Electrochemistry Volume 2

A Review of the Literature Published during 1970

By G. J. Hills

The Royal Society of Chemistry

Copyright © 1972 The Chemical Society
All rights reserved.
ISBN: 978-0-85186-017-6

Contents

Chapter 1 Organic Electrochemistry — Synthetic Aspects,
Chapter 2 Electrochemistry of Molten Salts,
Chapter 3 Solid Metal Electrode Reactions,
Chapter 4 Ionic Double Layers and Adsorption,
Chapter 5 Membrane Phenomena,
Author Index, 287,

CHAPTER 1

Organic Electrochemistry — Synthetic Aspects

BY A. BEWICK AND D. PLETCHER

1 Introduction

Judged solely by the number of papers reviewed this year, it would seem that there remains a very high level of interest and activity in the area of organic electrosynthesis. Other criteria would, however, suggest that organic electro-chemists are beginning to turn to other fields, particularly biochemistry. There is certainly evidence that in industry the current economic depression has served to sharpen the criticisms that exciting, specific electrosynthetic reactions have failed to materialize. This disenchantment must, however, be rather premature since much of the published work is still being carried out by workers who are ill qualified in at least one of the essential areas, organic chemistry or electrochemistry. As a result, there are still many examples of electrolyses in which the electrochemical conditions have been completely uncontrolled or in which product analysis has not been made. There has been a definite trend towards better balanced and higher quality work as the interaction between organic chemists and electrochemists has increased, but the number of truly ‘bilingual’ workers is still very low.

It is also apparent that there is still no coherent understanding of the detailed mechanisms by which the various electrolysis conditions determine the efficiency and specificity of electrosynthetic reactions. Some of the changes produced by variation of solvent, electrolyte, and electrode material are not quantitatively, or in some cases qualitatively, understood and further systematic studies are required. On the other hand, it is now well established that electrochemistry is an excellent technique for the generation of the reactive intermediates of organic chemistry and their participation in electrode processes is well characterized.

It is a necessary preliminary to the development of new industrial scale electrosynthetic processes that electrochemical techniques should become established as routine tools among synthetic chemists at the laboratory level. Already during the past ten years a sufficient number of laboratory syntheses in which electrochemical methods offer distinct advantages have been developed. The missing factor which would lead to more widespread acceptance of these has been the lack of a textbook covering electrochemical techniques from the viewpoint of the organic electrochemist. Although a number of books are now appearing, these are all heavily slanted towards particular aspects. The most recent of these, by Mann and Barnes, gives an adequate summary of the reactions of organic compounds in non-aqueous solvents but the treatment tends to be rather non-critical and non-stimulating, and there is little guidance on the applications of electrochemical techniques. Another new review, which has appeared in a context where it could make considerable impact on the synthetic chemist, was restricted in content and failed to demonstrate the range of techniques of modern organic electrochemistry. A number of other reviews have also appeared.

2 Oxidations

A. Aromatic Hydrocarbons. — Dietz and Larcombe have reported the use of cyclic voltammetry to identify some new carbonium ion intermediates in the anodic oxidation of certain aromatic hydrocarbons. For a number of years it has been known that the dicarbanion formed by the reduction of aromatic hydrocarbons in aprotic media will abstract a proton from the environment to form a carbanion and this intermediate may, at more anodic potentials, be reoxidized to a radical, as outlined in Scheme 1. Dietz and Larcombe have shown that there is a parallel process in the oxidation of those aromatic hydrocarbons which form stable cation-radicals. When studying the cyclic voltammetry of 9,10-diphenylanthracene (DPA), they observed that if the potential sweep included the second irreversible oxidation process, a new reduction peak was observed at more negative potentials. They attributed this peak to the reduction of the cation (1) formed in the sequence shown in Scheme 2, where Nu is an unidentified nucleophile, perhaps the solvent, traces of water, or the anion of the inert electrolyte. This behaviour was observed in several solvent-lectrolyte systems (not acetonitrile) and in all cases the new peak occurred at a potential 82 [+ or -] 6 mV more negative than the DPA-DPA·+ couple; the similarity in reduction potential is expected since the lowest unfilled orbital of the cation (1) is non-bonding. A reduction peak attributable to the reduction of a nucleophiledication adduct is also observed in the cyclic voltammetry of 1,3,6,8-tetraphenylpyrene and perylene. In every case this peak is irreversible, probably since the equilibrium (Scheme 3) lies well to the right, as it must do if the cation-radical is stable.

