Support and Size Effects of Ruthenium Catalysts with a Chiral Modifier for Asymmetric Hydrogenation of Aromatic Ketones

A series of supported Ru catalysts with achiral modifier triphenylphosphine (TPP) and chiral modifier (1R,2R)-1,2-diphenylethylenediamine [(1R,2R)-DPEN] were employed for the asymmetric hydrogenation of aromatic ketones. The textural and structural properties of the catalysts were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, N2 physisorption, transmission electron microscopy, CO2 and NH3 temperature-programmed desorptions, inductively coupled plasma atomic emission spectrometry, and ultraviolet−visible spectroscopy. Studies revealed that the enantiomeric excess (ee) followed this order according to the oxide supports used: MgO > γ-Al2O3 > CeO2 ≫ ZnO > SiO2, which was correlated with their surface basicity. Moreover, the highest ee value and intrinsic activity for the asymmetric hydrogenation of acetophenone were attained over the catalyst with a mean Ru nanoparticle diameter of 4.4 nm, while those for the asymmetric hydrogenation of 1-acetonaphthone were obtained over a catalyst with 6.2 nm nanoparticle diameter. The catalyst system consisting of Ru/MgO, TPP, and (1R,2R)-DPEN did not show a significant decrease in ee value after several recycles. The results indicate that there is a size matching effect between the Ru particles and the substrates, besides the acid/base influence of the oxide supports on the reaction.