(423c) Bestowing Chirality on Solid Surfaces

It has been shown that heterogeneous hydrogenation catalysts such as platinum and nickel can be made enantioselective by the use of molecular modifiers. Two mechanisms have been proposed for this based on the formation of one-to-one complexes between the modifier and the reactant and on the chiral templating of the surface via the formation of ordered adsorbate suprastructures. Both mechanisms have been explored in our laboratory using surface sensitive techniques. The adsorption of cinchona alkaloids from solution onto platinum surfaces, the prototypical example of the first, has been detailed using reflection-absorption infrared spectroscopy (RAIRS) in-situ at the liquid-solid interface. Parameters such as adsorption geometry, reversibility of adsorption, and the effect of gases and solvents were investigated. Correlations were identified between the catalytic performance of the cinchona modifiers and their adsorption properties, and between the latter and the structural parameters of those molecules. For the second mechanism, the templating of Pt(111) surfaces with chiral 2-butanol, 2-methylbutanoic acid, and 1-(1-naphthyl)ethylamine adsorbates has been probed under vacuum using enantiopure propylene oxide as a probe. Evidence for both one-to-one complexation and suprastructure ordering was obtained in the latter two cases. The relative importance of each mechanism in terms of the relative importance of enthalpic versus entropic factors in all these examples will be discussed.