Sample records for azirines
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Wang, Fei; Zhu, Na; Chen, Pinhong; Ye, Jinxing; Liu, Guosheng
2015-08-03
A novel method for convenient access to CF3-containing azirines has been developed, and involves a copper-catalyzed trifluoromethylazidation of alkynes and a photocatalyzed rearrangement. Both terminal and internal alkynes are compatible with the mild reaction conditions, thus delivering the CF3-containing azirines in moderate to good yields. The azirines can be converted into various CF3-substituted aziridines. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
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Substituent effects on the photolysis of methyl 2-carboxylate substituted aliphatic 2 H-azirines
Gómez-Zavaglia, Andrea; Kaczor, Agnieszka; Cardoso, Ana L.; Pinho e Melo, Teresa M. V. D.; Fausto, Rui
2007-05-01
In this study, the UV induced photochemical reactions of two 2 H-azirines - methyl 2-chloro-3-methyl-2 H-azirine-2-carboxylate (MCMAC) and methyl 3-methyl-2 H-azirine-2-carboxylate (MMAC) - isolated in argon matrices are compared. For both compounds, irradiation with λ > 235 nm led to observation of two primary photoprocesses: (a) C sbnd C bond cleavage, with production of nitrile ylides (P1-type products), and (b) C sbnd N bond cleavage, with production of methylated ketene imines (P2-type products). However, subsequent photoprocesses were found to be different in the two cases. In MCMAC, both primary photoproducts were shown to undergo further reactions: P1-type products decarboxylate, giving [(1-chloroethylidene)imino]ethanide, which bears a C dbnd N +dbnd C - group (P3-type product); P2-type products decarbonylate, yielding a substituted ylidene methanamine (P4-type product). In MMAC, only P2-type primary photoproducts appeared to react, undergoing decarbonylation or decarboxylation (both reactions leading to P4-type products), whereas P1-type products were found to be non-reactive. The non-observation of any secondary photoproduct resulting from photolysis of P1-MMAC revealed the higher photostability of this species when compared with the corresponding photoproduct obtained from MCMAC. The C sbnd N photochemical cleavage is an unusual process in aliphatic 2 H-azirines. In the studied compounds, its preference over the commonly observed C sbnd C azirine-ring bond photocleavage is attributed to the presence of electron withdrawing substituents (methylcarboxy group in both azirines and also the chlorine atom in MCMAC), which accelerates intersystem crossing towards the triplet state from where the cleavage of the C sbnd N bond takes place. The lack of the chlorine atom in MMAC may be partially compensated by the significantly higher stabilization of the P2-type photoproduct derived from this molecule ( ca. -52 kJ mol -1) relatively to the reactant, when
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International Nuclear Information System (INIS)
Cao, Jun
2015-01-01
In the present work, the combined electronic structure calculations and dynamics simulations have been performed to explore photocleavages of 2-formyl-2H-azirine and isoxazole in the gas phase and the subsequent rearrangement reactions. The carbonyl n → π * transition induces a cleavage of the C—N single bond of 2-formyl-2H-azirine to yield β-formylvinylnitrene in open-shell singlet state. However, the n → π * excitation of the imine chromophore results in a cleavage of the C—C single bond, producing a nitrile ylide intermediate through an internal conversion to the ground state. β-formylvinylnitrene and nitrile ylide with the carbonyl group are easily transformed into 2-formyl-2H-azirine and oxazole, respectively. The N—O bond cleavages on both S 1 ( 1 ππ * ) and S 2 ( 1 n N π * ) of isoxazole are ultrafast processes, and they give products of 2-formyl-2H-azirine, 3-formylketenimine, HCN + CHCHO, and HCO + CHCHN. Both 2H-azirines and ketenimines were suggested to be formed from the triplet vinylnitrenes by intersystem crossing in the previous studies. However, our calculations show that the singlet β-formylvinylnitrene is responsible for the formation of 2-formyl-2H-azirine and 3-formylketenimine, and the singlet vinylnitrenes can play a key role in the photoinduced reactions of both 2H-azirines and isoxazoles
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Cao, Jun
2015-06-01
In the present work, the combined electronic structure calculations and dynamics simulations have been performed to explore photocleavages of 2-formyl-2H-azirine and isoxazole in the gas phase and the subsequent rearrangement reactions. The carbonyl n → π* transition induces a cleavage of the C—N single bond of 2-formyl-2H-azirine to yield β-formylvinylnitrene in open-shell singlet state. However, the n → π* excitation of the imine chromophore results in a cleavage of the C—C single bond, producing a nitrile ylide intermediate through an internal conversion to the ground state. β-formylvinylnitrene and nitrile ylide with the carbonyl group are easily transformed into 2-formyl-2H-azirine and oxazole, respectively. The N—O bond cleavages on both S1(1ππ*) and S2(1nNπ*) of isoxazole are ultrafast processes, and they give products of 2-formyl-2H-azirine, 3-formylketenimine, HCN + CHCHO, and HCO + CHCHN. Both 2H-azirines and ketenimines were suggested to be formed from the triplet vinylnitrenes by intersystem crossing in the previous studies. However, our calculations show that the singlet β-formylvinylnitrene is responsible for the formation of 2-formyl-2H-azirine and 3-formylketenimine, and the singlet vinylnitrenes can play a key role in the photoinduced reactions of both 2H-azirines and isoxazoles.
