AVS 50th International Symposium
    Surface Science Wednesday Sessions
       Session SS-WeP

Paper SS-WeP29
Direct Observation of Key Interactions Between the Chiral Modifier, Substrate Molecule and Pt(111) Surface in the Enantioselective Hydrogenation of Methyl Pyruvate

Wednesday, November 5, 2003, 11:00 am, Room Hall A-C

Session: Poster Session
Presenter: P.H. McBreen, Universite Laval, Canada
Authors: S. Lavoie, Universite Laval, Canada
M.-A. Laliberte, Universite Laval, Canada
P.H. McBreen, Universite Laval, Canada
Correspondent: Click to Email
Methyl and ethyl pyruvate are substrates in one of the rare examples of efficient enantioselective heterogeneous catalysis. The asymmetric hydrogenation of these alpha-ketoesters to the corresponding lactates on chirally-modified supported platinum catalysts is known as the Orito reaction. The daunting complexity of this reaction may be appreciated by considering the fact that interactions between the chiral modifier and the metal surface, between the pro-chiral substrate and the metal surface, between the modifier and the pro-chiral substrate, and between the pro-chiral substrate and adsorbed hydrogen, must all be taken into account. A similar list can be made for interactions involving the half-hydrogenated ketoester-and all this ignores the role of the solvent. It then seems improbable that the uhv surface science approach can be used to profitably attack such a complex system. However, we will show that a uhv RAIRS study of methyl pyruvate on chirally modified Pt(111) reveals key information of relevance to any discussion of the reaction mechanism of the Orito reaction. In particular the RAIRS study permits the direct observation of hydrogen bonding between methyl pyruvate and the modifier. Furthermore, it shows how the interaction modifies the geometry and thermal stability of the adsorbed pyruvate. The results also reveal subtle effects due to spectator species, due to the modifier coverage and due to the adsorption conformation of methyl pyruvate. The combined new information on the 1:1 modifier-pyruvate interaction and on the pyruvate adsorption geometry suggests an important revision to the most widely accepted mechanism for the Orito reaction. In addition, data taken in a comparative study of methyl pyruvate on Ni(111) and Pt(111)-systems which display very different surface chemistries-will be used to comment on possible reasons why nickel is inactive for the Orito reaction.