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.

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As established by Baiker and co-workers, pantoylnaphthylethylamine (PNEA) is an efficient synthetic chiral modifier for the asymmetric hydrogenation of ketopantolactone (KPL) to pantolactone on supported Pt catalysts. We report a scanning tunneling microscopy (STM) study of PNEA and PNEAderived aminolactone species on Pt(111) and a reassignment of the relative stereochemistry of the modifier. Robust organic chemistry methods were used to establish that the structure of PNEA is R,S rather than R,R. The dissociative chemisorption of a fraction of PNEA adsorbed on Pt(111) yields two fragments that we attribute to a process involving C−N bond scission. We show that C−N bond scission occurs under hydrogenation conditions on PNEA-modified Pt/Al 2 O 3 catalysts, forming the aminolactone amino-4,4-dimethyldihydrofuran-2-one (AF). STM measurements on (S)-AF and 2,2,2-trifluoroacetophenone coadsorbed on Pt(111) show the formation of isolated 1:1 complexes. In contrast, measurements on coadsorbed (S)-AF and KPL show fluxional supramolecular AF/KPL assemblies. The possibility that such assemblies contribute to the overall enantioselectivity observed for PNEA-modified Pt catalysts is discussed.

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A study of the coadsorption of trifluoroacetic acid (TFA) and 2,2,2-trifluoroacetophenone (TFAP) on Pt(111) was carried out using scanning tunneling microscopy (STM) measurements. The investigation is based on literature reports that the conversion rate and the enantiomeric excess for the hydrogenation of TFAP on cinchona modified Pt/Al 2 O 3 are very sensitive to the presence of small amounts of TFA in the solvent. As previously reported, STM and density functional theory (DFT) studies show that TFAP forms aryl-CHÁÁÁO bonded dimers on Pt(111) at room temperature. In the present study, STM measurements show that TFA disrupts aryl-CHÁÁÁO bonding through insertion into TFAP dimers to form isolated trimolecular and bimolecular assemblies. Structural models are proposed for the TFAP-TFA aggregates. The implications of these structures for the rate and enantioselectivity of TFAP hydrogenation are discussed in terms of H-bonding activation of the carbonyl function. It is proposed that the acid additive operates on the enanantioselective hydrogenation of TFAP through the inhibition of the racemic reaction at sites remote from the chiral modifier. This action is assumed to occur in parallel with effects, already proposed in the literature, due to the formation of modifier-acid complexes and the protonation of the modifier.