Ligand-Assisted Carbonyl Bond Activation in Single Diastereomeric Complexes on Platinum

It is a significant challenge to relate ligand-assisted bond activation on metal surfaces to specific adsorption and intermolecular binding structures. To address this objective, we studied carbonyl bond activation in single chirality transfer complexes formed by methyl 3,3,3-trifluoropyruvate (i.e., MTFP) and (R)-1-(1-naphthyl)ethylamine (i.e., (R)-NEA) on a Pt(111) surface. The experiments combined reflectance absorbance infrared spectroscopy (RAIRS), scanning tunneling microscopy (STM), and density functional theory (DFT) methods. While STM measurements, in combination with DFT calculations, permit the study of single surface complexes, RAIRS is an ensemble technique that yields a composite spectrum resulting from an often heterogeneous distribution of molecular structures on the sampled surface. We show that the intrinsic thermal behavior of the MTFP/(R)-NEA/Pt(111) system facilitates meaningful comparison between single complex measurements by STM and ensemble measurements by RAIRS in that the vibrational signal can be attributed to a small number of complexation configurations, one of which has a high relative abundance. We take advantage of mode mixing in a ν(CF3) + ν(CO)keto vibration to detect a spectroscopic signature for complexation-induced carbonyl bond activation. A red-shift of the band correlates with DFT-predicted lengthening of the bridge-bonded carbonyl group. While the intensity of the shifted band is in the majority due to the most abundant complexation configuration, minority states produce line broadening. In addition to providing insight on rate-enhancement in enantioselective reactions on catalysts bearing chiral auxiliaries, the study contributes to the development of ligand control of reactivity and selectivity in heterogeneous catalysis

Isolating a Reaction Intermediate in the Hydrogenation of 2,2,2-Trifluoroacetophenone on Pt(111)

The isolation and identification of surface intermediates is of the utmost importance for the elucidation of mechanisms and selectivity patterns in heterogeneous catalysis. However, the metastable nature of reaction intermediates makes their detection and differentiation from other species challenging. This work reports a combined variable temperature scanning tunneling microscopy (VT-STM) and van der Waals-corrected density functional theory (opt88-vdW DFT) study showing that a hydroxy intermediate (hy-TFAP) formed in the hydrogenation of 2,2,2-tri­fluoro­aceto­phenone (TFAP) is trapped by parent TFAP to form a H-bonded bimolecular TFAP/hy-TFAP structure. The facile formation of the hydroxy intermediate, by residual hydrogen present in the ultrahigh vacuum chamber, was predicted based on a previous DFT study of the hydrogenation pathway for TFAP on Pt(111). The prediction is confirmed by comparison of calculated TFAP/TFAP and TFAP/hy-TFAP structures with STM images of bimolecular structures formed through TFAP adsorption and treatment at different temperatures

Aminolactone Chiral Modifiers for Heterogeneous Asymmetric Hydrogenation: Corrected Structure of Pantoyl-Naphthylethylamine, In-Situ Hydrogenolysis, and Scanning Tunneling Microscopy Observation of Supramolecular Aminolactone/Substrate Assemblies on Pt(111)

As established by Baiker and co-workers, pantoyl-naphthylethylamine (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 PNEA-derived 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 <i>R</i>,<i>S</i> rather than <i>R</i>,<i>R</i>. 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/Al₂O₃ catalysts, forming the aminolactone amino-4,4-dimethyldihydrofuran-2-one (AF). STM measurements on (<i>S</i>)-AF and 2,2,2-trifluoroacetophenone coadsorbed on Pt(111) show the formation of isolated 1:1 complexes. In contrast, measurements on coadsorbed (<i>S</i>)-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

Aminolactone Chiral Modifiers for Heterogeneous Asymmetric Hydrogenation: Corrected Structure of Pantoyl-Naphthylethylamine, In-Situ Hydrogenolysis, and Scanning Tunneling Microscopy Observation of Supramolecular Aminolactone/Substrate Assemblies on Pt(111)

As established by Baiker and co-workers, pantoyl-naphthylethylamine (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 PNEA-derived 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 <i>R</i>,<i>S</i> rather than <i>R</i>,<i>R</i>. 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/Al₂O₃ catalysts, forming the aminolactone amino-4,4-dimethyldihydrofuran-2-one (AF). STM measurements on (<i>S</i>)-AF and 2,2,2-trifluoroacetophenone coadsorbed on Pt(111) show the formation of isolated 1:1 complexes. In contrast, measurements on coadsorbed (<i>S</i>)-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

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 (<i>R</i>)-(+)-1-(1-naphthyl)­ethylamine, (<i>R</i>)-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 (<i>R</i>)-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 (<i>R</i>)-NEA