In the case of perylene, a further reversible couple is observed slightly negative to the potential of the hydrocarbon-cation-radical couple. The authors propose that this couple is due to system (2)/(3) in Scheme 4.

Jeftic and Adams have elucidated the overall reaction mechanism for the oxidation of benzo[a]pyrene at a Pt electrode in a series of aprotic solvents. In a detailed and careful study using cyclic voltammetry, a rotating disc electrode, coulometric and spectroscopic techniques, and product isolation, the authors consider the initial electron-transfer step, the intermediates in the reaction and their relative stability, and the final products. As may be seen from the Schemes 5 and 6, the major products are the dimer, and derived polymers, and a mixture of benzopyrene quinones formed by a complex series of electron transfers and hydrolyses by trace quantities of water present in the solvents.

The oxidation of 6-acetoxybenzo[a]pyrene is also discussed, the quinones being the only products; since this reaction does not proceed via a benzopyrene cation-radical, no dimerization or polymerization is observed.

A long series of papers on the oxidation of anthracenes has been produced by Parker. His main techniques are cyclic voltammetry, coulometry, and product identification and his two continuous themes are that (a) electrochemical substitution reactions generally take place by an e.c.e. mechanism and (b) many added reagents have the dual and competing roles of base and nucleophile to reactive intermediates produced at the electrode.

The oxidation of anthracene was studied in acetonitrile containing water, alcohols, acetic acid and acetate i0n. The products from the oxidation in the presence of water were shown to depend on the concentration of water in the system; at low water concentrations the product isolated was anthraquinone and as the water concentration was increased bianthrone became the major product. Finally, at very high water contents, a trimer compound is also produced, in which a 9,10-dihydroanthracene moiety separates two anthronyl units. Coulometry and cyclic voltammetry are used to confirm that the n-value for the oxidation actually decreases as the water content of the solvent is stepped up, and it is suggested that two factors may be important in explaining this surprising effect. They are (a) the relative importance of the water as a nucleophile and as a base may change with its concentration and (b) changes in the adsorption isotherms for anthracene and reaction intermediates may occur as the solvent becomes less anhydrous.

The product from the oxidation of anthracene in acetonitrile containing ethanol or acetic acid is bianthrone. In some earlier short communicaiions* it had been assumed that the bianthrone was formed by reaction between an intermediate and trace water in the system, although the n-value for the reaction and the actual mechanism had been disputed. Parker, however, points out that this mechanism is unlikely since it is difficult to believe that trace water could compete with a nucleophile as strong as an alcohol when the alcohol is present in a large excess and, anyway, the work in the presence of low concentrations of water had shown anthraquinone and not bianthrone to be the major product. Instead, he suggests that the mechanism for the production of bianthrone is that shown in Scheme 7, and supports this view by showing that, in acetonitrile containing ethanol or acetic acid, the 9-substituted anthracenes undergo a quantitative one-electron oxidation to bianthrone. The oxidation of anthracene in acetonitrilealcohol is reported to be an excellent preparative method for bianthrone.

The oxidation of 9,10-dihalogenoanthracenes’ and 9-phenylanthracene in acetonitrile has also been investigated. These species give, as the primary electrode intermediates, cation-radicals with sufficient stability to allow more detailed information to be obtained about the subsequent chemical reactions and the mechanism of substitution reactions. For example, the e.c.e. nature of the oxidation of 9,10-dihalogenoanthracenes may be definitely proved by a rotating disc experiment. A preparative scale electrolysis on the dihalogeno-anthracenes forms the bis-halohydrins (Scheme 8) which decompose to anthraquinone during isolation. The mechanism for the formation of anthra-quinone may be modified by the addition of a halogen acceptor such as cyclohexene. In the absence of an added nucleophile, the cation-radical formed by the oxidation of 9-phenylanthracene simply dimerizes to yield the 10,10′ dimer, while in the presence of ethanol or acetic acid the electrolysis product is the 9-substituted-9-phenyl-10-anthrone formed by the reactions set out in Scheme 9.

