Creating Chiral Surfaces for Enantioselective Heterogeneous Catalysis: R,R-Tartaric Acid on Cu(110)

Abstract

One of the most successful ways of inducing enantioselectivity in a heterogeneous catalytic system is by the adsorption of chiral “modifier” molecules on the reactive metal surface. However, little is known about the nature of the active sites present on the modified metal surface and how such modifiers bestow chirality to an achiral metal surface. In this paper we report the behavior of R, R -tartaric acid adsorption on a Cu(110) surface using high-resolution surface analytical techniques. R, R -Tartaric acid is known to be an extremely successful modifier molecule for the enantioselective hydrogenation of methyl acetoacetate, the simplest β-keto ester, to the R -enantiomer of the product molecule methyl 3-hydroxybutyrate. A combination of low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), and Fourier transform reflection−absorption infrared spectroscopy (FT-RAIRS) techniques has allowed us to demonstrate that a complicated adsorption phase diagram exists for this system. A rich variety of ordered overlayer structures are produced, in which preferred molecular forms, bonding and orientations of the chiral molecules are adopted, dependent on coverage, temperature and time. These different adlayers will clearly play a different role in the enantioselective reaction. Of particular interest is the fact that under certain conditions, the 2-dimensional order of the R, R -tartaric acid adlayer destroys all symmetry elements at the surface, leading to the creation of extended chiral surfaces! Such chiral surfaces may be an important factor in defining the active site in heterogeneous enantioselective reactions.