Electrochemistry Volume 7

A Review of Recent Literature

By H. R. Thirsk

The Royal Society of Chemistry

Copyright © 1980 The Chemical Society
All rights reserved.
ISBN: 978-0-85186-870-7

Contents

Chapter 1 Organic Electrochemistry – Synthetic Aspects By J. Grimshaw, 1,
Chapter 2 Membrane Phenomena By N. Lakshminarayanaiah, 40,
Chapter 3 The Application of A.C. Impedance Methods to Solid Electrolytes By W. I. Archer and R. D. Armstrong, 157,
Chapter 4 The Electrical Double Layer By S. K. Rangarajan, 203,
Author Index, 257,

CHAPTER 1

Organic Electrochemistry – Synthetic Aspects

BY J. GRIMSHAW

This Report covers material published during 1975. Papers dealing with physical organic chemistry, such as reaction mechanisms, which have a bearing on electro-chemical synthesis are included. Studies of radical-ions by e.p.r. have been excluded, as have papers on electrochemically initiated polymerization, electro-coating, and related technical fields.

Abbreviations used throughout this chapter are as follows: AN, acetonitrile; DME, 1,2-dimethoxyethane; DMF, dimethylformamide; DMSO, dimethyl sulphoxide; HMPT, hexamethylphosphoric triamide; THF, tetrahydrofuran:

1 General

The coverage of electro-organic synthesis in the Techniques of Chemistry series was completed in 1975. Anodic oxidation was surveyed in another book and chapters on electrochemistry have appeared in textbooks on the chemistry of quinones and of hydrazo-, azo-, and azoxy-groups. Other books have covered electrode kinetics, experimental electrochemistry, and electrochemical data for organic, organometallic, and biochemical substances. Reviews have appeared on general synthetic reactions, the synthesis of cyclic compounds, electroreduction, oxidation, the synthesis and reactions of organometallic compounds, and industrial electrosynthesis, including indirect electrochemical processes and reactor design. The use of ion-exchange membranes in electrochemical cells has been reviewed. Electrochemistry in thin layers of solution is discussed in a critical review and the application of electrochemistry to physical organic problems is discussed. IUPAC have published recommendations for sign conventions and the plotting of electro-chemical data.

A process for the purification of HMPT by fractional freezing, vacuum distillation, and drying over calcium oxide has been described, and the electrochemical properties of several other solvents have been evaluated. Oxydipropionitrile shows a large potential range for reductions at a mercury cathode but no reactions were studied in this solvent, and a possible limitation is that electrogenerated bases will cause elimination to give acrylonitrile. Ethylene carbonate is liquid at 40°C and shows a good range for oxidation and reduction: nitromethane and 1,2-dichloroethane are both satisfactory solvents for oxidation processes. Triethyl-n-hexyl-ammonium triethyl-n-hexylboride is a new ambient-temperature molten-salt solvent with a useful working range for reduction, but the solvent readily undergoes oxidation. A mixture of aluminium chloride (2 moles) and ethylpyridinium bromide (1 mole) is molten at ambient temperatures and forms a strong Lewis acid solvent that is useful for oxidation processes. Quinones solubilized in micelles formed in aqueous sodium dodecylsulphate show well-defined diffusion waves on polarography.

Advances in electronic apparatus for electrochemistry have been reviewed and new designs proposed for function generators and integrators. New designs for laboratory electrolysis cells are available. One of these is formed in a rolled sandwich construction of the two electrodes and a separator cloth. The reaction solution is pumped through a tube packed with the sandwich, so that the substrate is in contact with both the cathode and anode. If the latter situation can be tolerated, then this cell design gives high flow rates and current densities. Porous Teflon has been proposed as a diaphragm material.

Tungsten bronzes have been studied as electrode materials for use in both aqueous and aprotic solvents. They have a large reduction and oxidation range. Attempts have been made to improve the qualities of graphite as cathode material by coating it with mercury and by attaching, with chemical bonds, a surface layer of (S)-(-)-phenylalanine methyl ester, bonded through the amine nitrogen. The latter forms a chiral electrode surface which promotes the reduction of ketones to carbinols with partial asymmetric induction. Acetophenone afforded 1-phenyl-ethanol for which αD (c=3, CHCl3) was -7.2°. However, other workers were unable to repeat this claim of asymmetric reduction. The properties of platinized silica particles as a fluidized-bed electrode for the Kolbe reaction have been examined.

