Radical C─H-Aroylation of Allenes via Cooperative Photoredox and N-Heterocyclic Carbene Catalysis

This study demonstrates the use of cooperative photoredox and N-heterocyclic carbene (NHC) catalysis for sp² C─H acylation of allenes. The cascade comprises oxidative generation of an allene radical cation from an allene, its nucleophilic trapping to the corresponding allyl radical and highly regioselective cross coupling by a concomitantly reductively generated NHC-derived ketyl type radical. Ionic fragmentation of both the NHC and nucleophile ultimately yields the desired substituted allene. The organic photocatalyst, 4CzIPN, is highly effective in promoting both oxidative and reductive electron transfer steps. Tri- and tetra-substituted allenes can be obtained in good yields through such a cascade. Mechanistic studies-including radical trapping, acylazolium reactions, and Stern-Volmer quenching-support the proposed mechanism. Moreover, follow-up chemistry is conducted to demonstrate the synthetic value of the cascade products.