9 Electro-organic Chemistry By K. KORINEK and T. F. W. McKlLLOP lCl Ltd. Corporate Laboratory P.O. Box 11 The Heath Runcorn Cheshire 1 Introduction Although the literature in organic electrochemistry continues to expand at an ever-increasing rate much of the published work constitutes rather trivial ex- emplification. The emphasis throughout this review has been placed on those papers which indicate the utility of electrochemical methods in organic synthesis or in the study of organic systems. As in previous years a more comprehensive treatment of the subject is to be found in the Specialist Report on Electro- chemistry.’ The application of electrochemical methods to organometallic chemistry is a growing subject which has considerable promise as indicated in a brief review.2 The substantial number of papers involving organometallic electrochemistry have not been included in this or in previous reviews.Progress continues to be made in developing techniques for the characteriza- tion of intermediates and this is leading to a better understanding of their nature and role in electrode reactions. Advances in this subject formed the basis of a Faraday Discu~sion.~ Several booksL8 and a number of reviewsg-’ have also appeared dealing with a wide range of topics in organic electrochemistry. In addition several established review series have devoted chapters to organic electrode reaction^'^.'^ and to the electrochemistry of biologically interesting compounds.’ 1 ‘Electrochemistry’ ed. G. J. Hills (Specialist Periodical Reports) The Chemical Society London Vol.1 1970 Vol. 2 1972 Vol. 3 1973; ed. H. R. Thirsk Vol. 4 1974. 2 H. Lehmkuhl Synrhesis 1973 377. 3 Discussions of The Faraday Society Vol. 56. 4 ‘Techniques in Electrochemistry’ ed. E. Yeager and A. J. Salkind Wiley New York 1972. 5 ‘Organic Electrochemistry’ ed. M. M. Baizer Dekker New York 1973. 6 ‘Techniques of Electro-Organic Synthesis’ ed. A. Weissberger and N. L. Weinberg Wiley New York 1973. 7 A. P. Tomilov S. G. Mairanovskii M. Y. Fioshin and V. A. Sirnov ‘The Electro- chemistry of Organic Compounds’ Halsted Press New York 1972. 8 ”on-aqueous Electrolytes Handbook’ ed. G. J. Janz and R. P. T. Tomkins Academic Press New York Vol. 1 1972. 9 M. S. Bourbien and J.-J. Rameau Bull.SOC. chim. France 1973 1268. 10 G. Popp Eastman Organic Chemical Bulletin 1973 45 I. 11 L. Eberson and K. Nyberg Accounts Chem. Res. 1973 6 106. 12 L. A. Mirkind and Y. M. Tyurin Elektrosintez. Mekh. Org. Reakts. 1973 181. 13 A. A. Humffray in ‘Modern Aspects of Electrochemistry’ ed. J. O’M. Bockris and B. E. Conway Butterworths London 1972 Vol. 8. 14 M. Fleischmann and D. Pletcher Ado. Phys. Org. Chem. 1973 10 55. 15 ‘Electroanalytical Chemistry’ ed. A. J. Bard Dekker New York Vol. 6 1973. 285 K. Korinek and T. F. W. McKillop 2 Reduction Hydrocarbons.-Aromatic. In recent years the study of aromaticity and related phenomena has provided some intriguing problems for the physical-organic chemist. It is encouraging to see electrochemistry playing a useful part in such studies.For instance Breslow and Chu16 have dzscribed a novel thermo- dynamic method of determining the pK,'s of weak acids such as substituted cyclopropenes. This requires a measurement of the pK + for the corresponding cations and the reduction potentials for the conversion of cation through radical into anion. Although the method can only give approximate values the results for cyclopropenes lend support to the concept of the anions being destabilized or antiaromatic. Using the same approach but reversing the thermodynamic cycle the pK,+ for antiaromatic cations such as cyclopentadienyl can also be obtained.' ' The general utility of the method is shown by the use of a similar series of measurements to determine the acidity of a bicyclic trialkylhydrazine." Electron delocalization can be observed in some unsaturated systems in which one or more methylene groups interrupts conjugation the phenomenon being described as homoaromaticity.Anderson Broadhurst and Paquette' have studied the use of cyclic voltammetry and polarography in non-aqueous solvents in an attempt to quantify such-effects. Although this approach did not provide a polarographic criterion for homoaromaticity the authors indicate that the techniques may be useful in studying the ground-state conformations of poly- olefins. Activated Olefins. In recent years considerable effort has been expended in studying the mechanism of the industrially important