Experimental and theoretical studies have been made on the effect of adsorption of neutral molecules on electrochemical reactions. Cryptate complexes of alkali-metal ions are reduced at very negative potentials, but the potassium ion complex of kryptofix-[2,2,2] is strongly adsorbed at a mercury cathode from dilute solutions, which limits the use of this ion in conducting salts.

General studies on the properties of redox reactions have included a study of the reversible oxidation and reduction of four polynuclear hydrocarbons at pressures up to 2000 atm. The change in partial molar volume which accompanies the redox reaction can then be determined, and this gives information on the solvation changes which accompany electron transfer. When redox potential is determined by cyclic voltammetry, it is usually assumed that the ratio of diffusion coefficients for the redox species is sufficiently close to unity that its logarithm can be taken as zero. In a critical study of some aromatic radical-cations, the diffusion coefficient for the parent molecule was always found to be greater than for the cation, but the ratio could be taken as unity with sufficient accuracy. A linear relationship has been shown between values of the electron affinity and the polarographic half-wave potentials for some cyclic anhydrides. Polarography has been used to detect short-lived radicals and radicalcations that are generated by pulse radiolysis from anthracene, naphthalene, benzene, and acetone.

Further papers have appeared on the use of convolution potential sweep voltammetry in the determination of electrochemical reaction mechanisms, including the acetophenone pinacolization, the intramolecular cyclization of 1,3-dibenzoylpropane, and the coupling of 4-methylbenzylidenemalononitrile. The technique can also be used to determine standard electrode potentials where one part of the couple is unstable.

Ultraviolet spectroscopy has been used to study the intermediates in electrochemical reactions, and there is a developing interest in the application of resonance Raman spectroscopy to the detection of intennediates. Thin carbon films deposited on germanium prisms form optically transparent electrodes suitable for i.r. spectroelectrochemistry.

The contrasting colours of radical-ions and their neutral substrates have been made the basis of electrochromic display systems. Electrochemiluminescence continues to be examined.

2 Reduction

General. — Reduction of acetophenone in a chiral solvent, (S)-(+)-Me2NCH2-CH(OMe)CH(OMe)CH2NMe, gives the same ratio of meso to ([+ or -])-pinacols and the same degree of asymmetric induction in the ([+ or -])-pinacol as is obtained by photo-reduction of acetophenone in the same solvent. This strongly suggests that dimerization occurs by the same step in the two reactions; i.e., by combination of two radicals PhCH(OH)CH3. Cobalt(III) trisacetonylacetonate is destroyed on cathodic reduction, and the reaction in the presence of trimethyl-(-)-menthyl-ammonium perchlorate as supporting electrolyte was found to exhibit enantio-selectivity. The magnitude of this enantioselectivity varies systematically with potential and with electrolyte concentration.

A series of papers on mechanistic electrochemistry in liquid ammonia has appeared. Liquid ammonia has a low dielectric constant, very low acidity, and is a suitable medium for reduction. In the absence of added protonating agents, nitrobenzene and nitrosobenzene are reduced by two reversible one-electron steps to the radical-anion and the dianion. In the presence of isopropyl alcohol as a weak acid, the dianion of nitrobenzene adds one proton and rapidly decomposes to nitrosobenzene. The dianion of nitrosobenzene adds one proton to give an anionic species which can be reversibly oxidized to the parent nitrosobenzene. In the presence of strong acids such as ammonium ions, both compounds are reduced to phenylhydroxylamine. Quinoline is reduced in two one-electron steps, and the radical-anion dimerizes to a dianion, which can be re-oxidized to the parent quinoline. Diethyl fumarate, cinnamonitrile, and acrylonitrile show similar electrochemical behaviour in liquid ammonia to that in aprotic solvents. Dimerization occurs by combination of radical-anions, and the rate is increased by the presence of potassium ions.

Hydrocarbons. — A patent has been issued for the reduction of aromatic steroids in a mixture of liquid ammonia and THF at a steel cathode (Scheme 1). Trioxan has been suggested as a very useful solvent for the related reduction of benzene to cyclohexadiene at a mercury cathode. The reduction of naphthalene in AN to 1,4-dihydronaphthalene has been patented. 3-Hydroxyphenalenone (1) behaves in a manner like that of naphthalene on reduction in an aqueous buffer at a mercury cathode (Scheme 2), to give a dihydro-derivative. Related to the reduction of benzenoid compounds is the electrosynthesis of 2,5-dihydrothiophen-2-carboxylic acid by the reduction, over a mercury cathode, of the lithium salt of thiophen-2-carboxylic acid.

