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  • 1
    Electronic Resource
    Electronic Resource
    Restructuring during pretreatment of platinum/alumina for enantioselective hydrogenation (1999)
    Springer
    Catalysis letters 63 (1999), S. 121-126 
    Details
    ISSN: 1572-879X
    Keywords: enantioselective ; hydrogenation ; ketopantolactone ; cinchonidine ; catalyst pretreatment ; adsorbate‐induced restructuring
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The influence of reductive and oxidative heat treatment on the enantioselectivity of chirally modified Pt/alumina has been reinvestigated, using the hydrogenation of ketopantolactone as a test reaction. Enhancement in ee by 39–49% has been observed after treatment in hydrogen at 250–600°C, as compared to untreated or preoxidized catalysts. The changes in ee after reductive and oxidative treatments are reversible, and always the final treatment is decisive. A HRTEM study indicates that adsorbate‐induced restructuring of Pt crystallites during hydrogen treatment at elevated temperature can play a role in the selectivity improvement, but the changes are superimposed by the strong structure‐directing effect of the alumina support. The possible contribution of other effects (complete reduction of Pt n+ surface species, removal of impurities, or change of Pt particle size) could be excluded.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1019085525317
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  • 2
    Electronic Resource
    Electronic Resource
    Enantioselective hydrogenation of 2-methyl-2-pentenoic acid over cinchonidine-modified Pd/alumina (1996)
    Springer
    Catalysis letters 41 (1996), S. 199-202 
    Details
    ISSN: 1572-879X
    Keywords: enantioselective ; hydrogenation ; cinchonidine ; Pd/alumina ; methylpentenoic acid
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract A chiral alkanoic acid was prepared with up to 52% excess of the (S) enantiomer by hydrogenating anα,β-unsaturated carboxylic acid with a cinchonidine-Pd/Al2O3 catalyst system. Favourable conditions are: high surface hydrogen concentration (⩾ 60 bar hydrogen pressure, low catalyst concentration and apolar solvents), near ambient temperature and a cinchonidine/reactantmolar ratio of at least 0.4 mol%. It is proposed that high hydrogen solubility and the presence of 2-methyl-2-pentenoic acid reactant as dimers are advantageous for enantiodifferentiation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00811491
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  • 3
    Electronic Resource
    Electronic Resource
    Transient behavior of the enantioselective hydrogenation of a hydroxymethylpyrone (2000)
    Springer
    Catalysis letters 69 (2000), S. 129-132 
    Details
    ISSN: 1572-879X
    Keywords: enantioselective ; hydrogenation ; cinchonidine ; cinchonine ; palladium ; pyrone
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Various 2‐pyrone derivatives are important intermediates in the synthesis of biologically active compounds. Pd chirally modified by cinchona alkaloids has a potential in the enantioselective hydrogenation of 4‐hydroxy‐6‐methyl‐2‐pyrone to the corresponding 5,6‐dihydropyrone. A study of various parameters (solvent, temperature, pressure, concentration) and catalyst systems (Pd/alumina and Pd/titania, modified by cinchonidine or cinchonine) revealed striking variations of the reaction rate and enantioselectivity with conversion. This transient behavior is interpreted by the effect of competitive adsorption and hydrogenation of the substrate and modifier.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1019007014666
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  • 4
    Electronic Resource
    Electronic Resource
    Palladium-catalysed enantioselective hydrogenation of alkenoic acids. Role of isomerization (1999)
    Springer
    Catalysis letters 59 (1999), S. 95-97 
    Details
    ISSN: 1572-879X
    Keywords: enantioselective ; hydrogenation ; isomerization ; cinchonidine ; Pd/alumina ; alkenoic acid
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The enantioselective hydrogenation of 2-ethyl-propenoic acid over Pd/alumina modified with cinchonidine has been studied. The reaction, carried out in a batch reactor at 1 bar hydrogen pressure and room temperature, revealed that the isomerization of the C=C double bond is a competing side reaction. Double-bond migration and the subsequent hydrogenation of the two isomer alkenoic acids lowered the enantioselectivity drastically due to the formation of opposite enantiomers.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1019049311321
