Theoretical Studies of the Mechanism of Aerobic Alcohol Oxidation with Palladium Catalyst Systems

Density functional theory (DFT) was applied to a comprehensive mechanistic study of the Pd(II)-catalyzed oxidation of alcohols by molecular oxygen. Both parts of the catalytic cycle, i.e., the oxidative dehydrogenation of the substrate and the regeneration of the catalyst by the co-oxidant, molecular oxygen, were studied. The catalytic cycle under consideration consists of intramolecular deprotonation, β-hydride elimination, and migratory insertion steps, and it is relevant for a wide class of catalytic systems. In particular, a Pd(II) cyclometalated system was addressed and qualitatively compared with the Uemura system (Pd(OAc)2/pyridine) and with the Pd−carbene system. Geometries of the intermediate complexes and relative Gibbs free energies were identified along the proposed reaction path with the help of computational methods. The transition state for the β-hydride elimination, which is the highest point on the energy profile of the catalytic cycle, was identified.