The chemisorption of specific optically active compounds on metal surfaces can create catalytically active chirality transfer sites. However, the mechanism through which these sites bias the stereoselectivity of reactions (typically hydrogenations) is generally assumed to be so complex that continued progress in the area is uncertain. We show that the investigation of heterogeneous asymmetric induction with single-site resolution sufficient to distinguish stereochemical conformations at the submolecular level is finally accessible. A combination of scanning tunneling microscopy and density functional theory calculations reveals the stereodirecting forces governing preorganization into precise chiral modifier-substrate bimolecular surface complexes. The study shows that the chiral modifier induces prochiral switching on the surface and that different prochiral ratios prevail at different submolecular binding sites on the modifier at the reaction temperature.

The asymmetric hydrogenation of alpha-ketoesters on cinchona-modified supported platinum particles is a prototype reaction in heterogeneous chiral catalysis. The catalysis literature shows that the reaction is highly metal-specific, that it displays rate-enhancement with respect to the racemic reaction on the nonmodified surface, and that the observed stereoselectivity is a sensitive function of substrate and modifier structure. This set of observations has proven difficult to rationalize within the context of existing models for the mechanism of the Orito reaction. The most widely discussed mechanistic models are based on the formation of chemisorbed 1:1 complexes through H-bonding between the quinuclidine function of the cinchona modifier and the prochiral, keto-carbonyl, function of the substrate. Recent surface science studies, as well as advances in the area of C-H...O hydrogen bonding, suggest that chemisorption-induced polarization may lead to an aromatic-carbonyl H-bonding interaction between the aromatic anchor of the modifier and the coadsorbed substrate. By specifying that the aromatic C-H...O interaction is to the prochiral carbonyl and that it is accompanied by a H-bonding interaction between the ester carbonyl and the quinuclidine function, we show that it is possible to rationalize essentially all of the catalysis literature for the Orito reaction in terms of a single molecular mechanism. The generality of the proposed mechanistic model is demonstrated by addressing data from the literature for a representative range of substrates, modifiers, solvents, and metals. Results of catalytic tests on an asymmetric diketone substrate are presented in support of the model.

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 alpha-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.

Methyl pyruvate undergoes CH bond scission on Pt(111) at room temperature to trigger surface-mediated enol formation and subsequent self-assembly into enol superstructures. This process may be inhibited by performing the experiment below the temperature for CH bond scission or, at room temperature, by using a background pressure of H2. Superstructure formation is not due to a polymerization reaction. Hence, it is unlikely that rate enhancement of the enantioselective hydrogenation of methyl pyruvate on cinchona-modified Pt catalysts is simply due to the absence of a substrate polymerization reaction under reaction conditions.

Trifluoroacetophenone (TFAP) forms C O...H-C bonded dimers and trimers at room temperature on Pt(111). It is proposed that these systems mimic the prochiral carbonyl-chiral modifier interaction in the enantioselective hydrogenation of TFAP on cinchona-modified Pt catalysts. That is, the activation of TFAP in homomolecular assemblies at racemic sites is expected to be roughly the same as in the diastereomeric complex formed at chiral sites. This interpretation suggests a reason why alpha-phenyl ketones do not display a strong measured rate enhancement effect in the Orito reaction.