Kinetic analysis of the asymmetric hydrogenation of ß-keto esters over Α-amino acid-functionalized Pt nanoparticles

Analysis of the kinetics for the asymmetric hydrogenation of ß-keto esters over Pt nanoparticles (NPs) in the liquid-phase reveal a unique reaction pathway that is active on α-amino acid-functionalized Pt NPs but absent on “ligand-free” Pt NPs. Differences in both the apparent activation energies and the reaction orders with respect to organic reactant concentrations and the hydrogen partial pressure are interpreted through rate expressions derived from sequences of elementary steps. The hydrogenation proceeds by a classical Langmuir-Hinshelwood mechanism that sequentially adds two chemisorbed hydrogen atoms to the carbonyl group of the reactant on the surfaces of “ligand-free” Pt NPs. In contrast, the hydrogenation over ligand-functionalized Pt NPs appears to proceed by a concerted addition of two hydrogen atoms to the carbonyl group of the reactant mediated by the amino group of the α-amino acid ligand. Furthermore, the acidity and flexibility of the ligands likely influence their activation energies. Importantly, over ligand-functionalized Pt NPs no evidence for a background reaction on bare Pt ensembles was found, which implies that the origin of the enantiodifferentiation lies in two diastereomeric reaction pathways.