Rhodium‐Catalyzed (4+1) Cycloaddition between Benzocyclobutenones and Styrene‐Type Alkenes

Herein, we describe a unique one-carbon ring-expansion strategy to access multi-substituted 2-indanones from benzocyclobutenones and styrene-type olefins. The use of a cationic "ligandless" rhodium catalyst was the key for both high reactivity and selectivity towards the (4+1) product. Broad functional group tolerance, a good substrate scope, and scalability have been demonstrated. Computation studies reveal that the origin of the (4+1) selectivity is due to a facile β-H elimination pathway that reduces the overall barrier of the turnover-limiting C-C reductive elimination step.