(152d) In-Situ Vibrational Spectroscopic Investigation of Chirally-Modified Metal Catalysts for Enantioselective C=C Bond Hydrogenation

Solid-catalyzed liquid-phase reactions are poised to play an increasingly important role in the fine chemicals and pharmaceuticals industries. When compared to currently employed homogeneous catalytic methods, heterogeneous catalysts can offer some advantages, including ease of separation and handling, and possible reduced use of solvent. One very interesting class of catalyst is chirally-modified metal surfaces that can facilitate enantioselective hydrogenation. One of the most well studied catalysts from both a kinetic and surface chemistry standpoint is cinchonidine-modified Pt, which shows remarkable activity and selectivity for alpha-activated C=O hydrogenation reactions under mild conditions. In contrast, asymmetric C=C bond hydrogenation in α,β-unsaturated acids over similar catalysts has not been as successful, and there is a notable lack of detailed surface spectroscopic investigations. This talk will present the latest results from our laboratory addressing the surface chemistry relevant to α,β-unsaturated acid hydrogenation over cinchonidine-modified Pd. The in-situ vibrational techniques that are being employed are attenuated total reflection infrared spectroscopy (ATR-IRS), surface-enhanced Raman spectroscopy (SERS), and sum frequency spectroscopy (SFS). The adsorption of a representative reactant (e.g., (E)-2-methyl-2-penenoic acid) and its hydrogenation product onto Pd in typical solvents (e.g., methanol, 1,4-dioxane) will be investigated. In addition, their interaction with adsorbed cinchonidine will be examined under hydrogenation conditions. The spectroscopic data will be correlated with observed trends in catalytic rates and enantioselectivity for this system in order to elucidate key aspects of the reaction mechanism.