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

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.