Enantioselective Hydrogenation of Ketones over a Tartaric Acid-Modified Raney Nickel Catalyst: Substrate-Modifier Interaction Strength and Enantioselectivity

Abstract

Abstract Chiral (R,R)-tartaric acid and NaBr-doubly modified Raney nickel (TA-MRNi) is a promising heterogeneous catalyst for enantioselective hydrogenation of prochiral β-keto esters. To obtain deeper insights into the factors ruling the enantioselectivity, enantiodifferentiating hydrogenation of substituted ketones was studied over TA-MRNi and NaBr-modified RNi by use of combined individual-competitive hydrogenation techniques. Relative equilibrium adsorption constants of the substrates were estimated to evaluate their relative interaction strength with adsorbed tartaric acid moiety. DFT calculations were also performed to estimate the interaction energy through hydrogen bonding, providing clear support to the kinetic analysis and surface model. It is concluded with the enantioselective hydrogenation of ketones over TA-MRNi that the enantioselectivity increases as the substrate-modifier interaction strength increases: methyl acetoacetate (MAA) > acetylacetone (AA) ≈ 4-hydroxy-2-butanone (HB) > 2-octanone (2O).