Novel Evidence on the Role of the Nucleophilic Intermediate Complex in the Orito-Reaction: Unexpected Inversion in the Enantioselective Hydrogenation of 2,2,2-Trifluoroacetophenone on Pt-Cinchona Chiral Catalyst Using Continuous-Flow Fixed-Bed Reactor

Abstract

The enantioselective hydrogenation of 2,2,2,-trifluoroacetophenone over Pt/Al2O3 catalysts modified by cinchona alkaloids was investigated for the first time using continuous-flow fixed-bed reactor system in toluene/AcOH 9/1 solvent mixture in absence and presence of 0.1 v/v% trifluoroacetic acid (TFA). The enantioselective hydrogenations yielded the (R)-product in excess on Pt–CD, Pt–CN, Pt–QN and Pt–QD catalysts in toluene/AcOH mixture; consequently, unexpected inversion took place on the Pt–CN and Pt–QD catalysts. Hydrogenation in the presence of 0.1% (v/v) TFA follows the general rule of the Orito reaction, according to which the products formed in excess are (R)-alcohols on Pt–CD and Pt–QN and (S)-alcohols on Pt–CN and Pt–QD. Since there is no inversion in the presence of TFA, the observed unexpected inversion can be interpreted on the basis of the nucleophilic intermediate complex.