Enantioselective hydrogenation in supercritical fluids. Limitations of the use of supercritical CO2

Publisher Summary This chapter reviews the study of enantioselective hydrogenation in supercritical fluids. The Pt-catalyzed enantioselective hydrogenation of ethyl pyruvate to (R) -ethyl lactate is considerably faster in supercritical ethane than in the conventional apolar solvent toluene, whereas the enantioselectivity is unaffected. Complete catalyst deactivation is observed in CO 2 , which is shown by Fourier transform infrared spectroscopy (FTIR) to be because of the reduction of CO 2 to CO via reverse water–gas shift reaction. The catalyst could be regenerated by exposing it to ambient air, while hydrogen treatment is less efficient. This is the first evidence to the limitation of catalytic hydrogenations over Pt metals in supercritical CO 2 . The results suggest that care should be taken when applying supercritical CO 2 as solvent in hydrogenation reactions. Platinum metals catalyze the reduction of CO 2 to CO even at ambient temperature, and CO can poison the catalyst and contaminate the recycled CO 2 .