A full paper has appeared describing the advantages of drying solvents over alumina actually in the electrolysis vessel, so as to stabilize the dianions from aromatic hydrocarbons. Under these conditions anthracene, benzanthracene, chrysene, coronene, and perylene show reversible behaviour on cyclic voltammetry due to the formation ofradical-anions and dianions. Cyclo-octatetraene shows two reversible one-electron reduction steps under these conditions, and the rate of charge transfer for addition of the first electron depends on the supporting tetra-alkylammonium cation, being 103 times faster for Me4N+ than for Bu4N+. The activation barrier for addition of the first electron, due to a conformational effect, is not as important as previously considered; electrolyte double-layer effects are greater than any conformational effects.

Polarography of 1-phenylhex-1-yne in DMF shows a single four-electron wave during which hexylbenzene is formed. However, under the conditions of preparative electrolysis, the isomerization of acetylene to allene becomes important, and the dominant process is reduction of the allene (Scheme 3).

Activated Olefins. — A number of patents have appeared on the conversion of acrylonitrile into adiponitrile. A mechanistic study of the hydrodimerization of ethyl cinnamate and diethyl fumarate in DMF at room temperature and lower shows that the reaction proceeds in both cases via a radical-anion dimerization step. Alkali-metal ions (Li+, Na+, K+) greatly increase the rate of dimerization of dialkyl fumarates, ethyl cinnamate, and cinnamonitrile in DMF due to ion pairing with the radical-anions and then rapid dimerization of the ion pairs. The unsaturated nitrites (2; R = H) and (2; R = Me) undergo irreversible one-electron reduction, with dimerization and then cyclization, in DMF, with or without an added proton source (Scheme 4); reaction between two radical-anions is proposed as the dimerization step. The related nitrites (2; R = But) and (2; R =Ph) show reversible radical-anion formation in DMF and further reduction to the dianion (Scheme 5). On addition of a proton donor, two-electron reduction to the dihydro-compound occurs at the potential of the first wave, and an ECE mechanism has been proposed. Electroreduction of αβ-unsaturated nitriles in acidic aqueous solution leads to the production of amines.

Co-electrodimerization of carbonyl compounds with acrylonitrile in aqueous buffers leads to γ-hydroxy-nitriles, while a similar reaction with acrylic acid leads to γ-lactones (Scheme 6).

Carbonyl Compounds. — The mixed electrolytic reduction of 1,4-dimethylpyridinium methylsulphate and acetone leads to mixed coupling products (Scheme 7) along with products from reduction of the pyridine compound. Patents have been issued for the electrolytic preparation of pinacols from simple aliphatic ketones, and in one process the corresponding secondary alcohol is used as the solvent.

Two strikingly similar stereo- and enantio-selective hydrodimerization reactions have been described. Reduction of benzoin gives the racemic pinacol formed by threo-coupling between two molecules of the same enantiomeric configuration (Scheme 8). Hydrodimerization of the racemic tricyclic enone (3) also gives the pinacol by threo-coupling, between two molecules of identical enantiomeric configuration (Scheme 9). No other stereoisomers of the pinacol are formed in each case, although the enone also gives a mixture of ketols. Pinacol formation has been recorded during the reduction of thiophen-2,5-dicarboxaldehyde, 2-benzoyl-thiophen, 2-formylselenophen, 2-acetylselenophen, and acetylferrocene.

Studies on the rate of the hydrodimerization of benzaldehyde in sulpholan, using a rotating ring-disc electrode, have been interpreted as showing that there is dimerization of the radical-anion. The radical-anion of 4-nitrobenzaldehyde reacts too slowly for a rate constant to be determined using this technique. 4-Cyanobenzaldehyde undergoes dimerization by the same mechanism at high current densities but by an ECE mechanism at low current densities, where the chemical step is reaction between the radical-anion and a neutral molecule.

Reduction of amino-desoxybenzoins is dependent on the pH of the solution. If the pH is sufficiently acid that the amino-function is protonated, then cleavage of the carbon-nitrogen bond occurs, as shown in Scheme 10. In more alkaline solutions this reaction is suppressed, and reduction of the carbonyl group to secondary alcohol occurs, giving a mixture of stereoisomers. Griseofulvin is reduced to dihydrogriseofulvin in aqueous buffer solutions.

