Beyond Wittig: Phosphonium Ylides and Salts as Catalysts and Precursors in Radical Chemistry

Phosphonium ylides are classically recognized as nucleophilic reagents for carbonyl olefination. However, recent studies have revealed that their reactivity extends well beyond conventional two‐electron processes. They serve as catalysts in radical reactions and as sources of carbon‐centered radicals under thermal or photochemical activation. We give particular attention to the mechanistic distinctions among direct P─C bond homolysis, photoinduced single‐electron transfer (SET), and ylide radical‐cation‐mediated hydrogen‐atom transfer (HAT). Structurally related phosphonium salts are discussed separately as complementary radical precursors that undergo single‐electron activation and selective P─C bond fragmentation to generate carbon‐ or phosphorus‐centered radicals. The review also evaluates the experimental evidence used to support these mechanisms and identifies current limitations in substrate scope and catalytic efficiency. Future opportunities in catalyst design, asymmetric synthesis, electrochemical activation, and flow photochemistry are also outlined. These developments broaden the established reactivity profile of organophosphorus compounds and provide a foundation for more general and predictive single‐electron transformations.