Catalysis Letters, Vol.134, No.3-4, 264-269, 2010
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
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.
Keywords:Asymmetric hydrogenation;Platinum;Cinchona alkaloids;Trifluoroacetophenone;Continuous-flow fixed-bed reactor;Unexpected inversion