UC Riverside

The synthesis of enantiopure compounds in pharmaceuticals is a critical area given the various enantiomers of many drugs behave differently in the human body. One promising way to introduce enantioselectivity into heterogeneous catalysis is to add a small quantity of chiral modifiers in reaction mixtures. However, a gap in knowledge persists concerning the molecular-level understanding of the interaction between these chiral modifiers and reactants on solid catalyst surfaces. This research utilizes an in-situ infrared (IR) spectroscopy to monitor the adsorption behavior of chiral modifiers on solid surfaces from liquid solutions, drawing the correlations between adsorption mode and enantioselectivity in asymmetric hydrogenations.Adsorption behavior of 1-(1-naphthyl) ethylamine (1-NEA) from solution on Pt surfaces was studied via kinetic catalytic measurements, IR spectroscopy, and theoretical calculations. The research revealed that 1-NEA can undergo H−D exchange with D2 in solutions, mediated by Pt-supported catalysts. This reaction was general, observed with r- and s-1-NEA, r- and s-2-NEA, and cinchonine, on both Pt/SiO2 and Pt/Al2O3 catalysts. NEA adsorption and protonation through the amine N atom provide a mechanism explaining how the NEA molecule bestows enantioselectivity to Pt hydrogenation catalysts. Adsorption studies were also conducted with different types of chiral modifiers. It was found that both the naphthyl ring and amine group are crucial for adsorption on Pt surfaces. No adsorption evidence was observed with hydroxyl group functionalized molecules and molecules with a benzene ring. The kinetic measurements were made on the hydrogenation of ethyl pyruvate, catalyzed by chiral molecules modified Pt-supported catalysts, showed that chiral NEA molecules contributed to enantioselectivity, albeit not as effectively as cinchona alkaloids. Attempts to enhance enantioselectivity through the modification of Pt-supported catalysts with silylation agents yielded no significant increase. This research provides a comprehensive study of different chiral modifiers’ adsorption behaviors on Pt-supported catalysts at solid/liquid interfaces and their catalytic performance in enantioselectivity on ethyl pyruvate hydrogenations. This not only aids in understanding the adsorption nature of chiral modifiers on Pt surfaces but also assists in the design of a more efficient heterogeneous enantioselective catalysis.