Homogeneous and heterogeneous asymmetric reactions: Part 11 Sonochemical enantioselective hydrogenation of trifluoromethyl ketones over platinum catalysts

Enantioselective sonochemical hydrogenation of alpha,alpha,alpha-trifluorom ethyl ketones, namely 1,1,1-trifluoroacetophenone and 1,1,1-trifluoro-pheny lacetone was carried out over various platinum catalysts modified by cincho nidine in different solvents. Both compounds yielded the (R)-alcohol as maj or product. The reaction rates and the enantiomeric excesses were determine d over Pt/C, Pt/SiO2, Pt/K-10 and Pt/Al2O3 catalysts under conventional con ditions. Since Pt/Al2O3 exhibited the best catalytic performance the effect of ultrasound on the catalytic activity and enantioselectivity was tested using this catalyst applying different sonochemical pretreatments. These me thods included a pretreatment before the reaction in hydrogen and oxygen or both. The ultrasonic irradiation was found to be highly advantageous in th e case of trifluoroacetophenone, whereas only moderate changes were observe d using trifluoro-phenylacetone. After insonation of the catalysts the enan tioselectivity was considerably improved. Both the aerobic and anaerobic so nochemical pretreatments resulted in significant improvement in optical yie ld (up to 49% and 17% ee, at room temperature under 10 bar in 1,2-dichlorob enzene). In parallel, the hydrogenation rates increased to a small extent ( 1.1-1.2-fold increase) after hydrogenative ultrasonic pretreatment, whereas the rates decreased by a factor of 1.5-2 after aerobic insonation. To obta in more insight into the process, the effect of insonation time on the acti vity and enantioselectivity and actual changes of the catalyst were also st udied. (C) 1999 Elsevier Science B.V. All rights reserved.