Functionalization of Gold-Pincer Nanocluster for Asymmetric Catalysis.

Organic ligand-protected metal nanoclusters feature ultrasmall size, well-defined compositions, and diverse chiral structures. They have the potential to combine the advantages of asymmetric organocatalysis and nanometal catalysis. The major challenge is designing and synthesizing appropriate metal nanocluster structures for achieving high catalytic activity and excellent enantioselectivity. In this work, based on our developed metal-pincer nanoclusters and their unique di-coordination mode, we explored a functionalization method that allows flexible regulation of the spatial match/mismatch between the nanometal catalytic sites and the chiral organocatalytic sites of metal nanoclusters. A proline-functionalized Au nanocluster was constructed as the dominant catalyst. It enables asymmetric Michael addition of aldehydes and nitroalkenes with high TON and enantio-/diastereoselectivity, affording γ-nitroaldehydes that can be further transformed into important organic molecules. Mechanism studies revealed the catalytic synergy of the metal kernel and surface ligand, endowing the metal-pincer nanocluster catalyst with significantly higher efficiency than the individual proline and nanometal catalysts.