Origin of the rate enhancement and enantiodifferentiation in the heterogeneous enantioselective hydrogenation of 2,2,2-trifluoroacetophenone over Pt/alumina studied in continuous-flow fixed-bed reactor system

• Published in: Applied catalysis. A, General Vol. 382; no. 2; pp. 263 - 271
• Main Authors: Szőllősi, György, Cserényi, Szabolcs, Bucsi, Imre, Bartók, Tibor, Fülöp, Ferenc, Bartók, Mihály
• Format: Journal Article
• Language: English
• Published: Kidlington Elsevier B.V 15.07.2010; Elsevier
• Subjects: Aluminum oxide; Asymmetric hydrogenation; Catalysis; Catalysts; Chemistry; Cinchona alkaloids; Continuous-flow fixed-bed reactor; Exact sciences and technology; General and physical chemistry; Hydrogenation; Inversions; Origin of enantiodifferentiation; Origin of rate enhancement; Origins; Platinum; Reactors; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry; Toluene; Trifluoroactophenone; Trifluoroactophenone; Origin of rate enhancement; Cinchona alkaloids; Origin of enantiodifferentiation; Platinum; Asymmetric hydrogenation; Continuous-flow fixed-bed reactor; Binary compound; Cinchona; Enantioselectivity; Alcohol; Hydrogenation; Racemic; Alkaloid; Carboxylic acid; Dicotyledones; Angiospermae; Rubiaceae; Experimental data; Hydrocarbon; Toluene; Fixed bed reactor; Transition metal; Complexes; Equilibrium; Quinine; Heterogeneous catalysis; Quinidine; Benzenic compound; Spermatophyta; Acetic acid; Alumina; Catalyst
• ISSN: 0926-860X; 1873-3875
• DOI: 10.1016/j.apcata.2010.05.003

Based on the results of a study on the enantioselective hydrogenation of 2,2,2-trifluoroacetophenone over cinchona alkaloids modified Pt catalysts in the absence or in the presence of trifluoroacetic acid using continuous-flow fixed-bed reactor system and racemic – cinchona 1–cinchona 2–cinchona 1 hydrogenation series we propose reaction routes determined by the equilibrium between the electrophilic and nucleophilic intermediate complexes. A study on the origin of rate enhancement and enantiodifferentiation in the enantioselective hydrogenation of 2,2,2-trifluoroacetophenone (TFAP) over a Pt/alumina catalyst modified by cinchona alkaloids in toluene/acetic acid (AcOH) solvent mixture with and without trifluoroacetic acid (TFA) using continuous-flow fixed-bed reactor system is presented. The experimental data of the racemic – cinchona 1–cinchona 2–cinchona 1 hydrogenation series confirm the intrinsic nature of rate enhancement, namely the so-called “ligand acceleration” phenomenon. 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-cinchonidine and Pt-quinine and ( S)-alcohols on Pt-cinchonine and Pt-quinidine chiral catalysts. In toluene/AcOH mixture without TFA, unexpected inversion took place on the Pt-cinchonine and Pt-quinidine catalysts since the ( R)-product formed in excess instead of the ( S)-product. The observed unexpected inversion can be interpreted on the basis of the nucleophilic intermediate complex. Based on these observations we propose that in the hydrogenation of TFAP the reaction route involves the equilibrium of electrophilic and nucleophilic intermediate complexes, which was found to be dependent on the acid strength and concentration.