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Energy Technology Data Exchange (ETDEWEB)
Cao, Jun, E-mail: caojunbnu@mail.bnu.edu.cn [Guizhou Provincial Key Laboratory of Computational Nano-Material Sciences, Guizhou Normal College, Guiyang, Guizhou 550018, China and Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education, College of Chemistry, Beijing Normal University, Beijing 100875 (China)
2015-06-28
In the present work, the combined electronic structure calculations and dynamics simulations have been performed to explore photocleavages of 2-formyl-2H-azirine and isoxazole in the gas phase and the subsequent rearrangement reactions. The carbonyl n → π{sup *} transition induces a cleavage of the C—N single bond of 2-formyl-2H-azirine to yield β-formylvinylnitrene in open-shell singlet state. However, the n → π{sup *} excitation of the imine chromophore results in a cleavage of the C—C single bond, producing a nitrile ylide intermediate through an internal conversion to the ground state. β-formylvinylnitrene and nitrile ylide with the carbonyl group are easily transformed into 2-formyl-2H-azirine and oxazole, respectively. The N—O bond cleavages on both S{sub 1}({sup 1}ππ{sup *}) and S{sub 2}({sup 1}n{sub N}π{sup *}) of isoxazole are ultrafast processes, and they give products of 2-formyl-2H-azirine, 3-formylketenimine, HCN + CHCHO, and HCO + CHCHN. Both 2H-azirines and ketenimines were suggested to be formed from the triplet vinylnitrenes by intersystem crossing in the previous studies. However, our calculations show that the singlet β-formylvinylnitrene is responsible for the formation of 2-formyl-2H-azirine and 3-formylketenimine, and the singlet vinylnitrenes can play a key role in the photoinduced reactions of both 2H-azirines and isoxazoles.
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Maltsev, Alexander; Bally, Thomas; Tsao, Meng-Lin; Platz, Matthew S; Kuhn, Arvid; Vosswinkel, Michael; Wentrup, Curt
2004-01-14
Ar matrix photolysis of 1- and 2-naphthyl azides 3 and 4 at 313 nm initially affords the singlet naphthyl nitrenes, (1)()1 and (1)()2. Relaxation to the corresponding lower energy, persistent triplet nitrenes (3)()1 and (3)()2 competes with cyclization to the azirines 15 and 18, which can also be formed photochemically from the triplet nitrenes. On prolonged irradiation, the azirines can be converted to the seven-membered cyclic ketenimines 10 and 13, respectively, as described earlier by Dunkin and Thomson. However, instead of the o-quinoid ketenimines 16 and 19, which are the expected primary ring-opening products of azirines 15 and 18, respectively, we observed their novel bond-shift isomers 17 and 20, which may be formally regarded as cyclic nitrile ylides. The existence of such ylidic heterocumulenes has been predicted previously, but this work provides the first experimental observation of such species. The factors which are responsible for the special stability of the ylidic species 17 and 20 are discussed.
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Directory of Open Access Journals (Sweden)
Alexander F. Khlebnikov
2014-04-01
Full Text Available 3-Aryl-2H-azirines react with acylketenes, generated by thermolysis of 5-arylfuran-2,3-diones, to give bridged 5,7-dioxa-1-azabicyclo[4.4.1]undeca-3,8-diene-2,10-diones and/or ortho-fused 6,6a,12,12a-tetrahydrobis[1,3]oxazino[3,2-a:3′,2′-d]pyrazine-4,10-diones. The latter compounds, with a new heterocyclic skeleton, are the result of the coupling of two molecules of azirine and two molecules of acylketene and can be prepared only from 3-aryl-2H-azirines having no electron-withdrawing groups in the aryl substituent. Calculations at the DFT B3LYP/6-31G(d level for the various routes of bis[1,3]oxazino[3,2-a:3′,2′-d]pyrazine skeleton formation revealed a new domino reaction of 3-aryl-2H-azirines occurring in the presence of furandiones: acid-catalyzed dimerization to dihydropyrazine followed by consecutive cycloaddition of the latter to two molecules of acylketenes.
