Mechanistic study of C-H bond activation by O-2 on negatively charged Au clusters : alpha,beta-dehydrogenation of 1-methyl-4-piperidone by supported Au catalysts dagger

Au nanoparticles supported on the manganese oxide octahedral molecular sieve OMS-2 can efficiently catalyze alpha,beta-dehydrogenation of beta-N-substituted saturated ketones using O-2 as the terminal oxidant. However, despite the utility of this reaction, the active sites and the reaction mechanism remain unclear. Here, the reaction mechanism for the Au/OMS-2-catalyzed aerobic alpha,beta-dehydrogenation of 1-methyl-4-piperidone was investigated mainly by using density functional theory (DFT) calculations. From control experiments under various reaction conditions, we found that O-2 plays an important role in the alpha,beta-dehydrogenation over Au nanoparticles. Thus, we attempted to clarify the mechanism for the alpha,beta-dehydrogenation of 1-methyl-4-piperidone on Au nanoparticle catalysts by DFT calculations using Au cluster models. The reaction was found to cleave the C-H-alpha and C-H-beta bonds in that order. An O-2 molecule adsorbed on the negatively charged Au cluster caused by charge transfer from OMS-2 was found to be sufficiently activated to abstract the H-alpha atom in the 1-methyl-4-piperidone substrate. This indirect H-alpha abstraction by the activated O-2 was energetically more favorable than direct H-alpha abstraction by the Au cluster. The subsequent H-beta abstraction was found to be promoted by adsorbed oxygen species (i.e., HOO, OH, and O) formed after the H-alpha abstraction. The reaction mechanism proposed in this study provides general insight into the aerobic C-H bond activation by supported Au catalysts