Substrate adsorption on the cinchonidine-modified Pd/C during the enantio-differentiating hydrogenation as a vital stereocontrol factor

Abstract

Two regioisomeric α-phenyl-β-pyridylacrylic acids were hydrogenated with cinchonidine (CD)-modified Pd/C. The low ee obtained was attributed to the strong adsorption of the substrate, which caused desorption of CD from the catalyst surface. The ee was improved at the low substrate/CD ratio up to 82% for the 4-pyridyl isomer. The unmodified site in the CD-modified Pd catalyst was experimentally proved to be variable depending on the relative adsorption strength between the substrate and CD. At the low substrate/CD ratio, the ee with the 3-pyridyl isomer was also improved, but the highest ee was only 45%. The difference between the isomers suggests that the strong adsorption of the substrate on the metal surface may interrupt the interaction between CD and the substrate.