Enantioselective Hydrogenation of Ethyl Pyruvate Catalyzed by 1,2-Diphenyl-ethylenediamine-Modified Iridium Complex: Effect of Solvent (original) (raw)
Related papers
2006
A new reaction kinetic approach was used to describe the enantioselective hydrogenation of ethyl pyruvate over cinchona-Pt catalyst. The above reaction was considered as the sum of two parallel reactions: (i) racemic hydrogenation resulting in R-and S-product in equal amount and (ii) enantioselective hydrogenation leading to the exclusive formation of one of the two optically isomers. New terms such as acc diff and k e /k r (where k e and k r are the rate constants of the enantioselective and racemic hydrogenation, respectively) were introduced to characterize the relationship between the enantioselective and the racemic hydrogenation reactions. Results obtained show that the formation of R-product is rate accelerated, while the formation of S-product is decelerated. The results indicate also that the overall rate increase is a kinetic phenomenon and cannot be attributed to the suppression of the poisoning effect of CO or oligomers formed from ethyl pyruvate. The strong rate acceleration effect of achiral tertiary amines (ATAs) added to the reaction mixture was attributed to the decrease of the loss of modifier during the hydrogenation experiments.
Catalysis Today, 2008
In this work it is studied the enantioselective hydrogenation of ethyl pyruvate using a Pt/SiO 2 catalyst, modified with different chiral auxiliaries: (S)-(+)-1-aminoindan, (1R, 2S)-(+)-cis-1-amino-2-indanol and (S)-(+)-1-indanol. Cinchonidine modified system was taken as reference. It is analyzed the influence of the particle size of the catalyst, the molecular structure of the modifier and the nature of the solvent. The enantioselective hydrogenation of ethyl pyruvate resulted to be a structure-sensitive reaction, and accordingly, the Pt/SiO 2 (B) catalyst (dp = 6.5 nm) provided the best results. The modifier (S)-(+)-1-aminoindan presented an ee of 63%, whereas (S)-(+)-1-indanol gave a racemic mixture and (1R, 2S)-(+)-cis-1amino-2-indanol showed an effect of ''erosion of enantiomeric excess''. Concerning the solvent, a higher ee in 2-propanol was obtained when the modifier used was the (S)-(+)-1-aminoindan, while in toluene, n-heptane and acetic acid the performance of the catalytic systems was not good.
Catalysis Today, 2008
The enantioselective hydrogenation of 1-phenyl-1,2-propanodione (PPD) was investigated using cinchonidine-immobilised Pt/TiO 2 catalysts. Prior to metal deposition, TiO 2 was chirally modified by the direct anchoring of cinchonidine (CD) using trimethoxysilane as coupling agent (TMS-CD). The catalysts were prepared using a high H 2 pressure reduction-deposition method and were characterised by elemental analysis (C, H and N), TG, DRIFT, 13 C and 29 Si solid-state NMR, N 2 adsorption-desorption isotherms, XRD, XPS and HR-TEM. The catalytic activity was evaluated in a batch reactor at 298 K and 40 bar using cyclohexane as solvent with various cinchonidine concentrations. The results indicate that the enantioselectivity was sensitive to the CD surface concentration and the enantiomeric excess of the target product, 1-R-phenyl-1-hydroxy-2-propanone, was in the range of 25-51%. The best catalyst was the one supported on TiO 2 with a nominal content of 10 wt% TMS-CD. The effect of the H 2 pressure, the concentration of substrate, solvent and recyclability of the catalyst were studied. The results obtained confirmed that the variation of reaction conditions affects both the activity and enantioselectivity due to the substrate adsorption on the metal active sites. Concerning the solvent effect, the enantiomeric excess decreased non-linearly upon increasing the solvent dielectric constant; this was attributed to the interactions between solvents and TMS-CD on the surface. In the catalyst recycling studies, the enantiomeric excess decreased up to 40% after the 3rd reuse. The drop of activity and enantiomeric excess was attributed to the hydrogenation of the immobilised CD.
