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.

Chirally modified Pt catalysts are used in the heterogeneous asymmetric hydrogenation of α-ketoesters. Stereoinduction is believed to occur through the formation of chemisorbed modifier-substrate complexes. In this study, the formation of diastereomeric complexes by coadsorbed methyl 3,3,3-trifluoropyruvate, MTFP, and (R)-(+)-1-(1-naphthyl)ethylamine, (R)-NEA, on Pt(111) was studied using scanning tunneling microscopy and density functional theory methods. Individual complexes were imaged with sub-molecular resolution at 260 K and at room temperature. The calculations find that the most stable complex isolated in room-temperature experiments is formed by the minority rotamer of (R)-NEA and pro-S MTFP. The stereodirecting forces in this complex are identified as a combination of site-specific chemisorption of MTFP and multiple non-covalent attractive interactions between the carbonyl groups of MTFP and the amine and aromatic groups of (R)-NEA.

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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.

The chemical transformation and subsequent self-assembly of chiral alcohols on platinum was studied using three different pairs of prochiral ketones and their alcohol products. The ketones were chosen because they represent three different types of substrates in the asymmetric hydrogenation on chirally modified platinum catalysts. Scanning tunneling microscopy and high-resolution electron energy loss vibrational spectroscopy data were combined to show that methyl lactate transforms into the enol tautomer of methyl pyruvate on Pt(111) at room temperature. Specifically, the chiral alcohol undergoes dehydrogenation leading to the same adsorbed enol assemblies that are formed directly through the adsorption of the prochiral α-ketoester. Similarly, 1-phenylethanol transforms into assemblies of the enol tautomer of acetophenone. The interrelationship between surface reactivity and self-assembly was further explored by studying the oxidation of 1-phenyl-2,2,2-trifluoroethanol to form CH···O bonded 2,2,2-trifluoroacetophenone assemblies. In terms of catalytic function, these self-assembly processes provide insight on the optimization of the asymmetric hydrogenation of activated ketones on chirally modified platinum catalysts.