O2insertion into a palladium(ii)-hydride bond: Observation of mechanistic crossover between HX-reductive-elimination and hydrogen-atom-abstraction pathways

Abstract

The reaction of molecular oxygen with palladium(II)–hydrides is a key step in Pd-catalyzed aerobic oxidation reactions, and the mechanism of such reactions has been the focus of considerable investigation and debate. Here we describe the reaction of O₂ with a series of electronically varied Pd^II–H complexes of the type trans-(IMes)₂Pd(H)(O₂CAr), with different para-substituted benzoates as the ArCO₂⁻ ligand. Analysis of the oxygenation rates of these complexes revealed a non-linear Hammett plot, and further kinetic studies demonstrated that reaction of O₂ with the most electron-rich para-methoxybenzoate derivative proceeds via two parallel mechanisms, one initiated by rate-limiting reductive elimination of the carboxylic acid (HXRE) and the other involving hydrogen-atom abstraction by O₂ (HAA). DFT computational studies support these conclusions and reveal that the preferred mechanism for the O₂insertion reaction changes from HAA to HXRE as the para substituent on the benzoate ligand shifts from electron-donating to electron-withdrawing.