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.

A cylindrically configured plasma treatment system for Radio Frequency Glow discharges fed with ammonia was used to modify the internal surface of ePTFE arterial prostheses to improve their biocompatibility. For a better understanding of the effects of this type of treatment on the surface, RF-plasmas were also performed on PTFE films. The surface chemical composition was then characterized by XPS. The initial analyses showed that 15% of the surface atoms were replaced by nitrogen (N/C ratio of 0.3), whereas the F/C ratio decreased from 2 to 0.7-0.5 which indicates that the treated surfaces presented different chemical species, such as amine, imine, amide, acid groups, and insaturations. As XPS analyses could not lead directly to the nature of the N-species grafted on the surface (the chemical shifts being not significantly different), chemical derivatization was thus performed. Vapor phase chemical derivatization was carried out on model polymers to evaluate the reactivity and the selectivity of each reagent toward each of the expected functional groups.The results indicate that benzaldehyde derivatives were good derivatizing reagents for amine groups, whereas bromine was the reagent of choice to quantify the insaturations. Using these methods, the amine and alkene concentrations on the surface were found to increase according to plasma treatment time rising from their initial value of 3% for a 50 s plasma treatment to 6% for a 250 s plasma treatment. AFM studies on oriented Teflon films also demonstrated that the occurrence of chain breaking increased with plasma treatment time. It therefore appears that determining the treatment parameters should require the best compromise between several effects such as nitrogen grafting, amine and alkene formation, and chain breaking.

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.