Critical Size of Gold Nanocluster Catalysts for Aerobic α,β-Dehydrogenation of β-Aminoketones Arising from Their Bidentate Metalation

Recently, α,β-selective aerobic dehydrogenation of β-aminoketones, catalyzed by gold nanoparticles, has been developed as an environmentally benign method to synthesize β-enaminones, which are important building blocks for fine chemicals. A comprehensive understanding of the size-dependent catalytic activity of gold nanoparticles on the dehydrogenation of β-aminoketones is imperative to maximize the synthetic yield of β-enaminones. To address this issue, atomically precise Au25, Au144, and Au∼329 nanoclusters partially thiolated were synthesized on a carbon support and their activity in aerobic dehydrogenation was compared using 1-methyl-4-piperidone as a model substrate. The results demonstrated that Au144 and Au∼329 outperformed commercially available carbon-supported Au nanoparticle catalysts (>6 nm) and exhibited ∼9 and ∼11 times higher activity than Au25, respectively. This finding suggests that the maximum activity occurs at a size of ∼2.5 nm. The mechanistic studies revealed that the reduced activity of Au25 relative to Au144 and Au∼329 is attributable to the inhibition of bidentate adsorption of 1-methyl-4-piperidone onto a restricted exposed area on Au25.