Paper SS-ThM6
Enhanced Hydrogenation Activity and Diastereomeric Inter-actions of Methyl Pyruvate Co-adsorbed with R-1-(1-Naphthylethylamine) on Pd(111)

Thursday, November 10, 2016, 9:40 am, Room 104D

The diastereomeric interactions and reactivity of co-adsorbed methyl pyruvate (MP) and R-1-(1-naphthyl)ethylamine (NEA) are studied on a Pd(111) model single crystal catalyst using temperature-programmed desorption (TPD) and scanning tunneling microscopy (STM) with the aim of identifying the nature of the interactions and understanding the observed enhancement in hydrogenation activity of MP to methyl lactate on NEA-modified surfaces. The enhancement in hydrogenation activity of chirally modified sites is critical in designing heterogeneous enantioselective catalysts by mitigating the effect of unmodified racemic sites. TPD experiments of coadsorbed MP and hydrogen or deuterium on Pd(111) reveal that NEA accelerates the rates of both MP hydrogenation and H/D exchange. The measured STM images of docking complexes on Pd(111) are classified according to the angles between the long axes of MP and the naphthyl ring of NEA, which fall into well-defined ranges of 10 ± 5, 45 ± 5 and 70 ± 5º. Docking structures are modeled using first-principles density functional theory (DFT) calculations that include van der Waals interactions, and STM image simulations. Excellent agreement is found both between the shapes of the calculated and simulated images and their theoretical and experimental dihedral angle distributions, thereby confirming the validity of the calculations. Diastereomeric interactions between NEA and MP occur predominantly by binding of the carbon-carbon double bond of the enol tautomer of MP to the surface, while simultaneously optimizing C=O····H₂N hydrogen-bonding interactions. The combination of chiral-NEA driven diastereomeric docking with a tautomeric preference enhances the hydrogenation activity since C=C bonds hydrogenate more easily than C=O bonds. This model provides a rationale for the catalytic observations.