Photoredox-Catalyzed Single-Carbon Insertion for De Novo Indene Synthesis from Tetrasubstituted Alkenes.

A visible-light photoredox-catalyzed single-carbon insertion of tetrasubstituted alkenes is developed using hypervalent iodine diazo reagents. The reaction proceeds via a radical-polar crossover manifold under mild conditions, enabling intramolecular arene capture of photogenerated carbocations from acyclic precursors to afford polysubstituted indenes through a cascade cyclization. This strategy exploits weakly nucleophilic arenes as internal carbocation traps, enabling efficient de novo construction of fused carbocyclic frameworks. The reaction exhibits broad functional group tolerance and provides moderate to good yields. Mechanistic studies support a light-dependent radical-cation pathway, and the synthetic utility is demonstrated by scale-up and product derivatization.