Electrocyclization Reactions of Benzannulated 2-Azaheptatrienyl- and 4-Azanonatetraenyllithium Compounds: Synthesis of N-Acylated 2,3-Dihydro-1H-benzo[c]azepines and a (5,6-Dihydrobenzocycloocten-5-yl)amine

Abstract

Deprotonation of 2-alkenylbenzaldehyde N-benzyl­imines by lithium diisopropylamide and subsequent addition of pivaloyl chloride or methyl chloroformate yields 2-acylated 2,3-dihydro-1H-benzo[c]azepines. This reaction is interpreted as a cascade involving a 1,7-electrocyclization reaction and a subsequent 1,5-hydrogen shift. In contrast, 2-prop-1-enylbenzaldehyde N-allylimine leads to an N-acylated (5,6-dihydrobenzocycloocten-5-yl)amine upon deprotonation and addition of pivaloyl chloride. This transformation is interpreted as a reaction sequence consisting of a 1,7-electrocyclization, the addition of the electrophile, another deprotonation and a [2,3]-aza-Wittig rearrangement, which proceeds in two steps according to quantum chemical calculations.