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Journal ArticleDOI

Synthesis of γ-ketocarboxylic acids via reduction of γ-keto-α-hydroxycarboxylic acids with carbon monoxide catalyzed by a PdHCl system

20 Apr 1994-Journal of Molecular Catalysis (Elsevier)-Vol. 89, pp 93-100

Abstract: A PdHCl catalytic system is highly active in the synthesis of γ-ketoacids of type ArCOCH2CH2COOH via reduction with CO of the ketohydroxy acids ArCOCH2CHOHCOOH. Typical reaction conditions are: PCO: 20–30 atm; Pd/ substrate/H2O/HCl = 1/400–1000/800–3000/ 100–1000 (mol); temperature: 100–110°C; [Pd]: 10−3 to 10−2 M; solvent: dioxane; reaction time: 1–2 h. The reaction occurs in high yield only when the palladium precursor is used in combination with HCl and in the presence of H2O. Under the reaction conditions employed, the palladium(II) complex used as catalyst precursor decomposes to palladium metal. Pd/C is also highly active. It is proposed that the catalytic cycle proceeds through the following steps: (i) The chloride ArCOCH2CHClCOOH, which forms in situ from the starting substrate and HCl, undergoes oxidative addition to reduced palladium with formation of a catalytic intermediate having a Pd-[CH(COOH)CH2COPh] moiety. (ii) Interaction of H2O and CO on the metal yields an intermediate having also a carbohydroxy ligand, (HOOC)-Pd-[CH(COOH)CH2COPh]. (iii) This intermediate, after β-hydride abstraction from the carbohydroxy ligand, gives off CO2 and reductive elimination gives product PhCOCH2CH2COOH. Alternatively, HCl may react with the intermediate proposed in step (i), yielding directly the product and a Pd(II) species, which is reduced by CO to a Pd(0) species, which starts another catalytic cycle.
Topics: Palladium (57%), Catalytic cycle (55%), Catalysis (54%), Reductive elimination (54%), Oxidative addition (54%)

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Citations
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Journal ArticleDOI
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Journal ArticleDOI
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TL;DR: Opening the S‐pocket and simultaneous destabilization of the R‐pathway provides a potential general new strategy to enhance the S-selectivity of ThDP‐dependent enzymes.
Abstract: The thiamine diphosphate (ThDP)-dependent enzyme 2-succinyl-5-enolpyruvyl-6-hydroxy-3-cyclohexene-1-carboxylate synthase from Escherichia coli (EcMenD, E.C. 2.2.1.9) catalyzes the carboligation of α-ketoglutarate (α-KG) and various benzaldehyde derivatives with excellent chemo- as well as high R-selectivity (enantiomeric excess (ee) >93 %) to yield chiral α-hydroxy ketones. Based on the recently developed S-pocket concept, we engineered S-selective EcMenD variants by optimizing the steric properties and stabilization of the acceptor substrate in the S-pocket. Moreover, the moderate S-selectivity of the EcMenD variant I474A/F475G described recently for the carboligation of α-KG and benzaldehyde (ee=75 %) could be improved by selective destabilization of the R-pathway, which resulted in the variant I474A/F475G/R395Y (ee=85 % S). Subsequent investigation of the acceptor substrate range of this new variant revealed high S-selectivity especially with meta-substituted benzaldehydes, which gave access to 5-hydroxy-4-oxo-5-arylpentanoates with excellent enantioselectivities of up to 99 % ee S. Thus, opening the S-pocket and simultaneous destabilization of the R-pathway provides a potential general new strategy to enhance the S-selectivity of ThDP-dependent enzymes.

20 citations


Journal ArticleDOI
Abstract: PdCl2(PPh3)2, in combination with an extra amount of PPh3, is an excellent catalyst precursor for the hydrodechlorination of α-chloroacetophenone to acetophenone by hydrogen transfer from the H2OCO system. The reaction occurs with concomitant evolution of CO2. Under typical reaction conditions (50–70°C, 40–80 atm, substrate/Pd/P = 2000/1/50, H2O/substrate = 8–12/1), the reaction occurs in 70–80% yield in 2 h, using ethanol or dioxane as a solvent ([Pd] = 5 · 10−4 mol · l−1). When the catalyst precursor is employed without adding an additional amount of PPh3 extensive decomposition to metallic palladium occurs. Also Pd C is active in promoting the hydrodechlorination reaction. As expected the reaction rate increases upon increasing concentration of catalyst, carbon monoxide pressure and temperature. The yield is slightly influenced by the concentration of the substrate. The effect of the concentration of H2O is the most significant. In ethanol as a solvent at low concentration of water the reaction rate increases to reach a plateau above 6–7 · 10−2 mol · l−1 of water. On the basis of the fact that it is known that (i) the precursor is reduced to a Pd(0) species by the H2OCO system, even in the presence of hydrochloric acid, which is freed during the course of the hydrodechlorination reaction and that (ii) the starting α-chloroacetophenone oxidatively adds to Pd(0) to give Pd(CH2COPh)Cl(PPh3)2 (I) and that (iii) this complex reacts with hydrochloric acid to give acetophenone and PdCl2(PPh3)2 (II), it is proposed that the hydrodechlorination reaction proceeds via the intermediacy of a species analogous to complex (I) and that (II) is reduced to the Pd(0) complex through the intercation of CO and H2O with the metal center to give a species having a Pd-(COOH) moiety, which after β-hydride abstraction gives a palladium-hydride species with concomitant evolution of CO2. The hydride gives off a proton and reduces Pd(II) returning a Pd(0) species back to the catalytic cycle. We found also that complex (I) is reduced to a Pd(0) complex with formation of acetophenone through the action of H2O and CO. It is proposed that this reaction, which may be at the base of a different catalytic path, occurs via the intermediacy of a species having a HPd(CH2COPh) which, after reductive elimination of acetophenone give the Pd(0) complex starting a new catalytic cycle. In the case of the Pd C catalyzed hydrodechlorination it is suggested that H2O and CO interacts on the surface of the metal to give a hydride and evolution of CO2 and that this hydride displaces a chloride anion from α-chloroacetophenone absorbed on the catalytic surface to give the hydrodechlorination product.

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References
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Journal ArticleDOI
Abstract: A class of highly efficient homogeneous palladium catalyst systems has been developed for the production of perfectly alternating copolymers of carbon monoxide with ethylene. Mixtures of carbon monoxide, ethylene and propene are converted into the corresponding alternating carbon monoxide/olefin terpolymers in which C 3 units randomly replace ethylene units along the chain. The essential features of the new catalyst systems are that they are formed by the combination of an equimolar quantity of a suitable bidentate phosphine ligand with a palladium(II) species in which the counter anions are weakly coordinating. For a series of diphenylphosphinoalkanes of general formula Ph 2 P(CH 2 ) m PPh 2 the most efficient catalyst system for the production of high-molecular-weight polyketones is that with m = 3. High rates with conversions of more than one million molecules of carbon monoxide and ethylene per palladium center are obtained. In methanol, the majority of the polymer chains produced are polyketo-esters of general formula H(CH 2 CH 2 CO) n OMe; analyses of methanol-soluble oligomer fractions shows that diesters MeOCO(CH 2 CH 2 CO) n OMe and diketones H(CH 2 CH 2 CO) n CH 2 CH 3 are also formed. Two interlinked catalytic cycles are invoked to account for the formation of polyketones with keto-ester, diester and diketone end groups.

444 citations