-native states, not easily addressable by other methods. Enzymatic fragmentation of the polypeptide chain at the level of small peptides allows us to locate the covalent tag along the amino acid sequence, therefore enabling the construction of a map of solvent accessibility. Moreover, by subsequent MS/MS analysis of peptides, we demonstrate here the feasibility of attaining amino acid resolution in defining the target sites.

Gómez, Gabriela E.; Mundo, Mariana R.; Craig, Patricio O.; Delfino, José M.

2012-01-01

309

A Theoretical Study of Uranium(IV) Bis-Methyl Complexes: Towards the Predictive Formation of a Transient Uranium(IV) Carbene Complex  

Energy Technology Data Exchange (ETDEWEB)

In summary, we have shown that the first step of ortho C-H activation of pyridine N-oxide mediated by L{sub 2}U(IV)-(CH{sub 3}){sub 2} can occur with the three ligands L (Cp, CpTMS, and Cp*) to form an {eta}{sup 2}(C, O) ortho-metallated complex. Also, we have shown that for L=Cp and CpTMS it is possible to form a very stable uranium (VI) formaldehyde complex going through an uranium(IV) Schrock-type carbene intermediate. This is in contrast with the ligand Cp*: the formation of uranium(IV) formaldehyde complex is not possible with the Cp* owing to a high barrier at the H{sub 2}C-O repulsive coupling transition state. This high barrier can be explained by the steric hindrance of Cp* compared with Cp and CpTMS. Moreover, the electronic repulsion between the two negative charges (carbene and oxygen) is enhanced in the case of Cp*. (authors)

Yahia, A.; Castro, L.; Maron, L. [Univ Toulouse 3, Lab Phys and Chim Nanoobjets, INSA, F-31077 Toulouse (France); Yahia, A. [Univ Montpellier 2, Inst Chim Separat and Marcoule, UMR 5257, CEA, CNRS, ENSCM, Ctr Marcoule, F-30207 Bagnols Sur Ceze (France)

2010-07-01

310

A Theoretical Study of Uranium(IV) Bis-Methyl Complexes: Towards the Predictive Formation of a Transient Uranium(IV) Carbene Complex  

International Nuclear Information System (INIS)

In summary, we have shown that the first step of ortho C-H activation of pyridine N-oxide mediated by L2U(IV)-(CH3)2 can occur with the three ligands L (Cp, CpTMS, and Cp*) to form an ?2(C, O) ortho-metallated complex. Also, we have shown that for L=Cp and CpTMS it is possible to form a very stable uranium (VI) formaldehyde complex going through an uranium(IV) Schrock-type carbene intermediate. This is in contrast with the ligand Cp*: the formation of uranium(IV) formaldehyde complex is not possible with the Cp* owing to a high barrier at the H2C-O repulsive coupling transition state. This high barrier can be explained by the steric hindrance of Cp* compared with Cp and CpTMS. Moreover, the electronic repulsion between the two negative charges (carbene and oxygen) is enhanced in the case of Cp*. (authors)

2010-01-01

311

On the stability of perfluoroalkyl-substituted singlet carbenes: a coupled-cluster quantum chemical study.  

Science.gov (United States)

A series of trifluoromethyl-substituted carbenes R-C(:)-CF3 (R = NMe2, OMe, F, PMe2, P(NMe2)2, P(N(Pr-i)2)2, SMe, Cl); (dimethylamino)(perfluoroalkyl)carbenes Me2N-C(:)-R (R = CF3, C2F5, n-C3F7, i-C3F7, and t-C4F9) and symmetrically substituted carbenes R-C(:)-R (R = NMe2, OMe, F, PMe2, SMe, Cl) have been investigated by means of quantum chemistry methods. Different levels of approximation were used, including the CCSD(T) approach also known in quantum chemistry as the "golden standard", in combination with three different basis sets (TZVP, cc-pVDZ, cc-pVTZ). Relative stabilities of carbenes have been estimated using the differences between the singlet and triplet ground state energies (?EST) and energies of the hydrogenation reaction for the singlet and triplet ground states of the carbenes. The latter seem to correlate better with stability of carbenes than the ?EST values. The (13)C NMR chemical shifts of the methylidene carbon indicate the more high-field chemical shift values in the known, isolable carbenes compared to the unstable ones. This is the first report on the expected chemical shifts in the highly unstable singlet carbenes. Using these criteria, some carbene structures from the studied series (as, for instance, Me2N-C(:)-CF3, Me2N-C(:)-C3F7-i) are proposed as good candidates for the experimental preparation. PMID:24471524

Rozhenko, Alexander B; Schoeller, Wolfgang W; Leszczynski, Jerzy

2014-02-27

312

Cyclic (Alkyl)(amino)carbene Gold(I) complexes: A Synthetic and Structural Investigation  

Digital Repository Infrastructure Vision for European Research (DRIVER)

A series of mono- and dicarbene gold(I) complexes of types Au(CAAC)(Cl) [CAAC = cyclic (alkyl)(amino)carbene] (1) and [Au(CAAC)2]+[X]- (X = Cl, AuCl2) (2) have been prepared through reaction of AuCl(SMe2) with free carbenes a-e, and structurally characterized by single X-ray diffraction studies (1a, 1b, 2d, 2e). In addition two new free cyclic (alkyl)(amino)carbenes (c and e) have been synthesized.

Frey, Guido D.; Dewhurst, Rian D.; Kousar, Shazia; Donnadieu, Bruno; Bertrand, Guy

2008-01-01

313

Nitrene-carbene-carbene rearrangement. Photolysis and thermolysis of tetrazolo[5,1-a]phthalazine with formation of 1-phthalazinylnitrene, o-cyanophenylcarbene, and phenylcyanocarbene.  

Science.gov (United States)

1-Azidophthalazine 9A is generated in trace amount by mild FVT of tetrazolo[5,1-a]phthalazine 9T and is observable by its absorption at 2121 cm(-1) in the Ar matrix IR spectrum. Ar matrix photolysis of 9T/9A at 254 nm causes ring opening to generate two conformers of (o-cyanophenyl)diazomethane 11 (2079 and 2075 cm(-1)), followed by (o-cyanophenyl)carbene (3)12, cyanocycloheptatetraene 13, and finally cyano(phenyl)carbene (3)14 as evaluated by IR spectroscopy. The two carbenes (3)12 and (3)14 were observed by ESR spectroscopy (D|hc = 0.5078, E|hc = 0.0236 and D|hc = 0.6488, E|hc = 0.0195 cm(-1), respectively). The rearrangement of 12 ? 13 ? 14 constitutes a carbene-carbene rearrangement. 1-Phthalazinylnitrene (3)10 is observed by means of its UV-vis spectrum in Ar matrix following FVT of 9 above 550 °C. Rearrangement to cyanophenylcarbenes also takes place on FVT of 9 as evidenced by observation of the products of ring contraction, viz., fulvenallenes and ethynylcyclopentadienes 16-18. Thus the overall rearrangement 10 ? 11 ? 12 ? 13 ? 14 can be formulated. PMID:24304417

Høj, Martin; Kvaskoff, David; Wentrup, Curt

2014-01-01

314

Structural and spectroscopic comparison of methylaminophenylcarbenepentacarbonyltungsten(0) with tungsten carbonyl carbene complexes lacking a heteroatom  

International Nuclear Information System (INIS)

Crystallographic and spectroscopic data for (CO)5WC(NHCH3)C6H5 provide evidence that there is comparatively little metal-carbene ?-bonding in this and other zerovalent tungsten carbonyl carbene complexes. The W-Csub(car) bond length of 2.19(2) A indicates that its bond order is significantly lower than 2.0. Resonances in the 13C n.m.r. spectrum for CO w.r.t. tetramethylsilane at 195 and 215 ppm and an asymmetric CO stretching frequency of 2060 cm-1, in addition to structural similarities, imply analogous bonding in other zero-valent tungsten carbonyl carbene complexes, including those lacking a heteroatom bonded to the carbene carbon. (orig.)

1984-02-01

315

Characterization of Tunable Radical Metal-Carbenes: Key Intermediates in Catalytic Cyclopropanation  

Digital Repository Infrastructure Vision for European Research (DRIVER)

A new class of radical metal-carbene complex has been characterized as having Fischer-like orbital interactions and adjacent ? acceptor stabilization. Density Functional Theory (DFT) along with Natural Bond Orbital (NBO) analysis and Charge Decomposition Analysis (CDA) has given insight into the electronics of this catalytic intermediate in an open-shell cobalt-porphyrin, [Co(Por)], system. The complex has a single bond from the metal to the carbene and has radical character with localized s...

Belof, Jonathan L.; Cioce, Christian R.; Xu, Xue; Zhang, X. Peter; Space, Brian; Woodcock, H. Lee

2011-01-01

316

Protonolysis of a Ruthenium-Carbene Bond and Applications in Olefin Metathesis  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The synthesis of a ruthenium complex containing an N-heterocylic carbene (NHC) and a mesoionic carbene (MIC) is described wherein addition of a Brønsted acid results in protonolysis of the Ru-MIC bond to generate an extremely active metathesis catalyst. Mechanistic studies implicate a rate-determining protonation step to generate the metathesis active species. The NHC/MIC catalyst was found to have activity exceeding current commercial ruthenium catalysts.

Keitz, Benjamin K.; Bouffard, Jean; Bertrand, Guy; Grubbs, Robert H.

2011-01-01

317

Iron in the Service of Chromium – the ortho-Benzannulation of trans, trans-Dienyl Fischer Carbene Complexes  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Chromium Fischer carbene complexes with trans, trans-dienyl substituents on the carbene carbon will react with diiron nonacarbonyl to give 2-alkoxycyclohexa-2,4-dienone iron tricarbonyl complexes and/or 2-alkoxyphenols in excellent yields. In the presence of silica gel or base, the cyclohexadienone complex will suffer loss of the iron and aromatization to give 2-alkoxyphenols. The formation of 2-alkoxyphenols from dienyl chromium carbene complexes is a known process (ortho-benzannulation) tha...

Lian, Yiqian; Wulff, William D.

2005-01-01

318

A versatile methodology for the regioselective c(8)-metalation of purine bases.  

Science.gov (United States)

Purine nucleobases are excellent ligands for metal ions, forming normally coordinative Werner-type bonds by utilizing the N donor atoms of the nucleobase skeleton. Here we show that purines such as 8-chlorocaffeine and 8-bromo-9-methyladenine react with [Pt(PPh3)4] under oxidative addition of the C(8)-halogen bond to the metal center. The resulting Pt(II) complexes feature a C(8)-bound ylidene ligand. Protonation of the ylidene at the N(7/9)-atom yields complexes bearing a protic N-heterocyclic carbene ligand derived from the purine base with an NMe,NH-substitution pattern. PMID:24823250

Brackemeyer, Dirk; Hervé, Alexandre; Schulte To Brinke, Christian; Jahnke, Mareike C; Hahn, F Ekkehardt

2014-06-01

319

Three-coordinate nickel carbene complexes and their one-electron oxidation products.  

Science.gov (United States)

The synthesis and characterization of two new carbene complexes, (dtbpe)Ni?CH(dmp) (1; dtbpe = 1,2-bis(di-tert-butylphosphino)ethane; dmp = 2,6-dimesitylphenyl) and (dippn)Ni?CH(dmp) (2; dippn = 1,8-bis(di-iso-propylphosphino)naphthalene), are described. Complexes 1 and 2 were isolated by photolysis of the corresponding side-bound diazoalkane complexes, exemplified by (dtbpe)Ni{?(2)-N2CH(dmp)} (3). The carbene complexes feature Ni-C distances that are short and Ni-C-C angles at the carbene carbon that are intermediate between 120° and 180° (155.7(3)° and 152.3(3)°, respectively). The difference between the two carbenes became obvious when their reactivity toward 1-electron oxidizing agents was studied: the oxidation of 1 led to an internal rearrangement and the formation of a nickel(I) alkyl [{?(2)-P,C-di-tert-butylphosphino-di-tert-butyl-PCH(dmp)ethane}Ni][BAr(F)4] (4), while the oxidation of 2 allowed the isolation of an unrearranged product, formulated as the cationic nickel(III) carbene complex[(dippn)Ni?CH(dmp)][BAr(F)4] (6). Both oxidations are chemically reversible and the respective reductions lead to the neutral carbene complexes, 1 and 2. PMID:24716462

Iluc, Vlad M; Hillhouse, Gregory L

2014-04-30

320

Central-to-Axial Chirality Transfer in the Benzannulation Reaction of Optically Pure Fischer Carbene Complexes in the Synthesis of Allocolchicinoids  

Digital Repository Infrastructure Vision for European Research (DRIVER)

A method for the synthesis of allocolchicinoids is explored that involves the benzannulation reaction of Fischer chromium carbene complexes with alkynes. The benzannulation reaction is employed to install the aromatic C-ring via the reaction of an ?,?-unsaturated carbene complex in which the carbene complex is attached to a seven-membered ring that is to become the B-ring of the allocolchicinoids. Two different regioisomeric series can be accessed depending on which position the carbene com...

Vorogushin, Andrei V.; Wulff, William D.; Hansen, Hans-ju?rgen

2008-01-01

 
 
 
 
321

Cobalt azide complexes with a tris(carbene)borate ligand scaffold.  

Science.gov (United States)

The four-coordinate Co(II) complex, (azido-?N)[1,1,',1''-(phenylboranetriyl)tris(3-tert-butyl-1H-imidazol-2-ylidene)]cobalt(II), [Co(C27H38BN6)(N3)], (1), denoted PhB(t-BuIm)3CoN3, was prepared by the reaction of the corresponding chloride complex with NaN3. One-electron oxidation results in the isolation of the five-coordinate Co(III) complex, bis(azido-?N)[1,1,',1''-(phenylboranetriyl)tris(3-tert-butyl-1H-imidazol-2-ylidene)]cobalt(III), [Co(C27H38BN6)(N3)2], (2), denoted PhB(t-BuIm)3Co(N3)2. Attempts to prepare cobalt nitrides by thermolysis or photolysis of these complexes were unsuccessful. PMID:24005501

Scepaniak, Jeremiah J; Margarit, Charles G; Bontchev, Ranko P; Smith, Jeremy M

2013-09-01

322

Correlation between functionality preference of Ru carbenes and exo/endo product selectivity for clarifying the mechanism of ring-closing enyne metathesis.  

Science.gov (United States)

Functionality preferences of metathesis Ru carbenes to various alkenes and alkynes with electronic and steric diversity were determined by using time-dependent fluorescence quenching. The functionality preferences depend not only on the properties of multiple bonds but also on the ligands on Ru. There was a good correlation between functionality preference and the metathesis reaction outcome. The correlation between functionality preference and exo/endo product ratio offers a solution to resolve the mechanistic issue related with alkene- vs alkyne-initiated pathway in ring-closing enyne metathesis. The correlation indicates the preference is likely to dictate the reaction pathway and eventually the outcome of the reaction. The Ru catalyst favoring alkyne over alkene provides more endo product, indicating that the reaction mainly initiates at the alkyne. By changing the substitution pattern, the preference can be reversed to give an exclusive exo product. PMID:23944769

Lee, Ok Suk; Kim, Kyung Hwan; Kim, Jinwoo; Kwon, Kuktae; Ok, Taedong; Ihee, Hyotcherl; Lee, Hee-Yoon; Sohn, Jeong-Hun

2013-09-01

323

The reactivity game: theoretical predictions for heavy atom tunneling in adamantyl and related carbenes.  

Science.gov (United States)

The possibility of carbon atom tunneling at cryogenic temperatures for carbene-based ring expansion of adamantane analogues calls for a delicate balance of reactivity to experimentally detect the transpiring reaction. An overly reactive carbene will precipitously decay; an excessively stable carbene will not tunnel. Nevertheless, the factors that affect the quantum-mechanical tunneling (QMT) reactivity - mass, barrier height and width - are strikingly different from the classical "over the barrier" thermal mechanism. Herein, comparisons with experimental values and predictions on measurable rate constants for novel carbene systems are presented by way of small curvature tunneling (SCT) computations. Adamantane, noradamantane and bisnoradamantane have a significantly different C-C bond strain and reactivity, which can be modulated by tinkering with the carbene substituent atom (H, Cl or F) to obtain an observable lifetime of the reactant. The influence of barrier heights and widths, kinetic isotope effects (KIEs), the detection of the tunneling-determining atoms (TDA) and the comparisons with hydrogen-based reactions are discussed with the objective of finding the physical limits for QMT. PMID:24590008

Kozuch, S

2014-05-01

324

Probing the nature and extent of stabilization within foiled carbenes: homoallylic participation by a neighboring cyclopropane ring.  

