Influence of support acid–base properties on the platinum-catalyzed enantioselective hydrogenation of activated ketones

The influence of support acidity and basicity was investigated in the enantioselective hydrogenation of methyl benzoylformate and ketopantolactone on cinchonidine–modified Pt/Al2O3–SiO2 and Pt/Al2O3–Cs2O catalysts. Two series of flame-derived 4.7 wt.% Pt/Al2O3 catalysts in which the acid–base properties of the support were systematically varied by introducing SiO2 (5–80 wt.%) or Cs2O (0.25–10 wt.%) were applied. Addition of SiO2 improved the enantioselectivity with a maximum at 30 wt.%. Enantioselectivity correlated well with the acidity of the catalysts characterized by TPD of NH3 and with the selectivity to hydrogenolysis of methyl cyclohexyl ketone to ethylcyclohexane. On the contrary, doping with Cs was detrimental to the formation of the (R)-alcohols and the drop in enantioselectivity could unambiguously be attributed to the basicity of the support characterized by TPD of CO2. The critical impact of support ionicity on the electronic properties of Pt, and thus on the adsorption and interaction of the reaction components on the metal surface, was further proven by the good correlation between the enantioselectivities of all catalysts and the ratio of CO adsorbed in bridged to linear (B/L) geometry. The practical importance of our findings is demonstrated by the best ee (94 ± 0.5%) achieved so far in the industrially relevant hydrogenation of ketopantolactone to (R)-pantolactone.