Preparative Scale Electrolysis Research Articles

Electrochemical Oxidation of Tetrabutylammonium Salts of Aliphatic Carboxylic Acids in Acetonitrile

The anodic oxidation of a series of tetrabutylammonium aliphatic carboxylates has been performed in acetonitrile on glassy carbon electrodes. The electrochemical behavior was studied without the interference of the anodic oxidation of the solvent. The cyclic voltammetry analysis shows that the electron transfer and the decarboxylation are stepwise. The coulometric analysis shows that the overall mechanism is monoelectronic. However preparative scale electrolysis shows the intervention of two electron-transfer steps leading to carbocations, which then react with the acetonitrile and the carboxylate itself to form -acylamides as principal products. Two types of adsorption of the carboxylate ions and the alkyl carbocations were proposed to occur selectively, namely, on a small fraction of active sites or on the functional groups existing on the glassy carbon surfaces. © 2002 The Electrochemical Society. All rights reserved.

Ru(CH3CN)3Cl3, preparation and use as a mediator for the electrooxidation of hydrocarbons

Hydrogenation of ruthenium chloride in acetonitrile yields complexes of the type Ru(CH3CN)nCl6−n of which three are isolated (n=2, 3, 4). Their formation is traced by voltammetry. Ru(CH3CN)2Cl4 and Ru(CH3CN)3Cl3 have been characterized by single-crystal X-ray diffraction. Voltammetry shows that Ru(CH3CN)3Cl3 acts as a mediator for oxidation of cyclohexene, methylcyclohexene, 1-tetralol and tetralin. Its role in tetralin oxidation is illustrated by preparative scale electrolysis. The compounds Ru(CH3CN)2Cl4 and Ru(CH3CN)4Cl2 did not react with any of the mentioned hydrocarbons.

Electrochemical reductive carbon-to-carbon coupling of σ-ethynyl complexes of transition metals

The electrochemical properties of σ-ethynyl complexes of chromium subgroup metals were studied by cyclic voltammetry and preparative-scale electrolysis. The redox cycle of C5H5(CO)3CrC=CPh was shown to give the bis-carbyne complex (η5-C5H5)(CO)2Cr≡C-C(Ph)=C(Ph)-C-Cr(CO)2(η5-C5H5) formedvia the reductive Cβ−Cβ coupling of ethynyl moieties. The influence of the nature of the metal atom and the ligand environment on the course of this reaction was considered.

Mechanism of the electrochemical reduction of benzyl chlorides catalysed by Co(salen)

The electrochemical reduction of benzyl and 4-(trifluoromethyl)benzyl chlorides catalysed by Co(salen) (H 2salen, N,N′-bis(salicylidene)-ethane-1,2-diamine) was studied in acetonitrile. Electrogenerated (Co 1(salen)) − reacts with the halide to give an organocobalt(III) complex. Further one-electron reduction of the latter yields an unstable intermediate that undergoes rapid decomposition by cleavage of the Co C bond. The mechanism of bond breaking in the one-electron-reduced organocobalt(II) complex was investigated. The results of preparative-scale electrolysis on solutions containing Co(salen) and benzyl chloride, performed under different experimental conditions, in particular in the presence of radical or carbanion scavengers, indicate homolytic cleavage of the Co C bond.

Oxidation-induced ring expansion of a 2-phosphino-2H-azirine

Abstract Oxidation of the 2-phosphino-2H-azirine 1 by preparative scale electrolysis quantitatively leads to the four-membered phosphonium heterocycle 3, while addition of tetrachloro-o-benzoquinone to 1 produces an eight-membered heterocycle 4 with 60 % yield. In both cases, the first step is the formation of a transient phosphorus centered radical cation 1•+, which induces the cleavage of the C N bond and the ring expansion reaction of the azirine moiety affording the four-membered ring intermediate 2•+.

Electrochemical behaviour of three- to nine-membered cyclic polysilanes

The electrochemical behaviour of cyclic polysilanes, namely, hexamesitylcyclotrisilane (Mes2Si)3 (I), 1,2,3,4-tetra-tert-butyltetramethyl-cyclotetrasilane [t-Bu(Me)Si]4 (II), decaethylcyclopentasilane (Et2Si)5 (III), decapropylcyclopentasilane (Pr2Si)5 (IV), dodecamethylcyclohexasilane (Me2Si)6 (V), tetradecaethylcycloheptasilane (Et2Si)7 (VI), hexadecamethylcyclooctasilane (Me2Si)8 (VII) and octadecamethylcyclononasilane (Me2Si)9 (VIII), have been studied by cyclic voltammetry and preparative scale electrolysis. The effects of electrolyte, ring size and nature of substitutes both on oxidation peak potential and upon reactivity as well as style of product are discussed.