reductive coupling of activated olefins [Ann.Reports (B) 1971 68 2991. Saveant and co-workers2' have now reported in full their studies on 13 compounds in solvents of low acidity (acetonitrile dimethylformamide and alkaline ethanol). The kinetics are derived from the variation in peak potential in linear-sweep voltammetry with sweep rate initial concentrations and the amount of proton donor in solution. Where protonation is unimportant the dimerization occurs purely by radical coupling of two anion-radicals and although some ambiguity still exists in the interpretation of the kinetics it would seem that radical coupling still predominates in media of higher acidity. These results are in complete agree- ment with those reported previously by Bard and co-workers. The French group have also published two valuable theoretical papers.The first paper2' treats the interference of solution electron transfer with electrode electron l6 R. Breslow and W. Chu J. Amer. Chem. SOC. 1973 95 41 1. R. Breslow and S. Mazur J. Amer. Chem. SOC. 1973 95 584. S. F. Nelson and R. T. Landis jun. J. Amer. Chem. SOC. 1973 95 5422. I' L. B. Anderson M. J. Broadhurst and L. A. Paquette J. Amer. Chem. SOC. 1973,95 2198. 2o E. Lamy L. Nadjo and J. M. Saveant J. Electroanalyt. Chem. Interfacial Electrochem. 1973 42 189. 21 C. P. Andrieux L. Nadjo and J. M. Saveant J. Electroanalyt. Chem. Interfacial Electrochem. 1973 42 223. Elect ro-organic Chemistry 287 transfer in the particular case of electrohydrodimerization and derives diagnostic criteria of mcchanism and procedures for determining rate constants from linear- sweep voltammetry rotating-disc voltammetry and classical polarography.In the second paper22 the authors consider the effect of proton-transfer reactions on the same systems. Although these papers are concerned specifically with electrohydrodimerization the approach employed is of general interest. If electrochemical reduction is effected on a mixture of two activated olefins cross-coupling reactions may occur and it is studies of these reactions which have provided some of the evidence against the radical-radical coupling mechan- ism. Puglisi and Bard23 have reported their investigations on the reduction of dimethyl fumarate in the presence of cinnamonitrile and acrylonitrile.Various voltammetric and coulometric techniques using rotating ring disc electrodes were used to study solutions in NN-dimethylformamide with tetrabutylammon- ium iodide as supporting electrolyte. The authors conclude that contrary to previous reports crossed coupling competes with direct dimerization only when the ratio of non-reduced olefin to reduced olefin is very large and that crossed coupling is more efficient when both olefins are reduced. Although evidence was found for oxidation-reduction reactions in solution under certain conditions it seems that radical-radical coupling is still the major mechanistic pathway. Carbony1s.-The presence of the electrode surface allows the possibility of introducing stereochemical control during electrochemical reactions.Although well recognised few attempts have been made to utilize this possibility. Stemming largely from an interest in the detailed mechanism involved in the reduction of carbonyls several papers have appeared substantiating and extending Horner’s previous studies in which asymmetric synthesis of the carbinol is obtained by the addition of an optically active compound to the reduction medium. Scheme 1 shows a simplified picture of the possible steps involved in the reduction of a carbonyl-containing compound to the carbinol(1) and pinacol(2). Horner and S~hneider*~ have reported their studies on the reduction of acetophenone in which they examined the ratio of carbinol to pinacol and the optical yield of carbinol as a function of temperature electrode material solvent pH potential and the structure and configuration of the optically active base electrolyte.Mercury and cadmium seem to be the preferred electrodes and as expected lower temperatures tend to give higher optical yields. The effects obtained with different solvent systems and the increase in optical yield on addition of HCl up to 0.2mol1-’ are less easy to explain. Kariv Terni and Gileadi2’ have also studied the reduction of acetophenone but in this case they employed pre-reduced quinidine as the optically active agent. Considerably enhanced asymmetric induction occurs using the more strongly adsorbed alkaloids 22 L. Nadjo and J. M. Saveant J. Electroanalyt. Chem. Interfacial Electrochem. 