Science.gov (United States)

Oxadiazoline 6 was synthesized to generate endo-tricyclo[3.2.1.0(2,4)]octan-8-ylidene (3) by either photolysis or thermolysis. Diastereomer 6a thermally decomposed twice as fast as 6b. Carbene 3 was trapped stereoselectively by acrylonitrile and diethylamine in high yields. It behaved as a nucleophilic carbene with electron-poor alkenes, like acrylonitrile, but as an electrophile with very electron-rich species, such as diethylamine. However, when the reactions were performed in cyclohexane and cyclohexene, isomerization of 3 was favored. Replacement of the double bond in 7-norbornenylidene (1) by the single bond in the endo-fused cyclopropane unit of carbene 3 led to similar outcomes. Carbene 3 rightfully belongs to the family of foiled carbenes. PMID:23586674

Apeland, Ingrid Malene; Kählig, Hanspeter; Lorbeer, Eberhard; Brinker, Udo H

2013-05-17

325

trans-Bis[1-(2-benzamidoethyl-3-(2,4,6-trimethylphenylimidazol-2-ylidene]dichloridopalladium(II  

Directory of Open Access Journals (Sweden)

Full Text Available In the title compound, [PdCl2(C21H23N3O2], the PdII atom is located on an inversion centre and is coordinated in a slightly distorted square-planar environment by the chloride and N-heterocyclic carbene (NHC ligands in mutual trans positions. There are several hydrogen-bonding interactions, the most significant of which is a hydrogen bond between the amide moiety of the NHC and the chloride ligand. These hydrogen-bond interactions form a three-dimensional network.

Stefan Warsink

2012-08-01

326

Identification of carbene particles forming during decomposition of molybdenum neopentyl derivatives  

International Nuclear Information System (INIS)

Method of chromatomass-spectrometry was used to study MoCl5 interaction with neopentylmagnesium bromide at -78 and 20 deg C in the medium of dibutyl ether with successive addition of benzaldehyde to the system. It is shown that the main products of reactions are tetramethylhexane with 45-47 and 3-7% yield respectively, as well as ?-neopentylstyrene and ?-neopentylbutyl ether. If reaction is conducted at -78 deg C the major part of formed carbenes is used for reaction with benzaldehyde. The yield of ?-neopentylstyrene decreases at 20 degC. This is conditioned by carbene reactions with the solvent or other system components. The obtained data are the direct experimental corroboration of carbene particle formation during decomposition of molybdenum neopentyl derivatives

1989-01-01

327

Synthesis and catalytic properties of molybdenum polynuclear silicon-containing carbene complexes  

International Nuclear Information System (INIS)

Molybdenum carbene silicon-containing complexes PhMe2Si-CH=Mo(NAr)(OR)2 (1), Ph2Si[CH=Mo(NAr)(OR)2]2 (2) and (RO)2(ArN)Mo=CH-(SiMe2)2-CH=Mo(NAr)(OR)2 (Ar=2,6-Pr2iC6H3; R=CMe2CF3) were prepared by the addition reaction of R'-CH=Mo(NAr)(OR)2 (R'=But, PhMe2C) with silicon-containing vinyl reagents. Structures of 1, 2 complexes and known compound PhMe2C-CH=Mo(NAr)(OCMe2CF3)2 are established by X-ray structural analysis. Catalytic properties of silicon-containing carbene complexes are studied. Catalytic activity of the mentioned complexes depends on the nature of substitute near carbene atom of carbon

2007-02-01

328

Density Functional Theory Study of Mechanism of Cycloaddition Reaction Between Dimethyl-Silylene Carbene and Acetone  

Science.gov (United States)

The mechanism of the cycloaddition reaction between singlet dimethyl-silylene carbene and acetone has been investigated with density functional theory, From the potential energy profile, it can be predicted that the reaction has two competitive dominant reaction pathways. The presented rule of this reaction: the [2+2] cycloaddition effect between the ? orbital of dimethyl-silylene carbene and the ? orbital of ?-bonded compounds leads to the formation of a twisty four-membered ring intermediate and a planar four-membered ring product; The unsaturated property of C atom from carbene in the planar four-membered ring product, resulting in the generation of CH3-transfer product and silicic bis-heterocyclic compound.

Lu, Xiu-hui; Xiang, Ping-ping; Shi, Le-yi; Han, Jun-feng; Lian, Zhen-Xia

2010-04-01

329

{alpha},{beta}-Unsaturated Fischer carbene complexes as chemical multitalents  

Energy Technology Data Exchange (ETDEWEB)

The well established reaction of {alpha},{beta}-unsaturated Fischer carbenechromium complexes 6(R{sup 1} = H) with alkynes normally proceeds with carbonyl insertion to yield 4-alkoxyphenols 9. Led by the incidental formation of a cyclopentadiene 3 from certain {beta}-aminosubstituted complexes 6(X = NR{sub 2}{sup 3}, R{sup 1} = cPr) the authors have studied the influences of the nature of substituents (R{sup 1}, X on 6; R{sub L}, R{sub S} in the alkyne; R{sup 3} in the amino group), solvents, and temperature on the outcome of the reaction. Imino substitution on complexes 6 leads to 2H-pyrroles 1, a free primary amino group (X = NH{sub 2}) to pyridines 5, and bulky substituents R{sup 1} to cyclopenta[b]pyrans 8 with double insertion of an alkyne. Eventually, appropriate conditions have been developed which permit to selectively prepare either 3-alkoxy-5-(dialkylamino)cyclopentadienes 3 (as synthetic equivalents of cyclopentenones 4), 5-(dialkylaminomethylene)cyclopent-2-enones 7, 3-alkoxy-2-(1{prime}-morpholino-1{prime}-alkenyl)cyclopent-2-enones 10, and 2-acyl-3-(dialkylamino)cyclopent-2-enones 11 from easily accessible carbene complexes 6 (X = NR{sub 2}{sup 3}) in high yields. Mechanistic aspects and implications of these novel transformations will be discussed.

Meijere, A. de [Institut fuer Organische Chemie der George-August-Universitaet Goettingen (Germany)

1995-12-31

330

Energy barrier by cyclopropylchlorocarbene rearrangement measured by direct observation of the carbene in laser flash spectroscopy  

Energy Technology Data Exchange (ETDEWEB)

A transient absorption in the 250-265-nm range is assigned to cyclopropylchlorocarbene generated from the laser flash photolysis of 3-chloro-3-cyclopropyldiazirine. Confidence in this assignment was obtained from the following: (a) the decay of this absorption matches the growth of the pyridinium ylide: (b) the values for the rate constant for reaction of the carbene with 1-hexane obtained by monitoring the carbene at 250 nm and the ylide at 370 nm are similar. The kinetic parameters for the rearrangement of cyclopropylchlorocarbene to 1-chlorocyclobutene are log A = 11.1 and E{sub a} = 7.4 kcal/mol.

Liu, M.T.H.; Bonneau, R. (Universite de Bordeaux, Talence (France))

1989-10-19

331

Total Synthesis of (±)-Phomactin B2 via an Intramolecular Cyclohexadienone Annulation of a Chromium Carbene Complex  

Digital Repository Infrastructure Vision for European Research (DRIVER)

A total synthesis of (±)-phomactin B2 is described which has as its key step the intramolecular cyclohexadienone annulation of a Fischer carbene complex. The requisite carbene complex was prepared from geraniol in 11 steps and 12 % overall yield. The key cyclohexadienone annulation produced both rings of the [9.3.1] pentadecane ring system of phomactin B2 in a single step in 60% yield and as a 4:1 mixture of diastereomers. The major diastereomer was taken on to the natural product in a serie...

2007-01-01

332

Comparative reactivity of different types of stable cyclic and acyclic mono- and diamino carbenes with simple organic substrates.  

Science.gov (United States)

A series of stable carbenes, featuring a broad range of electronic properties, were reacted with simple organic substrates. The N,N-dimesityl imidazolylidene (NHC) does not react with isocyanides, whereas anti-Bredt di(amino)carbene (pyr-NHC), cyclic (alkyl)(amino)carbene (CAAC), acyclic di(amino)carbene (ADAC), and acyclic (alkyl)(amino)carbene (AAAC) give rise to the corresponding ketenimines. NHCs are known to promote the benzoin condensation, and we found that the CAAC, pyr-NHC, and ADAC react with benzaldehyde to give the ketone tautomer of the Breslow intermediate, whereas the AAAC first gives the corresponding epoxide and ultimately the Breslow intermediate, which can be isolated. Addition of excess benzaldehyde to the latter does not lead to benzoin but to a stable 1,3-dioxolane. Depending on the electronic properties of carbenes, different products are also obtained with methyl acrylate as a substrate. The critical role of the carbene electrophilicity on the outcome of reactions is discussed. PMID:24634983

Martin, David; Canac, Yves; Lavallo, Vincent; Bertrand, Guy

2014-04-01

333

Metal template controlled formation of [11]ane-P2C(NHC) macrocycles.  

Science.gov (United States)

The synthesis of N-heterocyclic carbene-diphosphine macrocycles by metal template assisted cyclization reactions has been explored. Attempts to prepare the facial tungsten tricarbonyl precursor complex containing an NH,NH-functionalized carbene and a suitable diphosphine resulted in displacement of the coordinated carbene and the isolation of the corresponding diphosphine tungsten tetracarbonyl [3]. The Re(I) chloro tetracarbonyl complex bearing an NH,NH-functionalized carbene ligand [5] can be prepared and is a suitable precursor for the subsequent formation of the carbene-diphosphine tricarbonyl intermediate [H(2)-6]Cl bearing reactive 2-fluoro substituents at the phosphine-phenyl groups. Two of these fluoro substituents are displaced by a nucleophilic attack upon deprotonation of the coordinated NH,NH-functionalized carbene resulting in new C-N bonds resulting in the partially coupled intermediate, [10], followed by the desired complex with the macrocyclic ligand [8]Cl. Compounds [H-7]Cl and [8]Cl are also formed during the synthesis of [H(2)-6]Cl as a result of spontaneous HF elimination. Complex [8](+) may be converted to the neutral dicarbonyl chloro analog [11] by action of Me(3)NO. Related chemistry with analogous manganese complexes is observed. Thus, from the NH,NH-functionalized carbene manganese bromo tetracarbonyl [12], the diphosphine manganese carbene tricarbonyl cation [H(2)-13] may be readily prepared which provides the macrocyclic carbene-diphosphine tricarbonyl cation [14](+) following base promoted nucleophilic intramolecular displacement of fluoride. Again, [14](+) is converted to the neutral bromo dicarbonyl upon reaction with Me(3)NO. All complexes with the exception of the reaction intermediate [10] have been characterized by spectroscopic and analytical methods in addition to X-ray crystallographic structure determinations for complexes [3], [5], [H(2)-6]Cl, [H(2)-6][9], [8]Cl, [10], [11], [12], and [14]Br. PMID:19067517

Kaufhold, Oliver; Stasch, Andreas; Pape, Tania; Hepp, Alexander; Edwards, Peter G; Newman, Paul D; Hahn, F Ekkehardt

2009-01-14

334

An efficient synthetic route to stable bis(carbene)borylenes [(L1)(L2)BH].  

Science.gov (United States)

Two-electron reduction of bis(carbene) boronium salts allows for the preparation of unsymmetrically substituted nucleophilic boron derivatives of type (L1)(L2)BH, which are characterized by X-ray crystallography. A single electron reduction of the same starting materials leads to the corresponding boron-centered radical cations (L1)(L2)BH?(+), X(-). PMID:24909943

Ruiz, David A; Melaimi, Mohand; Bertrand, Guy

2014-06-24

335

Stereoselective [4 + 1] annulation reactions with silyl vinylketenes derived from Fischer carbene complexes.  

Science.gov (United States)

Stable silyl vinylketenes were prepared via the thermal reaction of Fischer carbene complexes with triisopropylsilyl- or tert-butyldimethylsilyl-substituted alkynes. The ability of these silyl vinylketenes to participate with carbenoid reagents in [4 + 1] annulation reactions was investigated. The best results were obtained with diazomethane and substituted diazomethane reagents, which provided cyclopentenone products in excellent yields and essentially complete stereoselectivity. PMID:16901142

Moser, William H; Feltes, Laura A; Sun, Liangdong; Giese, Matthew W; Farrell, Ryan W

2006-08-18

336

Intramolecular transannulation of alkynyl triazoles via alkyne-carbene metathesis step: access to fused pyrroles.  

Science.gov (United States)

An intramolecular Rh-catalyzed transannulation reaction of alkynyl triazoles has been developed. This method allows efficient construction of various 5,5-fused pyrroles, including tetrahydropyrrolo and spiro systems. The method demonstrates excellent functional group compatibility. A rhodium carbene-alkyne metathesis mechanism is proposed for this transformation. PMID:24093728

Shi, Yi; Gevorgyan, Vladimir

2013-10-18

337

Vinyl dihydropyrans and dihydrooxazines: cyclizations of catalytic ruthenium carbenes derived from alkynals and alkynones.  

Science.gov (United States)

A novel synthesis of 2-vinyldihydropyrans and dihydro-1,4-oxazines (morpholine derivatives) from alkynals and alkynones has been developed. The cyclizations require a mild generation of catalytic ruthenium carbenes from terminal alkynes and (trimethylsilyl)diazomethane followed by trapping with carbonyl nucleophiles. Mechanistic aspects of the new cyclizations are discussed. PMID:24798093

Cambeiro, Fermín; López, Susana; Varela, Jesús A; Saá, Carlos

2014-06-01

338

A Dual Lewis Base Activation Strategy for Enantioselective Carbene-Catalyzed Annulations  

Science.gov (United States)

A dual activation strategy integrating NHC catalysis and a second Lewis base has been developed. NHC-bound homoenolate equivalents derived from ?,?-unsaturated aldehydes combine with transient reactive o-quinione methides in an enatioselective formal [4+3] fashion to access 2-benzoxopinones. The overall approach provides a general blueprint for the integration of carbene catalysis with additional Lewis base activations modes.

Izquierdo, Javier; Orue, Ane; Scheidt, Karl A.

2013-01-01

339

Targeting cytotoxicity and tubulin polymerization by metal-carbene complexes on a purine tautomer platform.  

Science.gov (United States)

This communication describes the synthesis, structural investigation and tubulin binding of purine rare imino-tautomer based Ag(i) and Hg(ii)-carbene complexes. These complexes exhibit cytotoxicity through tubulin interaction by binding to a site close to the GTP binding site. PMID:24854090

Khanna, Shruti; Jana, Batakrishna; Saha, Abhijit; Kurkute, Prashant; Ghosh, Surajit; Verma, Sandeep

2014-07-14

340

Role of carbene complexes in initiation and chain propagation in double bond redistribution reactions  

International Nuclear Information System (INIS)

A study has been made of the role of carbene complexes of tungsten in initiation and propagation of the ring-opening polymerization of cycloolefins. Data are given on polymerization of cyclopentene and cycloocterdiene-1,5 in the presence of the system tungsten chloride-diazo-compound (DAC)

1976-05-01

 
 
 
 
341

Singlet-triplet splittings and barriers to Wolff rearrangement for carbonyl carbenes.  

Science.gov (United States)

High-level ab initio calculations at the G3(MP2)//B3-LYP level have been used to study carbomethoxychlorocarbene and related halogenocarbenes and carbonyl carbenes. Initial calculations at the more accurate W1' level on the subset CH(2), HCCl, HCF, CCl(2), and CF(2) provide support for the reliability of G3(MP2)//B3-LYP for this type of problem. The W1' calculations also suggest that the experimental S-T splitting is slightly underestimated for HCCl and CF(2) and substantially underestimated for CCl(2), in keeping with other recent high-level studies. Whereas the parent carbonyl carbenes, namely formylcarbene, carbohydroxycarbene, and carbomethoxycarbene, are all predicted to have triplet ground states, their chloro and fluoro derivatives are predicted to have singlet ground states. In particular, carbomethoxychlorocarbene is predicted to have a singlet ground state, with the singlet-triplet splitting estimated as -16.0 kJ mol(-)(1). The barriers to Wolff rearrangement of the singlet carbonyl carbenes generally (but not always) correlate with the exothermicity accompanying the production of ketenes. In the case of the parent carbonyl carbenes, for which the rearrangement reaction is most exothermic, the barriers lie between about 10 and 30 kJ mol(-)(1), whereas for the less exothermic rearrangements of the chloro- and fluoro-substituted carbonyl carbenes, the Wolff rearrangement barriers increase significantly to between 58 and 75 kJ mol(-)(1). The calculated barrier for carbomethoxychlorocarbene is 58.2 kJ mol(-)(1). PMID:11414840

Scott, A P; Platz, M S; Radom, L

2001-06-27

342

Reactions of ligands similar to molecular nitrogen and being capable to take its coordination place in complexes  

International Nuclear Information System (INIS)

Reactions of ligands similar relative to molecular nitrogen in complexes of transition metals are djscussed. Isonitriles, carbon monoxide, carbon dioxide, olefins, acetylenes, cyanide, nitrils, nitrogen monoxide, azide are considered as investigated ligands. It is concluded that, as a rule, both for coordination of N2 and its analogs metal complexes rich with electrons and able to stabilize even sUch unstable particles as carbenes or carbynes formed under the effect of electrophilic reagents on coordinated isonitrils are most suitable. Reactions of introduction and oxidation of complexes with N2 analogs are considered

1983-01-01

343

Enantioselective Addition of Silicon Nucleophiles to Aldimines Using a Preformed NHC-Copper(I) Complex as the Catalyst.  