Electrochemical Oxidation of 4‐Ethenyl[22]paracyclophane: Cyclic Voltammetry, Electrosynthesis of Polymers, Analysis by Liquid Chromatography, and Mass Spectrometry

Abstract4‐Ethenyl[22]paracyclophane is oxidized electrochemically at a platinum electrode in acetonitrile/0.1 M tetra‐n‐butylammonium hexafluorophosphate. Its behavior in electro‐oxidation is compared to that of styrene. Cyclic voltammetry proves diffusion controlled, direct one‐electron oxidation of the hydrocarbon. The product of preparative scale electrolysis is analyzed by elemental analysis and various spectroscopic techniques. Bond formation occurs at the vinyl group, resulting in analogues of polystyrene. Polymers are separated on a reversed phase chromatography column, and online coupling to electrospray ionization mass spectrometry as well as a combination of size exclusion chromatography and MALDI‐MS prove the formation of species up to the undecamer. The determination of the molecular product masses allows the assignment of the polymer end groups. A mechanistic hypothesis for the radical cation induced electropolymerization of the monomer is developed from the electro‐chemical and structural results.

Electro-organic chemistry of fullerenes. Part 1. Indirect cathodic reduction of vic-dihalides and perfluoroalkyl halides using C 60 as mediator. Cyclic voltammetric study and preparative-scale electrolysis

Although a number of electrochemical studies on the redox behaviour of C 60 and its derivatives have been carried out, there has been no report dealing with C 60 as a redox mediator for preparative-scale indirect cathodic reduction of organic molecules. In this work, C 60 was studied as a redox mediator by both cyclic voltammetry and preparative-scale electrolysis. It was suggested that meso-1,2-dibromo-1,2-diphenylethane could be reduced by electrogenerated C 60 3− while erythro-ethyl-2,3-dibromo-3-phenylproponate and 4meso-diethyl 2,3-dibromosuccinate could be reduced by electrogenerated C 60 2− by cyclic voltammetric studies of C 60 and these bromides. Cyclic voltammetric study also showed that perfluoroalkyl iodide and bromide could be reduced catalytically by electrogenerated C 60 2− and C 60 3− respectively. Preparative indirect cathodic reduction of these vic-dibromides using a catalytic amount (0.02 to 0.04 mol per mol substrate) of C 60 as mediator provided the corresponding olefines in good yield and with high current efficiencies. However, preparative indirect cathodic reduction of perfluorohexyl halides failed because of the strong passivation of the cathode. Thus, C 60 was shown to work as an efficient mediator for preparative-scale indirect cathodic reduction. It is noted that C 60 is a potential power-variable organic reagent since the reducing power of C 60 anion can be easily changed electrochemically without any modification of the C 60 molecule.

Electrochemically activated reaction of nucleophilic substitution in polyfluorovinyl halides under the action of the cyclopentadienylirondicarbonyl anion [CpFe(CO)2

The reaction of polyfluorovinyl halides RCF=CFX (R =t-C4F9, X = F, CI, or Br; R = Ph, X = F, Cl) with the [CpFe(CO)2]− anion has been studied by cyclic voltammetry and preparative-scale electrolysis. The electrochemical activation of vinyl halides made it possible to obtain the products of nucleophilic substitution for all substrates investigated independently of their configuration and the nature of the halogen.

Derivatization of surfaces and self-inhibition in irreversible electrochemical reactions: Cyclic voltammetry and preparative-scale electrolysis

Self-inhibition of irreversible electrochemical reactions is discussed under the assumption that the portion of the surface that is covered by the adsorbing or electrode-derivatizing species blocks the electrode reaction completely. The decrease in the cyclic voltammetric current upon repetitive cycling and the peak characteristics during the first cycle are described as a function of a single dimensionless parameter that is a measure of the competition between the surface and solution reactions and of the matching between the diffusion flux of the substrate and the area occupied by the inhibiting species. The time dependence for the growth of the inhibiting film in preparative-scale electrolysis is also established. Possible extensions of the model in terms of the reaction scheme and the inhibition mechanism are discussed.

