Elucidation of Active Sites and Mechanism during Enantioselective Hydrogenation of Ethyl Pyruvate

Gary Attard

School of Chemistry, Main Building, Cardiff University, Cardiff, Wales, UK

The enantioselective hydrogenation of ethyl pyruvate using supported Pt modified by cinchona alkaloids is one of the most remarkable of all heterogeneous catalytic reactions and continues to provide a benchmark by which our understanding of surface chirality at catalyst surfaces may be measured. A key factor necessary for the optimisation of the performance of such catalysts is the identification of the active surface site giving rise to the greatest value of enantioselective excess (ee) in the final product mixture. One surface sensitive technique that has proved most fruitful in helping identify the different adsorption sites present at a supported Pt catalyst is cyclic voltammetry. Using a mixture of thermal and chemical treatments of the supported Pt catalyst, including selectively decorating the various Pt surface sites (steps or terraces) with inert or reactive adatoms, it is possible to correlate variations in catalyst activity and enantioselectivity with particular adsorption sites. Our conclusion from these electrochemical and hydrogenation studies is that edge sites (possibly chiral corner kinks) at the metal-support boundary are crucial to the achievement of optimal values of ee. Furthermore, it is demonstrated that ethyl pyruvate hydrogenation on extended Pt{100} terraces leads to a reduction in overall ee. It is suggested that in order to account for these observations, Pt{100} terrace sites present at the catalyst surface must either be giving rise to purely racemic reaction or, more likely,inversion of ee. Prospects for future catalyst design in the light of these findings will be discussed.