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, beta-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 beta-hydride elimination, which is the highest point on the energy profile of the catalytic cycle, was identified.