Fundamental aspects of enantioselective heterogeneous catalysis: a NEXAFS study of methyl pyruvate and (S)-(−)-1-(1-naphthyl) ethylamine on Pt{1 1 1}

Abstract

Near-edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy have been used to characterise the adsorption and reactivity of methyl pyruvate and (S)-(−)-1-(1-napthyl) ethylamine ((S)-NEA) on Pt{111}. Methyl pyruvate polymerises at room temperature yielding chains that contain CO bonds and no CC bonds; the CO bonds are inclined at 64±5° with respect to the metal surface. Polymerisation probably occurs by an aldol condensation that involves elimination of methanol. This mechanism is in excellent accord with the intramolecular bonding deduced from the NEXAFS results. These findings suggest that irreversible deactivation during start-up or steady state operation of Pt catalysts during enantioselective hydrogenation of alkyl pyruvates can be due to hydrogen starvation, which results in polymerisation of the prochiral reactant. For (S)-NEA, the naphthalene ring is inclined at 46±5° with respect to the metal surface. It is proposed that (S)-NEA binds to the surface via contributions from both the aromatic π-system and the lone pair of electrons on the amine nitrogen atom.