Examples have been given of the reduction of carboxylic acid to primary alcohol in acidic aqueous buffers, reduction of pyridine-2-carboxylic acid and -2,6-dicarboxylic acid, and the reduction of an amide function (see Scheme 11). The reduction of oxalic acid in aqueous solution to glyoxalic acid is the subject of a patent.

Nitro- and Nitroso-compounds. — Reduction of the two nitro-groups in 2,4-dinitro-phenol and 2,4-dinitrotoluene to amine has been examined. 3-Nitro-4-hydroxy-coumarin is also smoothly reduced to the corresponding amino-compound. Reduction of α-nitrocinnamic acid methyl ester in acid solution gives (+ or -])-phenyl-alanine. Reduction of 2-nitro-2 -isothiocyanatobiphenyl causes electrochemically initiated intramolecular cyclization (Scheme 12), and the product depends on the pH of the solution. In acidic solution, condensation between the generated hydroxylamino-function and the isothiocyanato-group to give (4) is rapid, but in neutral or alkaline solution this condensation is suppressed, and the isothiocyanato-group undergoes reduction. Further reactions then lead to the dihydrobenzo-[c]cinnoline (5).

A detailed mechanistic study of the reduction of nitrosobenzene in DMF is available. The anion from 1,1-dinitroethane in aqueous alkaline solution undergoes reversible one-electron reduction. At more negative potentials an irreversible reduction process occurs.

Other Nitrogen-containing Compounds. — X-Ray crystallography has been used to define the structure of the dihydroquinaldine dimer (6) obtained from cathodic reduction of quinaldine (Scheme 13). The old process for reduction of indoles to their dihydro-derivatives at a lead cathode in 20% sulphuric acid has been revived in a recent patent.

Reduction of the C=N function in aqueous medium to its dihydro-derivative has been observed for a number of heterocyclic systems. Thus reduction of 7-methyl-guanosine (7) in acid medium leads to reduction of the imidazole ring (Scheme 14). Reduction of 1,4-benzodiazepines leads to their dihydro-derivatives. The cyclopropyl ring in prazepam (8) remains intact during this process, as shown in Scheme 15. Reduction of lorazepam (9) in a buffer of pH 10.4 can be terminated at the dihydro-stage, but the initial product loses water in a slow step (see Scheme 16), so that the product that is isolated corresponds to apparently simple replacement of a hydroxy-group by hydrogen. This product will undergo further reduction of the C=N function.

Reduction of the C=N function is the electrochemical step in a potentially useful preparation of amines from amides. The amide is first converted into its O-methyl ether (10), which is reduced in AN at a mercury cathode, an amine being the final product (Scheme 17).

In contrast with the work just described, an extensive study of the reduction of N-benzylidene-4-toluidinein a solvent mixture of MeOH-MeOAc-H2O indicates that the products are the stereoisomeric hydro-dimers as well as the dihydro-compound. Those benzodiazepines which give only the dihydro-compound can be regarded as Schiff’s bases from benzophenone and an alkylamine; it would be useful to extend the study of Schiffs bases to detect any systematic variation of hydrodimer, dihydro-product yields. The yield of hydro-dimer from N-benzylidene-4-toluidine depends upon the availability of protons in the double layer and is increased by using a hydrophobic supporting electrolyte. Typical results are 44% hydro-dimer using KOAc and 58% hydro-dimer using BU4PBr as electrolyte and a mercury cathode, the yield decreasing at copper, lead, or glassy carbon. The ratio of ([+ or -])-: meso-hydro-dimer is in the range 0.9-1.1:1.

Benzyltrimethylammonium salts give bibenzyl as the principle product from reduction in HMPT at an aluminium cathode. The yield of bibenzyl falls when the water content of the solvent rises above 0.25 mol l-1, and toluene becomes the principal product. Use of a platinum cathode in HMPT leads to mixtures of bibenzyl and toluene; toluene is the only product from a platinum cathode in DMSO, DMF, or DME. Toluene is formed by reduction of the intermediate benzyl radical to the carbanion and then proton abstraction. Under these conditions the solution becomes sufficiently basic to promote the Sommelet–Hauser rearrangement of the substrate to form NN‘-dimethyl-o-toluidine.

(Continues…)Excerpted from Electrochemistry Volume 7 by H. R. Thirsk. Copyright © 1980 The Chemical Society. Excerpted by permission of The Royal Society of Chemistry.
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