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Directory of Open Access Journals (Sweden)
Nikolai V. Rostovskii
2015-03-01
Full Text Available Strained azirinium ylides derived from 2H-azirines and α-diazoketones under Rh(II-catalysis can undergo either irreversible ring opening across the N–C2 bond to 2-azabuta-1,3-dienes that further cyclize to 2H-1,4-oxazines or reversibly undergo a 1,5-cyclization to dihydroazireno[2,1-b]oxazoles. Dihydroazireno[2,1-b]oxazoles derived from 3-aryl-2H-azirines and 3-diazoacetylacetone or ethyl diazoacetoacetate are able to cycloadd to acetyl(methylketene generated from 3-diazoacetylacetone under Rh(II catalysis to give 4,6-dioxa-1-azabicyclo[3.2.1]oct-2-ene and/or 5,7-dioxa-1-azabicyclo[4.3.1]deca-3,8-diene-2-one derivatives. According to DFT calculations (B3LYP/6-31+G(d,p, the cycloaddition can involve two modes of nucleophilic attack of the dihydroazireno[2,1-b]oxazole intermediate on acetyl(methylketene followed by aziridine ring opening into atropoisomeric oxazolium betaines and cyclization. Azirinium ylides generated from 2,3-di- and 2,2,3-triaryl-substituted azirines give rise to only 2-azabuta-1,3-dienes and/or 2H-1,4-oxazines.
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[3 + 2]-Cycloadditions of nitrile ylides after photoactivation of vinyl azides under flow conditions
Directory of Open Access Journals (Sweden)
Stephan Cludius-Brandt
2013-08-01
Full Text Available The photodenitrogenation of vinyl azides to 2H-azirines by using a photoflow reactor is reported and compared with thermal formation of 2H-azirines. Photochemically, the ring of the 2H-azirines was opened to yield the nitrile ylides, which underwent a [3 + 2]-cycloaddition with 1,3-dipolarophiles. When diisopropyl azodicarboxylate serves as the dipolarophile, 1,3,4-triazoles become directly accessible starting from the corresponding vinyl azide.
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α-Diazo oxime ethers for N-heterocycle synthesis.
Choi, Subin; Ha, Sujin; Park, Cheol-Min
2017-06-01
This Feature Article introduces the preparation and synthetic utility of α-diazo oxime ethers. α-Oximino carbenes are useful synthons for N-heterocycles, and can be easily prepared from α-diazo oxime ethers as precursors. We begin with the preparation of α-diazo oxime ethers and their application in [3+2] cycloaddition. It turns out that the nature of metals bound to carbenes plays a crucial role in modulating the reactivity of α-oximino carbenes, in which copper carbenes smoothly react with enamines, whereas the less reactive enol ethers and nitriles require gold carbenes. In Section 3.2, a discussion on N-O and C-H bond activation is presented. Carbenes derived from diazo oxime ethers show unique reactivity towards N-O and C-H bond activation, in which the proximity of the two functionalities, carbene and oxime ether, dictates the preferred reaction pathways toward pyridines, pyrroles, and 2H-azirines. In Section 3.3, the development of tandem reactions based on α-diazo oxime ethers is discussed. The nature of carbenes in which whether free carbenes or metal complexes are involved dissects the pathway and forms different types of 2H-azirines. The 2H-azirine formation turned out to be an excellent platform for the tandem synthesis of N-heterocycles including pyrroles and pyridines. In the last section, we describe the electrophilic activation of 2H-azirines with vinyl carbenes and oximino carbenes. The resulting azirinium species undergo rapid ring expansion rearrangements to form pyridines and pyrazines.
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Grote, Dirk; Sander, Wolfram
2009-10-02
The photochemistry of a series of fluorinated p-iodophenyl azides 2 has been investigated using matrix isolation IR and EPR spectroscopy. In all cases, the corresponding phenylnitrenes 1 were formed as primary photoproducts. Further irradiation of the nitrenes 1 resulted in the formation of azirines 3, ketenimines 4, and nitreno radicals 5. The yield of 5 depends on the number of ortho fluorine substituents: with two ortho fluorine atoms the highest yield is observed, whereas without fluorine atoms the yield is too low for IR spectroscopic detection. The interconversion between the isomers 1, 3, and 4 proved to be rather complex. If the fluorine atoms are distributed unsymmetrically, two isomers of azirines 3 and ketenimines 4 can be formed. The yields of these isomers depend critically on the irradiation conditions.
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Carra, Claudio; Nussbaum, Rafael; Bally, Thomas
2006-06-12
2,6-Difluorophenylnitrene was reinvestigated both experimentally, in Ar matrices at 10 K, and computationally, by DFT and CASSCF/CASPT2 calculations. Almost-pure samples of both neutral rearrangement products (the bicyclic azirine and the cyclic ketenimine) of a phenylnitrene were prepared and characterized for the first time. These samples were then subjected to X-irradiation in the presence of CH2Cl2 as an electron scavenger, which led to ionization of the neutral intermediates. Thereby, it was shown that only the phenylnitrene and the cyclic ketenimine yield stable radical cations, whereas the bicyclic azirine decays to both of these compounds on ionization. The cyclic ketenimine yields a novel aromatic azatropylium-type radical cation. The electronic structure of the title compound is discussed in detail, and its relation to those of the iso-pi-electronic benzyl radical and phenylcarbene is traced.
