Nitrenes azepines from

Addition to aromatic systems is probably the primary step in a number of more complex nitrene reactions such as the formation of azepines from carbethoxynitrene and benzene and a number of... 

Much of the work on deoxygenation of aromatic nitro and nitroso compounds by tervalent phosphorus reagents has been reviewed.3 Triethyl phosphite has been used frequently, but diethyl methylphosphonite was found to be superior for the deoxygenation of nitro compounds.3,263 The isolation of azepines from such reactions (Scheme 42) indicated the initial formation of arylnitrenes. However, a marked influence of the nucleophile upon the direction of the apparent migration of the nitrene—away or toward an ortho substituent—was noted (Scheme 42).264 Evidence that the same intermediates are involved in the azepine forming reactions from phenyl azides and nitro-benzenes was obtained by a careful examination of the azepines formed from meta-substituted derivatives (Scheme 43). The ratios of the azepines 216 and... 

Pyridines are observed in some of these reactions, and the substituent pattern is that expected from nitrene-carbene interconversion Small yields of azepine have been recorded in the photolysis of 158 i ). The bicyclic intermediate 44 [Eq. (64)] has been postulated frequently over the last twenty years. A wavelength dependence of the formation of azepines has been observed i ) the yield of azepine increases with the energy of the light and with the concentration of the trapping agent (diethylamine). At the same time, the yields of competitive processes of the nitrene (carbazole formation from 2-biphenylylnitrene p-cyanophenyl-hydrazine from p-cyanophenylnitrene) decrease. The rate of the azepine-forming... 

The consensus of opinion on the formation of azepines from aryl azides suggests that an intermediate singlet nitrene is in equilibrium with a didehydroazepine, which itself may be in equilibrium with a azepinocarbene. This has received further support form the investigations of Murata and co-workers, who have isolated both the azomethine ylide 1 and the... 

In agreement with this, Berwick obtained azepines from the photolyses of o-azidoacetophenone as well as from 3-methylanthranil in piperidine. Notably, the azepines obtained from anthranils correspond to a migration of the nitrene away from the acyl substituent (see 212). In contrast,... 

On the basis of ab initio MO studies on the nitrenium ion 23 derived from 1//-azepine, it has been concluded that the singlet nitrene is a planar, fully conjugated C2v system with substantial 67r-aromatic character. 

Photolysis of aryl azides in amine solution, with a tertiary amine as cosolvent to promote stabilization of the singlet nitrene, has met with some success. For example, the yield of 2-piperidino-3 W-azepme. obtained by the photolysis of phenyl azide in piperidine, is increased from 35 to 58% in the presence of A A /V. /V -tetramethylethylenediamine (TMLDA).180 Also, an improved yield (36 to 60 %) of A,(V-diethyl-3W-azepin-2-amine (38, R = Et) can be obtained by irradiating phenyl azide in triethylamine, rather than in dicthylaminc, solution.181 Photolysis (or thermolysis) of phenyl azide in TMEDA produces, in each case, 38 (R = Et) in 40% yield.181 In contrast, irradiation of phenyl azide in aniline with trimethylamine as cosolvent furnishes jV-phenyl-377-azepin-2-amine (32, R = Ph) in only low yield (2%).35... 

Formation of l-aryl-l//-azepines is rare and occurs only with those arylnitrenes made sufficiently electrophilic by an electron-withdrawing, e.g. CN, N02 or CF3, ortho or para substituent. Even so, these docile nitrenes attack only electron-enriched arenes (e.g. mesity-lene or Af,Af-dimethylaniline) and are of minor synthetic importance (B-73MI51600). More reactive are 7r-deficient heteroarylnitrenes, and moderate yields (15-40%) of 1-heteroaryl-l//-azepines, e.g. (228 R=4,6-dimethoxy-l,3,5-triazin-2-yl), may be obtained by the photodecomposition of 2-azido-4,6-dimethoxy-l,3,5-triazine in a variety of aromatic substrates (81BCJ301). Interestingly, intramolecular insertion of arylnitrenes into arenes is more common and has been used for the synthesis of fused azepines, e.g. the azepinoindoles (229) from o-azidodiphenylmethanes (81JCS(P1)1132). 

An interesting development of this azepine synthesis is the intramolecular addition of acylnitrenes to arenes, as exemplified in Scheme 27 for the nitrene (231) generated from benzyl azidoformate by spray pyrolysis (80CC1161). 

Photogenerated nitrenes can undergo cycloaddition with alkenes intermolecular reaction leads to aziridine products (5.38), and intramolecular reaction in vinyl azides gives azirines (5.39). The bicyclic azirine from phenyl azide has not been isolated, but it is the intermediate that best accounts for the formation of a substituted azepine when this azide is irradiated in the presence of a secondary amine (5.401. 

Azepines (228) result from spontaneous valence-bond isomerization of azanorcaradienes (227) which are themselves made by reaction of arenes with nitrenes (see CHEC 5.16.4.1.3). 

The synthesis of medium-ring nitrogen heterocycles, such as azepines, azocines, and azonines, has been reviewed <1991T9131>. 1/7-Azepines 321 result from spontaneous valence-bond isomerization of azanorcaradienes 320 (Scheme 163), which are themselves made by reaction of arenes with nitrenes. Oxepins are prepared by an analogous method (323 324) the starting material is made from the dibromide 322 (Scheme 164) <1964AGE510>. 

The vapor-phase pyrolysis of phenyl azide yields azobenzene in 72 % yield . The pyrolysis in aniline solution leads to the production of dibenzamil , identified as an azepine by modern techniques in benzene and p-xylene, the major products are azobenzene and aniline, presumably originating from reactions of PhN in hydrocarbon solvents the thermolysis produces aniline, alkyl-anilines, azobenzene and polymer . It has been shown that aniline and iV-alkylanilines arise from triplet state nitrene reactions with the solvent. In the gas-phase thermolysis of phenyl azide at low pressures aniline and azobenzene are the major products whereas at higher pressures the formation of l-cyano-1,3-cyclopentadiene (or 2-anilino-7H-azepine in the presence of aniline) predominates . It was deduced that aniline and azobenzene arise from triplet nitrene reactions and that at high pressures, a hot singlet nitrene is formed, which undergoes intramolecular insertion to form an azocyclopropene intermediate, viz. 

When conventional rearrangements are not possible numerous reactions attributable to insertion of nitrenes or of abstraction of hydrogen from the environment to the same species have been reported. An early example was the attack on the solvent to give an azepine derivative (reaction 69) found when phenyl azide was... 

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