Electrochemical carbonylation of organoiron methyl complex: a study of reaction intermediates

Abstract The one-electron reduction of CpFe(CO)2CH3 has been investigated by voltammetry and Fourier transform IR spectroelectrochemistry. The reduction initiates the insertion of CO ligand in the FeCH3 bond. The dissociation of a CO group proceeds in a parallel reaction. Reaction intermediates, the acyl derivative and released CO, form the radical anion of a complex CpFe(CO)2(COCH3) which is able to reduce the parent compound. The reversible redox potential − 1.8 V of CpFe(CO)2(COCH3) allows the regeneration of its radical anion which drives a catalytic cycle. The lifetime of intermediates is shortened by side reactions, one of which is the migration of the acyl group from the central atom to the cyclopentadienyl ring. This explains the apparent discrepancy between products observed in preparative scale electrolysis and the absence of catalytic effects in routine voltammetric experiments.

Electrochemically induced aromatic nucleophilic substitution in p-diiodobenzene with [( η5-C 5H 5)Fe(CO) 2] − and [( η5-C 5H 5)W(CO) 3] −-anions

The reactions of p-diiodobenzene with [( η 5-C 5H 5)Fe(CO) 2] − and [( η 5-C 5H 5W(CO) 3] − were studied using a complex of electrochemical techniques (cyclic voltammetry, rotating ring-disk electrode and preparative scale electrolysis). For [( η 5-C 5H 5)Fe(CO) 2] − it was shown that aromatic nucleophilic substitution of either one or two iodine atoms in p-diiodobenzene for η 5-cyclopentadienylirondicarbonyl can be performed through electrochemical activation of the aryl halide. The ratio of ( η 5-C 5H 5)Fe(CO) 2C 6H 4I and ( η 5-C 5H 5)Fe(CO) 2C 6H 4Fe(CO) 2( η 5-C 5H 5) was shown to depend strongly on the electrolysis conditions: electrode potential electrolysis duration, etc. The yield of ( η 5-C 5H 5)Fe(CO) 2C 6H 4Fe(CO) 2( η 5-C 5H 5) can be considerably increased when ( η 5-C 5H 5)Fe(CO) 2C 6H 4I is taken as a starting compound. For [( η 5-C 5H 5)W(CO) 3I − the main product of the reaction with p-diiodobenzene is ( η 5-C 5H 5)W(CO) 3I.

A General Mechanistic Scheme for Intramolecular Electrochemical Hydrocyclizations. Mechanism of the Electroreductive Cyclization of .omega.-Keto .alpha.,.beta.-unsaturated Esters

A systematic procedure is suggested for formulating possible mechanisms by which electrochemical hydrocyclization and dehydrocyclization can occur. A combination of linear sweep voltammetry (LSV) and chemical and electrochemical arguments can be used to reduce the mechanistic possibilities to a small set or even a unique mechanism. The procedure is applied to the reductive cyclization of ω-keto-α,β-unsaturated esters. Of the several hundred mechanistic possibilities for this process, the combination of LSV and chemical arguments can reduce the list to four; preparative-scale electrolysis of a specially designed substrate permits assignment of a unique mechanism, the e-P-d-c-p path, at low concentrations of electroactive substance

Electrochemically induced aromatic nucleophilic substitution with [(η 5-C 5H 5)Fe(CO) 2] − and [(η 5-C 5H 5Mo(CO) 3] − anions

The reaction of low S N(Ar)-active and non-active aryl halides p-XC 6H 4Hal (X  CH 3, H, Cl, COCH 3, CN or NO 2; Hal  I or Br) with [(η 5-C 5H 5)Fe(CO) 2] − and [(η 5-C 5H 5)Mo(CO) 3] − is studied using a number of electrochemical methods (cyclic voltammetry, rotating-ring-disk-electrode technique and preparative-scale electrolysis). It is shown that the electrode catalysis enables one to carry out aromatic nucleophilic substitution which generally leads to σ-aryl derivatives of (η 5-cyclopentadienyl) irondicarbonyl. Nevertheless, a detailed study of the mechanism of the reduction and oxidation of [(η 5-C 5H 5)Fe(CO) 2C 6H 4X- p] at a Pt electrode reveals that the irreversibility of their reduction processes imposes certain restrictions on performing electrode-initiated aromatic nucleophilic substitution. This results in the occurrence of the “electrochemical activation window”, i.e.. limitation of the potential region where electrochemically induced aromatic nucleophilic substitution is possible. For the [(η 5-C 5H 5)Mo(CO) 3] − anion, we failed to obtain its σ-aryl derivatives using electrochemical activation of aryl halides. The main product of the reaction is (η 5-C 5H 5)Mo(CO) 3Hal.