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Nallagangula, Madhu; Namitharan, Kayambu
2017-07-07
First simultaneous generation and utilization of both copper-triazole and -ketenimine intermediates in copper-catalyzed sulfonyl azide-alkyne cycloaddition reactions is achieved for the one-pot synthesis of triazolopyrimidines via a novel copper-catalyzed multicomponent cascade of sulfonyl azides, alkynes, and azirines. Significantly, the reaction proceeds under very mild conditions in good yields.
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Carra, Claudio; Nussbaum, Rafael; Bally, Thomas
2006-01-01
2,6-Difluorophenylnitrene was reinvestigated both experimentally, in Ar matrices at 10 K, and computationally, by DFT and CASSCF/CASPT2 calculations. Almost-pure samples of both neutral rearrangement products (the bicyclic azirine and the cyclic ketenimine) of a phenylnitrene were prepared and characterized for the first time. These samples were then subjected to X-irradiation in the presence of CH₂Cl₂ as an electron scavenger, which led to ionization of the neutral intermediates. Thereby, it...
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Xue, Jiadan; Luk, Hoi Ling; Eswaran, S V; Hadad, Christopher M; Platz, Matthew S
2012-06-07
The photochemistry of 4-methoxycarbonylphenyl azide (2a), 2-methoxycarbonylphenyl azide (3a), and 2-methoxy-6-methoxycarbonylphenyl azide (4a) were studied by ultrafast time-resolved infrared (IR) and UV-vis spectroscopies in solution. Singlet nitrenes and ketenimines were observed and characterized for all three azides. Isoxazole species 3g and 4g are generated after photolysis of 3a and 4a, respectively, in acetonitrile. Triplet nitrene 4e formation correlated with the decay of singlet nitrene 4b. The presence of water does not change the chemistry or kinetics of singlet nitrenes 2b and 3b, but leads to protonation of 4b to produce nitrenium ion 4f. Singlet nitrenes 2b and 3b have lifetimes of 2 ns and 400 ps, respectively, in solution at ambient temperature. The singlet nitrene 4b in acetonitrile has a lifetime of about 800 ps, and reacts with water with a rate constant of 1.9 × 10(8) L·mol(-1)·s(-1) at room temperature. These results indicate that a methoxycarbonyl group at either the para or ortho positions has little influence on the ISC rate, but that the presence of a 2-methoxy group dramatically accelerates the ISC rate relative to the unsubstituted phenylnitrene. An ortho-methoxy group highly stabilizes the corresponding nitrenium ion and favors its formation in aqueous solvents. This substituent has little influence on the ring-expansion rate. These results are consistent with theoretical calculations for the various intermediates and their transition states. Cyclization from the nitrene to the azirine intermediate is favored to proceed toward the electron-deficient ester group; however, the higher energy barrier is the ring-opening process, that is, azirine to ketenimine formation, rendering the formation of the ester-ketenimine (4d') to be less favorable than the isomeric MeO-ketenimine (4d).
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Banert, Klaus; Chityala, Madhu; Hagedorn, Manfred; Beckers, Helmut; Stüker, Tony; Riedel, Sebastian; Rüffer, Tobias; Lang, Heinrich
2017-08-01
Solutions of azidomethylidenemalononitrile were photolyzed at low temperatures to produce the corresponding 2H-azirine and tricyanomethane, which were analyzed by low-temperature NMR spectroscopy. The latter product was also observed after short thermolysis of the azide precursor in solution whereas irradiation of the azide isolated in an argon matrix did not lead to tricyanomethane, but to unequivocal detection of the tautomeric ketenimine by IR spectroscopy for the first time. When the long-known "aquoethereal" greenish phase generated from potassium tricyanomethanide, dilute sulfuric acid, and diethyl ether was rapidly evaporated and sublimed, a mixture of hydronium tricyanomethanide and tricyanomethane was formed instead of the previously claimed ketenimine tautomer. Under special conditions of sublimation, single crystals of tricyanomethane could be isolated, which enabled the analysis of the molecular structure by X-ray diffraction. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
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Balucani, Nadia
The observation of N-containing organic molecules and the composition of the haze aerosols, as determined by the Aerosol Collector and Pyrolyser (ACP) on-board Huygens, are clear indications that some chemistry involving nitrogen active forms and hydrocarbons is operative in the upper atmosphere of Titan. Neutral-neutral reactions involving the first electronically excited state of atomic nitrogen, N(2D), and small hydrocarbons have the right prerequisites to be among the most significant pathways to formation of nitriles, imines and other simple N-containing organic molecules. The closed-shell products methanimine, ethanimine, ketenimine, 2H-azirine and the radical products CH3N, HCCN and CH2NCH can be the intermediate molecular species that, via addition reactions, polymerization and copolymerization form the N-rich organic aerosols of Titan as well as tholins in bulk reactors simulating Titan's atmosphere.
