Photon-driven radical hydro-phosphoniumylation of unactivated olefins

Phosphonium salts are widely applied in organic synthesis, catalysis, materials science, and medicinal chemistry. Hydro-phosphoniumylation of alkene is one of the most powerful and straightforward methodologies for phosphonium salt synthesis. However, the established phospha-Michael reaction is limited to electronically activated olefins, and unactivated alkenes are not reactive. Herein, we report a photocatalytic and redox-neutral protocol for the efficient addition of phosphines and CF₃COOH to various unactivated olefins, which would be thermodynamically unfavorable under thermochemical conditions. The reaction commences with the generation of a phosphine radical cation (PRC) through the single-electron oxidation of phosphine by an excited photocatalyst. PRC adds to alkene in a kinetically barrierless manner. The method exhibits a broad substrate scope for both phosphines and alkenes. β-Deuterated phosphonium salts, whose synthesis is difficult by other methods, could also be accessed by this reaction with CF₃COOD/D₂O. Mechanistic and density functional theory (DFT) studies support a radical addition mechanism for P–C bond formation.