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  • 5
    Electronic Resource
    Electronic Resource
    Enantioselective hydrogenation of ethyl pyruvate. Influence of oxidative treatment of cinchonidine-modified platinum catalyst and hemiketal formation in alcoholic solvents (1994)
    Springer
    Catalysis letters 29 (1994), S. 115-124 
    Details
    ISSN: 1572-879X
    Keywords: enantioselective hydrogenation ; Pt/alumina ; cinchonidine ; ethyl pyruvate ; protonation ; hemiketal formation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The reasons for the increase in the rate and enantiomeric excess after oxidative (aerobic) treatment of Pt/alumina in ethanol have been investigated. It is demonstrated that this treatment results in the formation of acetic acid and consequently in the protonation of the quinuclidine n1 of cinchonidine. This favours the cinchonidine-pyruvate interaction and improves enantioselectivity. In addition, the reaction rate is enhanced due to acid catalysis of the carbonyl reduction. NMR and UV measurements indicate the rapid transformation of ethyl pyruvate to the corresponding hemiketal in primary alcohols as solvents. It is shown that the possible involvement of this hemiketal (and that formed between cinchonidine and ethyl pyruvate) as an intermediate in the pyruvate hydrogenation mechanism can be excluded.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00814258
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  • 6
    Electronic Resource
    Electronic Resource
    Enantioselective hydrogenation of α-ketoesters over Pt/alumina modified with cinchonidine: theoretical investigation of the substrate-modifier interaction (1994)
    Springer
    Catalysis letters 23 (1994), S. 271-279 
    Details
    ISSN: 1572-879X
    Keywords: Enantioselective hydrogenation ; α-ketoesters ; α-hydroxyesters ; cinchonidine ; modifier ; Pt/alumina ; molecular modelling
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Enantio-differentiation in the asymmetric hydrogenation of α-ketoesters to α-hydroxyesters over platinum catalysts modified with cinchona-alkaloid modifiers occurs through interaction of the ketoester with the cinchona modifier. The structure of the probable transition complex has been calculated for the system methyl pyruvate (substrate) cinchonidine (modifier) using molecular mechanics and quantum chemistry techniques at both ab initio and semiempirical levels. The calculations suggest that protonated cinchonidine is energetically more likely to interact with the substrate and that the crucial interaction occurs via hydrogen bonding of the quinuclidine nitrogen and the oxygen of the α-carbonyl moiety of methyl pyruvate. In this complex the methyl pyruvate is transformed into a half-hydrogenated species which is adsorbed on the platinum surface and on hydrogenation yields the product methyll actate. Theoretical studies indicate that adsorption of the complex leading to (R) -methyl lactate is energetically more favourable than that of the corresponding complex which yields (S) -methyl lactate, which may be the key for the enantio-differentiation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00811362
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  • 7
    Electronic Resource
    Electronic Resource
    Enantioselective hydrogenation of ethyl pyruvate in supercritical fluids (1995)
    Springer
    Catalysis letters 34 (1995), S. 1-9 
    Details
    ISSN: 1572-879X
    Keywords: enantioselective hydrogenation ; supercritical fluids ; ethyl pyruvate ; cinchonidine ; Pt/alumina ; CO-poisoning
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The enantioselective hydrogenation of ethyl pyruvate to (R)-ethyl lactate has been studied using gases under supercritical conditions as solvents. The catalyst was a 5 wt% Pt/alumina modified with cinchonidine. In supercritical ethane the reaction time could be reduced by a factor of 3.5 compared to toluene under similar conditions, without any loss in enantioselectivity. A further advantage of ethane is that the enantioselectivity remains high even at high catalyst/reactant ratio, which is interesting in view of a possible application of a continuous fixed-bed reactor for this reaction. A strong catalyst deactivation was observed in supercritical CO2, which is due to the reduction of CO2 on Pt as indicated by FTIR.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00808315
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