O-2 insertion into a palladium(II)-hydride bond: Observation of mechanistic crossover between HX-reductive-elimination and hydrogen-atom-abstraction pathways

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 O-2 with a series of electronically varied Pd-II-H complexes of the type trans-(IMes)(2)Pd(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 O-2 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 O-2 (HAA). DFT computational studies support these conclusions and reveal that the preferred mechanism for the O-2 insertion reaction changes from HAA to HXRE as the para substituent on the benzoate ligand shifts from electron-donating to electron-withdrawing.