Red. Trav. Chim. Pays-Bas 113, 465-474 (1994) SSDl 0 1 6 5 -0 5 1 3 ( 9 4 ) 0 0 0 3 8 -4 Abstract.A LEED and XPS study of the adsorption of naphthalene, quinoline, and 10,11-dihydrocinchonidine on Pt(ll1) at 300K has shown that only naphthalene forms an ordered ad-layer, and that quinoline and the alkaloid adsorb in a disordered state and without decomposition. These experiments do not support the hypothesis of ordered adsorption of alkaloid that forms the basis of the template model for the interpretation of enantioselectivity in Pt-catalysed pyruvate hydrogenation. The model is accordingly reviewed. Molecular modelling studies show that a highly specific 1 : 1 interaction between cinchonidine (or cinchonine) and pyruvate interprets the observed sense of the enantioselectivity, provided relative energy relationships derived for purely intermolecular interactions are valid for the same molecules in the adsorbed state. Moreover, the 'product' of this 1: 1 interaction is a satisfactory precursor to the H-bonded state considered responsible for the greatly enhanced rate that always accompanies enantioselective reaction over cinchona-modified Pt. The previously published dependencies of optical yield on ( a ) surface concentration of adsorbed cinchonidine modifier, and ( 6 ) modifier composition for mixtures of quinine and quinidine, are shown to be in quantitative agreement with the proposed 1 : 1 interaction model and at variance with the ordered adsorption model. Catalysts modified and used under strictly anaerobic conditions show negligible activity and enantioselectivity demonstrating that oxygen plays a crucial role in successful catalyst preparation. XPS experiments confirm that adsorption of cinchonidine from air-saturated ethanolic solution on Pt(l11) provides an adlayer containing both alkaloid and adsorbed oxygen. (S)-(-)-l-benzyl-pyrrolidine-2-rnethanol, various configurations of ephedrine, D-and L-histidine and the methyl esters of D-and L-tryptophan have been examined as modifiers for supported Pt. Although there is evidence that these compounds can provide chiral direction to pyruvate hydrogenation, rate enhancement is slight and enantioselectivity is correspondingly low.

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The interaction of 10,ll-dihydrocinchonidine (DHC) with a Pt(ll1) surface has been studied using X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and low-energy electron diffraction (LEED). From a saturated ethanolic solution DHC is chemisorbed in multilayers whereas from more dilute solutions adsorption is limited to a monolayer. When exposed to ethanol, the multilayers are removed leaving a strongly chemisorbed monolayer of DHC. A comparison of the LEED patterns of the structurally related compounds naphthalene, quinoline, and DHC shows that only naphthalene forms an ordered adlayer (3 x 3) whereas quinoline and DHC are adsorbed in a disordered state and without decomposition. The enhanced hydrogenation rate and the optical yield for the methyl pyruvate hydrogenation reaction at a cinchona alkaloidmodified EUROPT-1 catalyst, and the absence of ordered adsorption of the alkaloid on a Pt(ll1) surface, support the 1 : 1 interaction model for the alkaloid and pyruvate.Data are also reported for the interaction of Pt(ll1) with ethanol, both in the liquid and vapour state; dissolved oxygen is shown to influence the surface chemistry and lead to the formation of surface acetate species.

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