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International Nuclear Information System (INIS)
Cao, Jun; Xie, Zhi-Zhong; Yu, Xiaodong
2016-01-01
In the present work, the combined electronic structure calculations and surface hopping simulations have been performed to investigate the excited-state decay of the parent oxazole in the gas phase. Our calculations show that the S_2 state decay of oxazole is an ultrafast process characterized by the ring-opening and ring-closure of the five-membered oxazole ring, in which the triplet contribution is minor. The ring-opening involves the O−C bond cleavage affording the nitrile ylide and airine intermediates, while the ring-closure gives rise to a bicyclic species through a 2−5 bond formation. The azirine and bicyclic intermediates in the S_0 state are very likely involved in the phototranspositions of oxazoles. This is different from the previous mechanism in which these intermediates in the T_1 state have been proposed for these phototranspositions.
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Energy Technology Data Exchange (ETDEWEB)
Cao, Jun [Guizhou Provincial Key Laboratory of Computational Nano-material Science, Guizhou Education University, Guiyang, Guizhou 550018 (China); Guizhou Synergetic Innovation Center of Scientific Big Data for Advanced Manufacturing Technology, Guizhou Education University, Guiyang 550018 (China); Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education, College of Chemistry, Beijing Normal University, Beijing 100875 (China); Xie, Zhi-Zhong [Department of Chemistry, School of Chemistry, Chemical Engineering and Life Sciences, Wuhan University of Technology, Wuhan 430070 (China); Yu, Xiaodong, E-mail: yuxdhy@163.com [Department of Architecture and Chemical Engineering, Tangshan Polytechnic College, Tangshan 063020 (China)
2016-08-02
In the present work, the combined electronic structure calculations and surface hopping simulations have been performed to investigate the excited-state decay of the parent oxazole in the gas phase. Our calculations show that the S{sub 2} state decay of oxazole is an ultrafast process characterized by the ring-opening and ring-closure of the five-membered oxazole ring, in which the triplet contribution is minor. The ring-opening involves the O−C bond cleavage affording the nitrile ylide and airine intermediates, while the ring-closure gives rise to a bicyclic species through a 2−5 bond formation. The azirine and bicyclic intermediates in the S{sub 0} state are very likely involved in the phototranspositions of oxazoles. This is different from the previous mechanism in which these intermediates in the T{sub 1} state have been proposed for these phototranspositions.
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International Nuclear Information System (INIS)
Gillings, N.M.; Gee, A.D.; Inoue, O.
1999-01-01
In an attempt to elucidate the contribution of the extent of nitrogen protonation on the in vivo binding of methamphetamine in the brain, the enantiomers of [N-methyl- 11 C]β,β-difluoroamphetamine (4) were prepared for use in positron emission tomography (PET) studies. Thus, the enantiomers of β,β-difluoroamphetamine were prepared from trans-β-methylstyrene, via bromination, conversion into the azirine, fluorination and resolution as the tartrate salts. (R)- and (S)-β,β-difluoroamphetamine (3) were then each labelled with carbon-11 (t 1/2 =20.4 min) by N-methylation of the corresponding homochiral β,β-difluoroamphetamine with [ 11 C]methyl iodide. The labelled products were each synthesised, purified and formulated in 35 min, starting from [ 11 C]carbon dioxide in 15-16% decay-corrected radiochemical yield, with a radiochemical purity of >99% and specific radioactivity of 50-150 GBq μmol -1 at end of synthesis
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3,4,5,6-Tetrafluorophenylnitren-2-yl: a ground-state quartet triradical.
