Chelating Base Effects in Palladium-Mediated Activation of Molecular Oxygen

Abstract

We elucidate here the mechanism for the reaction of molecular oxygen with palladium-hydride complexes in toluene using quantum mechanics (B3LYP/LACVP** with the PBF polarizable continuum solvent model) for ((−)sparteine)-PdII(H)OAc. Here we focus specifically on two classes of pathways: (1) those proceeding through Pd0 and (2) those involving direct insertion of oxygen into the Pd–H bond. In particular, we present calculated potential energy surfaces and structures of the ((−)-sparteine)PdH system in which the OAc ion is substituted for Cl. We find that the acetate ligand has the ability to act as a base while chelating the Pd, making an external base unnecessary and significantly lowering the energy barrier involved in the Pd^0 pathway. This switches the calculated preference to the reductive elimination pathway proceeding through Pd^0 (ΔΔH^(‡) = 1.0 kcal/mol, ΔΔG^(‡) = −7.7 kcal/mol). The results presented herein demonstrate the ability to steer the reaction pathway through the choice of ancillary ligands. We expect that this strategy might contribute significantly to the development of new catalytic systems utilizing molecular oxygen as the stoichiometric oxidant.

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