Vincent Demers-Carpentier

Direct observation of molecular preorganization for chirality transfer on a catalyst surface.

Scanning tunneling microscopy and theoretical calculations shed light on an asymmetric heterogeneous catalyst. 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.

Stereodirection of an α-Ketoester at Sub-molecular Sites on Chirally Modified Pt(111): Heterogeneous Asymmetric Catalysis

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.

Single-chiral-catalytic-surface-sites: STM and DFT study of stereodirecting complexes formed between (R)-1-(1-naphthyl)ethylamine and ketopantolactone on Pt(111)

The formation of bimolecular complexes on metal surfaces through interaction between a single chemisorbed chiral molecule and a single chemisorbed prochiral substrate molecule can be considered as a preorganization step toward chirality transfer. In the case of asymmetric hydrogenation on chirally modified platinum catalysts, the metal surface dissociates H2 and provides atomic hydrogen for the desymmetrization step. Along the reaction path, the combined chemisorption and intermolecular interactions in the assembly formed between the modifier and the substrate determine which enantiomer is formed in excess. In this study, we use DFT calculations and STM measurements to describe chemisorption and intermolecular interactions in isolable structures formed between single ketopantolactone and single (R)-1-(1-naphthyl)ethylamine molecules on Pt(111). The study reveals several distinct complexation geometries at the sub-molecular level as well as the stereodirecting forces operating in the most abundant bimolecular assemblies. The comparison of theoretical and experimental data strongly suggests that partial hydrogenation of KPL occurs under the experimental conditions and that some of the most abundant complexes are formed by the hydroxy intermediate.

Scanning Tunneling Microscopy Measurements of the Full Cycle of a Heterogeneous Asymmetric Hydrogenation Reaction on Chirally Modified Pt(111)

The hydrogenation of a prochiral substrate, 2,2,2-trifluoroacetophenone (TFAP), on Pt(111) was studied using room-temperature scanning tunneling microscopy (STM) measurements. The experiments were carried out both on a clean surface and on a chirally modified surface, using chemisorbed (R)-(+)-1-(1-naphthyl)ethylamine, ((R)-NEA), as the modifier. On the nonmodified surface, introduction of H2 at a background pressure of ∼1 × 10(-6) mbar leads to the rapid break-up of TFAP dimer structures followed by the gradual removal of all TFAP-related images. During the latter step, some monomers display an extra protrusion compared to TFAP in dimer structures. They are attributed to a half-hydrogenated intermediate. The introduction of H2 to a mixture of (R)-NEA and TFAP on Pt(111) leads to the removal of TFAP without any change in the population of the modifier, as required for an efficient chirally modified catalyst.