Visible-Light Unlocked Carbene Insertion and Radical Release in a Structurally Constrained Pincer Phosphorus Compound.

Geometrically constrained phosphines have attracted significant attention for their ability to mediate reactions traditionally associated with transition metals. Here, we show that visible-light excitation unlocks new transition-metal-like reactivity in a well-studied ONO-pincer phosphine. Photochemical generation of α-siloxy carbenes enables rapid P─C bond formation, affording bicyclic phosphines with complete diastereocontrol. These phosphine intermediates readily engage with electrophiles to form air- and moisture-stable phosphoranes, which, upon subsequent irradiation, undergo selective P─C bond homolysis, releasing carbon-centered radicals while regenerating the initial phosphine framework. Time-resolved EPR spectroscopy and DFT calculations reveal that geometric constraint is crucial for accessing both carbene-insertion rearrangement and P─C bond homolysis pathways not observed with conventional phosphines. Together, these findings establish a rare light-driven phosphine → phosphorane →phosphine (P(III) → P(V) → P(III)) reactivity loop and demonstrate how structural constraint enables main-group centers to perform elementary steps analogous to transition metals.