Highly stereocontrolled [2+2] cycloaddition versus unprecedented imino-ene reactions of imino-ketenimines

N-[(2-Benzylideneamino)phenyl]-C-methylketenimines undergo intramolecular cyclization via two different reaction pathways, a [2+2] cycloaddition and a rare imino-ene reaction, to yield azeto[1,2-a]benzimidazoles and 2-(α-styryl)benzimidazoles, respectively. The results are interpreted in terms of a two-step mechanism involving two stereoisomeric conjugated betaines as intermediates.