Enantioselective hydrogenation of ethyl pyruvate catalysed by cinchonine-modified Pt/Al2O3: tilted adsorption geometry of cinchonine

Abstract

The enantioselective hydrogenation of ethyl pyruvate (EtPy) was studied on Pt-alumina catalyst modified by cinchonine (CN) and for comparison by cinchonidine (CD) in toluene and in AcOH. The effects of the modifiers concentration on the reaction rate and the enantioselectivity were examined. Using the Engelhard 4759 catalyst under mild experimental conditions (room temperature, hydrogen pressure 1 bar) in the case of CN (S)-ethyl lactate (EtLt) formed in excess (ee) (in AcOH ee max $ 88%; in toluene ee max $ 72%). In the case of CD (R)-EtLt formed in excess (in AcOH ee max $93%; in toluene ee max $ 84%). The results of H-D exchange measurements and results of modifier mixtures suggest that the compounds responsible for chiral induction are different intermediates, which structure depends mostly on the acidic or non-acidic nature of the hydrogenation medium. The proposed structure of intermediate responsible for enantioselection is an 1:1 CN-or CD-EtPy surface complex in which the quinoline skeleton of CD approximately parallel on the Pt surface while the quinoline plane of CN being tilted relative to the Pt surface under identical experimental conditions.

Enantioselective Hydrogenation of N‐Acetyldehydroamino Acids over Supported Palladium Catalysts

“…A lower ee value was also obtained with cinchonine (7) as modifier, in accordance with the results usually obtained in the enantioselective hydrogenations over Cinchona-modified catalysts. [23,24] As expected the major enantiomer formed in presence of 7 had the opposite absolute configuration as obtained in presence of 6, and the addition of 1 also resulted in inversion of the enantioselectivity. Significantly decreased values were obtained when the C-6'-methoxy-substituted Cinchona alkaloids quinine (8) or quinidine (9) were used; in the presence of the latter the racemic product was obtained.…”

Kinetic Aspects of Nonlinear Phenomena in Heterogeneous Enantioselective Catalysis

“…Balanced equations determine the relationships between adsorbed intermediates, for example, equality of the sum of species coverage to unity. It follows from equation (2) that there are 11 steps, 1 balanced equation, 6 intermediates (Z, ZM 1 , ZM 2 , ZM 1 AH 2 , ZM 2 AH 2 , ZAH 2 ), and therefore 6 basic routes, out of which N (1) and N (3) describe formation of R-products with involvement of modifiers M 1 and M 2 respectively, while N (2) and N (4) correspond to formation of S products with these modifiers. Racemic reactions are taken care of by inclusion of routes N (5) and N (6) .…”

“…In heterogeneous catalysis few publications have addressed enantioselective hydrogenation in the presence of a mixture of two chiral catalyst modifiers [3][4][5][6][7] also called as chiral auxiliaries. The experiments with modifier mixtures have been utilized to extract mechanistic information about competitive modifier adsorption, adsorption strength, etc.…”

“…The experiments with modifier mixtures have been utilized to extract mechanistic information about competitive modifier adsorption, adsorption strength, etc. Typically a mixture of two modifiers, which are diastereomers [3,4], (e.g., cinchonine (CN) and cinchonidine) two completely different chiral modifiers or more recently enantiomers [7] have been used simultaneously in the same hydrogenation experiment. This approach has been named as nonlinear phenomena to separate it from NLE observed in homogeneous catalysis.…”

Asymmetric cascade reaction of 2-nitrophenylpyruvates over chirally modified platinum catalyst

“…Although, one may speculate that this could occur due to alterations in the adsorption mode of the cinchona alkaloids as a function of coverage [122][123][124], the most plausible explanation is that for the adsorption of the intermediate product 4b in order to obtain 5b, the surface of the metal should be only partially covered by the modifier. This confirms that it is highly probable that cyclization occurs on the metal surface, as was suggested previously.…”