Adsorption States and Modifier−Substrate Interactions on Pt(111) Relevant to the Enantioselective Hydrogenation of Alkyl Pyruvates in the Orito Reaction

• Published in: Journal of the American Chemical Society Vol. 125; no. 51; pp. 15756 - 15757
• Main Authors: Lavoie, S, Laliberté, M.-A, McBreen, P. H
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 24.12.2003
• Subjects: Adsorption and desorption kinetics; evaporation and condensation; Chemistry; Condensed matter: structure, mechanical and thermal properties; Exact sciences and technology; General and physical chemistry; Physics; Solid-fluid interfaces; Solid-gas interface; Surface physical chemistry; Surfaces and interfaces; thin films and whiskers (structure and nonelectronic properties); Chemisorption; Infrared spectroscopy; Enantioselectivity; Adsorption site; Organic adsorbate; Reaction path; Adsorption capacity; Experimental study; Gas solid adsorption; Platinum; Surface reactions; Transition elements; Gas solid interface; Metallic adsorbent; Ketoester; Crystal faces; Modified material
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja036700w

Reflectance FTIR spectroscopy (RAIRS) was used to study the chemisorption and intermolecular interactions of methyl pyruvate and (±)-1-(1-naphthyl)ethylamine (NEA) on Pt(111). NEA serves, in this study, as a tractable model of a chiral modifier in the asymmetric hydrogenation of α-dicarbonyls on alkaloid-modified platinum surfacesthe Orito reaction. The results show the presence of a majority enediolate state on the clean surface. A perpendicularly adsorbed trans conformation state is populated at close to full-monolayer coverage on the clean surface. The latter state desorbs at 185 K. The enediolate undergoes dissociation at 230 K. NEA displays hydrogen-bond association at high coverages. Coadsorption studies show that NEA inhibits the formation of the enediolate state. Multilayer methyl pyruvate shows a clear hydrogen-bond interaction with chemisorbed NEA, leading to a reorientation of the ethylamine group. The high-coverage trans-chemisorbed methyl pyruvate state also hydrogen-bonds to chemisorbed NEA. The latter interaction renders the trans state stable to above 300 K. A new schematic mechanism for the Orito reaction is proposed on the basis of these data.