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

The sonochemical enantioselective hydrogenation of various prochiral carbon yl compounds to the corresponding (R)-hydroxy derivatives over different pl atinum catalysts using cinchonidine as chiral modifier is described. The ca rbonyl derivatives studied were trifluoromethyl ketones and alpha- and beta -ketoesters. The sonochemical pretreatment of the catalyst-modifier system was found to be mostly advantageous in increasing the optical yield. Moreov er, the ultrasonically promoted reactions provided the best enantiomeric ex cesses (ee%) ever obtained in this heterogeneous catalytic system (methyl p yruvate 95 ee%, ethyl 4-phenyl-2-oxobutyrate 95 ee%, and ethyl benzoylforma te 92 ee%). In a few cases, however, the insonation did not result in enhan cement in enantioselectivities. On the basis of the comparison with convent ional reactions the limitations of the ultrasonic irradiation in asymmetric hydrogenations are also drawn. Chirality 11:470-474, 1999. (C) 1999 Wiley- Liss, Inc.