Donor–acceptor interactions in the enantioselective hydrogenation of α-ketoesters

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

“…Since no inversion occurred in the hydrogenation of TFAP using AcOH, which would suggest a change in reaction mechanism, we interpreted the unexpected inversion on the basis of postulating a nucleophilic IC [54][55][56][57]62], also proposed by other authors [63][64][65][66]. The proposed nucleophilic IC is supported by earlier studies: (1) under the conditions of the Orito reaction NMR investigations support multipoint interaction between the substrate and chiral modifier; (2) the nucleophilic IC is identifiable in liquid phase by NMR and can also be quantified; (3) IC concentration decreases with increasing acid concentration; (4) there is a correlation between the concentration of this complex and the enantioselectivity of the hydrogenation [67][68][69].…”

Density Functional Theory Study of Benzene Adsorption on Small Pd and Pt Clusters

“…Indeed, this may considerably diverge from previous reported calculations, usually done on extended surfaces. Our motivation is related to the mechanism of ester enantioselective hydrogenation , that occurs on supported Pt and Pd catalysts in the presence of chiral modifiers such as the cinchona alkaloids. The enantioselective hydrogenation of pyruvate esters catalyzed by Pt modified by the cinchona alkaloids cinchonidine and cinchonine has received much attention recently, as one of the rare effective heterogeneous enantioselective catalysts. − The most widely accepted reaction model for the ester hydrogenation, the so-called 1:1 interaction model, is based on the hypothesis that the cinchona alkaloids adsorb intact on the Pt catalyst surface via the quinoline π-electron system. ,, It has been generally accepted that the adsorption of such modifiers on a Pt or Pd surface, via the quinoline moiety, provides a chiral adjacent site at which selective enantioface adsorption of pyruvate occurs with subsequent hydrogenation affording preferentially one enantiomer.…”

The Role of Modifier Structure in Heterogeneous Enantioselective Hydrogenation: One-to-One Interactions of 1-Phenyl-1,2-propanedione and Methyl Pyruvate with Modifiers on the Pt(111) Surface

“…Heterogeneous enantioselective hydrogenation of methyl/ethyl pyruvate to methyl/ethyl lactate over cinchona alkaloid modified platinum catalysts is an extensively researched topic. ,,,− ,,− Spectroscopic and theoretical data support the model assumption that the modifier (e.g., cinchonidine) is strongly adsorbed via its quinoline moiety oriented preferentially parallel to the Pt surface. − The quinoline part of the modifier is adsorbed even under hydrogenation conditions. ,, The studies on adsorption of α-keto esters on a metal surface indicate that adsorption is affected by the presence of coadsorbed hydrogen and that lone-pair- and π-bonded α-keto esters possibly coexist on the surface. ,,− The π-bonded species adopting the s- cis conformation has been suggested to be relevant in the enantiodifferentiating diastereomeric 1:1 transition state complex between a reactant and a modifier. , 1:1 Complexes between a prochiral reactant, methyl pyruvate, and a chiral modifier, ( S )-(−)-1-(1-naphthyl)ethylamine, on Pt(111) in the presence of coadsorbed hydrogen has been observed by scanning tunneling microscopy (STM) . In situ attenuated total reflection (ATR) infrared spectroscopy studies during the enantioselective hydrogenation of ethyl pyruvate over cinchonidine-modified Pt/Al 2 O 3 catalyst show the preferential adsorption of ethyl pyruvate as an s- cis conformer and indicate a hydrogen bond between th...…”