Science.gov (United States)

A remaining major challenge in the asymmetric addition of silicon nucleophiles to typical prochiral acceptors, the enantioselective 1,2-addition to aldimines, is addressed. Activation of the Si?B bond in the silicon pronucleophile by a copper(I) alkoxide with McQuade's chiral six-membered N-heterocyclic carbene as a supporting ligand releases the silicon nucleophile, which adds to various aldimines with high levels of enantioselectivity. The new method provides a catalytic asymmetric access to ?-silylated amines. PMID:24700790

Hensel, Alexander; Nagura, Kazuhiko; Delvos, Lukas B; Oestreich, Martin

2014-05-01

344

Phosphine modified cobalt hydroformylation  

Energy Technology Data Exchange (ETDEWEB)

An ongoing challenge in phosphine modified cobalt hydroformylation is the fundamental understanding of the electronic and steric properties of phosphine ligands that influence the selectivity and activity of the catalytic reaction. A series of acyclic and cyclic phosphines have been prepared and tested in phosphine modified cobalt hydroformylation of 1-octene. Molecular modelling on a series of phospholanes showed some interesting theoretical and experimental correlations. We also evaluated the use of N-heterocyclic carbenes as an alternative for phosphines in modified cobalt hydroformylation. (orig.)

Rensburg, H. van; Tooze, R.P.; Foster, D.F. [Sasol Technology UK, St. Andrews (United Kingdom); Janse van Rensburg, W. [Sasol Technology, Sasolburg (South Africa)

2006-07-01

345

Towards New Generations of Metathesis Metal-Carbene Pre-catalysts  

Science.gov (United States)

: A short general introduction combined with some historical milestones in the field of olefin metathesis is presented followed by an overview of recent representatives of metal carbene initiators. This paper attempts to relief the many superb contributions and overwhelming work invested in intelligent design and innovative synthesis in this area. Despites of recent advances there is still a great interest in the generation of new, better performing, and more environment friendly metathesis.

Allaert, Bart; Dieltiens, Nicolai; Stevens, Chris; Drozdzak, Renata; Dragutan, Ileana; Dragutan, Valerian; Verpoort, Francis

346

Generation of the first representatives of the Schrock complexes of heavy carbene analogs  

International Nuclear Information System (INIS)

The reactions of vanadocene, Cp2V, or decamethylvanadocene, Cp*2V, with 7,7-dimethyl-7-silanorbornadiene or 7,7-dimethyl-7-germanorbornadiene derivatives were studied by ESR in organic solvents. The reactions afforded the first representatives of Schrock complexes of heavy carbene analogs with transition metals. It is shown that decamethylvanadocene can be used as a paramagnetic trap for short-lived dimethylgermylene

2005-03-01

347

Formation of carbene and carbine particles during decomposition of molybdenum benzyl derivatives  

International Nuclear Information System (INIS)

A qualitative composition of hydrocarbon products of interaction of benzyl magnesium chlorides and MoCl_5 in diethylene and dibutyl ethers has been studied. Hydrocarbon reaction products mainly consist of toluene and dibenzyl and small quantities of totan and stilbene. Total residue after the reaction contains some quantity of ''benzyl'' groups bound with Mo or Mg. Carbene and carbine particles disappear at the expense of secondary reactions with organo-metallic compounds of Mo or Mg

1980-01-01

348

Polymer complexes.. XXXX. Supramolecular assembly on coordination models of mixed-valence-ligand poly[1-acrylamido-2-(2-pyridyl)ethane] complexes  

Science.gov (United States)

The build-up of polymer metallic supramolecules based on homopolymer (1-acrylamido-2-(2-pyridyl)ethane (AEPH)) and ruthenium, rhodium, palladium as well as platinum complexes has been pursued with great interest. The homopolymer shows three types of coordination behaviour. In the mixed valence paramagnetic trinuclear polymer complexes [( 11)+( 12)] in the paper and in mononuclear polymer complexes ( 1)-( 5) it acts as a neutral bidentate ligand coordinating through the N-pyridine and NH-imino atoms, while in the mixed ligand diamagnetic poly-chelates, which are obtained from the reaction of AEPH with PdX 2 and KPtCl 4 in the presence of N-heterocyclic base consisting of polymer complexes ( 9)+( 10), and in monouclear compounds ( 6)-( 8), it behaves as a monobasic bidentate ligand coordinating through the same donor atoms. In mononuclear compounds ( 13)+( 14) it acts as a monobasic and neutral bidentate ligand coordinating only through the same donor atoms. Monomeric distorted octahedral or trimeric chlorine-bridged, approximately octahedral structures are proposed for these polymer complexes. The poly-chelates are of 1:1, 1:2 and 3:2 (metal-homopolymer) stoichiometry and exhibit six coordination. The values of ligand field parameters were calculated. The homopolymer and their polymer complexes have been characterized physicochemically.

El-Sonbati, A. Z.; El-Bindary, A. A.; Diab, M. A.

2003-02-01

349

DFT STUDY ON THE MECHANISM OF THE ADDITION REACTION BETWEEN CARBENE AND GLYCINE  

Scientific Electronic Library Online (English)

Full Text Available SciELO Chile | Language: English Abstract in english The mechanism of addition reaction between the singlet carbene and glycine has been investigated at the B3LYP/6-311+G* level of theory, including geometry optimization and vibrational analysis for the involved stationary points on the potential energy surface. Computational results suggest that ther [...] e are two reaction pathways (1) and (2) during the addition process. In the pathway (1), carbene attacks the C=O bond from the H¹C¹C² side of glycine to form an intermediate (IM1), which is a barrier-free exothermic reaction. Then, IM1 isomerizes to a product (Pro1) via a transition state (TS1) with a potential barrier of 25.7 kJ/mol. Similarly, in the pathway (2), carbene attacks the C=O bond from the H²C¹C² side of glycine to form an intermediate IM2. Subsequently, IM2 isomerizes to Pro2 via TS2, where the Pro2 and Pro1 are enantiomers actually. The calculated potential barrier of 51.3 kJ/mol is higher than that of the pathway (1). Correspondingly, the reaction energy for the both pathways is -258.5 kJ/mol. Additionally, the atoms in molecules (AIM) theory has also been performed to characterize the bonding interaction and structural features for the addition reaction.

TAN, XIAOJUN; WANG, WEIHUA; LI, PING; GU, JINSONG; WANG, GUORONG; WANG, DANGSHENG; LI, TAO.

350

Explicit role of dynamical and nondynamical electron correlation on singlet-triplet splitting in carbenes  

International Nuclear Information System (INIS)

Density functional theoretical studies have been performed on carbene systems to determine the singlet-triplet splitting and also to explore the role of electron correlation. Using an approximate method of separation of dynamical and nondynamical correlation, it is found that dynamical and nondynamical electron correlation stabilizes the singlet state relative to the triplet for halo carbenes in both BLYP and B3LYP methods. Calculations performed on higher homologues of methylene suggest that beyond CH(CH3), both the electron correlations have leveling effect in stabilizing the singlet state relative to the triplet. It has also been observed while dynamical electron correlation fails to provide any substantial degree of stabilization to the singlet states of higher homologues of methylene in B3LYP method, an opposite trend is observed for nondynamical counterpart. Among the larger systems studied (9-triptycyl)(?-naphthyl)-carbene has the highest stability of the triplet state whereas bis-imidazol-2-ylidenes has the most stable singlet state. Interestingly, the values of the dynamical electron correlation for each state of each system studied are different for the two methods used. The reason behind this apparent discrepancy lies in the fact that the coefficients of the LYP part in B3LYP and BLYP functionals are different

2007-02-14

351

Explicit role of dynamical and nondynamical electron correlation on singlet-triplet splitting in carbenes  

Energy Technology Data Exchange (ETDEWEB)

Density functional theoretical studies have been performed on carbene systems to determine the singlet-triplet splitting and also to explore the role of electron correlation. Using an approximate method of separation of dynamical and nondynamical correlation, it is found that dynamical and nondynamical electron correlation stabilizes the singlet state relative to the triplet for halo carbenes in both BLYP and B3LYP methods. Calculations performed on higher homologues of methylene suggest that beyond CH(CH{sub 3}), both the electron correlations have leveling effect in stabilizing the singlet state relative to the triplet. It has also been observed while dynamical electron correlation fails to provide any substantial degree of stabilization to the singlet states of higher homologues of methylene in B3LYP method, an opposite trend is observed for nondynamical counterpart. Among the larger systems studied (9-triptycyl)({alpha}-naphthyl)-carbene has the highest stability of the triplet state whereas bis-imidazol-2-ylidenes has the most stable singlet state. Interestingly, the values of the dynamical electron correlation for each state of each system studied are different for the two methods used. The reason behind this apparent discrepancy lies in the fact that the coefficients of the LYP part in B3LYP and BLYP functionals are different.

Seal, Prasenjit [Department of Chemistry, University of Calcutta, 92, A.P.C. Ray Road, Kolkata - 700 009 (India); Chakrabarti, Swapan [Department of Chemistry, University of Calcutta, 92, A.P.C. Ray Road, Kolkata - 700 009 (India)], E-mail: swapanchem@yahoo.co.in

2007-02-14

352

?3-Chlorido-tris(bis{1-[2-(dimethylaminoethyl]-3-methylimidazol-2-ylidene}silver(I dichloride  

Directory of Open Access Journals (Sweden)

Full Text Available In the crystal structure of the title compound, [Ag3Cl(C8H15N36]Cl2, the AgI ion, which is located on a twofold rotation axis, exists in a T-shape coordination environment. Two carbene C atoms of the N-heterocyclic carbene (NHC ligands are bonded tightly forming a slightly bent [Ag(NHC2]+ cation [C—Ag—C angle = 162.80?(18°]. Three of these complex cations are further aggregated by one bridging chloride anion, which is lying on a threefold rotoinversion axis and is only loosely binding to the Ag+ ions. The N atom of the amine group is not engaged in any coordinative bond.

Sebastian Leitner

2012-03-01

353

{3,3?-Bis[(anthracen-9-ylmethyl]-1,1?-[(ethane-1,2-diyldioxybis(ethane-1,2-diyl]bis(imidazol-2-ylidene}mercury(II bis(hexafluoridophosphate acetonitrile disolvate  

Directory of Open Access Journals (Sweden)

Full Text Available In the title compound, [Hg(C42H38N4O2](PF62·2CH3CN, the HgII cation lies on a twofold axis which is also the internal symmetry element of the complete cationic complex. The HgII cation is coordinated by two symmetry-related C(carbene atoms [Hg—C = 2.058?(9?Å] in a nearly linear geometry, with a C—Hg—C angle of 175.8?(5°. There are weak intermolecular C—H...F interactions in the crystal packing between an F atom of a hexafluoridophosphate anion and a –CH2– group of the bis-N-heterocyclic carbene ligand.

Di-Si Bai

2012-03-01

354

Concomitant nitrene and carbene insertion accompanying ring expansion: spectroscopic, X-ray, and computational studies.  

Science.gov (United States)

Reinvestigation of the thermolysis of azido-meta-hemipinate (I) yielded, in addition to known II, unusual products III and IV. These products are formed via a rare intramolecular nitrene insertion into an adjacent methoxy C-H bond followed by an intermolecular reaction during a ring-expansion and a ring-extrusion reaction followed by a carbene insertion. The structures of the new compounds were confirmed using a battery of techniques, including HRMS (ESI-QTOF) and 2D NMR as well as X-ray crystallography for compound IV. Density functional theory methods were used to support the proposed mechanism of formation of the products. PMID:24410290

Kaur, Divneet; Luk, Hoi L; Coldren, William; Srinivas, Pavan M; Sridhar, Lakshetti; Prabhakar, Sripadi; Raghunathan, Partha; Guru Row, T N; Hadad, Christopher M; Platz, Matthew S; Eswaran, Sambasivan V

2014-02-01

355

Elucidation of carbene ambiphilicity leading to the discovery of reversible ammonia activation.  

Science.gov (United States)

An N,N'-diamidocarbene (DAC) was found to activate a broad range of primary as well as secondary aliphatic and aromatic amines. The relative rates measured for the insertion of the DAC into the primary amines were consistent with an electrophilic activation mechanism; in contrast, the DAC functioned as a nucleophile upon treatment with secondary aryl amines. Collectively, these results constituted the first ambiphilic process for an isolable carbene. By comparison, an analogous diaminocarbene was found to serve exclusively as a nucleophile under similar conditions and led to the discovery of the first organic reagent to reversibly activate ammonia. PMID:24303890

Moerdyk, Jonathan P; Blake, Garrett A; Chase, Daniel T; Bielawski, Christopher W

2013-12-18

356

Laboratory detection of a second carbon chain carbene - Butatrienylidene, H2CCCC  

International Nuclear Information System (INIS)

From laboratory measurement of 45 lines in its rotational spectrum, butatrienylidene, H2C4, has been identified, the second carbene with a known microwave spectrum in the sequence formed by the addition of successive carbon atoms to a linear cumulene backbone. With a dipole moment computed to be high (4.5 D), H2C4 is a good candidate for interstellar and circumstellar detection. This paper presents a determination of the rotational and leading centrifugal distortion constants and a tabulation of possible astronomical lines in the radio and far-IR. 7 refs

1990-01-01

357

A Serine-Substituted P450 Catalyzes Highly Efficient Carbene Transfer to Olefins In Vivo  

Science.gov (United States)

Genetically encoded catalysts for non-natural chemical reactions will open new routes to sustainable production of chemicals. We designed a unique serine-heme ligated cytochrome “P411” that catalyzes efficient and selective carbene transfers from diazoesters to olefins in intact Escherichia coli cells. The mutation C400S in cytochrome P450BM3 gives a signature ferrous-CO Soret peak at 411 nm, abolishes monooxygenation activity, raises the resting state FeIII/II reduction potential, and significantly improves NAD(P)H-driven cyclopropanation activity.

Coelho, Pedro S.; Wang, Z. Jane; Ener, Maraia E.; Baril, Stefanie A.; Kannan, Arvind A.; Arnold, Frances H.; Brustad, Eric M.

2013-01-01

358

Synthesis of hydronaphthalenes through coupling of enyne carbonyl compounds that contain pendant alkene groups with Fischer carbene complexes  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The coupling of enyne carbonyl compounds that contain pendant alkene groups with Fischer carbene complexes to afford furans that contain pendant alkene groups is described. Subsequent intramolecular Diels-Alder reactions are effective in select cases, resulting in hydronaphthalene systems after dehydration. Although the Diels-Alder event is thermodynamically unfavorable, the overall transformation of alkene-furans to dihydronaphthalenes is a favorable process.

Kumar-patti, Rajesh; Duan, Shaofeng; Camacho-davila, Alejandro; Waynant, Kris; Dunn, Kenneth A.; Herndon, James W.

2010-01-01

359

Selective arylation, alkenylation, and cyclization of dibromonaphthols, using visible light, via carbene intermediates.  

Science.gov (United States)

The photoreactivity of several 3-substituted-1,6-dibromo-2-naphthols has been investigated in neat acetonitrile in the presence of diluted Et3N and in aqueous buffered acetonitrile (pH 8, phosphate buffered), using visible light (450 nm). Hydrobromic acid loss in the presence of the base, for the unsubstituted naphthol, or heterolytic C-Br cleavage directly from the naphtholates, for the more acid 3-substutited naphthols (R = COOCH3, CONH2, CONMe2), generates electrophilic carbene intermediates, which have been successfully trapped by molecular oxygen, pyrrole, acrylonitrile, ethyl vinyl ether, and allyltrimethylsilane. Product distribution analysis reveals three types of products arising from (i) arylation, (ii) alkenylation, and (iii) cyclization reactions. The generation and the reactivity of alpha-ketocarbene intermediates, as electrophilc diradicals, has been supported by laser flash photolysis, with the detection of both the carbene (lambda(max) 510 nm) and 1,2-naphthoquinone-O-oxide (R = CONMe2, lambda(max) 600 nm) in the presence of O2. PMID:19522477

Verga, Daniela; Doria, Filippo; Mella, Mariella; Freccero, Mauro

2009-08-01

360

Insights into the strength and nature of carbene···halogen bond interactions: a theoretical perspective.  

