An energetic explanation of the kolbe electrosynthesis

Sasaki, Kazuo; Nagaura, Shigeo

doi:10.1016/0013-4686(66)87065-2

Cited by 12 publications

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“…Kolbe electrolysis is defined as the one-electron oxidation of carboxylate ions (RCO 2 – ), generating alkyl radicals after decarboxylation. , A one-electron oxidation mediated decarboxylation of carboxylic acids possessing aromatic substituent(s) at the α-position has been described for a cytochrome P450 system . The resulting carboxyl radical (R–COO • ) readily decomposes to afford carbon dioxide and an alkyl radical, where the rate of decarboxylation ( k dec ) of R–COO • is strongly modulated by the structure of the R moiety, with k dec of C 6 H 5 –CH 2 –COO • about 50 times larger than k dec of C 6 H 5 –COO • . , The redox potential values ( E m vs NHE at pH 7) of 1.33 V for (CysS • + H + + e – )/CysSH and 1.40 V for (R • + CO 2 + e – )/R–COO – (R = C 6 H 5 ; no available data for HO–C 6 H 4 –CH 2 ) make it thermodynamically feasible that, in the first step of the decarboxylation reaction, the thiyl radical Cys503-S • could oxidize the substrate’s carboxylate anion to a radical (Figure ). Since according to continuum electrostatic calculations Glu505 and the substrate share a proton, the generated thiolate could be protonated by this residue (at a 3 Å distance).…”

Section: Resultsmentioning

confidence: 99%

Structural Basis for a Kolbe-Type Decarboxylation Catalyzed by a Glycyl Radical Enzyme

et al. 2011

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4-Hydroxyphenylacetate decarboxylase is a [4Fe-4S] cluster containing glycyl radical enzyme proposed to use a glycyl/thiyl radical dyad to catalyze the last step of tyrosine fermentation in clostridia. The decarboxylation product p-cresol (4-methylphenol) is a virulence factor of the human pathogen Clostridium difficile . Here we describe the crystal structures at 1.75 and 1.81 Å resolution of substrate-free and substrate-bound 4-hydroxyphenylacetate decarboxylase from the related Clostridium scatologenes . The structures show a (βγ)(4) tetramer of heterodimers composed of a catalytic β-subunit harboring the putative glycyl/thiyl dyad and a distinct small γ-subunit with two [4Fe-4S] clusters at 40 Å distance from the active site. The γ-subunit comprises two domains displaying pseudo-2-fold symmetry that are structurally related to the [4Fe-4S] cluster-binding scaffold of high-potential iron-sulfur proteins. The N-terminal domain coordinates one cluster with one histidine and three cysteines, and the C-terminal domain coordinates the second cluster with four cysteines. Whereas the C-terminal cluster is buried in the βγ heterodimer interface, the N-terminal cluster is not part of the interface. The previously postulated decarboxylation mechanism required the substrate's hydroxyl group in the vicinity of the active cysteine residue. In contrast to expectation, the substrate-bound state shows a direct interaction between the substrate's carboxyl group and the active site Cys503, while His536 and Glu637 at the opposite side of the active site pocket anchor the hydroxyl group. This state captures a possible catalytically competent complex and suggests a Kolbe-type decarboxylation for p-cresol formation.

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Section: Resultsmentioning

confidence: 99%

Structural Basis for a Kolbe-Type Decarboxylation Catalyzed by a Glycyl Radical Enzyme

et al. 2011

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“…This electrochemical reaction is interesting from a technological point of view as the oxidation is carried out at the electrode, requiring no auxiliary compounds and thus, in theory, improving atom efficiency. [124][125][126] The reaction however is generally agreed to proceed via the formation of a carboxyl radical and subsequent decarboxylation into an alkyl radical. 9.…”

Section: Kolbe Reactionmentioning

confidence: 99%

“…Mechanistically, this reaction is disagreed upon in its basic elements. [124][125][126] The reaction however is generally agreed to proceed via the formation of a carboxyl radical and subsequent decarboxylation into an alkyl radical. Sufficiently stable radicals at this step can go on to form n-1 length alkanes, alkenes or alcohols, but with the correct substrate a radical coupling reaction can occur, creating 2n-2 length carbon backbones.…”

