Ultrasonics in asymmetric syntheses. Sonochemical enantioselective hydrogenation of prochiral C=O groups over platinum catalysts

The sonochemical enantioselective hydrogenation of various prochiral carbonyl compounds to the corresponding (R)-hydroxy derivatives over different platinum catalysts using cinchonidine as chiral modifier is described. The carbonyl derivatives studied were trifluoromethyl ketones and α- and β-ketoesters. The sonochemical pretreatment of the catalyst-modifier system was found to be mostly advantageous in increasing the optical yield. Moreover, the ultrasonically promoted reactions provided the best enantiomeric excesses (ee%) ever obtained in this heterogeneous catalytic system (methyl pyruvate 95 ee%, ethyl 4-phenyl-2-oxobutyrate 95 ee%, and ethyl benzoylformate 92 ee%). In a few cases, however, the insonation did not result in enhancement in enantioselectivities. On the basis of the comparison with conventional reactions the limitations of the ultrasonic irradiation in asymmetric hydrogenations are also drawn.