Science.gov (United States)

Halogen-bonding, a noncovalent interaction between a halogen atom X in one molecule and a negative site in another, plays critical roles in fields as diverse as molecular biology, drug design and material engineering. In this work, we have examined the strength and origin of halogen bonds between carbene CH? and XCCY molecules, where X?=?Cl, Br, I, and Y?=?H, F, COF, COOH, CF?, NO?, CN, NH?, CH?, OH. These calculations have been carried out using M06-2X, MP2 and CCSD(T) methods, through analyses of surface electrostatic potentials V S(r) and intermolecular interaction energies. Not surprisingly, the strength of the halogen bonds in the CH?···XCCY complexes depend on the polarizability of the halogen X and the electron-withdrawing power of the Y group. It is revealed that for a given carbene···X interaction, the electrostatic term is slightly larger (i.e., more negative) than the dispersion term. Comparing the data for the chlorine, bromine and iodine substituted CH?···XCCY systems, it can be seen that both the polarization and dispersion components of the interaction energy increase with increasing halogen size. One can see that increasing the size and positive nature of a halogen's ?-hole markedly enhances the electrostatic contribution of the halogen-bonding interaction. PMID:23455928

Esrafili, Mehdi D; Mohammadirad, Nafiseh

2013-06-01

 
 
 
 
361

Palladium-Catalyzed Heck Coupling Reaction of Aryl Bromides in Aqueous Media Using Tetrahydropyrimidinium Salts as Carbene Ligands  

Directory of Open Access Journals (Sweden)

Full Text Available An efficient and stereoselective catalytic system for the Heck cross coupling reaction using novel 1,3-dialkyl-3,4,5,6-tetrahydropyrimidinium salts (1, LHX and Pd(OAc2 loading has been reported. The palladium complexes derived from the salts 1a-f prepared in situ exhibit good catalytic activity in the Heck coupling reaction of aryl bromides under mild conditions.

?smail Özdemir

2010-01-01

362

Carbene Spin Multiplicity in Solution Probed Using Time-Resolved EPR Spectroscopy  

Scientific Electronic Library Online (English)

Full Text Available SciELO Mexico | Language: English Abstract in spanish Espectroscopia de resonancia paramagnética electrónica (EPR por las siglas en inglés) fue utilizada para detectar los radicales libres formados mediante reacciones de abstracción de átomos de hidrógeno de diversos carbenos generados fotoquímicamente en soluciones de n-hexano y a temperatura ambiente [...] . En particular, se describe el análisis de tres carbenos que fueron generados por fotoexcitación de un bifenilo con dos grupos diazo. Se identificaron señales EPR de dos radicales alquilo y uno bencílico. La estructura de dichos radicales se confirmó mediante un proceso de simulación computacional utilizando parámetros espectroscópicos (factores-g, constantes de acoplamiento hiperfino) reportados en la literatura. Experimentos previos de absorción óptica han llevado a la conclusión de que los carbenos estudiados presentan un estado basal de tipo triplete, lo que se confirma al observar las señales EPR de emisión a campo bajo pero de absorción a campo alto. Además, se encontró que los espectros presentan un espín fuertemente polarizado como consecuencia de un mecanismo CIDEP, y que la intensidad de las señales está relacionada con los rendimientos cuánticos asociados a la producción de los carbenos iniciales. Como era de esperarse, un tercer sustrato, que reacciona vía una reacción concertada de tipo Wolff a través de un reordenamiento del intermediario singulete producido en el estado excitado, no dio lugar a ninguna señal EPR. Así mismo, cuando las reacciones de los tres carbenos se llevan a cabo en acetonitrilo, la multiplicidad del espín se invierte de modo que se genera un estado basal.singulete, por lo que no se observan señales EPR. Los resultados descritos en este trabajo son consistentes con aquellos obtenidos mediante fotólisis láser. Abstract in english Time-resolved (CW) EPR spectroscopy at X-band has been used to detect free radicals created by H-atom abstraction reactions of photochemically generated carbenes in n-hexadecane solutions at room temperature. Three different carbenes were studied, formed via loss of nitrogen after photo-excitation o [...] f a substituted biphenyl diazo precursor. The TREPR signals from two different alkyl radicals and the benzylic-type radical from the carbene are detected for two of the three diazo substrates. The radicals were unambiguously identified by spectral simulation using literature parameters (g-factors and hyper-fine coupling constants). Previous transient optical absorption spectroscopy experiments have concluded that these two carbenes have triplet ground states, which is confirmed by the low field emissive, high field absorptive phase of the TREPR signals reported here. The spectra are strongly spin polarized by the Radical Pair Mechanism of CIDEP, and their overall intensities track with the measured quantum yields for carbene production. A third substrate, known to react via a concerted Wolff reaction or excited state rearrangement via a singlet state, showed no TREPR signal, as expected. Reactions run in acetonitrile, where the spin multiplicity is reversed to give a singlet ground state, did not lead to detectable TREPR signals for all three substrates. The observations are consistent with previously published laser flash photolysis results.

Wang, Jin; Lebedeva, Natalia V.; Platz, Matthew S.; Forbes, Malcolm D. E..

363

6-(3,5-Dimethyl-1H-pyrazol-1-yl)-2,2'-bipyridine as ligand for actinide(III)/lanthanide(III) separation.  

Science.gov (United States)

With the aim of better understanding the selectivity of the established system 2,6-ditriazinylpyridine (BTP) for actinide(III)/lanthanide(III) separations, a related model system was synthesized and studied. The N donor complexing ligand 6-(3,5-dimethyl-1H-pyrazol-1-yl)-2,2'-bipyridine (dmpbipy) was synthesized having a fused N heterocycle ring structure modified from the BTP partitioning ligand, and its extraction performance and selectivity for trivalent actinide cations over lanthanides was evaluated. X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS), and time-resolved laser fluorescence spectroscopy (TRLFS) results show that 1:1 complexes are formed, unlike the 1:3 complex for BTP systems. The equilibrium constant for curium complex formation with dmpbipy was determined to be log K = 2.80, similar to that for nitrate. The Gibbs free energy, ?G(20 °C), of 1:1 Cm-dmpbipy formation in n-octan-1-ol was measured to be -15.5 kJ/mol. The dmpbipy ligand in 1-octanol does not extract Am(III) Eu(III) from HNO(3) but was found to extract Am(III) with limited selectivity over Eu(III) (SF(Am(III)/Eu(III)) ? 8) dissolved in 2-bromohexanoic acid and kerosene at pH > 2.4. PMID:20849125

Girnt, Denise; Roesky, Peter W; Geist, Andreas; Ruff, Christian M; Panak, Petra J; Denecke, Melissa A

2010-10-18

364

Synthesis of 1,3-dithiin dithioortho esters from the reaction of Fischer carbenes and 3H-1,2-dithiole-3-thiones.  

Science.gov (United States)

[reaction: see text] Tricyclic 4-ethyl-5-thioxo-3H,5H-bis[1,2]dithiolo[3,4-d][4,3-b]pyrrol-3-one and monocyclic 3H-1,2-dithiole-3-thione derivatives reacted with Fischer carbene complexes, giving 1,3-dithiin dithioortho esters through insertion of the carbenic carbon into the S-S bond next to the thiocarbonyl function of the substrate. PMID:16408999

Granados, Alejandro M; Kreiker, Jeronimo; de Rossi, Rita H; Fuertes, Pedro; Torroba, Tomás

2006-01-20

365

Theoretical study of addition reactions of heavy carbenes to carbon and boron nitride nanotubes.  

Science.gov (United States)

In an effort to gain insight into the activation energies and reaction enthalpies of the chemical functionalization of carbon and boron nitride nanotubes, calculations using density functional theory have been carried out for the cycloaddition of a heavy carbene to a single-walled carbon (SWCNT; C(130)H(20)) and a boron nitride (SWBNNT; B(65)N(65)H(20)) nanotube. The (CH(3))(2)X + SWCNT and (CH(3))(2)X + SWBNNT (X = C, Si, Ge, Sn, and Pb) reactions are the subject of the present study. All the stationary points were determined at the B3LYP/LANL2DZ level of theory. The major conclusions that can be drawn from this work are as follows: (i) Considering both the activation barrier and reaction enthalpy based on the model calculations presented here, it is found that the order of (CH(3))(2)X reactivity is X = C > Si > Ge > Sn > Pb, irrespective of whether cycloaddition is to a SWCNT or a SWBNNT sidewall. That is to say, (CH(3))(2)C and (CH(3))(2)Si can readily add to the sidewalls of SWCNT and SWBNNT, whereas (CH(3))(2)Ge, (CH(3))(2)Sn, and (CH(3))(2)Pb are unreactive. (ii) Since the chemical reactivities of SWCNT and SWBNNT sidewalls closely resemble those of the small C(16)H(10) and B(8)N(8)H(10) molecules, at least in a qualitative sense, the use of the above small molecules as models is sufficient to provide qualitatively correct results. (iii) Our theoretical observations indicate that all the (5,5) SWCNT and SWBNNT cycloadducts favor opened rather than closed three-membered ring structures. (iv) The theoretical investigations demonstrate that the singlet-triplet splitting of the carbene species (R(2)X) as well as that of the small model molecules can be used as a diagnostic tool to predict the addition reactivities of carbene analogues and sidewalls of various nanotubes, respectively. Moreover, the results obtained in this work allow a number of predictions to be made. PMID:16853811

Su, Ming-Der

2005-11-24

366

Rhodium-mediated activation of an alkane-type C–H bond  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Abnormal C4-bonding of N-heterocyclic carbenes effectively modulates the electron density at rhodium and allows for the selective cleavage of an unactivated C(sp3)–H bond, whereas no such intramolecular C–H bond breaking is observed when the carbene binds normally through the C2 carbon.

Kru?ger, Anneke; Neels, Antonia; Albrecht, Martin

2010-01-01

367

Low-spin hexacoordinate Mn(III): synthesis and spectroscopic investigation of homoleptic tris(pyrazolyl)borate and tris(carbene)borate complexes.  

Science.gov (United States)

Three complexes of Mn(III) with "scorpionate" type ligands have been investigated by a variety of physical techniques. The complexes are [Tp(2)Mn]SbF(6) (1), [Tp(2)*Mn]SbF(6) (2), and [{PhB(MeIm)(3)}(2)Mn](CF(3)SO(3)) (3a), where Tp(-) = hydrotris(pyrazolyl)borate anion, Tp*(-) = hydrotris(3,5-dimethylpyrazolyl)borate anion, and PhB(MeIm)(3)(-) = phenyltris(3-methylimidazol-2-yl)borate anion. The crystal structure of 3a is reported; the structures of 1 and 2 have been previously reported, but were reconfirmed in this work. The synthesis and characterization of [{PhB(MeIm)(3)}(2)Mn]Cl (3b) are also described. These complexes are of interest in that, in contrast to many hexacoordinate (pseudo-octahedral) complexes of Mn(III), they exhibit a low-spin (triplet) ground state, rather than the high-spin (quintet) ground state. Solid-state electronic absorption spectroscopy, SQUID magnetometry, and high-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy were applied. HFEPR, in particular, was useful in characterizing the S = 1 spin Hamiltonian parameters for complex 1, D = +19.97(1), E = 0.42(2) cm(-1), and for 2, D = +15.89(2), E = 0.04(1) cm(-1). In addition, frequency domain Fourier-transform THz-EPR spectroscopy, using coherent synchrotron radiation, was applied to 1 only and gave results in good agreement with HFEPR. Variable-temperature dc magnetic susceptibility measurements of 1 and 2 were also in good agreement with the HFEPR results. This magnitude of zero-field splitting (zfs) is over 4 times larger than that in comparable hexacoordinate Mn(III) systems with S = 2 ground states. Complexes 3a and 3b (i.e., regardless of counteranion) have a yet much larger magnitude zfs, which may be the result of unquenched orbital angular momentum so that the spin Hamiltonian model is not appropriate. The triplet ground state is rationalized in each complex by ligand-field theory (LFT) and by quantum chemistry theory, both density functional theory and unrestricted Hartree-Fock methods. This analysis also shows that spin-crossover behavior is not thermally accessible for these complexes as solids. The donor properties of the three different scorpionate ligands were further characterized using the LFT model that suggests that the tris(carbene)borate is a strong ?-donor with little or no ?-bonding. PMID:23259486

Forshaw, Adam P; Smith, Jeremy M; Ozarowski, Andrew; Krzystek, J; Smirnov, Dmitry; Zvyagin, S A; Harris, T David; Karunadasa, Hemamala I; Zadrozny, Joseph M; Schnegg, Alexander; Holldack, Karsten; Jackson, Timothy A; Alamiri, Ahmad; Barnes, Diane M; Telser, Joshua

2013-01-01

368

Generation of carbene complexes in decomposition of organometallic compounds of transition metals  

International Nuclear Information System (INIS)

A method of forming carbine complexes initiating polymerization of cycloolephines with use being made of a redox system is considered herein. It has been established that one of such systems may involve decomposition of compounds of the general formula Rsub(3)ZCHsub(2)WXsub(n). There is described the decomposition of an organo-metallic compound, accompanied by formation of radicals (CH3)SicH2WCL5 initiating polymerization of cycloolephines at temperatures down to -78 deg C. The decomposition products are identified by gas-liquid chromatography, paramagnetic resonance and mass-spectrometry techniques. The composition of the decomposition products is given. It is concluded that some organo-metallic compounds of transition metals may serve as sources of carbenes in solutions at low temperatures

1975-08-21

369

The U=C Double Bond: Synthesis and Study of Uranium Nucleophilic Carbene Complexes  

International Nuclear Information System (INIS)

Treatment of U(BH4)4 with 1 or 3 equiv of Li2(SCS). 1.5Et2O, 1, afforded the actinide carbene complexes U(?-SCS)3[U(BH4)3]2 (4) and U(?-SCS)3[Li(Et2O)]2 (6), respectively [SCS = (Ph2P = S)2C]. In THF, complex 4 was transformed into the mononuclear derivative (SCS)U(BH4)2(THF)2 (5). The multiple bond character of the uranium-carbon bond was first revealed by the X-ray crystal structures of the three complexes. The U=C bond in these complexes present a nucleophilic character, as shown by their reaction with a carbonyl derivative. Finally, DFT calculations prove the involvement of both 5f and 6d orbitals in both the ? and the ? U-C bonds. (authors)

2009-01-01

370

Redox behaviour of cymantrene Fischer carbene complexes in designing organometallic multi-tags.  

Science.gov (United States)

A series of Group?7 Fischer carbene complexes, [Cp(CO)2 Mn(I) =C(OEt)Ar] (Cp=cyclopentadienyl, Ar=Th=thienyl (1?a), Ar=Fu=furyl (2?a), Ar=Fc=ferrocenyl (3?a)) and biscarbene complexes, [Cp(CO)2 Mn?C(OEt)?Ar'?(OEt)C?Mn(CO)2 Cp] (Ar'=Th'=2,5-thienylene (1?b), Ar'=Fu'=2,5-furylene (2?b), Ar'=Fc'=1,1'-ferrocendiyl (3?b)) was synthesized and characterized. Chemical oxidation of [Cp(CO)2 Mn?C(OEt)Fc] (3?a) and isolation of the oxidised species [3?a][PF6 ] possessing a Mn(II) centre proved possible below -30?°C in dichloromethane solution. The ESR spectrum of the transiently stable radical cation, [3?a][PF6 ], confirmed the presence of a low-spin Mn(II) centre characterized by a rhombic g tensor (gx =1.975, gy =2.007 and gz =2.130) in frozen dichloromethane at 77?K with (55) ?Mn hyperfine coupling constants A1 , A2 and A3 of 115, 33 and 43?G, respectively. Electrochemical studies demonstrated the influence of the Ar substituent on the oxidation potential. All complexes showed that the redox potentials of carbene double bond reduction and Mn(I) oxidation were dependent on the type of Ar group, but only 3?b showed resolved oxidations for the two Mn(I) centres. Surprisingly, Mn(I) oxidation occurs at lower potentials than ferrocenyl oxidation. Density functional theory (DFT) calculations were carried out to delineate the nature of the species involved in the oxidation and reduction processes and clearly confirm that oxidation of Mn(I) is favoured over that of ferrocene. PMID:24644017

Bezuidenhout, Daniela I; van der Westhuizen, Belinda; Swarts, Pieter J; Chatturgoon, Teshica; Munro, Orde Q; Fernández, Israel; Swarts, Jannie C

2014-04-22

371

A search for thermal isomerization of olefins to carbenes: Thermal generations of the silicon-nitrogen double bond  

Energy Technology Data Exchange (ETDEWEB)

The first part of this thesis will search for the thermal isomerization of olefins to carbenes which is predicted to be a high energy process by calculations and has only been observed in a few strained olefins. The possibility of thermal isomerization of simple olefins to carbenes will be explored. Substitution of a silyl group on the double bond of an olefin allows a potential intermediate which has a {beta}-radical to the silyl group during the cis-trans isomerization. The effects of a trimethylsilyl group on this isomerization are the subject of this study. The second part of this thesis will include the generation and chemistry of intermediates containing a silicon-nitrogen double bond. The isomerization of parent silanimine to the aminosilylene was calculated to be a high energy process. New approaches to the silicon-nitrogen double bond will also be presented. 92 refs., 12 figs., 11 tabs.

