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†
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–Hα and C–Hβ 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 Hα atom in the 1-methyl-4-piperidone substrate. This indirect Hα abstraction by the activated O2 was energetically more favorable than direct Hα abstraction by the Au cluster. The subsequent Hβ abstraction was found to be promoted by adsorbed oxygen species (i.e., HOO, OH, and O) formed after the Hα abstraction. The reaction mechanism proposed in this study provides general insight into the aerobic C–H bond activation by supported Au catalysts.