Grote, Dirk; Finke, Christopher; Kossmann, Simone; Neese, Frank; Sander, Wolfram
2010-04-19
The photochemistry of 2-iodo-3,4,5,6-tetrafluorophenyl azide (7 d) has been investigated in argon and neon matrices at 4 K, and the products characterized by IR and EPR spectroscopy. The primary photochemical step is loss of a nitrogen molecule and formation of phenyl nitrene 1 d. Further irradiation with UV or visible light results in mixtures of 1 d with azirine 5 d', ketenimine 6 d', nitreno radical 2 d, and azirinyl radical 9. The relative amounts of these products strongly depend on the matrix and on the irradiation conditions. Nitreno radical 2 d with a quartet ground state was characterized by EPR spectroscopy. Electronic structure calculations in combination with the experimental results allow for a detailed understanding of the properties of this unusual new type of organic high-spin molecules. Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
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Energy Technology Data Exchange (ETDEWEB)
Gillings, N.M.; Gee, A.D. [PET Centre, Aarhus University Hospital, Aarhus C (Denmark); Inoue, O. [School of Allied Health Sciences, Osaka University Medical School, Osaka (Japan)
1999-04-01
In an attempt to elucidate the contribution of the extent of nitrogen protonation on the in vivo binding of methamphetamine in the brain, the enantiomers of [N-methyl-{sup 11}C]{beta},{beta}-difluoroamphetamine (4) were prepared for use in positron emission tomography (PET) studies. Thus, the enantiomers of {beta},{beta}-difluoroamphetamine were prepared from trans-{beta}-methylstyrene, via bromination, conversion into the azirine, fluorination and resolution as the tartrate salts. (R)- and (S)-{beta},{beta}-difluoroamphetamine (3) were then each labelled with carbon-11 (t{sub 1/2}=20.4 min) by N-methylation of the corresponding homochiral {beta},{beta}-difluoroamphetamine with [{sup 11}C]methyl iodide. The labelled products were each synthesised, purified and formulated in 35 min, starting from [{sup 11}C]carbon dioxide in 15-16% decay-corrected radiochemical yield, with a radiochemical purity of >99% and specific radioactivity of 50-150 GBq {mu}mol{sup -1} at end of synthesis.
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Capture of an elusive nitrile ylide as an intermediate in isoxazole-oxazole photoisomerization.
Nunes, Cláudio M; Reva, Igor; Fausto, Rui
2013-11-01
The unimolecular photochemistry of 3,5-dimethylisoxazole (1) induced by a narrow-band tunable UV laser was studied using low-temperature matrix isolation coupled with infrared spectroscopy. Monomers of 1 were isolated in argon matrices at 15 K and characterized spectroscopically. Irradiation of matrix-isolated 1 at λ = 222 nm (near its absorption maximum) led to the corresponding 2H-azirine 3 and ketenimine 6 as primary photoproducts and also to nitrile ylide 4 and 2,5-dimethyloxazole (5). The photoproducts were identified (i) by comparison with infrared spectra of authentic matrix-isolated samples of 3 and 5 and (ii) using additional irradiations at longer wavelengths (where 1 does not react) which induce selective photoisomerizations of 4 and 6. In particular, irradiation with λ = 340 nm led to the unequivocal identification of the nitrile ylide anti-4, which was transformed into oxazole 5. The details of the 1,5-electrocyclization of the carbonyl nitrile ylide 4 and its structural nature (propargyl-like versus allene-like geometry) were also characterized using theoretical calculations. Thus, the elusive carbonyl nitrile ylide 4 was captured and characterized for the first time as an intermediate in the isoxazole-oxazole photoisomerization.
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Flash Vacuum Pyrolysis of Azides, Triazoles, and Tetrazoles.
Wentrup, Curt
2017-03-08
Flash vacuum pyrolysis (FVP) of azides is an extremely valuable method of generating nitrenes and studying their thermal rearrangements. The nitrenes can in many cases be isolated in low-temperature matrices and observed spectroscopically. NH and methyl, alkyl, aralkyl, vinyl, cyano, aryl and N-heteroaryl, acyl, carbamoyl, alkoxycarbonyl, imidoyl, boryl, silyl, phosphonyl, and sulfonyl nitrenes are included. FVP of triazoloazines generates diazomethylazines and azinylcarbenes, which often rearrange to the energetically more stable arylnitrenes. N 2 elimination from monocyclic 1,2,3-triazoles can generate iminocarbenes, 1H-azirines, ketenimines, and cyclization products, and 1,2,4-triazoles are precursors of nitrile ylides. Benzotriazoles are preparatively useful precursors of cyanocyclopentadienes, carbazoles, and aza-analogues. FVP of 5-aryltetrazoles can result in double N 2 elimination with formation of arylcarbenes or of heteroarylcarbenes, which again rearrange to arylnitrenes. Many 5-substituted and 2,5-disubstituted tetrazoles are excellent precursors of nitrile imines (propargylic, allenic, or carbenic), which are isolable at low temperatures in some cases (e.g., aryl- and silylnitrile imines) or rearrange to carbodiimides. 1,5-Disubstituted tetrazoles are precursors of imidoylnitrenes, which also rearrange to carbodiimides or add intramolecularly to aryl substituents to yield indazoles and related compounds. Where relevant for the mechanistic understanding, pyrolysis under flow conditions or in solution or the solid state will be mentioned. Results of photolysis reactions and computational chemistry complementing the FVP results will also be mentioned in several places.
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Linear ketenimines. Variable structures of C,C-dicyanoketenimines and C,C-bis-sulfonylketenimines.