1973,44 327. ’’ V. J. Puglisi and A.J. Bard J. Electrochem. SOC. 1973 120 748. 24 L. Horner and R. Schneider Tetrahedron Letters 1973 3133. 25 E. Kariv H. A. Terni and E. Gileadi Electrochim. Acta 1973 18 433. K. Korinek and T F. W. McKillop OH I R' -C-R2 I H OH (1) I R' -G-R2 (4) Scheme 1 but essentially the same trends are observed. The authors interpret their results as evidence for an adsorbed complex between the alkaloid and acetophenone or one of its reactive intermediates (3) or (4).One can probably take the conclusions a step further however and this has been done recently by a French group who studied the reduction of phenylglyoxalic acid to mandelic acid in aqueous con- ditions on a mercury cathode.26 As these authors point out the final stereo- chemistry of the alcohol must depend essentially on the environment of the carbanion (4)during its life-time and therefore it is the influence of the alkaloid on the carbanion which is of critical importance.Such a mechanism would also lend itself better to an understanding of the effect of pH and addition of other solvents. Not surprisingly if adsorption is important this same group reports that the efficiency of the stirring has a profound effect on the optical yield higher optical yields being obtained as the stirring is improved. Andrieux and Saveant2' have continued their study of the reduction of M-substituted ketones and have reported the successful pinacolization of tri- fluoroacetophenone using acetonitrile as solvent and a mixture of lithium and tetraethylammonium perchlorates as supporting electrolytes.The yield obtained (35%) is double that achieved by photochemical methods. Traditionally the electrochemist has endeavoured to keep the system as simple as possible when studying an organic compound and this has undoubtedly restricted his view of the potential use to which electrochemical methods may be put. Perhaps indicative of a change in attitude is a paper by Lund and Simonet28 in which they study the reduction of some ketones and imines in the presence of a less easily reduced alkyl halide. Since reduction leads to radical anions and 26 M. Jubault E. Raoult and D. Peltier Compt. rend. 1973 277 C 583. 21 C. P. Andrieux and J. M. Saveant Bull. SOC. chim. France 1973 2090. 28 H.Lund and J. Simonet Bull. SOC.chim. France 1973 1843. Electro-organic Chemistry 289 possibly dianions it is hardly surprising that in the presence of good electro- philes substitution reactions ensue. In such a complex system there are clearly a number of competing reaction pathways and as the authors indicate their studies are at a preliminary stage yet it is quite conceivable that substitution reactions of synthetic use could be derived from such an approach. After ail the organic chemist usually has to contend with complex systems and competing react ions. Reductive Cleavage of Halides.-The first step in the reduction of haIogeno-aromatic compounds is the formation of the corresponding radical anion. The stability of this radical anion and the chemistry which results from its formation depend on a variety of factors such as the medium employed the electrochemical condition of reactions and the nature and position of the halide and any other substituent present.Indeed some of the papers dealing with this subject provide elegant examples of the sophistication and experimental subtlety needed to cope with such molecular perversity. For example Hawley and co- worker~~~ have examined the decomposition of the radical anions obtained from 2- 3- and 4-fluorobenzonitrile in NN-dimethylformamide. As shown in Scheme 2 each compound behaves differently 2-fluorobenzonitrile (5) giving rise to benzonitrile (6) via a dimerization and disproportionation sequence whereas 3-fluorobenzonitrile (7) undergoes elimination of the cyano-group followed by hydrogen abstraction to give fluorobenzene (8).Benzonitrile and 4,4'-dicyanobiphenyl(I 0) are the products of reduction of 4-fluorobenzonitrile (9). The biphenyl is believed to result from dimerization of the fluorobenzonitrile radical anion followed by rapid loss of the two fluoride ions while benzonitrile is formed by loss of fluoride ion and hydrogen abstraction. Alwair and Grimshaw have also been interested in the factors which control the carbon-halogen fragmentation of halogenoaromatics and have reported their findings in two papers dealing with 4-(chlorostyryl)pyridines30and deriva- tives of quinoline quinoxaline and phena~ine.