Photoredox-Catalyzed [3+2] annulation of Aromatic Amides with Olefins via Iminium Intermediates

Abstract

Despite the preliminary success of transition metal-catalyzed [3+2] annulation of amides with olefins, the corresponding radical-type [3+2] annulation remains a laborious challenge. Herein we report the first photoredox-catalyzed radical-type [3+2] annulation of aromatic amides with olefins. We established an approach to generate unprecedented iminium radicals by reducing the oxyiminium intermediates, formed in situ from corresponding amides with Tf2O, via photoredox catalysis. The [3+2] annulation was achieved via stepwise radical process, instead of forming linear products via other pathways as previously reported. This annulation protocol exhibits excellent functional group tolerance, and a diversity of substrates are united under the photoredox conditions, affording iminium products that can be in situ diversified into 1-indanones, enamines and amines. Mechanistic investigations indicate reduction of the oxyiminium intermediate to the iminium radicals by excited-state of the photocatalyst initiates the catalytic cycle.