Direct Synthesis of in-Situ Chirally Modified Palladium Nanocrystals without Capping Agents and Their Application in Heterogeneous Enantioselective Hydrogenations

Shape-controlled metal nanocrystals possess the advantages of specific atomic arrangement on the surface and uniform size, which can act as ideal model catalysts for heterogeneous enantioselective catalytic studies. Nevertheless, capping agents are commonly retained on the surface of the as-synthesized metal nanocrystals, which may hinder the necessary interaction between the substrate and the chiral modifiers coadsorbed on the metal surface for chiral recognition. In this paper, we directly synthesized dendritic or cubic palladium nanocrystals in-situ modified by chiral modifiers such as cinchonidine (CD) or S-proline through a one-pot strategy by replacing the conventional capping agents with the chiral modifiers. The as-prepared CD-modified Pd nanodendrite catalyst already exhibits enantioselectivity in the asymmetric hydrogenation of (E)-α-phenylcinnamic acid without subsequent chiral modification processes. The in-situ S-proline-modified Pd nanocube catalyst shows higher enantioselectivity than nanocubes synthesized with polyvinylpyrrolidone (PVP) or cetyltrimethylammonium bromide (CTAB) as traditional capping agents in asymmetric hydrogenation of acetophenone. Chiral modifiers have dual functions (both shape-control agents and catalytic functional molecules), which can not only eliminate the adverse effect of traditional capping agents (such as PVP) on heterogeneous enantioselective hydrogenations but also simplify the follow-up chiral modification step of metal catalysts, providing a simple and efficient synthetic protocol to directly prepare in-situ chirally modified metal nanocrystal catalysts.