~ The authors conclude that the stability of the carbon-halogen bond in a halogenated radical anion depends upon the strength of this bond and the redox potential of the substrate-radical anion couple.Further they have shown that within a given series the rate of cleavage can be correlated with the free electron density in the radical anion at the carbon terminus as calculated by Hiickel MO methods. Of more interest to the organic chemist is a paper by Nelson Carpenter and Leo32 dealing with the reduction of monohalogenated nitrobenzenes in non- aqueous media. As well as studying the mechanistic features these authors have attempted to examine the synthetic possibilities already demonstrated or potentially obtainable from the reduction of halogenonitrobenzenes (Scheme 3). 29 K. H. Houser D. E. Bartak and M. D. Hawley J.Amer. Chem. Soc. 1973,95 6033. 30 K. Alwair and J. Grimshaw J.C.S. Perkin It 1973 1150. I K. Alwair and J. Grimshaw J.C.S. Perkin It 1973 181 I. " R. F. Nelson A. K. Carpenter and E. T. Seo J. Electrochem. Soc. 1973 120 206. K. Korinek and T. F. W.McKillop .bcN][N=cwc=N] dimerize, e (5) disproportion I FacN -* (7) [.=CmC=N] Scheme 2 Electro-organic Chemistry 29 1 X = p-Br;o-Br AN-TEAP b -2.0 v CD3CN-TEAP b AN-TEAP I CN NO aprotic media I b AN = MeCN; TEAP = Et4N+C104-Scheme 3 The electroreduction of simple alkyl halides is of considerable importance in the production of lead alkyls and this has given rise to detailed mechanistic studies which are now forming the basis of synthetic methods for other organo-metallic compounds.In the context of organic chemistry however it is the reduction of dihalogenated compounds leading to unsaturated or cyclized products which holds most interest. The concertedness or otherwise of 1,3-elimination reactions has been one of the most controversial subjects in organic chemistry in recent years and this has been equally true of electrochemical 1,3-eliminations [Ann.Reports (B) 1971 68,2041. K Korinek and T. F. W.McKillop Fry and Britt~n~~ have now reported in full their results obtained from the reduction of meso-and dl-2,4-dibromopentane (1 1) in DMSO using a mercury cathode. That both isomers yield essentially the same amounts of the cis-and trans-dimethylcyclopropanes (1 2) and (13) argues strongly against a concerted mechanism.The authors have extended their investigation by preparing the (+)-(2S,4S) stereoisomer of (1 1). On reduction this yielded the (-)-(lR,2R)-form of (13) of high optical purity which the authors interpret as cyclization occurring from the carbanion via a semi-VV transition state with inversion. Azizullah and Grim~haw~~ have studied the reduction on mercury of endo-2 endo-6-dibromobornane (14) in which it is impossible to achieve the semi-W transition state. Aqueous ethanol and NN-dimethylformamide were the solvents used with either tetrapropylammonium perchlorate or tetraethylammonium bromide as supporting electrolytes and under these conditions the cyclized compound tricyclene (15) was usually the major product with bornane (16) as the only other hydrocarbon produced in significant quantity.The authors showed that bornane arises by stepwise reduction of each bromo- group to the carbanion which is then protonated and that tricyclene does not arise by any solvolytic process. Clearly tricyclene must arise either by front-face displacement of a bromine ion by the carbanion or via a more concerted process possibly involving a biradical ; the authors prefer the latter explanation. Until recently the reduction of am-dibromides has always been discussed in terms of non-adsorbed intermediates yet it is well known that chemisorption of radical intermediates occurs to a significant extent on mercury cathodes. Brown and Gonzalez3’ have re-examined the reduction in non-aqueous media of.1,3-dibromopropane and 174-dibromobutane on mercury and aluminium cathodes. Product analysis and kinetic measurements have shown that reduction on mercury proceeds through adsorbed radicals whereas on aluminium products are best explained in terms of free-radical reactions initiated by solvated electrons. 33 A. J. Fry and W. E. Britton J. Org. Chem. 1973 38 4016. 34 Azizullah and J. Grimshaw J.C.S. Perkin I 1973 425. 35 0. R. Brown and E. R. Gonzalez J. Electroanalyt. Chem. Interfacial Electrochem. 1973 43 215. Electro-organic Chemistry 293 Nitro- and Nitrosocompounds.