Interaction of optically active tartaric acid with a nickel-silica catalyst: Role of both the modification and reaction media in determining enantioselectivity

An enantioselective Ni/SiO2 catalyst has been prepared by modification with aqueous, methanolic, ethanolic, and butanolic solutions of (R)-(+)-tartaric acid (TA) and used in the asymmetric hydrogenation of a prochiral beta-keto ester (methyl acetoacetate) to a beta-hydroxy ester ((R)-(-)-methyl S-hydroxybutyrate). The effects of systematically varying the modification time and temperature, modifier concentration, and the incorporation of NaBr as a comodifier on the uptake of TA and on the ultimate activity and enantioselectivity are discussed. The nature of the interaction of TA with the surface nickel metal is considered and data on the corrosive leaching of nickel from the catalyst surface are presented. The degree of TA adsorption was found to increase and the extent of nickel dissolution to decrease with the following sequence of modifier solvents: water, methanol, ethanol, l-butanol. Changes in hydrogenation activity and enantioselectivity are reported as a function of the reaction time, temperature, and the reactant to catalyst molar ratio. Modification of the catalyst with TA not only induced enantioselectivity but also promoted the rate of hydrogenation where the alcoholic treatments yielded the highest activities and selectivities. The reaction exhibits typical Langmuir-Hinshelwood behavior and reaction orders with respect to methylacetoacetate and hydrogen concentrations and apparent activation energies are provided. The effect of the reaction solvent on product composition is discussed, and a linear relationship between the apparent rate and dielectric constant of the reaction medium, in the case of alcoholic solvents, is presented. The action of thiophene, acetic acid, and water as additives is identified, and catalyst durability is addressed.