Photochemical reactions of azo compounds. III. Photochemical cyclodehydrogenations of substituted azobenzenes

Abstract

2,4,6-Trimethylazobenzene has been shown to undergo photochemical cyclodehydrogenation in sulphuric acid to give 2,4-dimethylbenzo[c]cinnoline and 1,2,4-trimethylbenzo[c]cinnoline. The formation of the former involves cyclization with the loss of a methyl substituent : the formation of the latter involves cyclization with the migration of a methyl substituent. The reaction involves a photochemical disproportionation: 4-(4-aminophenyl)-2,4,6-trimethylcyclohexa-2,5-dienone was also isolated, having been formed by acid-catalysed rearrangement (and hydrolysis) from 2,4,6-trimethylhydrazobenzene. Substituted azobenzenes having a carboxy, iodo, or chloro substituent in the 4-, 3-, or 2-position have also been irradiated and the resulting benzo[c]cinnolines and benzidines isolated and characterized. Irradiation of 2-substituted azobenzenes has been found to give the parent benzo[c]cinnoline as well as the 4-substituted benzo[c]-cinnoline, showing that carboxy-, iodo-, and chloro-substituents can be ejected as well as methyl substituents. The irradiation of azobenzene-3-carboxylic acid has given 1-hydroxybenzo[c]cinnoline-10-carboxylic acid lactone in addition to benzo[c]-cinnoline-3-carboxylic acid.