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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

TL;DR: In this paper, 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(COO H)CH2COPh] moiety.
About: This article is published in Journal of Molecular Catalysis.The article was published on 1994-04-20 and is currently open access. It has received 8 citations till now. The article focuses on the topics: Palladium & Catalytic cycle.
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Journal ArticleDOI
TL;DR: In this article, the authors used the water soluble catalyst Pd(TPPTS)3 (TPPTS = sodium salt of trisulfonated triphenylphosphine, P(C6H4-m-SO3Na)3) for renewable 5-hydroxymethylfurfural (HMF) carbonylation.
Abstract: Carbonylation of renewable 5-hydroxymethylfurfural (HMF) under aqueous phase catalytic conditions using the water soluble catalyst Pd(TPPTS)3 (TPPTS = sodium salt of trisulfonated triphenylphosphine, P(C6H4-m-SO3Na)3) was investigated. Pd(TPPTS)3 was easily prepared in situ via complexation of PdCl2 in an aqueous TPPTS solution and reduction with carbon monoxide. Using the Pd(TPPTS)3 catalyst at 70°C, 5 bar CO pressure and [Pd] = 150 ppm chemoselective carbonylation of HMF was observed to yield 5-formylfuran-2-acetic acid (FFA) as the sole carbonylation product; the only by-product was 5-methylfurfural (MF). The formation of MF under these conditions amounts to a new type of catalytic and very selective reduction with CO, formally equivalent to hydrogenolysis of an alcohol group without using H2. This is surprising since one would expect the water gas shift reaction. Both the activity and selectivity of HMF carbonylation were strongly influenced by the TPPTS Pd molar ratio; maximum efficiency being observed for PPTS Pd = 6 . The nature of the anion of the added acid markedly influenced the selectivity. Acids of weakly or non-coordinating anions, such as H3PO4, CF3COOH, p-CH3C6H4SO3H, H2SO4, and HPF6 afforded mainly carbonylation. The selectivity decreased dramatically with acids of strongly coordinating anions such as HBr and HI. With the latter the only product observed was MF. Replacement of TPPTS by ligands containing less −SO3Na groups such as disulfonated triphenylphosphine (TPPDS) or disulfonated tris(p-fluorophenyl)phosphine (TFPPDS) gives rise to a dramatic drop in the catalytic activity. Using palladium catalysts modified with monosulfonated triphenylphosphine (TPPMS) only traces of FFA and MF were obtained. Pd(TPPTS)3 in aqueous media similarly catalyses the selective carbonylation of benzyl alcohol to phenylacetic acid. In sharp contrast, classical hydrophobic Pd PPh 3 catalysts are inactive in this carbonylation reaction in organic solvents. A catalytic cycle is proposed to explain the observed results.

38 citations

Journal ArticleDOI
TL;DR: Asymmetric mixed carboligation reactions of α-ketoglutarate with different aldehydes were explored with the thiamine diphosphate dependent enzymes SucA, Kgd and MenD to selectively deliver chiral δ-hydroxy-γ-keto acids with moderate to excellent stereoselectivity.

31 citations

Journal ArticleDOI
TL;DR: In this article, a sustainable procedure for the synthesis of various alkyl arylacetates from benzyl alcohols has been developed, with palladium as the catalyst and organic carbonates as the green solvent and in situ activator.

23 citations

Journal ArticleDOI
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
TL;DR: In this paper, it was shown that PdCl2(PPh3)2 is an excellent catalyst precursor for the hydrodechlorination of α-chloroacetophenone to acetophenone by hydrogen transfer from the H2OCO system.
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.

13 citations

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