Section: Kolbe Reactionmentioning

confidence: 99%

Deoxygenation of biobased molecules by decarboxylation and decarbonylation – a review on the role of heterogeneous, homogeneous and bio-catalysis

et al. 2015

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Use of biomass is crucial for a sustainable supply of chemicals and fuels for future generations. Compared to fossil feedstocks, biomass is more functionalized and requires defunctionalisation to make it suitable for use. Deoxygenation is an important method of defunctionalisation. While thermal deoxygenation is possible, high energy input and lower reaction selectivity makes it less suitable for producing the desired chemicals and fuels. Catalytic deoxygenation is more successful by lowering the activation energy of the reaction, and when designed correctly, is more selective. Catalytic deoxygenation can be performed in various ways. Here we focus on decarboxylation and decarbonylation. There are several classes of catalysts: heterogeneous, homogeneous, bio-and organocatalysts and all have limitations. Homogeneous catalysts generally have superior selectivity and specificity but separation from the reaction is cumbersome. Heterogeneous catalysts are more readily isolated and can be utilised at high temperatures, however they have lower selectivity in complex reaction mixtures. While bio-catalysts can operate at ambient temperatures, the volumetric productivity is lower. Therefore it is not always apparent in advance which catalyst is the most suitable in terms of conversion and selectivity under optimal process conditions. Here we compare classes of catalysts for the decarboxylation and decarbonylation of biobased molecules and discuss their limitations and advantages. We mainly focus on the activity of the catalysts and find there is a strong correlation between specific activity (turn over frequency) and temperature for metal based catalysts (homogeneous or heterogeneous). Thus one is not more active than the other at the same temperature. Alternatively, enzymes have a higher turnover frequency but drawbacks (low volumetric productivity) should be overcome.

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“…Solvent oxidation at low current densities has been proposed to result from relatively low coverages of the electrode surface by carboxylate ions, which adsorb on the anode at higher positive polarizations [25] . Under standard conditions, this limitation at lower potentials is at odds with the need of low current density for the production of RS−OMe and suppression of RR products.…”

Section: Resultsmentioning

confidence: 99%

Anodically‐Generated Alkyl Radicals Derived from Carboxylic Acids as Reactive Intermediates for Addition to Alkenes

Ding

Orazov

2023

ChemElectroChem

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Electrochemically driven CÀ C coupling has the potential to reduce the cost and environmental impact of some organic syntheses currently accomplished through thermochemical methods. Here, we use electrochemical oxidation of carboxylic acids as a source of reactive carbon-centered radicals that enable radical addition to alkenes in the anode boundary layer. We demonstrate an optimization of reaction conditions to suppress the thermodynamically favored, but synthetically undesirable radical self-coupling in favor of radical addition to styrene. In methanol solvent, 88 % selectivity and 72 % Faradaic efficiency for targeted functionalized benzenes are achieved. For low current densities, iridium anodes outperform platinum, gold, palladium, and glassy carbon anodes. With constant potential or constant current electrolyses, the deposition of organic by-products on the catalyst surface leads to anode passivation. We show that periodic cathodic current pulses effectively regenerate the catalyst. Lastly, we confirm the role of free radicals in the reaction mechanism with a radical trap.

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An energetic explanation of the kolbe electrosynthesis

Cited by 12 publications

References 0 publications

Structural Basis for a Kolbe-Type Decarboxylation Catalyzed by a Glycyl Radical Enzyme

Structural Basis for a Kolbe-Type Decarboxylation Catalyzed by a Glycyl Radical Enzyme

Deoxygenation of biobased molecules by decarboxylation and decarbonylation – a review on the role of heterogeneous, homogeneous and bio-catalysis

Anodically‐Generated Alkyl Radicals Derived from Carboxylic Acids as Reactive Intermediates for Addition to Alkenes

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