In a paper with Eberson, Parker has extended the work, previously reported as short communications, on the steric factors which control whether, during the oxidation of aromatic hydrocarbons and the debromination of 9,10-dibromoanthracene, certain heterocyclic compounds act as nucleophiles or bases. It is shown that cyclic voltammetry may be used to distinguish the two reactions.

Oxidation of anthracenes in acetonitrile containing acetate ion, or methanol containing methoxide ion, has been shown to produce the 9,lO-disubstituted-9,10-dihydroanthracene and Parker has reported a study of the stereochemistry of these reactions.’ He found that in all cases the trans isomer was favoured in the anode reaction, and the ratio of trans to cis varied between three and infinity for different anthracenes; conversely the chemical oxidation of the anthracenes, by lead tetra-acetate, lead to a 50 : 50 mixture. Although in the case of the acetate ion the stereochemical preference could be explained by formation of a cyclic acetoxonium ion, this is not so in the case of methoxylation and it would appear that the electrode must affect the stereochemistry of the products.

A number of papers have considered the anodic oxidation of alkyl-substi-tuted aromatic hydrocarbons. In the non-nucleophilic medium methylene chloride-tetrafluoroborate, the products of controlled potential oxidation of durene, mesitylene, and p-xylenehave been studied. The major product from durene is the diphenylmethane (4); it is isolable in organic yields as high as 85 % and it is thought to arise by electrophilic attack of the trimethyl-benzyl cation on a further durene molecule. The major product from the oxidation of mesitylene is the biphenyl (9, i.e. coupling between nuclei has occurred. Some trimeric nuclear coupled product and polymeric material is also formed. The author suggests that this coupling of nuclei arises from the one-electron oxidation of the substrate (Scheme lo), while the side-chain-nucleus coupling observed for durene occurs via an initial two-electron oxidation of the substrate as shown above. However, the marked difference between the products from mesitylene and durene is surprising and further work on these systems is clearly warranted. p-Xylene oxidation yields mainly polymer but some side-chain-nucleus coupled product is also produced.

Two papers have reported studies of the oxidation of alkyl-substituted benzenes in acetic acid and both conclude that when the hydrocarbon is not discharged at a less positive potential than the anion of the inert electrolyte, the products are best explained by two co-existing mechanisms involving oxidation of the anion and of the hydrocarbon. The first paper reports the oxidation of toluene in acetic acid containing acetate, nitrate, or tolylsate ions while the second paper considers the oxidation of mesitylene in the presence of nitrate ion. In both papers the conclusions are reached mainly from a careful analysis of the products, although the interpretation of the results in the former paper is complicated by the use of constant current electrolyses. The evidence for the discharge of the hydrocarbon is the isolation of some nuclear-substituted acetates which occur in fairly high yields at low conversions. However, the major products are formed by substitution in the side-chain and the isolation of bibenzyls is evidence for benzyl radicals as intermediates. The presence of toluene in the nitrateacetic acid system does not change the current-potential curve of the inert electrolytesolvent system which is strong evidence for discharge of the nitrate ion. Thus the dual mechanism shown in Scheme 11 is postulated. Nuclear substitution is favoured by the presence of free acetate ion when an acetate assisted concerted mechanism has been proposed. It is also pointed out that the situation is different when the hydrocarbon is oxidized at potentials well below that of the anion; in these cases nuclear substitution is favoured.

The oxidation of an aqueous emulsion of cumene at a platinum electrode has been shown to lead to fracture of a carbon-carbon bond and high yields of benzaldehyde (up to 80%) with some acetophenone. The optimum conditions were with sodium hydroxide as the base electrolyte and a current density of 2-4 A dm-2.

