On the reaction of N-vinyliminophosphoranes. Part 7. A short new 1-aza-azulene synthesis

Abstract

Thermal reaction of triphenyl(vinylimino)phosphorane derivatives with several tropones have been studied to provide a short new route to the 1-aza-azulene ring system. (1-Phenylvinylimino)-, vinylimino-, and a mixture of (1-butylvinylimino)- and (1-methylpent-1-enylimino)-triphenylphosphorane were treated with tropone and alkylated tropones in an enamine alkylation process, and the subsequent aza-Wittig reaction resulted in the formation of 1,8-dihydrocyclohepta [b] pyrroles. The pyrrole derivatives were easily dehydrogenated by nickel peroxide or manganese dioxide to give 1-azaazulene derivatives. In a similar fashion, the iminophosphoranes were treated with 2-bromotropone or with 2-chlorotropone in the presence of triethylamine to give 1-aza-azulenes in a single step. The 1-azaazulenes, which were halogenated on the seven-membered ring, were also prepared conveniently by the reaction of triphenyl(1-phenylvinylimino)phosphorane with 2,7-dibromo-, 2,4,7-tribromo-, and 2,3,5,7-tetrachlorotropone, albeit in modest yields.