Zhang, Xianping.

1990-09-21

372

Supramolekulare N-heterocyclische Carben Liganden: Synthese, Charakterisierung und Anwendung von Übergangsmetall-NHC-Calix[4]aren-Komplexen  

Digital Repository Infrastructure Vision for European Research (DRIVER)

In the present work different substituted imidazolium salts based on benzenes and calix[4]arenens were synthesized. In this connection, a proximal imidazolium substituted calix[4]arene was described for the first time. In the following it was shown that imidazolium substituted calix[4]arene could easily be converted into the corresponding silver-NHC-calixarene-complexes which were successfully used as supramolecular carbene transfer reagents in the silver-NHC-complex mediated transmetallation...

2006-01-01

373

Experimental Evidence for Cobalt(III)-Carbene Radicals: Key Intermediates in Cobalt(II)-Based Metalloradical Cyclopropanation  

Digital Repository Infrastructure Vision for European Research (DRIVER)

New and conclusive evidence has been obtained for the existence of cobalt(III)-carbene radicals that have been previously proposed as the key intermediates in the underlying mechanism of metalloradical cyclopropanation by cobalt(II) complexes of porphyrins. In the absence of olefin substrates, reaction of [Co(TPP)] with ethyl styryldiazoacetate was found to generate the corresponding cobalt(III)-vinylcarbene radical that subsequently dimerizes via its gamma-radical allylic resonance form to a...

2011-01-01

374

One-pot three-component synthesis of quinoxaline and phenazine ring systems using Fischer carbene complexes  

Science.gov (United States)

Summary One-pot three-component coupling of o-alkynylheteroaryl carbonyl derivatives with Fischer carbene complexes and dienophiles leading to the synthesis of quinoxaline and phenazine ring systems has been investigated. This involves the generation of furo[3,4-b]pyrazine and furo[3,4-b]quinoxaline as transient intermediates, which were trapped with Diels–Alder dienophiles. This is the first report on furo[3,4-b]pyrazine intermediates.

Roy, Priyabrata

2010-01-01

375

One-pot three-component synthesis of quinoxaline and phenazine ring systems using Fischer carbene complexes  

Directory of Open Access Journals (Sweden)

Full Text Available One-pot three-component coupling of o-alkynylheteroaryl carbonyl derivatives with Fischer carbene complexes and dienophiles leading to the synthesis of quinoxaline and phenazine ring systems has been investigated. This involves the generation of furo[3,4-b]pyrazine and furo[3,4-b]quinoxaline as transient intermediates, which were trapped with Diels–Alder dienophiles. This is the first report on furo[3,4-b]pyrazine intermediates.

Priyabrata Roy

2010-05-01

376

One-pot three-component synthesis of quinoxaline and phenazine ring systems using Fischer carbene complexes  

Digital Repository Infrastructure Vision for European Research (DRIVER)

One-pot three-component coupling of o-alkynylheteroaryl carbonyl derivatives with Fischer carbene complexes and dienophiles leading to the synthesis of quinoxaline and phenazine ring systems has been investigated. This involves the generation of furo[3,4-b]pyrazine and furo[3,4-b]quinoxaline as transient intermediates, which were trapped with Diels–Alder dienophiles. This is the first report on furo[3,4-b]pyrazine intermediates.

Priyabrata Roy; Binay Krishna Ghorai

2010-01-01

377

Metal nanoparticles functionalized with metal-ligand covalent bonds  

Science.gov (United States)

Metal-organic contact has been recognized to play important roles in regulation of optical and electronic properties of nanoparticles. In this thesis, significant efforts have been devoted into synthesis of ruthenium nanoparticles with various metal-ligand interfacial linkages and investigation of their electronic and optical properties. Ruthenium nanoparticles were prepared by the self-assembly of functional group onto bare Ru colloid surface. As to Ru-alkyne nanoparticles, the formation of a Ru-vinylidene (Ru=C=CH--R) interfacial bonding linkage was confirmed by the specific reactivity of the nanoparticles with imine derivatives and olefin at the metal-ligand interface, as manifested in NMR, photoluminescence, and electrochemical measurements. Interestingly, it was found the electronic coupling coefficient (beta)for strongly depend upon such metal-ligand interfacial bonding. Next, such metal-ligand interfacial bonding was extended to ruthenium-nitrene pi bonds on ruthenium colloids, which were investigated by XPS. The nanoparticles exhibited a 1:1 atomic ratio of nitrogen to sulfur, consistent with that of sulfonyl nitrene fragments. In addition, the nanoparticle-bound nitrene moieties behaved analogously to azo derivatives, as manifested in UV-vis and fluorescence measurements. Further testimony of the formation of Ru=N interfacial linkages was highlighted in the unique reactivity of the nanoparticles with alkenes by imido transfer. Extensive conjugation between metal-ligand interfacial bond results in remarkable intraparticle charge delocalization on Ru-alkynide nanoparticles, which was manipulated by simple chemical reduction or oxidation. Charging of extra electrons into the nanoparticle cores led to an electron-rich metal core and hence red-shift of the triple bond stretching mode, lower binding energy of sp hybridized C 1s and dimmed fluorescence of nanoparticles. Instead, chemical oxidation resulted in the opposite impacts on these properties. By taking advantage of such extensively conjugated metal-ligand bonding and effective intraparticle charge delocalization of ruthenium nanoparticles, Ru=carbene nanoparticles functionalized with multiple moieties by olefin metathesis reactions was further exploited for metal ion sensing. When the nanoparticles were co-functionalized with 1-vinylpyrene and 4-vinylbenzo-18-crown-6, upon the binding of metal ions into the crown ether cavity, the emission intensity of the nanoparticle fluorescence from the conjugation of vinylpyrene was found to diminish, with the most significant effects observed with K+ ions. In the case of ruthenium nanoparticles co-functionalized with pyrene and histidine derivative moieties through Ru=carbene pi bonds. The selective complexation of the histidine moiety with transition metal ions led to marked diminishment of the emission intensity from conjugation of pyrene. Of all the metal ions tested, the impacts were much more drastic with Pb2+ , Co2+ and Hg2+ than with Li +, K+, Rb+, Mg2+ Ca 2+ and Zn2+ ions. These were ascribed to the selective binding of 18-crown-6 to potassium ions or complexation of histidine derivative to transition metal ions, where the metal ions led to polarization of the nanoparticle core electrons to the metal surface and hence impeded intraparticle charge delocalization. Functionalization of semiconductor with metal nanoparticles could be exploited to remarkably enhance their photo catalytic performance. Before this exploration, in the last chapter, the impacts of the TiO2 nanocrystalline structure on the photocatalytic activity were then examined by using the reduction of methylene blue in water. It was found that in the presence of anatase and brookite crystalline phase, TiO2 nanotube arrays exhibited the highest photo catalytic activity. This is ascribed to synergistic coupling of the anatase and brookite crystalline domains, which led to effective charge separation upon photoirradiation.

Kang, Xiongwu

378

A coordination compound of Ge(0) stabilized by a diiminopyridine ligand.  

Science.gov (United States)

Reduction of the cationic Ge(II) complex [dimpyrGeCl][GeCl3] (dimpyr=2,6-(ArN=CMe)2NC5H3, Ar=2,6-iPr2C6H3) with potassium graphite in benzene affords an air sensitive, dark green compound of Ge(0), [dimpyrGe], which is stabilized by a bis(imino)pyridine platform. This compound is the first example of a complex of a zero-valent Group?14 element that does not contain a carbene or carbenoid ligand. This species has a singlet ground state. DFT studies revealed partial delocalization of one of the Ge lone pairs over the ?*(C=N) orbitals of the imines. This delocalization results in a partial multiple-bond character between the Ge atom and imine nitrogen atoms, a fact supported by the X-ray crystallography and IR spectroscopy data. PMID:24488623

Chu, Terry; Belding, Lee; van der Est, Art; Dudding, Travis; Korobkov, Ilia; Nikonov, Georgii I

2014-03-01

379

Synthesis and reactions of cyclovalence isomers of azo-keto-carbenes  

International Nuclear Information System (INIS)

Novel types of cycloaddition products with an azomethine imine functionality have been prepared from ?-azo-?'-diazo ketones with a carbon chain of variable length between the azo- and the keto-group; the reaction is induced by catalytical amounts of rhodium(II) acetate and occurs with the concomitant extrusion of dinitrogen. The synthesis of these cyclic azomethine imines succeeded in the course of the intramolecular reaction of the azo nitrogen atoms with a carbene/carbenoid carbon atom, in situ generated from the ?-diazoketone functionality; this is a novel cyclization reaction. Some of the resulting cyclization products are stable and have been isolated, others could only be trapped with dipolarophiles as [3+2] cycloadducts. The ring-size of the heterocyclic products depends on the one hand on the length of the carbon-chain (for n = 0, 1, 2) between the carbonyl carbon atom and the quaternary aliphatic or aromatic carbon atom that blocks the tautomerization of the azo-group in the starting material. On the other hand, the ring size depends on which of the two nitrogen atoms of the azo-group undergoes the ring closure with the carbene/carbenoid carbon atom generated from the ?-diazoketon functionality in the course of the reaction. By far the most serious problem in the preparation of the cyclic azomethine imines is the synthesis of the required ?-azo-?'-diazo ketones. A so far unknown property of the azo-group is its intramolecularly directed nucleophilicity toward ketenes, which emerge from acid chlorides and anhydrides, or from ?-diazo ketones in the course of the Wolff rearrangement. This complicated the approach to the required ?-azo-?'-diazo ketones via these functionalities as precursors and with the desired chain length between the azo- and diazo-keto-groups. Nevertheless, these problems could be overcome by using alternative strategies. Utilizing ?-azo-?'-diazo ketones a largely commonly applicable approach to endocyclic and N-exocyclic azomethine imines was worked out, which results from the so far unknown intramolecular reaction of azo-nitrogen atoms on carbenoids (rhodium-stabilized carbenes). Work up and isolation of the different reaction products turned out not to be without difficulty. Most of the novel and in solution partly unstable compounds that were characterized in this thesis would not have been isolable in the conventional manner (chemical separation processes, column chromatography). However, this has been managed by employing the radial chromatography upon optimizing some parameters; apparently, this is a not widely known chromatographic separation method that enables fast separations with high efficiency. To clarify the question, why only certain azomethine imines react with special dipolarophiles, quantum-mechanical calculations of the reactants' frontier orbitals have been carried out; this resulted in a satisfactory concordance with the FMO theory by Fukui. Likewise, molecular calculations rationalize the observed stereo- and regioselectivity as well as the kinetic and thermodynamic reaction control in the course of cycloaddition reactions of azomethine imines. (author)

2001-01-01

380

Enlarging the ? System of Phosphorescent (C^C*) Cyclometalated Platinum(II) NHC Complexes.  

Science.gov (United States)

Cyclometalated (C^C*) platinum(II) N-heterocyclic carbene (NHC) complexes are emerging as a new class of phosphorescent emitters for the application in organic light-emitting devices (OLEDs). We present the synthesis of six new complexes of this class to investigate the influence of extended ? systems. Therefore, six different NHC ligands with a varying number of additional phenyl substituents were used in combination with the monoanionic acetylacetonate (acac) ligand to obtain complexes of the general formula [(NHC)Pt(II)(acac)]. The complexes were fully characterized by standard techniques and advanced spectroscopic methods ((195)Pt NMR). For all complexes the solid-state structure determination revealed a square-planar coordination of the platinum atom. Absorption and emission spectra were measured in thin amorphous poly(methyl methacrylate) films at room temperature. Four compounds emit in the blue-green region of the visible spectrum with quantum yields of up to 81%. PMID:24866934

Tronnier, Alexander; Pöthig, Alexander; Metz, Stefan; Wagenblast, Gerhard; Münster, Ingo; Strassner, Thomas

2014-06-16

 
 
 
 
381

rac-cis-Dicarbonylchlorido{1-[2-(diphenylphosphanyl-?Pbenzyl]-3-(phenyl-?C1imidazol-2-ylidene-?C2}ruthenium(II dichloromethane monosolvate  

Directory of Open Access Journals (Sweden)

Full Text Available In the title compound, [Ru(C28H22N2PCl(CO2]·CH2Cl2, the RuII atom exhibits a distorted octahedral coordination geometry. The N-phenyl group of the ligand has undergone orthometalation; as a result, the tridentate phosphane-functionalized N-heterocyclic carbene ligand is coordinating in a meridional fashion. This complex is of interest with respect to transfer hydrogenation catalysis and also provides an example of C—H activation behavior in late transition metal complexes. The dichloromethane solvent molecule is disordered over two sets of sites with an occupancy ratio of 0.873?(14:0.127?(14.

Gregory J. Domski

2012-09-01

382

Theoretical study on the mechanism of extraction reaction between silylene carbene and its derivatives and thiirane  

Science.gov (United States)

The mechanism of the sulfur extraction reaction between singlet silylene carbine and its derivatives and thiirane has been investigated with density functional theory (DFT), including geometry optimization and vibrational analysis for the involved stationary points on the potential energy surface. The energies of the different conformations are calculated by B3LYP/6-311G( d, p) method. From the potential energy profile, it can be predicted that the reaction pathway of this kind consists in two steps: (1) the two reactants firstly form an intermediate through a barrier-free exothermic reaction; (2) the intermediate then isomerizes to a product via a transition state. This kind of reactions has similar mechanism: when the silylene carbene and its derivatives [X2Si=C: (X = H, F, Cl, CH3)] and thiirane approach each other, the shift of 3 p lone electron pair of S in thiirane to the 2 p unoccupied orbital of C in X2Si=C: gives a p ? p donor-acceptor bond, thereby leading to the formation of intermediate (INT). As the p ? p donor-acceptor bond continues to strengthen (that is the C-S bond continues to shorten), the intermediate (INT) generates product (P + C2H4) via transition state (TS). It is the substituent electronegativity that mainly affect the extraction reactions. When the substituent electronegativity is greater, the energy barrier is lower, and the reaction rate is greater.

Lu, X. H.; Che, X.; Shi, L.; Han, J. F.; Lian, Z. X.; Li, Y. Q.

2010-12-01

383

All-electron relativistic multireference configuration interaction investigation of fluoroiodo carbene.  

Science.gov (United States)

We present herein the first all-electron relativistic internally contracted multireference configuration interaction with Davidson correction (icMRCI+Q) study on the low-lying states of fluoroiodo carbene, CFI, which contains the most electronegative element (fluorine) and the heavy halogen (iodine). The potential energy surface (PES) of the first excited singlet state (Ã(1)A?) of CFI was carefully examined along the C-I bond distance at the icMRCI+Q/ANO-RCC level, while the other two geometric parameters were optimized at every C-I bond length in contrast to fixing them at the equilibrium values. A reliable barrier height of the Ã(1)A? state was determined to be 625 cm(-1) by our high-level icMRCI+Q calculations with large ANO-RCC basis set and with inclusion of the spin-orbit coupling, core-valence correlation, and zero-point-energy. Finally, the electronic states of CFI with vertical transition energy up to 6 eV were studied. The calculation presented here will provide more comprehensive results about the structure and behavior of electronic states of CFI radical. PMID:24611930

Sun, Erping; Lv, Hang; Shi, Dandan; Wei, Changli; Xu, Haifeng; Yan, Bing

2014-04-01

384

Rhodium (II) carbene C-H insertion in water and catalyst reuse; Insercao C-H de carbenoides de rodio em agua e reutilizacao do catalisador  

Energy Technology Data Exchange (ETDEWEB)

A five-session laboratory experiment is described for the synthesis of a beta-lactam via Rh(II) catalysed intramolecular C-H insertion of a alpha-diazo-alpha-ethoxycarbonyl acetamide. The metallo-carbene, responsible for the C-H bond activation, was generated from the diazo substrate and the catalyst Rh{sub 2}(OAc){sub 4}. The high stability and solubility of the catalyst and the exclusive C-H insertion of the Rh-carbene allows the synthesis of this important heterocycle in water and the catalyst reutilization. (author)

Candeias, Nuno R.; Gois, Pedro M.P.; Afonso, Carlos A.M. [Instituto Superior Tecnico, Lisboa (Portugal)]. E-mail: carlosafonso@ist.utl.pt

2007-07-01

385

Novel 7-phenylsulfanyl-1,2,3,4,10,10a-hexahydro-pyrazino[1,2-a]indoles as dual serotonin 5-HT2C and 5-HT6 receptor ligands  

DEFF Research Database (Denmark)

Novel 7-phenylsulfanyl-1,2,3,4,10,10a-hexahydro-pyrazino[1,2-a]indoles are synthesized using a six-step protocol. Notably, the synthesis route make use of a new and improved ring-closing methodology for the assembly of the hexahydro-pyrazino[1,2-a]indole scaffold, which is based on intramolecular C-H insertion of a carbene. The compounds act as dual serotonin 5-HT2C- and 5-HT6-ligands.