Asymmetric Hydrogenation of Ethyl Pyruvate: Relationship between Conversion and Enantioselectivity
Journal of Catalysis, 1996
The hydrogenation of ethyl pyruvate over cinchonidine-modified Pt exhibited intriguing transient behavior at the beginning of the reaction, and a period of rising rate and enantioselectivity was linked to conversion of substrate for a wide range of reaction temperatures and initial substrate concentrations. A "reaction-driven equilibration" of the surface environment for optimal enantioselective catalysis was proposed for these reactions carried out under conditions where moderate enantioselectivities are observed. c 1996 Academic Press, Inc. We were remiss in Ref. in neglecting to note that Wells' group had made this observation prior to that of our work.
Journal of Catalysis, 2002
Using the Engelhard 4759 catalyst in acetic acid under mild experimental conditions (room temperature, hydrogen pressure 1 bar, DHCD concentration 0.01 mM/L) an optical yield of 92% can be achieved. The high enantioselectivity is accompanied by the following turnovers: EtPy/DHCD > 43,000, EtPy/Pt surface > 1000, TOF = 1-2 s −1 , and DHCD/Pt surface ratio = 0.0072. The enantioselectivity reducing factor is identified by ESI-MS as the gradual hydrogenation of the quinoline skeleton of DHCD that becomes more pronounced with increasing temperature and hydrogenation time. The discovery of oxonium cations, the extremely low DHCD/Pt surface ratio, and the new data obtained by the Pt black + alumina mixture made possible an interpretation of the mechanism of the heterogeneous enantioselective hydrogenation of α-ketoesters. c 2002 Elsevier Science
Journal of Catalysis, 2001
The previously proposed model for reactant-modifier interaction in the enantioselective hydrogenation of activated carbonyl compounds over platinum chirally modified by cinchona alkaloids has been extended to platinum modified by synthetic pyrrolidinylnaphthyl-ethanol modifiers. As in the case of cinchonidine, the most used modifier, the model predicts enantiomeric excess in nearly quantitative agreement with experiment. Excellent agreement is achieved despite the fact that structural assumptions had to be made and the platinum surface was not explicitly taken into account. The one-to-one interaction between modifier and reactant was calculated at the ab initio level. A comparison of the results for different modifiers leads to the conclusion that steric repulsion caused by the anchoring group plays an important role in the enantiodifferentiating interaction. The favoured formation of the (R)product is traced to the fact that the pro-(S) complex leading to the (S)-product upon hydrogenation is more destabilised due to repulsive interaction than the pro-(R) complex. The model calculations are a useful tool for designing effective modifiers and for gaining insight into the mechanism of enantiodifferentiation.
Catalysis Communications, 2008
The effect of the addition of various nitrogen containing and condensed aromatic compounds on both the rate and the enantioselectivity in the hydrogenation of over supported cinchonidine-Pt catalysts has been investigated. The results show that preadsorbed quinoline and acridine cannot be fully replaced from the surface of platinum by cinchonidine. It has been found that the addition of these compounds increases both the reaction rate and the enantioselectivity (both ee max and ee end values). The observation that these additives have no negative effect on the enantioselectivity indicates that generally accepted mechanistic models need some corrections.
Tetrahedron: Asymmetry, 1993
The room temperature and atmospheric pressure hydrogenation of ethyl pyruvate over Pt!A1203 catalysts modified by varying amounts of dihydrocinchonidine was examined. Data were obtained which showed that the hydrogenation occurred on the comer atoms and adatoms on the Pt crystallites in the catalyst. The formation of(S) ethyl lactate was observed when very low concentrations of the alkaloid modifier were used while at higher modifier concentrations the (R) lactate was produced. The formation of the (R) lactate was accompanied by an increase in the hydrogenation rate. A working hypothesis was formulated to explain these results and to serve as a model for the design of future experiments. This model suggested that the initial adsorption of the dihydrocinchonidine takes place on the face atoms adjacent to the comer atoms on the metal crystallite. This will place the chiral portion of the alkaloid close to the comer atom active she. Ensntioselective pyruvate adsorption would be facilitated by a hydrogen bond between the C, OH of the alkaloid and the ethoxy oxygen of the pymvate and hydrogenation of the keto group will lead to (S) lactate formation. In order to place the chiral portion of the modiier near an adatom it is proposed that the alkaloid is adsorbed in an edge-on manner on the face near the adatom active site. In this way the modifier can interact directly with the pyruvate to change its adsorption characteristics. This will lead to an increase in reaction rate and the formation of the (R) lactate.