-Although the electroreduction of nitro-substituents to amines is an extremely convenient and well-studied procedure organic chemists continue to avoid its use often preferring more complicated and less specific chemical methods.Perhaps by examining one or two cases from among the numerous recent examples organic chemists will be encouraged to try electrochemical methods. A very straightforward preparation of p-aminobenzyl cyanide from the corresponding nitro-compound has been des- ~ribed.~~ Using a mercury cathode and a solvent system of 1 1 4N-HCl in EtOH 95 % chemical yield can be obtained with almost 100 % current efficiency. Tryptophan can be readily produced by reduction of indolyl nitroacrylate in 89 % EtOH the best yields being achieved at pH 1-3 with a lead ~athode.~' Provided that a suitable ortho-substituent is present the reduction of aromatic nitro-groups can often be directed towards cyclized products as is the case with some a-(o-nitropheny1)ketones (17).38 These compounds usually show two discrete waves by polarography.Acidic or slightly basic solutions of these compounds on electrolysis at the first reduction potential using a mercury cathode give quantitative yields of the hydroxyindole (18) by a four-electron reduction as shown in Scheme 4. At higher potentials however a six-electron R' R' R' 1-H20 I H reduction occurs leading directly to the indoles (19). Although chemical reduc- tion methods have been employed it can be extremely difficult to stop reduction at the hydroxylamine stage if hydroxyindole is required. Carbon-Nitrogen and Nitrogen-Nitrogen Double Bonds-As in previous years an enormous amount of work has been carried out on the reduction of various heterocyclic systems especially those which are of biological interest.Unfor-tunately they deal solely with detailed examinations of the electrochemical 36 P. E. Iversen J. H. P. Utley and S. 0. Yeboah Org. Prep. Proced. Internat. 1973 5 129. '' 1. A. Avrutskaya K. K. Babievskii V. M. Belikov E. V. Zaporozhets and M. Ya. Fioshchin Elektrokhimiya 1973 9 1363. 38 R. Hazard and A. Tallec Bull. SOC.chim. France 1973 3040. 294 K. Korinek and T. F. W.McKillop mechanisms which are of little general interest to the organic chemist. One of the few exceptions is a short paper by Grimshaw and Tro~ha-Grimshaw~~ on the reduction of some styrylpyrazole derivatives. The polarogram of 5-phenyl-3-styrylpyrazole (20) in anhydrous NN-dimethylformamide shows two discrete waves and by controlled-potential reduction at the first wave in moist NN-dimethylformamide reasonable yields of 5-phenyl-3-( 2-phenylethy1)pyrazole (21) were obtained.The diphenyl-substituted pyrazole (22) however showed three waves in the polarogram and on reduction at a potential near the diffusion plateau of the first two waves afforded the tetrahydro-product (23). The authors point out that these electrochemical reductions are an alternative to catalytic hydrogenation and may be especially valuable in compounds containing some other functionality prone to catalytic reduction. CH,CH,Ph 3Ph J--i I I H +4e mCHzCH2Ph I I Ph Ph (23) In recent years one or two examples have been reported which indicated that electrochemical reduction of oximes yielded opposite stereochemistry to that obtained by dissolving metal reductions.A cautionary note is sounded by Allen- mark and Helgee4' who have studied the reduction of 1,2-(~-ketotetramethylene)-ferrocene oxime. They found that electrolysis gives better yields of amine but at the expense of selectivity. With both electrochemical and sodium in ethanol reductions however the endo-isomer predominates. Since the earlier reports offered considerable promise to the organic chemist it would seem well worth- while trying to determine the various factors involved in such reductions. Reduction of Bondsinvolving Sulphur.- The reductive cleavage of carbon-sulphur bonds continues to receive attention.For instance Japanese workers have reported the conversion of derivatives of S-methylmethionines into a-amino- butyric acid with reasonable yields.41 Thioethers and tertiary sulphonium 39 J. Grimshaw and J. Trocha-Grimshaw J.C.S. Perkin I 1973 1275. 40 S. Allenmark and B. Helgee Acta Chem. Scand. 1973 27 1816. 41 T. Iwasaki M. Miyoshi M. Matsuoka and K. Matsumoto Chem. and Ind. 1973 1163. Electro -organ ic Chemistry ion derivatives of /3-mercaptopyruvic acid provide an interesting series of com- pounds in which reduction can occur either at the carbonyl group or with cleavage of the a-s~bstituent.