Mechanistic study of C–H bond activation by O2 on negatively charged Au clusters: α,β-dehydrogenation of 1-methyl-4-piperidone by supported Au catalysts

Abstract

Au nanoparticles supported on the manganese oxide octahedral molecular sieve OMS-2 can efficiently catalyze α,β-dehydrogenation of β-N-substituted saturated ketones using O₂ 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 α,β-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 O₂ plays an important role in the α,β-dehydrogenation over Au nanoparticles. Thus, we attempted to clarify the mechanism for the α,β-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^α and C–H^β bonds in that order. An O₂ 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^α atom in the 1-methyl-4-piperidone substrate. This indirect H^α abstraction by the activated O₂ was energetically more favorable than direct H^α abstraction by the Au cluster. The subsequent H^β abstraction was found to be promoted by adsorbed oxygen species (i.e., HOO, OH, and O) formed after the H^α abstraction. The reaction mechanism proposed in this study provides general insight into the aerobic C–H bond activation by supported Au catalysts.