Finnerty, Justin; Mitschke, Ullrich; Wentrup, Curt
2002-02-22
C,C-dicyanoketenimines 10a-c were generated by flash vacuum thermolysis of ketene N,S-acetals 9a-c or by thermal or photochemical decomposition of alpha-azido-beta-cyanocinnamonitrile 11. In the latter reaction, 3,3-dicyano-2-phenyl-1-azirine 12 is also formed. IR spectroscopy of the keteniminines isolated in Ar matrixes or as neat films, NMR spectroscopy of 10c, and theoretical calculations (B3LYP/6-31G) demonstrate that these ketenimines have variable geometry, being essentially linear along the CCN-R framework in polar media (neat films and solution), but in the gas phase or Ar matrix they are bent, as is usual for ketenimines. Experiments and calculations agree that a single CN substituent as in 13 is not enough to enforce linearity, and sulfonyl groups are less effective that cyano groups in causing linearity. C,C-bis(methylsulfonyl)ketenimines 4-5 and a C-cyano-C-(methylsulfonyl)ketenimine 15 are not linear. The compound p-O2NC6H4N=C=C(COOMe)2 previously reported in the literature is probably somewhat linearized along the CCNR moiety. A computational survey (B3LYP/6-31G) of the inversion barrier at nitrogen indicates that electronegative C-substituents dramatically lower the barrier; this is also true of N-acyl substituents. Increasing polarity causes lower barriers. Although N-alkylbis(methylsulfonyl)ketenimines are not calculated to be linear, the barriers are so low that crystal lattice forces can induce planarity in N-methylbis(methylsulfonyl)ketenimine 3.
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Dehydrophenylnitrenes: matrix isolation and photochemical rearrangements.
Sander, Wolfram; Winkler, Michael; Cakir, Bayram; Grote, Dirk; Bettinger, Holger F
2007-02-02
The photochemistry of 3-iodo-2,4,5,6-tetrafluorophenyl azide 8 and 3,5-diiodo-2,4,6-trifluorophenyl azide 9 was studied by IR and EPR spectroscopy in cryogenic argon and neon matrices. Both compounds form the corresponding nitrenes as primary photoproducts in photostationary equilibria with their azirine and ketenimine isomers. In contrast to fluorinated phenylnitrenes, ring-opened products are obtained upon short-wavelength irradiation of the iodine-containing systems, indicative of C-I bond cleavage in the nitrenes or didehydroazepines under these conditions. Neither 3-dehydrophenylnitrene 6 nor 3,5-didehydrophenylnitrene 7 could be detected directly. The structures of the acyclic photoproducts were identified by extensive comparison with DFT calculated spectra. Mechanistic aspects of the rearrangements leading to the observed products and the electronic properties of the title intermediates are discussed on the basis of DFT as well as high-level ab initio calculations. The computations indicate strong through-bond coupling of the exocyclic orbital in the meta position with the singly occupied in-plane nitrene orbital in the monoradical nitrenes. In contrast to the ortho or para isomers, this interaction results in low-spin ground states for meta nitrene radicals and a weakening of the C1-C2 bond causing the kinetic instability of these species even under low-temperature conditions. 3,5-Didehydrophenylnitrenes, on the other hand, in which a strong C3-C5 interaction reduces coupling of the radical sites with the nitrene unit, might be accessible synthetic targets if the intermediate formation of labile monoradicals could be circumvented.
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Aziridines in the synthesis of {sup 11}C- and {sup 18}F-labelled compounds
Energy Technology Data Exchange (ETDEWEB)
Gillings, N.M
1998-07-01
Racemic [4-{sup 11}C]aspartic acid, [4-{sup 11}C]asparagine and 2,4-diamino[4-{sup 11}C]butyric acid were synthesised by the ring-opening of an N-activated aziridine-2-carboxylate with [{sup 11}C]cyanide, followed by preparative HPLC and hydrolysis/reduction. These labelled amino acids arise from nucleophilic attack at the {beta}-carbon of the aziridine ring. A radioactive by-product of ca. 25% was attributed to the product of {alpha}-attack. Several N-activated 2-aryl aziridines were synthesised for the attempted synthesis of {beta}-[{sup 18}F] fluorophenylalanine and {beta}-[{sup 18}F]fluorodopa. Ring-opening with [{sup 18}F]fluoride showed no evidence of {beta}-fluorinated products and it is proposed that attack occurs exclusively at the {alpha}-carbon, giving the corresponding {alpha}-[{sup 18}F]fluoro-{beta}-amino acids. Further evidence for this was the reaction of the {beta}-unsubstituted N-activated aziridine-2-carboxylate with [{sup 18}F]fluoride. This reaction was totally regiospecific and afforded exclusively the {alpha}-substituted product, {alpha}-[{sup 18}F]fluoro-{beta}-alanine. Aziridine precursors were resolved by chiral HPLC. On labelling the chiral aziridines, however, racemic {sup 11}C- and {sup 18}F-labelled amino acids were obtained. This was attributed to racemisation of the initially formed ring-opened products. The use of [{sup 11}C]methyl lithium as a nucleophile for aziridine ring-opening was investigated. Reaction was expected to occur at low temperature, thus potentially avoiding racemisation. No products corresponding to aziridine ring-opening with [{sup 11}C]methyl lithium were, however, observed. A difluorinated analogue of amphetamine was synthesised by fluorination of an azirine (via an aziridine). This racemic compound was resolved as its chiral tartarate salts and subsequently labelled by methylation with [{sup 11}C]methyl iodide, giving the novel compound {beta}, {beta}-difluoro[N-methyl-{sup 11}C]methamphetamine in high
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Aziridines in the synthesis of 11C- and 18F-labelled compounds
International Nuclear Information System (INIS)
Gillings, N.M.