~~ In aqueous or aqueous-alcoholic media at a mercury electrode the sulphonium salts undergo carbon-sulphur cleavage whereas the direction of reduction with thioethers R’SCH2-CO-C0,R2 depends on the nature of the R’ group.When R’is aryl cleavage of the S-CH bond still occurs but when R’ is alkyl reduction of the carbonyl group takes place. A similar dependence on the nature of R has been found in the reduction of a-arylamino-esters which can proceed with high yields.43 With tosylates cleavage can be observed at the sulphur-oxygen bond rather than the carbon-sulphur bond. Gerdi144 has examined the reduction of various ditosylates in aprotic media such as NN-dimethylformamide with a particular interest in the intramolecular cyclization shown in Scheme 5. Not surprisingly Scheme 5 the reaction occurs most efficiently for n = 2 to give high yields of ethylene oxide.From all the data obtained the author concludes that it is the orientation of the molecule at the electrode which determines the efficiency of ring closure. Perhaps the most interesting paper to appear recently involving electro- chemistry and organic sulphur compounds is that involving a new synthesis of s~lphones.~’ Sulphur dioxide is reduced in aprotic media at a platinum electrode to give the blue radical anion which reacts with alkyl halides to give the sulphone as shown in Scheme 6. The yields can be as high as SO-SO% and provided that SO2 + e-+ S02-Sol-+ RX -+ RSOl + X-RSOl + S03-+ RS0,-+ SO2 RSO2-+ RX -+ R-SO,-R + X-Scheme 6 the electrochemical conditions can be kept simple this could be the preferred route to various sulphones.With certain dihalides e.g. (24) the sultine (25) is the product of electrosynthesis but this can easily be converted into sulphone (26) by treatment with silica gel in ethanol. 42 J. Moiroux and M. B. Fleury Electrochim. Acta 1973 18 691. 43 K. Matsumoto M. Suzuki T. Iwasaki and M. Miyoshi J. Org. Chem. 1973,37 2731. 44 R. Gerdil Helv. Chim. Acta 1973 56 1859. 45 D. Knittel and B. Kastening J. Appl. Electrochem. 1973 3 291. K. Korinek and T. F. W.McKillop o",'40 \ 0 Miscellaneous.-The possible involvement of hydrated electrons in many cathodic reduction processes has been the subject of considerable debate in recent years. An earlier claim to have measured the concentration of hydrated electrons near an electrode using an optical method has now been challenged and has resulted in a dial~gue.~~,~' Bewick and Avaca have studied the physical and mechanistic aspects of formation of electrons generated in HMPA.48 Another intermediate of possible synthetic interest which is electrochemically generable is the superoxide ion.Mayeda and Bard4' have used this intermediate in a novel way to produce singlet oxygen. Superoxide ion from oxygen and ferricenium ion from ferrocene are produced alternately in the same acetonitrile solution by pulsing between the reduction potential of oxygen and the oxidation potential of ferrocene. The reaction sequence shown in Scheme 7 then occurs in solution leading to singlet oxidation which was identified by chemical quench- ing. Fer % [Fer].+ [Fer].' + 01-* Fer + '02 Scheme 7 The separate electroreductions of ethylene and carbon dioxide have been well studied. Gambino and Silvestri'' have recently described the production of succinic acid and oxalic acid when reduction is carried out in the presence of both gases in aprotic media. By controlling the partial pressures they showed that electrosynthesis can be directed towards either of the two acids. This is an intriguing system both from the mechanistic point of view and from its implica- tions for commercial processes and it clearly merits further study. 3 Oxidatious Hydrocarbons.-A great deal of interest has recently been shown in the anodic oxidation of aromatic compounds in non-nucleophilic media. Many different solvents and a variety of experimental conditions were used to obtain information 4h A.Bewick B. E. Conway and A. M. Tuxford J. Electroanalyt. Chem. Interfacial Electrochem. 1973 42 App. 11-15. 47 D. C. Walker J. Electroanalyt. Chem. Interfacial Electrochem. 1973,42 App. 17-18. L. A. Avaca and A. Bewick J. Electroanalyt. Chem. Interfacial Electrochem. 1973,41 395. 49 E. A. Mayeda and A. J. Bard J. Amer. Chem. SOC. 1973,95,6223. 