Photocatalytic Phosphine-Mediated Deoxygenative [3 + 2] Cycloaddition of α,β-Unsaturated Carbonyls and Alkenes

Tertiary phosphine is a powerful nucleophile that enables numerous synthetically important reactions. However, the exclusive reliance on a two-electron pathway has restricted the development of phosphine chemistry. We report here a visible light photocatalytic phosphine-mediated deoxygenative [3 + 2] cycloaddition of α,β-unsaturated carbonyls and activated alkenes, providing facile access to polysubstituted cyclopentenes from readily accessible starting materials in promising yields with good substrate scope. Mechanistic investigations corroborate the transient involvement of a phosphine radical cation and consequent α-addition (α to electron-withdrawing groups) to C-C double bonds due to the inherent electrophilicity of the radical cation, as opposed to the traditional nucleophilic β-addition of phosphine. The ensuing single electron transfer reduction, Michael addition, and 1,4-proton transfer cascade furnish a phosphorus ylide, which undergoes an intramolecular Wittig reaction to finish the construction of the cyclopentenes. This work demonstrates that tertiary phosphine can serve as a convenient reagent for carbonyl deoxygenation and act as a traceless directing group for controllable cross-coupling of two distinct alkenes, providing new avenues to expand the frontiers of phosphine chemistry and synthetic chemistry.