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

Enantioselective sonochemical hydrogenation of α,α,α-trifluoromethyl ketones, namely 1,1,1-trifluoroacetophenone and 1,1,1-trifluoro-phenylacetone was carried out over various platinum catalysts modified by cinchonidine in different solvents. Both compounds yielded the (R)-alcohol as major product. The reaction rates and the enantiomeric excesses were determined over Pt/C, Pt/SiO2, Pt/K-10 and Pt/Al2O3 catalysts under conventional conditions. 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 methods included a pretreatment before the reaction in hydrogen and oxygen or both. The ultrasonic irradiation was found to be highly advantageous in the case of trifluoroacetophenone, whereas only moderate changes were observed using trifluoro-phenylacetone. After insonation of the catalyst, the enantioselectivity was considerably improved. Both the aerobic and anaerobic sonochemical pretreatments resulted in significant improvement in optical yield (up to 49% and 17% ee, at room temperature under 10 bar in 1,2-dichlorobenzene). 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 obtain more insight into the process, the effect of insonation time on the activity and enantioselectivity and actual changes of the catalyst were also studied.