Diastereocontrol in Hydroxyl-Directed Hydrogenation: Deciphering the Size-Selectivity Relationship of Supported Pd Nanoparticles

Stereoselective hydrogenation provides a concise way for the synthesis of compounds with multiple stereocenters, but stereoselectivity control is particularly challenging on the surface of bare supported metal nanoparticles (NPs). In this work, a series of supported Pd NPs were synthesized and used for the diastereoselective hydrogenation of (−)-terpinen-4-ol. It is found that diastereoselectivity is strongly related to the size of Pd: Pd NPs with a big size favor hydrogenation from the hydroxyl face, and a diastereomeric ratio (dr) of 95:5 was obtained with a Pd size of 27.9 nm. Kinetic and calculation results suggest that the highly coordinated Pd sites assist in stabilizing the transition state when the substrate is adsorbed from the hydroxyl face. Thus, the observed diastereoselectivity could be related to the different ratios of highly coordinated Pd sites over Pd NPs of different sizes. We hope the insights obtained in this work could provide fundamental principles for dealing with structure sensitive reactions.