CO2-facilitated radical sequential (3 + 2) annulation of 1,6-enynes via cooperation of sulfinate catalysis and photocatalysis†

Reported here is the CO₂-facilitated radical sequential (3 + 2) annulation of 1,6-enynes that proceeds under the concerted catalysis of sulfinate and a photocatalyst (PC) to construct benzo-fused tricyclic scaffolds, that is tetrahydrofluorenes and their N-heterocyclic analogues, in generally good yields. This CO₂-facilitated method allows the stable aromatic groups of 1,6-enynes to undergo radical addition-induced dearomatization and rearomatization under metal-, external oxidant- and base-free conditions and features a broad scope of 1,6-enyne substrates, as demonstrated by more than 60 examples including a series of 1,6-enyne derivatives of complex bioactive compounds. Importantly, CO₂ as a green additive is disclosed to be essential for achieving this reaction with good efficiency, demonstrating a new role of CO₂ in this transformation.