XPS and LEED studies of 10,11-dihydrocinchonidine adsorption at Pt(111). Implications for the role of cinchona alkaloids in enantioselective hydrogenation

Abstract

The interaction of 10,11-dihydrocinchonidine (DHC) with a Pt(111) 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 × 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(111) surface, support the 1 : 1 interaction model for the alkaloid and pyruvate.

Data are also reported for the interaction of Pt(111) 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.