Cyclisations of ortho-substituted N-arylbenzimidoyl nitrenes. Part 1. Cyclisations with ortho-alkyl substituents: skeletal rearrangements and [1,9]alkyl migrations

Abstract

Photolysis of the N-(N-arylbenzimidoyl)sulphimides (2a)–(2c) and (2e), in which the ortho-positions of the N-aryl group carry alkyl substituents, gives the cyclopenta[d]pyrimidines (8), together with the carbodi-imides (6). Photolysis of the corresponding tetrazoles (3) gives the same products. The cyclopenta[d]pyrimidines and carbodi-imides are also produced when the tetrazoles (3), and the oxadiazolone (5), are subjected to flash vacuum pyrolysis. It is proposed that the cyclopenta[d]pyrimidines are formed by cyclisation of N-arylbenzimidoyl nitrene intermediates to give 3aH-benzimidazoles, which then undergo skeletal rearrangement by successive [1,5]-vinyl and -imidoyl shifts. 1-(2-Methyl-1-naphthyl)-5-phenyltetrazole (4) undergoes an analogous reaction when photolysed, giving the pyrimidine (13).

In the flash pyrolysis of the tetrazoles (3) and the oxadiazolone (5), benzimidazoles are also isolated in low yields. These products can also be accounted for by postulating the intermediacy of 3aH -benzimidazoles, which subsequently undergo [1,9] methyl shifts, or, with tetrazole (3c), elimination of formaldehyde.