Enantioselective hydrogenation of β-dehydroamino acids on a cinchonidine-modified palladium catalyst (original) (raw)
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Enantioselective Hydrogenation ofN-Acetyldehydroamino Acids over Supported Palladium Catalysts
Advanced Synthesis & Catalysis, 2007
The enantioselective hydrogenation of two N-acetyldehydroamino acids over Cinchona alkaloidmodified, supported palladium catalysts has been studied. Moderate enantioselectivities, up to 36 %, were obtained in the hydrogenation of 2-acetamidocinnamic acid over cinchonidine-modified Pd/TiO 2 under low hydrogen pressure. Increase in the pressure or use of benzylamine as additive led to a gradual decrease in the enantiomeric excess and eventually inversion of the sense of the enantioselectivity. On the contrary, the optical purity of the product resulting from the hydrogenation of 2-acetamidoacrylic acid was significantly increased by addition of benzylamine to the reaction mixture. Enantiomeric excess values up to 58 % and 60 % were obtained over Pd/Al 2 O 3 modified by cinchonidine and cinchonine, respectively. These optical purities are the best obtained in the hydrogenation of dehydroamino acid derivatives over chirally modified heterogeneous metal catalysts.
Catalysis Letters
The enantioselective hydrogenation of (E)-2,3-diphenylpropenoic acids substituted by hydroxyl group has been studied over Pd/Al2O3 catalyst modified by cinchonidine. The effect of the acidic hydroxyl substituents was compared with that of the methoxy group in the same position. The para-hydroxyl substituent on the 3-phenyl ring had similar effect on the enantioselectivity as the methoxy group, whereas the meta positioned decreased the optical purity of the saturated acid. This was explained by different origin of the increase in the enantioselectivity obtained in the presence of electron releasing substituents in these positions. Although, the para-hydroxyl group on the 2-phenyl ring had beneficial influence on the enantioselectivity of the hydrogenation of the mono-substituted acid, in the presence of fluorine or hydroxyl group on the 3-phenyl ring the effect of the two substituents was not additive. This study demonstrated that the cinchonidine-modified Pd catalyst is appropriate ...
Journal of Molecular Catalysis A: Chemical, 2005
The enantioselective hydrogenation of four a,b-unsaturated carboxylic acids has been studied over cinchona alkaloid-modified supported Pd catalyst in a high-pressure continuous-flow system using a fixed-bed reactor. The hydrogenation of the aliphatic substrates resulted in products of slightly lower enantioselectivities as obtained in batch reactors under similar reaction conditions. The optical purity of the product formed in the hydrogenation of a-phenylcinnamic acid was over 70%, exceeding the values obtained in slurry reactor in the same solvent. It was shown that the enantioselectivity increasing effect of benzylamine is kept in the fixed-bed reactor. Based on the results obtained by changing the modifier concentration in the feed we suggest that the interaction of the substrates with cinchonidine may occur in the liquid phase and the complex formed is adsorbed on the Pd surface. Thus, relatively high concentration of such complex and consequently modifier amount is necessary for obtaining good enantioselectivities in the hydrogenation of a,b-unsaturated carboxylic acids. #
Journal of Catalysis, 1999
The mechanism of enantiodifferentiation in the hydrogenation of alkenoic acids over cinchona-modified Pd has been investigated using the tiglic acid → 2-methyl-butanoic acid transformation as test reaction. Application of simple derivatives of cinchonidine, modified at the (C-9)-OH and/or the quinuclidine nitrogen, proved that both functional groups are involved in the enantiodiscriminating step. Addition of a strong base (1,8-diazabicyclo[5.4.0]undec-7-ene, DBU) to tiglic acid prior to hydrogenation revealed that one cinchonidine molecule interacts with a dimer of tiglic acid on the metal surface. Ab initio calculations corroborate the existence of an energetically favored acid dimer-cinchonidine intermediate stabilized by hydrogen bonding, involving both the OH and the quinuclidine nitrogen of cinchonidine.
Catalysis Communications, 2008
The hydrogenation of trifluoromethyl substituted prochiral a,b-unsaturated carboxylic acids has been studied over cinchona alkaloid modified palladium heterogeneous catalysts. Low enantioselectivities were obtained in the hydrogenation of all three test compounds, 2trifluoromethylacrylic acid, 4,4,4-trifluoro-3-methyl-2-butenoic acid and (E)-4,4,4-trifluoro-3-phenyl-2-butenoic acid, respectively. Significant increase in the enantioselectivity, up to 43%, was obtained in the hydrogenation of 4,4,4-trifluoro-3-methyl-2-butenoic acid by using benzylamine as additive. The presented results showed for the first time the possibility of enantioselective hydrogenation of a-unsubstituted b-disubstituted a,b-unsaturated carboxylic acids in the cinchonidine modified palladium catalytic system, that resulted in optically enriched saturated acids having the chiral center in b position.
Journal of Molecular Catalysis A: Chemical, 2006
Commercial 5wt.% Pd-containing catalysts doped with cinchonidine were investigated in the enantioselective hydrogenation of (E)-2-methyl-2-butenoic acid (P=4.0MPa of H2, T=298K) with the aim of determining how the activity and enantioselectivity may be affected by specifications of the catalytic system (metal dispersion, support acidity and surface area).
Catalysis Communications, 2009
The hydrogenation of two 2,3-diarylpropenoic acids bearing heteroaromatic substituents, i.e. (E)-2-phenyl-3-(2-furyl)propenoic acid and (E)-2-(3-pyridyl)-3-phenylpropenoic acid was studied over cinchonidine modified Pd catalyst for the first time. The 2-furyl substituted compound was selectively hydrogenated to 2-phenyl-3-(2-furyl)propionic acid in the presence of cinchonidine in up to 73% ee. The hydrogenation of the acid bearing 3-pyridyl substituent afforded lower ee as compared to (E)-2,3diphenylpropenoic acid, reaching up to 61% in presence of benzylamine additive. The differences in the results were assumed to be due to the different adsorption strengths and modes of these acids and deviations in their interactions with the adsorbed modifier.
Catalytic asymmetric hydrogenation of β-substituted α,β,γ,δ-unsaturated amino acids
Tetrahedron Letters, 1999
The Rh-DuPHOS and Rh-BPE catalyzed hydrogenation of IS-substituted ¢x,[~,y,8-unsaturated amino acids establishes two contiguous stereogenic centers simultaneously. Both high regioselectivity and good to excellent enantioselectivity have been demonstrated in this process leading to I~-branched allyl glycine derivatives. Enamide geometry was found to influence stereoselectivity. Studies aimed at defining the scope and limitations of this process are described.