Computational Study of the Palladium-Catalyzed Tandem Transformation of Phosphinyl Allene to Benzo[ b ]phosphole Oxide: Key Roles of the Palladacycle Structure and Ancillary Ligands

Abstract

The ortho selectivity of C–H activation in the palladium-catalyzed annulation of phosphinyl allenes to form benzo[b]phosphole oxides has previously been attributed to coordinative assistance from the adjacent P═O moiety. In this study, comprehensive structural and energetic analyses reveal a different mechanistic origin. The observed ortho C–H activation is governed primarily by the preferred six-membered twisted-boat conformation of the palladacyclic transition state. The presumed P═O···Pd coordination does not exert a directing effect; instead, it distorts the favorable palladacyclic geometry and raises the transition-state energy to a prohibitive level, making its involvement in C–H activation infeasible. A closer inspection of the elementary steps in this cascade reaction shows that ancillary ligands modulate two key processes in opposite directions: they elevate the oxidative addition barrier while markedly stabilizing the C–H activation transition state. As a result, the overall rate-limiting step depends sensitively on the electron count (14e– or 16e–) of the active Pd(0) species, as well as the identity of the ancillary ligand. Collectively, these findings support a strain- and ligand-controlled mechanism rather than the previously assumed heteroatom-directed C–H activation model, providing new insights into palladium-catalyzed allene annulation and heterocycle construction.