Evidence for a two steps electron transfer in the electrochemical reduction of dibromodifluoromethane at carbon

▪ In the presence ot ethyl acrylate the height of the peak reduction of CF 2Br 2 (P 1∘) is lowered (P 1). The ratio P 1/P 1∘ can be reduced to 1/2, depending with concentration of the olefin and speed of the cathodic scan. From these data it is possible to derive the kinetic parameter of the reaction of the intermediate radical ∘CF 2Br on the olefin, yielding BrCF 2CH 2CH 2CO 2Et. Reduction of CF 2Br 2 in the presence of ethyl acrylate. ▪ Depending with the potential choosen for preparative scale electrolysis, the three compounds (1), (2), (3) are obtained. These experiments outline the occurence of an initial monoelectronic electrochemical transfer, during the reduction of CF 2Br 2, through the trapping of the intermediate radical ∘CF 2Br in the presence of an activated olefin.

ChemInform Abstract: Outer-Sphere Dissociative Electron Transfer to Organic Molecules: A Source of Radicals or Carbanions? Direct and Indirect Electrochemistry of Perfluoroalkyl Bromides and Iodides.

As an example of the general problem posed in the title, the reduction of CF3Br, CF3I, and CF3-(CF2)n-I (n = 5, 7) by outer-sphere heterogeneous (glassy-carbon electrodes) and homogeneous (aromatic anion radicals) reagents is investigated, in aprotic solvents containing tetralkylammonium salts, by cyclic voltammetry and preparative-scale electrolysis.

ChemInform Abstract: Electrolytically‐Induced Allene ‐ Alkyne Isomerizations.

AbstractThe preparative scale electrolysis of the allenes (I) in the presence of diethyl malonate as a proton donor results in the uptake of 2.0‐2.1 electron/molecule of (I).

Product distribution in preparative-scale electrolysis: Part VIII. Transition between fast and slow chemical kinetics. Application to mechanisms involving one or two first-order consecutive chemical steps

The variations of the product distribution in preparative-scale electrolysis with the various experimental parameters are an important tool for both mechanistic studies and yield optimization in cases where competing chemical reactions are coupled with the initial electron transfer steps. Two limiting situations have been treated so far, involving respectively fast chemical reactions taking place in a thin reaction layer, thinner than the diffusion layer, or, conversely, slow chemical reactions taking place exclusively outside the diffusion layer. The intermediate case of moderately fast reactions and thus the passage from one situation to the other is investigated for two reaction schemes involving one and two irreversible follow-up chemical steps. The variations of the product concentrations with time are discussed as functions of a minimal number of dimensionless rate parameters.

Catalysis of 1,2-dibromo-1,2-diphenylethane reduction on platinum and carbon felt electrodes coated by polypyrrole films containing 4,4′-bipyridinium groups: Effects of film thickness, film composition and comparison with homogeneous catalysis

The catalysis of 1,2-dibromo-1,2-diphenylethane reduction on platinum and carbon felt electrodes coated by popypyrrole films containing viologen (4,4′-bipyridinium) groups has been investigated. The catalytic activities of polymeric films prepared by electropolymerization of differently substituted pyrrole monomers have been compared, on the basis of cyclic voltammetry data. On the other hand, carbon felt appeared to be an excellent material for the preparation by electropolymerization of modified electrodes fit for use in preparative scale electrolysis. We have also compared dibromoalkane reductions on platinum-modified electrodes and with viologen species in solution.

Electrochemical carboxylation of ketene S,S-acetals

The cathodic reduction of ketene S,S-acetals in aprotic media has been investigated. Because of instability of the radical anion the reduction steps are irreversible. In presence of carbon dioxide the half-wave potentials are shifted to more positive values and preparative scale electrolysis yields 2-alkylthio-3-acyl-acrylic acids. The pathway of the electrochemical substitution is discussed as subsequent addition and elimination steps.