1998-01-01
Racemic [4- 11 C]aspartic acid, [4- 11 C]asparagine and 2,4-diamino[4- 11 C]butyric acid were synthesised by the ring-opening of an N-activated aziridine-2-carboxylate with 11 C]cyanide, followed by preparative HPLC and hydrolysis/reduction. These labelled amino acids arise from nucleophilic attack at the β-carbon of the aziridine ring. A radioactive by-product of ca. 25% was attributed to the product of α-attack. Several N-activated 2-aryl aziridines were synthesised for the attempted synthesis of β-[ 18 F] fluorophenylalanine and β-[ 18 F]fluorodopa. Ring-opening with [ 18 F]fluoride showed no evidence of β-fluorinated products and it is proposed that attack occurs exclusively at the α-carbon, giving the corresponding α-[ 18 F]fluoro-β-amino acids. Further evidence for this was the reaction of the β-unsubstituted N-activated aziridine-2-carboxylate with [ 18 F]fluoride. This reaction was totally regiospecific and afforded exclusively the α-substituted product, α-[ 18 F]fluoro-β-alanine. Aziridine precursors were resolved by chiral HPLC. On labelling the chiral aziridines, however, racemic 11 C- and 18 F-labelled amino acids were obtained. This was attributed to racemisation of the initially formed ring-opened products. The use of [ 11 C]methyl lithium as a nucleophile for aziridine ring-opening was investigated. Reaction was expected to occur at low temperature, thus potentially avoiding racemisation. No products corresponding to aziridine ring-opening with [ 11 C]methyl lithium were, however, observed. A difluorinated analogue of amphetamine was synthesised by fluorination of an azirine (via an aziridine). This racemic compound was resolved as its chiral tartarate salts and subsequently labelled by methylation with [ 11 C]methyl iodide, giving the novel compound β, β-difluoro[N-methyl- 11 C]methamphetamine in high specific activity for in vivo binding studies using positron emission tomography. The non-radioactive reference compound was also
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Gritsan, N P; Gudmundsdóttir, A D; Tigelaar, D; Zhu, Z; Karney, W L; Hadad, C M; Platz, M S
2001-03-07
Laser flash photolysis (LFP, Nd:YAG laser, 35 ps, 266 nm, 10 mJ or KrF excimer laser, 10 ns, 249 nm, 50 mJ) of 2-fluoro, 4-fluoro, 3,5-difluoro, 2,6-difluoro, and 2,3,4,5,6-pentafluorophenyl azides produces the corresponding singlet nitrenes. The singlet nitrenes were detected by transient absorption spectroscopy, and their spectra are characterized by sharp absorption bands with maxima in the range of 300-365 nm. The kinetics of their decay were analyzed as a function of temperature to yield observed decay rate constants, k(OBS). The observed rate constant in inert solvents is the sum of k(R) + k(ISC) where k(R) is the absolute rate constant of rearrangement of singlet nitrene to an azirine and k(ISC) is the absolute rate constant of nitrene intersystem crossing (ISC). Values of k(R) and k(ISC) were deduced after assuming that k(ISC) is independent of temperature. Barriers to cyclization of 4-fluoro-, 3,5-difluoro-, 2-fluoro-, 2,6-difluoro-, and 2,3,4,5,6-pentafluorophenylnitrene in inert solvents are 5.3 +/- 0.3, 5.5 +/- 0.3, 6.7 +/- 0.3, 8.0 +/- 1.5, and 8.8 +/- 0.4 kcal/mol, respectively. The barrier to cyclization of parent singlet phenylnitrene is 5.6 +/- 0.3 kcal/mol. All of these values are in good quantitative agreement with CASPT2 calculations of the relative barrier heights for the conversion of fluoro-substituted singlet aryl nitrenes to benzazirines (Karney, W. L. and Borden, W. T. J. Am. Chem. Soc. 1997, 119, 3347). A single ortho-fluorine substituent exerts a small but significant bystander effect on remote cyclization that is not steric in origin. The influence of two ortho-fluorine substituents on the cyclization is pronounced. In the case of the singlet 2-fluorophenylnitrene system, evidence is presented that the benzazirine is an intermediate and that the corresponding singlet nitrene and benzazirine interconvert. Ab initio calculations at different levels of theory on a series of benzazirines, their isomeric ketenimines, and the transition