Ultrasonics in chiral metal catalysis.: Effects of presonication on the asymmetric hydrogenation of ethyl pyruvate over platinum catalysts

Chiral sonochemical hydrogenation of ethyl pyruvate to the corresponding ethyl lactate was carried out over various platinum catalysts in different solvents under moderate hydrogen pressure (atmospheric and 10 atm). The reaction rates and the enantiomeric excesses were determined over Pt/C, Pt/Al2O3, Pt/SiO2 and Pt/K-10 catalysts in various solvents both under conventional and sonochemical conditions. The effect of ultrasounds on the catalytic activity and enantioselectivity was tested applying sonochemical pretreatment before the reaction, The ultrasonic irradiation was found to be highly advantageous in these hydrogenations. After the presonication of the catalysts, the enantioselectivity was highly improved over Pt/SiO2 and Pt/K-10 catalysts. In addition, reactions took place in quantitative yield with complete chemoselectivity and the hydrogenation rates increased by one order of magnitude despite the very mild (atmospheric hydrogen pressure, room temperature) experimental conditions. To clarify the macroscopic effect of ultrasounds on the catalysts transmission electron microscopic measurements were carried out. The sonochemical pretreatment decreases the metal particle size of the catalysts producing in some cases more selective surface environment for the enantioselective hydrogenation.