Krogsgaard-Larsen, Niels; Jensen, Anders A.

2010-01-01

386

Efficient near-UV emitters based on cationic bis-pincer iridium(III) carbene complexes.  

Science.gov (United States)

We report on the photophysical studies of two cationic near-UV emitters based on bis-pincer Ir(III) carbene complexes: [Ir(nBu)(C(NHC)(Me)CC(NHC))2]X, where Ir(nBu)(C(NHC)(Me)CC(NHC)) is (4,6-dimethyl-1,3-phenylene-?C(2))bis(1-butylimidazol-2-ylidene) and X = I(-) or PF6(-)). The compounds are highly emitting in deaerated CH3CN solution with emission maxima at 384 and 406 nm, and photoluminescence quantum yields of 0.41 and 0.38, for [Ir(nBu)(C(NHC)(Me)CC(NHC))2]I and Ir(nBu)(C(NHC)(Me)CC(NHC))2]PF6, respectively. In order to gain deeper understandings into their structural and electronic features, as well as to ascertain the nature of the excited states involved into the electronic absorption processes, density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations have been performed on the ground and excited states of the closely related complex [Ir(Me)(C(NHC)(Me)CC(NHC))2](+). In the solid state, an emission at low energy is observed (?(max) = 500 nm) for both complexes. However, the intensity of the emission at high energy versus the intensity of the new emission at low energy is dependent on the nature of counterions. The origin of this emission is not completely clear, but the experimental data point to the formation of trapping sites induced by aggregation processes involving the interaction between the cationic emitter and the counterion. PMID:24093480

Darmawan, Noviyan; Yang, Cheng-Han; Mauro, Matteo; Raynal, Matthieu; Heun, Susanne; Pan, Junyou; Buchholz, Herwig; Braunstein, Pierre; De Cola, Luisa

2013-10-01

387

Ligand modeling and design  

Energy Technology Data Exchange (ETDEWEB)

The purpose of this work is to develop and implement a molecular design basis for selecting organic ligands that would be used tin applications for the cost-effective removal of specific radionuclides from nuclear waste streams.

Hay, B. [Pacific Northwest Lab., Richland, WA (United States)

1996-10-01

388

Ab initio Study of Mechanism of Forming Germanic Bis-Heterocyclic Compound Between Dichloro-Germylene Carbene (Cl2Ge = C:) and Formaldehyde  

Science.gov (United States)

The mechanism of the cycloaddition reaction of forming germanic bis-heterocyclic compound between singlet dichloro-germylene carbene and formaldehyde has been investigated with CCSD(T)//MP2/6-31G* method, from the potential energy profile, we predict that the reaction has two competitive dominant reaction pathways. The presented rule of this reaction: the 2p unoccupied orbital of the C atom in dichloro-germylene carbene insert the ? orbital of formaldehyde from oxygen side, resulting in the formation of intermediate. In the intermediate and between two reactants, because of the two bonding ? orbital in dichloro-germylene carbene and formaldehyde have occurred [2+2] cycloaddition reaction, forming two four-membered ring compounds in which Ge and O are in the opposite orientation and in the syn-position, respectively. Because of the unsaturated property of C atom from carbene in the two four-membered ring compounds, they further reacts with formaldehyde, resulting in the generation of two germanic bis-heterocyclic compounds.

Lu, Xiu-hui; Che, Xin; Lian, Zhen-xia; Li, Yong-qing

2010-08-01

389

Pull-push mechanism for the 1,2-hydrogen rearrangement of carbenes. Substituent and deuterium isotope effects for thermal decomposition of 1-phenyl-2-diazopropanes  

International Nuclear Information System (INIS)

Intramolecular and intermolecular deuterium isotope effects have been measured for the thermal carbene H rearrangements of three substituted 1-phenyl-2-diazopropanes-1-d in hexane at 250C. The isotope effects, k/sub H//k/sub D/, vary from ca. 1.2 to 1.5, increasing with electron-withdrawing substituents, for the formation of both cis- and trans-?-methylstyrenes. Product ratios were determined for these compounds as well as the corresponding undeuterated ones, permitting calculation of intermolecular primary and secondary isotope effects. In addition, the competition among the cis and trans products and the product of H migration from the terminal methyl group permitted determination of Hammett rho values of ca. -1 for the competition between benzylic and terminal migration. The results strongly indicate a ''pull-push'' mechanism, which can be pictured roughly as electrophilic attack on the C--H bond by the phantom p orbital of the carbene along with backside nucleophilic attack by the carbene unshared electron pair to push the H away and form the ? bond. The data are believed to be consistent only with a nonzero barrier for the carbene hydrogen rearrangement

1978-03-15

390

Molecular rectangles from platinum(II) and bridging dicarbene, diisocyanide and 4,4'-bipyridine ligands.  

Science.gov (United States)

A series of benzobisimidazolium salts have been prepared and treated with [PtCl2(dppe)] or [PtCl2(dmpe)] in the presence of sodium acetate to yield dinuclear dicarbene-bridged complexes of the type [(P^P)(X)Pt(benzodicarbene)Pt(X)(P^P)](X)2 [5]Br2-[11]I2. The reaction of compounds [7]Br2 and [11]I2 with AgPF6 in acetonitrile led to the exchange of the metal-bound bromo or iodo ligands for acetonitrile and the formation of complexes [(dmpe)(MeCN)Pt(benzodicarbene)Pt(MeCN)(dmpe)](PF6)4 [12](PF6)4 and [13](PF6)4. The dinuclear complex [13](PF6)4 reacts with 4,4'-bipyridine (4,4'-bipy) to give the molecular rectangle [15](PF6)8 featuring two bridging dicarbene and two bridging 4,4'-bipy ligands. Complex [13](PF6)4 also reacts with ?,?'-bis(triisopropylsiloxy)phenyl-1,4-diisocyanide 16 followed by hydrolysis of the Si-O bonds and intramolecular carbene formation to yield the molecular square [18](BF4)8 featuring bridging NR,NR- and NH,O-substituted dicarbene ligands. Reaction of the dinuclear complex [12](PF6)4 containing a sterically demanding N,N',N'',N'''-substituted dicarbene ligand under otherwise identical reaction conditions did not lead to any molecular rectangles. The molecular square [19](BF4)8 was synthesized from ?,?'-bis(triisopropylsiloxy)phenyl-1,4-diisocyanide 16 and [PtCl2(dmpe)] in the presence of AgBF4. It reacts to yield the molecular square [20](BF4)8 with four NH,O-substituted dicarbene ligands by Si-O bond cleavage and subsequent intramolecular cyclization. PMID:24022647

Schmidtendorf, Markus; Pape, Tania; Hahn, F Ekkehardt

2013-12-01

391

Transition metal-induced activation of alkynes leading to metal carbene species: synthetic application to new ?-conjugated molecules  

International Nuclear Information System (INIS)

The in situ generation of furylcarbene complexes by means of transition metals was applied to catalytic carbene reactions, such as Wittig-type olefination and cycloisomerization reactions. In the presence of [Rh(OAc)2]2 as a catalyst and triphenylphosphine as a carbene accepter, carbonyl-ene-ynes 1a reacted with benzaldehyde to give a 2-styrylfuran derivative 3a. The reaction involving phosphine-ylide formation followed by Wittig-type olefination with aldehydes was applied to synthesis of furfurylidene-containing p-extended conjugated molecules. A Ruthenium catalyst underwent cycloisomerization of 1,2-bis(carbonylenyl)acetylene 10a,b to afford 2,2'-bifuran structures. The crystal structure of 5,5'-diphenyl-2,2'-bifuran 11b was determined by X-ray crystallography. The ORTEP drawings of 11b exhibited transoid and highly planar structure. The emission bands of the bisfuran 11b were observed at 414 and 440 nm, its quantum yield (84%) was higher than that of a similar 5,5'-diphenyl-2,2'-bithiophene

2008-03-01

392

Transition metal-induced activation of alkynes leading to metal carbene species: synthetic application to new {pi}-conjugated molecules  

Energy Technology Data Exchange (ETDEWEB)

The in situ generation of furylcarbene complexes by means of transition metals was applied to catalytic carbene reactions, such as Wittig-type olefination and cycloisomerization reactions. In the presence of [Rh(OAc){sub 2}]{sub 2} as a catalyst and triphenylphosphine as a carbene accepter, carbonyl-ene-ynes 1a reacted with benzaldehyde to give a 2-styrylfuran derivative 3a. The reaction involving phosphine-ylide formation followed by Wittig-type olefination with aldehydes was applied to synthesis of furfurylidene-containing p-extended conjugated molecules. A Ruthenium catalyst underwent cycloisomerization of 1,2-bis(carbonylenyl)acetylene 10a,b to afford 2,2'-bifuran structures. The crystal structure of 5,5'-diphenyl-2,2'-bifuran 11b was determined by X-ray crystallography. The ORTEP drawings of 11b exhibited transoid and highly planar structure. The emission bands of the bisfuran 11b were observed at 414 and 440 nm, its quantum yield (84%) was higher than that of a similar 5,5'-diphenyl-2,2'-bithiophene.

Abo, T; Ohe, K [Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510 (Japan)], E-mail: ohe@scl.kyoto-u.ac.jp

2008-03-15

393

Synthesis, Characterization, and Photophysical and Electroluminescent Properties of Blue-Emitting Cationic Iridium(III) Complexes Bearing Nonconjugated Ligands.  

Science.gov (United States)

The development of pure-blue-to-deep-blue-emitting ionic phosphors is an ultimate challenge for full-color displays and white-light sources. Herein we report two series of short-wavelength light-emitting cationic iridium(III) complexes with nonconjugated ancillary and cyclometalating ligands, respectively. In the first series, nonconjugated 1-[(diphenylphosphino)methyl]-3-methylimidazolin-2-ylidene-C,C2' (dppmmi) is used as the ancillary ligand and 2-phenylpyridine (ppy), 2-(2,4-difluorophenyl)pyridine (dfppy), and 1-(2,4-difluorophenyl)-1H-pyrazole (dfppz) are used as cyclometalating ligands. In the second one, nonconjugated 2,4-difluorobenzyl-N-pyrazole (dfbpz) is used as the cyclometalating ligand and 3-methyl-1-(2-pyridyl)benzimidazolin-2-ylidene-C,C(2)' (pymbi) as the ancillary ligand. The synthesis and photophysical and electrochemical properties, together with the X-ray crystal structures of these complexes, have been investigated. At room temperature, blue-emitting complexes [Ir(ppy)2(dppmmi)]PF6 (1) and [Ir(dfppy)2(dppmmi)]PF6 (2; PF6(-) is hexafluorophosphate) show much larger photoluminescence quantum yields of 24% and 46%, respectively. On the contrary, for complexes [Ir(dfppz)2(dppmmi)]PF6 (3) and [Ir(dfbpz)2(pymbi)]PF6 (4), deep-blue luminescence is only observed at low temperature (77 K). Density functional theory calculations are used to rationalize the differences in the photophysical behavior observed upon changes of the ligands. It is shown that the electronic transition dipoles of cationic iridium complexes 1 and 2 are mainly confined to cyclometalated ligands ((3)MLCT and LC (3)?-?*) and those of complex 3 are confined to all of the ligands ((3)MLCT, LC (3)?-?*, and (3)LLCT) because of the high LUMO energy level of dfppz. The emission of 4 mainly originates from the central iridium(III) ion and cyclometalated ligand to ancillary ligand charge transfer ((3)MLCT and (3)LLCT), in contrast to commonly designed cationic complexes using carbene-type ancillary ligands, where emission originates from the cyclometalated main ligands. Solution-processed organic light-emitting diodes based on complexes 1 and 2 gave blue-green (498 nm) and blue (478 nm) electroluminescence with maximum current efficiencies of 3.8 and 3.4 cd A(-1), respectively. The results indicate that introducing nonconjugated ligands into cationic iridium complexes is an effective means of achieving short-wavelength light-emitting phosphors. PMID:24914469

Zhang, Fuli; Ma, Dongxin; Duan, Lian; Qiao, Juan; Dong, Guifang; Wang, Liduo; Qiu, Yong

2014-07-01

394

Bromo(eta4-cycloocta-1,5-diene)[1-(2,3,4,5,6-pentafluorobenzyl)-3-(2,4,6-trimethylbenzyl)benzimidazol-2-ylidene]rhodium(I).  

Science.gov (United States)

The title complex, [RhBr(C8H12)(C24H19F5N2)], has a distorted pseudo-square-planar geometry. The Rh-C bond distance between the N-heterocyclic ligand and the metal atom is 2.022 (3) angstroms. The angle between the carbene heterocycle and the coordination plane is 75.60 (11) degrees. It is shown that the average Rh-C(cyclooctadiene) distance is linearly dependent on the Rh-C(imidazole) distance in this type of compound. The crystal structure contains one intramolecular and two intermolecular types of C-H...F interactions, as well as one type of pi-pi stacking interaction. PMID:16763305

Dinçer, Muharrem; Ozdemir, Namik; Gülcemal, Süleyman; Cetinkaya, Bekir; Büyükgüngör, Orhan

2006-06-01

395

Reactivity of a (Bis-NHC)tricarbonylruthenium(0) Complex with Methyl Triflate and Methyl Iodide. Formation of Methyl- and Acetylruthenium(II) Derivatives: Experimental Results and Mechanistic DFT Calculations  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The ruthenium(0) complex [Ru{kappa C-2(2)-MeIm-(CH2)(3)ImMe}(CO)(3)] (1), Mehn(CH2)(3)ImMe = 1,3-bis(3-methylimidazol-2-yliden-1-yl)propane, which contains a chelating bis(N-heterocyclic carbene) ligand, reacts with MeOTf at room temperature to give the ionic ruthenium(II) methyl derivative [RuMe{kappa C-2(2)-MeIm(CH2)(3)ImMe}-(CO)(3)]OTf ([2]OTf), whereas an analogous reaction of 1 with MeI renders the neutral ruthenium(II) acetyl derivative [RuI{C(O)Me}{kappa C-2(2)-MeIm(CH2)(3)ImMe}(CO)(2)...

Cabeza, Javier A.; Damonte, Marina; Garcia-alvarez, Pablo; Perez-carreno, Enrique

2013-01-01

396

A cryptand-encapsulated germanium(II) dication.  

Science.gov (United States)

Unlike cations of metals such as sodium or calcium, oxidized silicon and germanium centers generally require strongly bound covalent ligands. We report the synthesis and characterization of a germanium(II) dication in the form of the salt (Ge.cryptand[2.2.2])(O3SCF3)2. The salt is isolated in 88% yield from the reaction of cryptand [2.2.2] and an N-heterocyclic carbene complex of GeCl(O3SCF3) as an air-sensitive, white solid. The crystal structure of the salt shows minimal interaction between the cryptand-encapsulated germanium(II) ion and the two -O3SCF3 counterions. These results suggest a widely expanded role of cryptands and related molecules in stabilizing nonmetallic cations. PMID:19039131

Rupar, Paul A; Staroverov, Viktor N; Baines, Kim M

2008-11-28

397

Dehydrocoupling reactions of dimethylamine-borane by Pt(II) complexes: a new mechanism involving deprotonation of boronium cations.  

Science.gov (United States)

Coordinatively unsaturated Pt(II) complex [Pt(I(t)Bu')(I(t)Bu)](+) stabilized by N-heterocyclic carbene (NHC) ligands dehydrogenates N,N-dimethylamineborane through a mechanism that involves hydride abstraction, assisted by an amine, to yield a platinum-hydride complex [PtH(I(t)Bu')(I(t)Bu)] with concomitant formation of the boronium cation [(NHMe2)2BH2](+). This latter species is very likely in equilibrium with the THF stabilized borenium cation [(NHMe2)(THF)BH2](+), bearing an acidic NH group that is able to protonate the platinum hydride [PtH(I(t)Bu')(I(t)Bu)] releasing H2, the amino borane H2B-NMe2 and regenerating the catalytic [Pt](+) species. PMID:23822667

Roselló-Merino, Marta; López-Serrano, Joaquín; Conejero, Salvador

2013-07-31

398

[2-Butyl-4-(4-tert-butylbenzyl-1,2,4-triazol-3-ylidene]chlorido[(1,2,5,6-?-cycloocta-1,5-diene]iridium(I  

Directory of Open Access Journals (Sweden)

Full Text Available In the title compound, [IrCl(C8H12(C17H25N3], the IrI ion has a distorted square-planar coordination geometry. The N-heterocyclic carbene ligand has an extended S-shaped conformation. The butyl group was refined using a two-part 1:1 disorder model. In the crystal, three unique weak C—H...Cl contacts are present. Two of these form a motif described as R21(6 in graph-set notation, while a third forms an R22(8 motif about a crystallographic inversion center. The result is a chain structure which extends parallel to the crystallographic a axis.

