Dynamic Inner-/Outer-Sphere Ligand Exchange in P(V)═O Reduction by Sacrificial Methods

Reduction of P(V)═O bonds via sacrificial methods holds broad prospects for industrial applications and mechanistic studies. Using our recently reported catalytic sacrificial system as a model, we investigated the mechanism of the oxygen atom transfer between P(V)═O substrates and sacrificial P(III) reagents through both computations and experiments. The results identified a distinctive inner-/outer-sphere ligand exchange process, wherein the phosphorus center selectively coordinates optimal inner-sphere ligands to reduce the activation barriers for two critical steps: P-O-P intermediate formation and its decomposition. This unusual mechanism explains the superior reactivity of the initiator diethyl 2-bromomalonate (DEBM). Systematic evaluation of other initiators reveals the generality of the ligand exchange, which may extend to other phosphonium-based ion-pair systems. Featuring thermodynamically driven reversible processes overall, the reaction shows diminished efficiency with trialkyl-substituted substrates. These findings provide key mechanistic insights into sacrificial P═O bond reduction and establish a foundation for rationally exploiting this ligand exchange in diverse chemical transformations.