Hydrogenation of acetophenone derivatives: Tuning the enantioselectivity via the metal–support interaction

The influence of acidic and basic supports on the enantioselective hydrogenation of acetophenone and its aryl-substituted derivates was studied. The Pt/Al2O3, Pt/Al2O3–SiO2, and Pt/Al2O3–Cs2O catalysts were prepared by single-step flame spray pyrolysis, and cinchonidine (CD) was used as chiral modifier. For all five aromatic ketones, the acidic support improved the ee, while the basic support diminished it. Opposite tendencies were observed for the reaction rate: acidic support lowered and basic support enhanced the rate of ketone conversion. In situ investigation of the hydrogenation of the quinoline ring of CD revealed for the first time that degradation of the modifier leads to remarkable variation in the substrate/modifier ratio. This effect and a possible catalyst deactivation due to aldol condensation of the ketones may be the origin of the decrease in ee with conversion. Following the diastereoselectivity during hydrogenation of the heteroaromatic ring of the quinoline unit of CD proved dominantly pro(S) adsorption mode on Pt/Al2O3 and Pt/Al2O3–SiO2, and pro(R) on Pt/Al2O3–Cs2O catalysts. Changes in the adsorption mode were minor during hydrogenation of aromatic ketones. This, together with the small effect of ketones on the hydrogenation rate of CD, is interpreted as an indication to weak substrate–modifier interactions, in contrast to the situation during the hydrogenation of α-ketoesters (methyl benzoylformate).