Origin of ligand acceleration in heterogeneous ethyl pyruvate hydrogenation

Ethyl pyruvate (EP) was hydrogenated in the presence and absence of cinchonidine (CD) over a broad concentration range of EP (0.01–2.0 mol L−1) using a commercial 5% Pt/Al2O3 catalyst (Strem). Ligand acceleration (LA) could be observed at a higher concentration (0.3–2.0 mol L−1), whereas at lower concentration, both enantioselective and racemic reactions have very similar reaction rates. Regardless of LA, a constant (84±2%84±2%) enantiomeric excess (ee) could be obtained. As CD concentration was increased, the ee increased from 0 to 80%, while the hydrogenation rate remained constant at a low EP concentration (0.01 mol L−1). Continuous fixed-bed reactor experiments revealed that LA can be linked to catalyst deactivation. In particular, EP causes catalyst deactivation, which is faster at higher reactant concentrations. CD restores catalyst activity and suppresses catalyst deactivation, resulting in higher reaction rates compared with racemic reactions carried out without CD.