Chiral Recognition on Catalytic Surfaces: Theoretical Insight in a Biomimetic Heterogeneous Catalytic System

Abstract

Docking of molecules to surface chiral sites generated by adsorbed chiral organic modifiers is a fundamental step in heterogeneous metal-catalyzed asymmetric hydrogenation. The understanding of such docking events is nonetheless limited by the technical difficulties in obtaining experimental submolecular information for complex adsorbed structures. In addition an accurate theoretical analysis is computationally very demanding due to the need of including the metal surface. The present investigation explores by means of density functional theory the supramolecular docking structures of ketopantolactone within the chiral sites formed by cinchonidine adsorbed on a platinum surface, a crucial step in the enantioselective hydrogenation of ketopantolactone over cinchonidine-modified platinum. The study is performed using periodic slabs exposing Pt(111) surfaces of (6 × 6) and (8 × 8) atoms with a depth of four layers and including complete relaxation of the first three. Twenty six docking sites are investigated thus covering the most complete configurational space until now. The implications of the physi- and chemisorption of ketopantolactone in the docking have been also explored, thus revealing the role of weak and strong adsorption of the substrate in the formation of precursor states for hydrogen uptake within a docking site. The calculations on the (8 × 8) periodic slab were performed in order to decouple the energy related to the adsorption of the substrate from the energy involved in the docking interactions with the adsorbed alkaloid. Such a surface is in fact apt to accommodate both the chiral surface site and the noninteracting substrate. The study revealed that the substrate likely approaches the chiral site either from solution or from the physisorbed state. The resulting scenario gives a basic understanding of the elements involved in the docking of substrates within the chiral sites formed by cinchona alkaloids on platinum, and constitutes a fundamental stage for the comprehensive clarification of enantioselectivity at chirally modified surfaces.