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 O2 with a series of electronically varied PdII–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.