Arylnitrenes intramolecular reactions

Whereas the production of arylnitrenes by the deoxygenation of nitrosobenzenes or nitro-benzenes by trivalent phosphorus reagents and their subsequent intramolecular ring expansion to 3//-azepines are well-known processes, the corresponding intermolecular reactions to form 1//-azepines have been exploited only on rare occasions and appear to be of little preparative value. For example, the highly electrophilic pentafluorophenylnitrene, obtained by deoxygenation of pentafluoronitrosobenzene with triethyl phosphite in benzene solution, produced a low yield (2-10%) of l-(pentafluorophenyl)-l//-azepine, which was isolated as its [4 + 2] cycloadduct with ethenetetracarbonitrile.169 With anisole as the substrate l-(pentafluorophenyl)-l//-azepin-2(3//)-one (16% bp 128 —130 C/0.4 Torr) was obtained. 

The intramolecular cyclization of the arylnitrenes derived from azido- or nitro-biphenyls to the adjacent aromatic ring has been well leviewed. - The reaction is a useful route to carbazoles, two recent examples of which are shown in Scheme IS. In Raphael s elegant proach to the indolocarbazole fam-... 

The intramolecular insertion reaction of arylnitrenes proceeds with retention of configuration at carbon. For example, heating the (S)-aryl azide (11 X = Na) in the vapor phase gives 2-ethyl-2-methylin-doline in 50- % yield in ca. 100% optical purity.The optical purity of the product is lower if the azide is heated in solution, or if the nitrene is generated from the corresponding nitro compoimd (11 X = NO2) with triethyl phosphite. ... 

Intramolecular substitution by arylnitrenes has been known for a long time and the cyclization of o-azidobiphenyls to carbazoles is representative of an important series of nitrene reactions that yield heterocycles. Only fairly recently has an intermolecular counterpart of these reactions been observed. Originally when phenyl azide was thermolyzed in benzene only azobenzene (11%), aniline (18%), and tars were obtained, and no diphenylamine was found. Abramovitch considered that phenylnitrene is not sufficiently electrophilic to attack benzene, so he decomposed a series of aryl azides (bearing strongly electron-withdrawing substituents, NO2, CN, CF3) in solvents activated... 

Intramolecular cyclization reactions of arylnitrenes by aromatic substitution (singlet), C-H aliphatic insertion, and attack on ortho nitrogen functions are usually very much more facile than their intermolecular counterparts. 

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