Aspects of enantioselective heterogeneous catalysis:: structure and reactivity of (S)-(-)-1-(1-naphthyl)ethylamine on Pt{111}

The molecular orientation, spatial distribution, and thermal behavior of the powerful chiral catalyst modifier precursor (S)-naphthylethylamine adsorbed on Pt{111} have been studied by NEXAFS, XPS, STM, and temperature programmed reaction. At 300 K, both in the presence and in the absence of coadsorbed hydrogen, the strongly tilted molecules do not form ordered arrays. These results constitute the first direct evidence against the template model and are at least consistent with the 1:1 interaction model of chiral induction in the enantioselective hydrogenation of alkyl pyruvates. Raising the temperature beyond 320 K (the temperature of enantioselectivity collapse) leads either to irreversible dimerization with hydrogen elimination or to dissociation of the ethylamine moiety, depending on whether coadsorbed H-a is present. Either way, the stereogenic center is destroyed. These findings provide the first direct clue as to the possible origin of enantioselectivity collapse, by a mechanism not previously considered. When NEA and methyl pyruvate are coadsorbed in the presence of H-a, STM reveals entities that could correspond to a 1:1 docking complex between the prochiral reactant and the chiral modifier.