(739d) Enantioselective Hydrogenation of α, β-Unsaturated Carboxylic Acid Over Cinchona-Modified Pd/Al2O3: a Spectroscopic and Kinetic Study

Enantioselective synthesis plays a key role in the industrial production of agrochemicals, fragrances, and pharmaceuticals. However, studies of C=C hydrogenation with modified catalyst are lagging behind. In this work, the adsorption of methyl pentenoic acid (MPeA), a prochiral α, β-unsaturated carboxylic acid on Pd catalyst, was studied. In addition, the hydrogenation of this molecule with/without the chiral modifier cinchonidine is explored, with the aim of understanding the reaction mechanism. The adsorption of MPeA has been studied by in-situ ATR-IR spectra on Pd/Al₂O₃ and Al₂O₃ surface. Several peaks appear upon exposure to acid solution, and these peaks remain after purging with pure solvent, indicating an irreversible adsorption on the Pd/Al₂O₃ surface. Detailed analysis of these vibrational spectra suggests that the acid adsorbs on surface both as monomeric and dimeric molecular species, and as a bridging bidentate carboxylate species. In addition, vibrational studies of cinchonidine-acid interactions in solution and on the surface were carried out.  Corresponding kinetic studies of the hydrogenation of MPeA in several solvents in a batch autoclave reactor were carried out. Methanol and dioxane exhibit good performance in terms of activity and selectivity, while dichloromethane (CH₂Cl₂) provides nearly no reactivity, indicating the reaction is strongly solvent-dependent. The data suggest that the reaction is first order with respect of H₂ under low pressure (<7.5 atm), while becoming H₂-independent for pressures larger than 20 atm, consistent with Langmuir-Hinshelwood type adsorption. The reaction order of the substrate MPeA up to around 40-50% conversion is close to 1, but increases to 2 as the reaction progresses further. The reaction rate was also found to decrease with increasing initial product/reactant ratio when product is added initially, suggesting that the product is strongly adsorbed on the surface. In the case of enantioselective hydrogenation, the highest e.e. value was found to be ca 30%. Furthermore, the presence of modifier had no effect on reaction mechanism (i.e., reaction orders of reactant and H₂ remained the same). However, the reaction rate decreased compared with racemic reaction, presumably due to blockage of active sites by the cinchonidine modifier.