Gary S. Nichol

2012-02-01

399

Effect of carbene fragments in tungsten complexes PhMe2E-CH=W(NAr)(OR')2 and Me3E-CH=W(NAr)(OR')2 (E=C, Si) on their catalytic properties in olefin permutations  

International Nuclear Information System (INIS)

New silicon-containing tungsten carbene complex is synthesized. Catalytic properties of silicon-containing tungsten carbene complexes Me3Si-CH=W(NAr)(OR')2 (1), PhMe2Si-CH=W(NAr)(OR')2 (2) and their hydrocarbon an logs Me3C-CH=W(NAr)(OR')2 (3), PhMe2C-CH=W(NAr)(OR')2 (4) (Ar=2,6-Pr2iC6H3, R'=CMe2CF3) are studied. It is shown that the nature of carbene fragment in tungsten catalysts has an influence on their catalytic activity. Silicon-containing carbene complexes are less active as compared with their carbon analogs. Structure of new compound 2 as well as known complexes 1, 3 and 4 is ascertained by the X-ray structure analysis

2008-09-01

400

3,3?-Dimethyl-1,1?-(methylenedi-p-phenylene)diimidazolium bis(hexafluorophosphate)  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The title N-heterocyclic carbene compound, C21H22N42+·2PF6?, crystallizes as an inversion twin. There are two independent N-heterocyclic carbene dications (A and B) and four independent hexafluorophosphate anions in the asymmetric unit. The cations are L-shaped with the benzene rings being inclined to one another by 88.82?(16)° in cation A and 87.03?(16)° in cation B. The imidazole rings make dihedral angles of 35.7?(2) and 32.83?(18)° with th...

Kun Huang; Ping Zhang; Da-Bin Qin

2010-01-01

 
 
 
 
401

3,5-Bis(3-methylimidazolium-1-ylmethyltoluene bis(hexafluorophosphate  

Directory of Open Access Journals (Sweden)

Full Text Available The asymmetric unit of the title N-heterocyclic carbene compound, C17H22N42+·2PF6?, consists of one N-heterocyclic carbene dication and two hexafluorophosphate anions. The two imidazole rings are twisted away from but to the same side of the central toluene ring, making dihedral angles of 76.69?(7 and 78.03?(7° with the central ring. In the crystal, the components are linked by C—H...F interactions, generating a three-dimensional network.

Hoong-Kun Fun

2010-03-01

402

Carbene complexes of zirconium. Synthesis, structure, and reactivity with carbon monoxide to affort coordinated ketene  

International Nuclear Information System (INIS)

Treatment of Cp2Zr(L)(CO) (Cp = C5H5; L = PMe3, CO) with Cp*2ZrH2 (Cp* = C5Me5) affords zirconium oxycarbene complexes, Cp2(L)Zr double bond CHO single bond Zr(X)Cp*2 (L = PMe3, X = H, I; L = CO, X = H), that represent some of the first examples of group 4 metal-to-carbon multiple bonding. The first X-ray diffraction structure determination of a zirconium carbene complex, that of Cp(PMe3)Zr double bond CHO single bond Zr(H)Cp*2.C6H6, is reported (C2/c, a = 27.318 (4) A, b = 19.895 (3) A, c = 19.932 (5) A ? = 132.188 (10)0, Z = 8) and shows a very short Zr-C bond length of 2.117 (7) A. Treatment of Cp2(CO)Zr double bond CHO single bond Zr(H)Cp*2 with CH3 I or Cp2(PMe3)Zr double bond CHO single bond Zr(I)Cp*2 with CO affords the zirconium substituted enediolate zirconacycle, Cp*2ZrOCH double bond C(Zr(I)Cp2)O, which has been characterized by an X-ray diffraction study (P21/c, a = 15.866 (4) A, b = 10.673 (3) A, c = 20.561 (4) A, ? = 105.5 (2)0, Z = 4). This complex most likely forms by coupling of the zirconoxycarbene and a carbonyl to give a metal-coordinated ketene intermediate that subsequently rearranges to the isolated product. An isotopic crossover experiment has demonstrated that the new carbon-carbon bond is formed in an intramolecular coupling step. The ketene intermediate can be trapped by dissolving Cp2(CO)Zr double bond CHO single bond Zr(H)Cp*2 in pyridine, giving Cp2(py)Zr(O double bond C double bond CHOZr(I)Cp*2). Treatment of the isolated ketene complex with CH3I in benzene gives the enediolate zirconocycle; in pyridine Cp2(py)Zr(O double bond C double bond CHOZr(I)Cp*2) can be observed spectroscopically. 38 references, 4 figures, 5 tables

1984-09-05

403

Theoretical studies of the insertion of carbenes in the zeolite framework: modification of the acidity and creation of chiral sites  

Scientific Electronic Library Online (English)

Full Text Available SciELO Brazil | Language: English Abstract in portuguese Foi realizado um estudo teórico da inserção de carbenos (CH2 e CHCHO) na estrutura cristalina da zeólita Y. O método our own n-layered integrated molecular orbital and molecular mechanics (ONIOM) M062x/6-31G(d,p):PM6 foi utilizado para descrever a inserção do metileno (CH2) nas ligações T-O (T = Si [...] ou Al) da rede resultando em quatro estruturas isômeras. Em todos os casos os cálculos indicaram uma alta exotermicidade para a inserção do metileno na rede cristalina, independentemente da multiplicidade do carbeno (singlete ou triplete). A inserção na ligação O-H ácida para formar um grupo metóxi adsorvido foi o processo mais favorável, mas este caminho não ocorre se a inserção é feita na zeólita desprotonada. A inserção do formil-carbeno (CHCHO) cria centros quirais na estrutura cristalina. A acidez da zeólita após a inserção dos carbenos foi calculada usando a energia de desprotonação. Os cálculos predizem a possibilidade de desenvolver zeólitas ácidas com centros quirais, que poderiam imitar enzimas, especialmente para uso em processos de química fina e transformação de biomassa. Abstract in english A theoretical study of the insertion of carbenes (CH2, CHCHO) in the framework structure of zeolite Y was carried out. The "our own n-layered integrated molecular orbital and molecular mechanics" (ONIOM) scheme M062x/6-31G(d,p):PM6 was used to describe the insertion of methylene (CH2) in the T-O bon [...] ds (T = Si, Al) of the framework yielding four isomeric structures. In all cases, calculations indicated a high exothermic process for the insertion of methylene in the framework, regardless of the spin multiplicity of the carbene (singlet or triplet). Insertion into the acidic O-H bond to afford an adsorbed methoxy group was the most favourable process, but this pathway does not occur if insertion is carried out on the deprotonated zeolite. Insertion of formylcarbene (CHCHO) creates chiral sites in the framework. The acidity of the zeolite structures after carbene insertion was calculated by means of the deprotonation energy. The calculations predicted the possibility of designing acidic zeolites with chiral sites that could mimic enzymes, especially for uses in fine chemical processes and biomass transformations.

Maria Beatriz S., Mota; Nilton, Rosenbach Jr.; Claudio J. A., Mota.

404

Synthesis of binuclear gold (I) biscarbene complexes by carbene transfers from tungsten to gold  

International Nuclear Information System (INIS)

The binuclear tungsten (0) biscarbene complexes 3 and 4 react with tetrachloroauric acid (stoechiometric ratio 1:2) to give the corresponding gold (I) complexes 5 and 6 involving the transfer of biscarbene ligands from two tungsten to two gold fragments. Preparation and spectroscopic data are described. (Author)

1984-01-01

405

Cocaine Receptor Binding Ligands.  

Science.gov (United States)

The invention is directed to a binding ligand for cocaine and other neurotransmitter receptors in the brain. Specifically, a novel family of compounds, represented by 3beta-(4-iodophenyl)-tropan-2beta-carboxylic acid methyl ester tartrate shows high bindi...

M. J. Kuhar F. I. Carroll J. W. Boja A. H. Lewin P. Abraham

1990-01-01

406

Possible formation of crystalline sodium carbene carbonate Na[subscript 2](CO)CO[subscript 3] at high pressure  

Energy Technology Data Exchange (ETDEWEB)

Combined experimental and ab-initio computational studies suggest that at pressures above 20 GPa and temperatures above 1500 K sodium formate deprotonizes under formation of Na{sub 2}{sup +} [C=O| CO{sub 3}{sup 2-}]. This new compound involves mesomeric C-O bonds with carbonate plus carbene units as limiting states and CO{sub 2} chains as intermediate. Sodium is six-fold coordinated by O-atoms. The formation of this new compound implies nucleophilic addition as reaction mechanism which is consistent with previously observed reduced stability of the H-O bond at these high pressures. The findings suggest further that the sextet state of carbon is energetically less unfavourable at high pressure than at ambient pressure.

Tschauner, O.; Kiefer, B.; McClure, J.; Sinogeikin, S. (UNLV); (CIW); (New Mexico State)

2010-05-24

407

Metal-metal interaction in Fischer carbene complexes: a study of ferrocenyl and biferrocenyl tungsten alkylidene complexes.  

Science.gov (United States)

A series of ferrocenyl (Fc = ferrocenyl; fc = ferrocen-1,1'-diyl) and biferrocenyl (Bfc = 1',1?-biferrocenyl; bfc = 1',1?-biferrocen-1,1?-diyl) mono- and biscarbene tungsten(0) complexes of the type [(CO)5W?C(OMe)R] (1, R = Fc; 3, R = Bfc) and [(CO)5W?C(OMe)-R'-(OMe)C?W(CO)5] (2, R' = fc; 4, R' = bfc) were synthesized according to the classical synthetic methodology by reacting W(CO)6 with LiR (R = Fc, fc, bfc), followed by a subsequent alkylation using methyl trifluoromethanesulfonate. Electrochemical investigations were carried out on these complexes to get a closer insight into the electronic properties of 1-4. The ferrocenyl and biferrocenyl moieties in 1-4 show reversible one-electron redox events. It was further found that the Fischer carbene unit is reducible in an electrochemical one-electron transfer process. For the tungsten carbonyl moieties, irreversible oxidation processes were found. In addition, charge transfer studies were performed on 1-4 using in situ UV-vis-NIR and infrared spectroelectrochemical techniques. During the UV-vis-NIR investigations, typical low energy transitions for the mixed-valent biferrocenyl unit were found. A further observed high energy NIR absorption is attributed to a metal-metal charge transfer transition between the tungsten carbonyl fragment and the ferrocenyl/biferrocenyl group in the corresponding oxidized states, which can be described as class II systems according to Robin and Day. This assignment was verified by infrared spectroelectrochemical studies. The electrochemical investigations are supported by density functional theory calculations. The structural properties of 1-4 in the solid state were investigated by single-crystal X-ray diffraction studies showing no substituent effects on bond lengths and angles. The biferrocenyl derivatives exhibit syn-conformation of the ferrocenyl and carbene building blocks. PMID:24299202

van der Westhuizen, Belinda; Speck, J Matthäus; Korb, Marcus; Friedrich, Joachim; Bezuidenhout, Daniela I; Lang, Heinrich

2013-12-16

408

Diastereotopic group selectivity and chemoselectivity of alkylidene carbene reactions on 8-oxabicyclo[3.2.1]- oct-6-ene ring systems.  

Science.gov (United States)

?-Hydroxyalkylidene carbenes, generated from thermolysis of ?,?-epoxy-N-aziridinylimines, undergo diastereotopic group selective 1,5 C–H insertion reactions on 2,4-dimethyl-8-oxabicyclo[3.2.1]oct-6-ene ring systems. Protection of a tertiary alcohol at C-3 of the bridged oxabicycle as a trimethylsilyl ether reverses the sense of diastereoselectivity. 1,5 C–H insertion into a methine adjacent to an OBn group, 1,5 O–R insertion into a tertiary alcohol (R = H) or silylether (R = TMS) at C-3 to form spirocyclic dihydrofurans, 1,2-rearrangement to an alkyne and fragmentation to a ketone are competing major pathways for 2-benzyloxy-substituted 8-oxabicyclo[3.2.1]oct-6-ene systems. Dihydrofuran formation is shown to be a result of substitution on the oxabicyclic ring system through comparison with other methods of alkylidene carbene formation. PMID:24175334

Munro, Kevin R; Male, Louise; Spencer, Neil; Grainger, Richard S

2013-10-21

409

Formation of organometallic compounds of carbene and carbine types in the interaction of RLi with neodymium and yttrium chlorides in solvating media  

International Nuclear Information System (INIS)

Studied is the interaction of neodymium and yttrium chlorides with benzyl lithium and neophyl lithium and shown is the formation of unusual organometallic compounds of the carbene and carbine types. The product yield is 60-100% from the theory (in calculation per RLi). On the basis of the data obtained by the method of infrared spectroscopy a conclusion is made that in synthesized compounds the benzyl grouping is connected with the metal by ternary bonds

1979-01-01

410

A Class of Phase-Transfer Catalyst with Interionic Strain: Insight into the Bonding of Disubstituted N- vs Carbene-Stabilized N(I)-Centered Cations.  

Science.gov (United States)

The straightforward synthesis of a class of nitrogen-based phase-transfer catalysts (PTCs) having markedly dissociated anions due to interionic donor-donor "ion pair strain" and use for catalyzing benzylation and benzylic fluorination is reported. Provided also is insight into the bonding of disubstituted N- vs so-called divalent carbene-stablized N(I)-centered cations and the unprecedented finding of a cyclopropenium based C-H···?aryl interaction. PMID:24857779

Mirabdolbaghi, Roya; Dudding, Travis; Stamatatos, Theocharis

2014-06-01

411

CAAC Boranes. Synthesis and characterization of cyclic (alkyl) (amino) carbene borane complexes from BF3 and BH3  

Digital Repository Infrastructure Vision for European Research (DRIVER)

In situ formation of two cyclic (alkyl) (amino) carbenes (CAACs) followed by addition of BF3•Et2O provided the first two examples of CAAC–BF3 complexes: 1-(2,6-diisopropylphenyl)-3,5,5-trimethyl-3-phenylpyrrolidin-2-ylidene trifluoroborane, and 2-(2,6-diisopropylphenyl)-3,3-dimethyl-2-azaspiro[4.5]decan-1-ylidene trifluoroborane. These CAAC–BF3 complexes are robust compounds that are stable to ambient laboratory conditions and sili...

Julien Monot; Louis Fensterbank; Max Malacria; Emmanuel Lacôte; Geib, Steven J.; Curran, Dennis P.

2010-01-01

412

Crystallization of protein–ligand complexes  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Methods presented for growing protein–ligand complexes fall into the categories of co-expression of the protein with the ligands of interest, use of the ligands during protein purification, cocrystallization and soaking the ligands into existing crystals.

2007-01-01

413

Radioprotection by DNA ligands.  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Molecular studies on the mechanism of radioprotection by Hoechst 33342 have suggested that radioprotective activity might be improved by addition of electron-donating substituents to the ligand. This paper reports the results of experiments with proamine, in which the ethoxy group of Hoechst 33342 has been replaced with a dimethylamino group. Clonogenic survival studies with V79 cells confirmed the expectation of increased radioprotective activity of proamine. Also, proamine is less cytotoxic...

Martin, R. F.; Broadhurst, S.; D Abrew, S.; Budd, R.; Sephton, R.; Reum, M.; Kelly, D. P.

1996-01-01

414

Multidentate ligand kinetics, (2)  

International Nuclear Information System (INIS)

The kinetics and mechanism of the ligand-substitution reactions of triethylenetetraminecadmium(II) ion (Cd(II)-trien) and tetraethylenepentaminecadmium(II) ion (Cd(II)-tetren) with ethylenediaminetetraacetatocuprate(II) ion (Cu(II)-edta) were studied spectrophotometrically at I = 0.2 M (1 M = 1 mol dm-3) and at 25.00C. The ligand-substitution reactions were catalyzed by traces of either edta or the polyamine (triethylenetetramine and tetraethylenepentamine). The substitution reactions were shown to proceed by means of a chain reaction mechanism where the chain-propagating steps are the reactions of the metal complexes with the free ligands. Each chain-propagating step was investigated. The ligand-substitution reactions of Cd(II)-trien and Cd(II)-tetren with edta were studied by the use of the chain reactions in the pH range of 8.8 - 10.2. The rate law is expressed as: -d ( CdR2+]/dt = kappa sub(L) ( CdR2+] ( edta4-] + kappa sub(HL) ( CdR2+] ( Hedta3-], where R refers to trien or tetren. The rate constants, kappa sub(L) and kappa sub(HL), were determined at 25.00C and I = 0.2 M as: kappa sub(L) = (1.3 +- 0.2) x 106 M-1s-1 and kappa sub(HL) = (6.5 +- 1.5) x 104 M-1s-1 for Cd(II)-trien, and kappa sub(L) = (3.6 +- 0.4) x 105 M-1s-1 for M-1s-1 for Cd(II)-tetren. (auth.)

