Complexities and Dynamics of the Enantioselective Active Site in Heterogeneous Catalysis

Abstract There is now renewed interest in utilising chirally modified metals as heterogeneous catalysts for enantioselective reactions. Effective systems have been produced by first modifying a metal surface with chiral molecules and, subsequently, conducting the enantioselective reaction on the modified surface. How stereocontrol is achieved by the chirally modified metal is still a matter of speculation and a lack of sensitive spectroscopic data of the modified surface has precluded the development of detailed models. In this work we report results relevant to the asymmetric hydrogenation of β-ketoesters over R,R-tartaric acid and S-alanine modified metal surfaces. Using a powerful range of surface spectroscopies on modified surfaces created under ultra-clean and controlled environments, we show that the adsorbed modifiers, R,R-tartaric acid and S-alanine, display rich and complex phase diagrams in which the chemical nature and 2-dimensional organisation of the chiral units varies considerably as a function of surface coverage and temperature. From this work it would seem that the models suggested for stereocontrol involving one-to-one interaction between the adsorbed chiral modifier and the reactant (substrate) are, perhaps, too simplistic and careful account needs to be taken of the complexities and dynamic interplay between different modifier phases at a surface in order to fully understand the nature and scope of stereocontrol.