While carbonium ion rearrangements are well known in anodic oxidations, cation-radical rearrangements are less common. Miller and Mayeda believe that they have observed a sigmatropic rearrangement of a cation-radical during the oxidation of 1,1,3-triphenylindene at a platinum electrode in a non-nucleophilic solvent, liquid sulphur dioxide, which contained methanol as a trapping agent (see Scheme 12). In methanol as solvent, the phenyl shift does not occur during the reaction, indicating that the solvolysis is too rapid for the rearrangement to take place.

The oxidation of a substituted bibenzyl to a phenanthrene has been reported. This reaction was shown, by means of cyclic voltammetry, to proceed via initial formation of a dihydrophenanthrene (Scheme 13). Cyclic voltammetry on the dihydrophenanthrene showed that it forms a relatively stable cation-radical. Since this species is not oxidized to the dication until more positive potentials, it would seem that the phenanthrene must arise by an e.c.e. mechanism, i.e. the initial electron transfer is followed by loss of a proton to form a radical which can then be oxidized further. This reaction may be taken to be evidence that benzyl cations are generally formed via a radical intermediate.

Shono and Matsumura have reported that the anodic oxidation of arylcyclopropanes in methanol leads to opening of the cyclopropane ring and the formation of ethers. The products are more consistent with initial electron transfer from the aromatic ring rather than the cyclopropane system and this mechanism is supported by the slopes of E1/2 – σ* plots.

The oxidation of ethylenes with various degrees of phenyl substitution has been discussed by Eberson and Parker. They carried out cyclic voltammetry in acetonitrile and preparative electrolyses in an acetonitrileacetic acid mixture and they conclude that the basic mechanism and the products are very dependent on the structure of the olefin. The initial electrode process may involve one or two electrons. Bard and Phelps have, however, disputed their claim that tetra-p-anisylethylene undergoes oxidation in a single, reversible, two-electron step; the data are interpreted in terms of two, reversible, one-electron steps separated only by a few mV. Tetrakis-(p-NN-dimethyl-aminophenyl) ethylene is reported to show behaviour which is consistent with a direct two-electron transfer.

Miller et al. have extended the work, previously reported in a preliminary communication, on the electrophilic substitution of aromatic hydrocarbons by the anodically generated iodine cation; they have used a dual approach. In the first, they oxidized mixtures of iodine and the aromatic hydrocarbon at the potential required for oxidation of the iodine. This led to a mono-iodinated product, but the yield was not high due to the further oxidation of the product and to acetamidation of the aromatic compounds (Scheme 14). In the second approach they added the hydrocarbon after the complete anodic oxidation of the iodine and the yields of mono-iodinated products were then very high. They believe that the electrophilic species is the N-iodonitrilium salt

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and this view was supported by the isolation of the corresponding amide after addition of water.

B. Aliphatic Hydrocarbons. — The anodic oxidation of cyclohexene-chloride ion mixtures in acetonitrile has been reported. At low potentials, where the chloride ion but not the cyclohexene is oxidized, the major product isolated is formed via chlorine evolution and reaction between the chlorine and the cyclohexene (see Scheme 15). Although this is the only product which could be obtained by chemical oxidation of the mixtures, it was shown that at potentials where the cyclohexene is discharged and when suitable concentrations of the reactants were used, the reaction

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could be carried out to give reasonable current yields of 3-chlorocyclohexene; this product is further strong evidence for the carbonium ion as an intermediate in the oxidation of hydrocarbons in aprotic media. The success of this allylic substitution process demonstrates an important feature of electrode reactions: the heterogeneous nature of the process and the consequent need for diffusion of species to the electrode surface introduces the possibility of controlling reactions by controlling the flux of each reactant to the electrode surface. The use of this principle may make possible a range of syntheses which cannot be carried out in homogeneous solution.

(Continues…)Excerpted from Electrochemistry Volume 2 by G. J. Hills. Copyright © 1972 The Chemical Society. Excerpted by permission of The Royal Society of Chemistry.
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