1978-01-01

415

Radiobiology with DNA ligands  

International Nuclear Information System (INIS)

The paper deals with the following topics: labelling of DNA ligands and other tumour-affinic compounds with 4.15-d 124I, radiotoxicity of Hoechst 33258 and 33342 and of iodinated Hoechst 33258 in cell cultures, preparation of 76Br-, 123I-, and 221At-labelled 5-halo-2'-deoxyuridine, chemical syntheses of boron derivatives of Hoechst 33258.III., Gadolinium neutron capture therapy

1997-01-01

416

Galanin receptors and ligands.  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The neuropeptide galanin was first discovered 30?years ago. Today, the galanin family consists of galanin, galanin-like peptide (GALP), galanin-message associated peptide (GMAP), and alarin and this family has been shown to be involved in a wide variety of biological and pathological functions. The effect is mediated through three GPCR subtypes, GalR1-3. The limited number of specific ligands to the galanin receptor subtypes has hindered the understanding of the individual effects of each r...

2012-01-01

417

Galanin Receptors and Ligands  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The neuropeptide galanin was first discovered 30?years ago. Today, the galanin family consists of galanin, galanin-like peptide (GALP), galanin-message associated peptide (GMAP), and alarin and this family has been shown to be involved in a wide variety of biological and pathological functions. The effect is mediated through three GPCR subtypes, GalR1-3. The limited number of specific ligands to the galanin receptor subtypes has hindered the understanding of the individual effects of each r...

2012-01-01

418

Cooperativity of anion⋯? and ?⋯? interactions regulates the self-assembly of a series of carbene proligands: Towards quantitative analysis of intermolecular interactions with Hirshfeld surface  

Science.gov (United States)

The cooperative effect of weak non-covalent forces between anions and electron deficient aromatics by ?⋯? stacking of a series of carbene proligands (1-3) have been thoroughly explored by crystallographic studies. Structural analysis revealed that the anion⋯? and ?⋯? interactions along with intermolecular hydrogen bonding mutually cooperate to facilitate the assembling of the supramolecular framework. The ?⋯? and corresponding anion⋯? interactions have been investigated in the title carbene proligands despite their association with counter ions. The presence of the anion in the vicinity of the ?-system leads to the formation of anion⋯?/?⋯?/?⋯anion network for an inductive stabilization of the assemblies. To assess the dimensionality of the supramolecular framework consolidated by cooperative anion⋯?/?⋯? interactions and hydrogen bonding, different substituent effects in the carbene backbone have been considered to tune these interactions. These facts show that the supramolecular framework based on these cooperative weak forces may be robust enough for application in molecular recognition. The investigation of close intermolecular interactions between the molecules via Hirshfeld surface analyses is presented in order to reveal subtle differences and similarities in the crystal structures. The decomposition of the fingerprint plot area provides a percentage of each intermolecular interaction, allowing for a quantified analysis of close contacts within each crystal.

Samanta, Tapastaru; Dey, Lingaraj; Dinda, Joydev; Chattopadhyay, Shyamal Kumar; Seth, Saikat Kumar

2014-06-01

419

[3,3]-Sigmatropic Rearrangement vs. Carbene Formation in Gold-Catalyzed Transformations of Alkynyl Aryl Sulfoxides: Mechanistic Studies and Expanded Reaction Scope  

Science.gov (United States)

Gold-catalyzed intramolecular oxidation of terminal alkynes with an arenesulfinyl group as the tethered oxidant is a reaction in gold chemistry of high impact, as it introduced to the field the highly-valued concept of gold carbene generation via alkyne oxidation. The proposed intermediacy of ?-oxo gold carbenes in these reactions, however, has never been substantiated. Detailed experimental studies suggest that the involvement of such reactive intermediates in the formation of dihydrobenzothiepinones is highly unlikely. Instead, a [3,3]-sigmatropic rearrangement of the initial cyclization intermediate offers a reaction path that can readily explain the high reaction efficiency and the lack of sulfonium formation. With internal alkyne substrates, however, the generation of a gold carbene species becomes competitive with the [3,3]-sigmatropic rearrangement. This reactive intermediate, nevertheless, does not proceed to afford the Friedel-Crafts type cyclization product. Extensive Density Functional Theory studies support the mechanistic conclusion that the cyclized product is formed via an intramolecular [3,3]-sigmatropic rearrangement instead of the previously proposed Friedel-Crafts type cyclization. With the new mechanistic insight, the product scope of this versatile formation of mid-sized sulfur-containing cycloalkenones has been expanded readily to various dihydrobenzothiocinones, a tetrahydrobenzocyclononenone, and even those without the entanglement of a fused benzene ring. Besides gold, Hg(OTf)2 can be an effective catalyst, thereby offering a cheap alternative for this intramolecular redox reaction.

Lu, Biao; Li, Yuxue; Wang, Youliang; Aue, Donald H.; Luo, Yingdong; Zhang, Liming

2013-01-01

420

Intermolecular and Regioselective Access to Polysubstituted Benzo- and Dihydrobenzo[c]azepine Derivatives: Modulating the Reactivity of Group 6 Non-Heteroatom-Stabilized Alkynyl Carbene Complexes.  

Science.gov (United States)

We highlight the versatility of non-heteroatom-stabilized tungsten-carbene complexes 3 synthesized in situ, which have been used in a modular approach to access 2-benzazepinium isolable intermediates 5. By employing very mild conditions, benzazepinium derivatives 5 have been obtained in high yield from simple compounds, such as acetylides 2, Fischer-type alkoxycarbenes 1, and phenylimines 4. The process, involving a formal [4+3] heterocycloaddition, occurs in a totally regioselective manner, which differs from the approach previously observed in similar procedures for other carbene analogues. This work, which involves three components, reveals a control of the reactivity of non-heteroatom-stabilized carbene complexes 3 ([4+3] vs. [2+2]-heterocycloaddition reactions) depending on the acetylide substitution pattern. The influence of the substitution pattern in the behavior of the complexes has been computationally analyzed and rationalized. Finally, elaboration of the 2-benzazepinium intermediates allows access to 3H-benzo[c]azepines 6 and 3H-1,2-dihydrobenzo[c]azepines 7-9 with high control of the substitution of the nine positions of the heterocycle. PMID:24757080

González, Jairo; Gómez, Aránzazu; Funes-Ardoiz, Ignacio; Santamaría, Javier; Sampedro, Diego

2014-06-01

 
 
 
 
421

Imidazoline receptors ligands  

Directory of Open Access Journals (Sweden)

Full Text Available Extensive biochemical and pharmacological studies have determined three different subtypes of imidazoline receptors: I1-imidazoline receptors (I1-IR involved in central inhibition of sympathicus that produce hypotensive effect; I2-imidazoline receptors (I2-IR modulate monoamine oxidase B activity (MAO-B; I3-imidazoline receptors (I3-IR regulate insulin secretion from pancreatic ?-cells. Therefore, the I1/I2/I3 imidazoline receptors are selected as new, interesting targets for drug design and discovery. Novel selective I1/I2/I3 agonists and antagonists have been recently developed. In the present review, we provide a brief update to the field of imidazoline research, highlighting some of the chemical diversity and progress made in the 2D-QSAR, 3D-QSAR and quantitative pharmacophore development studies of I1-IR and I2-IR imidazoline receptor ligands. Theoretical studies of I3-IR ligands are not yet performed because of insufficient number of synthesized I3-IR ligands.

Agbaba Danica

2012-01-01

422

Synthesis of azolyl carboximidamides as ligands for Zn(II) and Cu(II): application of the Zn(II) complexes as catalysts for the copolymerization of carbon dioxide and epoxides.  

Science.gov (United States)

A series of novel S,N-heterocyclic (thiazolyl) substituted carboximidamides 3 and 4 was synthesized in yields up to 82% from specific triazinium salts 1 and primary or secondary amines 2 which additionally bear pyridine or imidazole units. These carboximidamides are used as tailor-made ligands for the complexation of Cu(II) and Zn(II). The coordination behavior of 3 and 4 and the properties of the resulting metal complexes are affected a significant extent especially by the nature of these amine substituents. The most important structural feature of the novel complexes is that the ligation of the metal cations is achieved by a 1,3,5-triazapentadienyl anion system, compare the X-ray structure of the model complex Cu-4d. Analogous Zn(II) complexes 5, 6a, 6b, 6c, 7a, and 7b were obtained from carboximidamides 3, 4a, 4b, 4c, 4d, and 4e after reaction with diethylzinc. Interestingly, these Zn(II) complexes possess an intrinsic activity to catalyze the copolymerization of cyclohexene oxide and carbon dioxide to give polycarbonates 15 (TON up to 113; Turn Over Number: moles of substrate 14 consumed per moles of zinc. Molecular weights: up to 206.10(3) Da). Contaminations of 15 by polyethers are produced only in remarkably small amounts. PMID:16468787

Walther, Martin; Wermann, Kurt; Lutsche, Marion; Günther, Wolfgang; Görls, Helmar; Anders, Ernst

2006-02-17

423

Catalyst Design and Optimization within a Framework of Green Chemistry  

Digital Repository Infrastructure Vision for European Research (DRIVER)

This thesis is divided into two main parts. The first, being the evaluation of existing catalytic reactions under the modern aspects of “Green Chemistry”. Energy conservation, atom economy, minimization of waste, side product recycling and use of renewable feedstocks were optimized in systems evaluated, and comparisons to previously know systems made. The second part is focused on carbocyclic carbene synthesis and application of carbocyclic and remote N-heterocyclic carbene complexes in...

Tosh, Evangeline

2009-01-01

424

Galanin receptor ligands.  

Science.gov (United States)

The three galanin receptor subtypes (GalR1-3) belong to the G protein-coupled receptor superfamily. The widespread distribution of galanin and its receptors in the CNS and PNS and the numerous physiological and pharmacological effects of galanin (for review, cf. Vrontakis, 2002) render the three galanin receptors attractive drug targets. The industrial efforts, however, have not yet resulted in a wealth of receptor subtype specific agonists or antagonists with high affinity and selectivity. The present paper summarizes the properties of the galanin ligands used at the end of 2004 in the ca. 2000 publications and complements their pharmacological characterization with new data. PMID:15944002

Lu, Xiaoying; Lundström, Linda; Langel, Ulo; Bartfai, Tamas

2005-06-01

425

1. Medicinal chemistry of a small molecule drug lead: Tamoxilog 2. Electronic communication through ruthenium nanoparticles: Synthesis of custom ligands and nanoparticles  

Science.gov (United States)

1. Compound NSC-670224, previously shown to be toxic to Saccharomyces cerevisiae at low micromolar concentrations, potentially acts via a mechanism of action related to that of tamoxifen (NSC 180973), a widely utilized breast cancer drug. The structure of NSC-670224, previously thought to be a 2,4-dichloro arene, was established as the 3,4-dichloro arene, and a focused library of analogues were synthesized and biologically evaluated in conjunction with the UCSC Chemical Screening Center. The synthesis of a biotinylated affinity probe was also completed in order to extract the protein target(s) of NSC-670224 from yeast and human cell lines in collaboration with the Hartzog lab (UCSC MCD Biology) 2. Stabilization of ruthenium nanoparticles (Ru NPs) through carbene bound ligands has led to a simple and effective means to generate new materials with unique optoelectronic properties. The affinity of freshly prepared Ru NPs to diazo compounds, specifically octyl diazoacetate (ODA), provides a robust nanostructure that can be further functionalized via metathesis of terminal olefins to generate these unique materials. Carbene-stabilized Ru NPs have provided insights into the nature of extended conjugation and intraparticle charge delocalization through covalently bound probes (e.g., ferrocene and pyrene). The growing interest to study electronic communication through Ru NPs has lead to collaborative, multidisciplinary efforts between analytical (Shaowei Chen lab, UCSC), theoretical (Haobin Wang Lab, NMSU), and synthetic organic chemists (Konopelski Lab, UCSC). With this powerful collaboration, new methods to generate stabilized Ru NPs, testing theory with experiment, and efficient means to functionalize NPs have been investigated. The syntheses of custom ligands and their applications to nanoparticle-mediated electronic communication are reported.

Zuckerman, Nathaniel Benjamin

426

Ligand Identification Scoring Algorithm (LISA)  

Digital Repository Infrastructure Vision for European Research (DRIVER)

A central problem in de novo drug design is determining the binding affinity of a ligand with a receptor. A new scoring algorithm is presented that estimates the binding affinity of a protein-ligand complex given a three-dimensional structure. The method, LISA (Ligand Identification Scoring Algorithm), uses an empirical scoring function to describe the binding free energy. Interaction terms have been designed to account for van der Waals (VDW) contacts, hydrogen bonding, desolvation effects a...

Zheng, Zheng; Merz, Kenneth M.

2011-01-01

427

Macrocyclic G-quadruplex ligands  

DEFF Research Database (Denmark)

G-quadruplex stabilizing compounds have recently received increased interest due to their potential application as anticancer therapeutics. A significant number of structurally diverse G-quadruplex ligands have been developed. Some of the most potent and selective ligands currently known are macrocyclic structures which have been modeled after the natural product telomestatin or from porphyrin-based ligands discovered in the late 1990s. These two structural classes of G-quadruplex ligands are reviewed here with special attention to selectivity and structure-activity relationships, and with focus on the recent developments.

Nielsen, M C; Ulven, Trond

2010-01-01

428

Combining NHC-Cu and Brønsted base catalysis: enantioselective allylic substitution/conjugate additions with alkynylaluminum reagents and stereospecific isomerization of the products to trisubstituted allenes.  

Science.gov (United States)

All-catalytic route to trisubstituted allenes: The first examples of catalytic enantioselective allylic substitution reactions that involve alkyne-based nucleophiles and lead to products having tertiary stereogenic centers are followed by an exceptionally stereospecific amine-catalyzed isomerization to trisubstituted allenes (see picture; NHC = N-heterocyclic carbene). PMID:23864395

Dabrowski, Jennifer A; Haeffner, Fredrik; Hoveyda, Amir H

2013-07-22

429

Nickel-catalysed carboxylation of organoboronates.  

Science.gov (United States)

A nickel/N-heterocyclic carbene (NHC) catalysed carboxylation of aryl-, heteroaryl- and alkenylboronates, affording the corresponding carboxylic acids, has been developed. This transformation proceeds under one atmosphere of CO2 with a broad range of substrates and exhibits good functional group compatibility. PMID:24915842

Makida, Yusuke; Marelli, Enrico; Slawin, Alexandra M Z; Nolan, Steven P

2014-06-26

430

Catalytic MBH reaction of ?-substituted nitroalkenes with azodicarboxylates.  

Science.gov (United States)

An unprecedented N-heterocyclic carbene (NHC) catalyzed Morita-Baylis-Hillman (MBH) reaction of ?-substituted nitroalkenes and azodicarboxylates has been developed. Both ?-aryl and ?-alkyl nitroalkenes worked well for the reaction using 5 mol% of NHC catalyst, giving the desired ?-hydrazino-?,?-unsaturated nitroalkenes in good to excellent yields. PMID:23884292

Chen, Xiang-Yu; Xia, Fei; Ye, Song

2013-09-14

431

Synthesis of functionalized coumarins and quinolinones by NHC-catalyzed annulation of modified enals with heterocyclic C-H acids.  

Science.gov (United States)

N-Heterocyclic carbene (NHC) catalyzed lactonization and lactamization of 2-bromoenals with heterocyclic C-H acids proceeding via the ?,?-unsaturated acyl azolium intermediates is reported. The reaction furnished coumarin- or quinolinone-fused lactone/lactam derivatives. In addition, results of the enantioselective version of this reaction using chiral NHC are presented. PMID:24716576

Yetra, Santhivardhana Reddy; Roy, Tony; Bhunia, Anup; Porwal, Digvijay; Biju, Akkattu T

2014-05-01

432

From acetone metalation to the catalytic ?-arylation of acyclic ketones with NHC-nickel(II) complexes.  

Science.gov (United States)

Air-stable N-heterocyclic carbene-nickel(ii) complexes at concentrations as low as 1 mol% exhibit high catalytic activity for the ?-arylation of acyclic ketones and join a highly restricted list of nickel catalysts for this key reaction. Mechanistic investigations suggest a radical pathway. PMID:24668065

Henrion, Mickaël; Chetcuti, Michael J; Ritleng, Vincent