50 S. Gambino and G. Silvestri Tetrahedron Letters 1973 3025. Electro-organic Chemistry 297 on the inherent stability of cation radicals generated in these reactions. Greatly enhanced stability of radical cations has been observed at low temperatures in trifluoroacetic acid and in AlCl melts. Discussion about the mechanism of anodic oxidation of aromatic hydrocarbons has centred on two possible reaction mechanisms the eecc and the ecec mechanisms.It has also been shown that reactions of certain radical cations proceed to a dication (27) through a fast disproportionation equilibrium ArH-' ArH2+ + ArH (27) but the extent to which this reaction contributes to the overall process has been questioned. Parker and Hammerich have now shown5' that it is possible to observe irreversible behaviour for both the oxidation and reduction step of 4,4'-dimethoxybiphenyl thianthrene and 9,lO-di-p-anisylanthracenein aceto-nitrile nitromethane dichloroethane and other solvents containing a suspension of neutral alumina which is believed to remove the last traces of water. In solvents containing trifluoroacetic acid or the corresponding acid anhydride stable solutions of radical cations and dications were prepared.The dispropor- tionation constants calculated from the reversible electrode potentials are very dependent upon the solvent used and the cationic system in question. The mechanistic aspects of anodic substitution reactions in the presence of the nucleophile have been the subject of another paper by Parker and Jen~en.~~ Cyclic voltammograms of 9-phenylanthracene in the presence of pyridine have shown a pre-peak the height of which was in direct relation to the stoicheio- metric ratio of 9-phenylanthracene to pyridine. Parker had previously suggested that this observation could be consistent with a nucleophile-assisted electron transfer.Using a digital simulation technique it was found that an ecec mechan-ism gave substantial cathodic shifts of the peak potential for the oxidation of a substrate in the presence of a reactant and if the rate constants were sufficiently high not only a cathode shift but also a pre-peak was observed on simulated voltammograms. The experimental observations cannot therefore be used as evidence for an assisted mechanism. The details of the reaction mechanism are however only of limited importance in the practical applications of anodic aromatic substitution which has been reviewed by Eberson and Nyberg." The method offers a unique way of intro- ducing functional groups into cheap starting materials such as hydrocarbons. Another type of synthetically useful reaction the mixed oxidative coupling of aromatic compounds has been investigated by N~berg.~~ Oxidation of napththalene at a platinum anode in the presence of alkylbenzene in MeCN- MeC0,H (volume ratio 9 1)containing 0.1 M tetrabutylammonium tetrafluoro- borate formed two main products 1-arylnaphthalene and 1,l'-binaphthyl.The relative yield of arylnaphthalene follows the relative nucleophilicity of the 5' 0.Hammerich and V. D. Parker Electrochim. Acta 1973 18 537. 52 B. S. Jensen and V. D. Parker Electrochim. Acta 1973 18 665. 53 K. Nyberg Acta Chem. Scand. 1973 27 503. K. Korinek and T. F. W.McKillop alkylbenzene which suggests an electrophilic reaction between naphthalene cation radicals and alkylbenzenes.The preparative electrolysis of naphthalene in the presence of the more nucleophilic hydrocarbons isodurene and penta- methylbenzene produced the mixed coupled products 1-(2,3,4,6-tetramethyl- pheny1)naphthalene and l-(pentamethylpheny1)naphthalene in 42 and 56 % isolated yields respectively. The fact that the products of oxidative coupling are often more easily oxidized than the original substrates has been limiting the usefulness of oxidative cycliza- tions. Ronlan Hammerich and Parker have now reporteds4 a study of oxidative intramolecular cyclization of methoxybibenzyls and bis-(3-methoxyphenyl)- methane in acetonitrile and in acetonitrile containing trifluoroacetic acid. In acetonitrile the yields of dimerized products were low but in the presence of TFA the cation radicals of the coupled products were stable and often on subse- quent reduction produced substituted dihydrophenanthrene (28) or fluorene (29) in high yields.Me -OmO-M e MeOmOMe An interesting influence of halide ions (Cl- Br- I -) on the electroluminescence (ecl) of 9,lO-diphenylanthracene (DPA) has been reported by Kihata Sukihara and Honda.” They have used a controlled-potential double-step method in which the potential of the electrode is changed in a stepwise manner and have shown that in a solution containing DPA. -and no halide ions the ecl emission appears at a potential corresponding to the first anodic wave of DPA as shown in Scheme 8. In the presence of halide ions the ecl emission in acetonitrile DPAs-+ DPA.’ + 2DPA + hv Scheme 8 containing DPA.- appears at the potentials of the first anodic wave of the halide ion.It is suggested that (DPA-X-)* is an important intermediate in the ecl emission the mechanism being shown in Scheme 9. This mechanism is also DPAs-+ X,-+ DPA -X. + 2X-DPA -X. + DPAs--+ DPA -X-* + DPA DPA -X-* + DPA* + X-DPA* -+ DPA + hv Scheme 9 54 A. Ronlan 0. Hammerich and V. D. Parker J. Amer. Chem. Soc. 1973 95 7132. 55 T. Kihata M. Sukihara and K. Honda Electrochim. Acra 1973 18 639. Electro-organic Chemistry 299 supported by the low intensity of the ecl of the DPA-iodide system since the heavy atom will play a role in the energy-dissipation process of the excited species. The Southampton groups6 has reported studies on oxidation of fluoro-aromatic hydrocarbons in fluorosulphonic acid containing acetic acid (0.1 moll-I) which in this medium functions as a base.Most of the compounds studied have shown an initial one-electron oxidation to form a cation radical which is stable on the time-scale of cyclic voltammetry. Results of some prepara- tive studies have shown that fluorosulphonate esters may be prepared using this technique. One report on the oxidation of alkanes in similar media has already appeared showing that ap-unsaturated ketones are the major products. This work has now been extended” to show that in fluorosulphonic acid solutions which contain less of the added base the anodic oxidation of butane propane and ethane becomes possible. These experiments have clearly demonstrated that the proton-donating ability of the medium is determining the electrode and chemical behaviour of alkanes.A 50-60% yield of ctp-unsaturated ketone (30) was achieved in the anodic oxidation of cyclohexane in the presence of acetic propionic or hexanoic acid. 01.15 M RCOZH?YcoR High yields of acetamidated adamantyl derivatives were also achieved by the anodic oxidation of adamantane and 1-halogenoadamantanes in acetonitrile containing lithium perchlorate as supporting electrolyte.’’ Clark Fleischmann and Pletcher” have also reported a comparative study of the anodic oxidation of a series of aliphatic hydrocarbons in six solvents containing tetrafluoroborate as supporting electrolyte. Scheme 10 shows the steps involved in product forma- tion from propylene.It was found that the basicity of the solvent determines the stability of the cation radical to loss of proton and the degree to which the production of the starting material is important. The results have also suggested that in sulpholan and propylene carbonate protonation was much more important than in aceto- nitrile and nitroethane. Furthermore the nucleophilicity also determines the relative importance of the reactions of intermediates with the solvent. The strong apparent nucleophilicity of tetrafluoroborate is interesting. Reasonable yields (45 % on current) of a fluorinated product l-chloro-2-fluorocyclohexane 56 J. P. Coleman M. Fleischmann and D. Pletcher Electrochim. Acta 1973 18 331. ’’ J. Bertram J. P. Coleman M.Fleischmann and D. Pletcher J.C.S. Perkin ZI 1973 374. 58 V. R. Koch and L. L. Miller Tetrahedron Letters 1973 9 693. ’’ D. B. Clark M. Fleischmann and D. Pletcher J. Electroanalyr. Chem. Interfacial Electrochem. 1973 42 133. 300 K. Korinek and T. F. W.McKillop F F I I Me-CH-CHi -e-Hf~ Me-CH -CH,OAc BF4-T CH,=CH=CH,OAc Me-CH=CH 3 MekH-eH 3 H+ + CH CH2=CH-CH,F Me-CH=CH, li CH2 Me ’+ ‘MeH% Me2CHOAc Scheme 10 were also observed6’ in the oxidation of chloride ion in the presence of cyclo-hexene in methylene chloride-tetrabutylammonium tetrafluoroborate. Schafer and Koch6’ have reported on the anodic dimerization of substituted olefins containing both the electron-withdrawing group -CO-R and the electron-donating group -NH-R in methanolic sodium perchlorate solutions.The products of dimerization of enamino-ketones or enamino-esters were substituted pyrroles formed by radical dimerization of the primary oxidation product followed by the intracyclization and elimination of the dication (Scheme 11). X x* x-/ It H-C H-C-C-H II -2e C MeOH NaCIO,’ Me-c I &-Me I Me’ \NH NH NH I I I R R R X C-CHX II ‘C-Me -RNH \ bx C I Me N Me Me’ \NH NH I I I RR R Scheme 11 6o V. R. Koch L. L. Miller D. B. Clark M. Fleischmann T. Joslin and D. Pletcher J. Electroanalyt. Chem. Interfacial Electrochem. 1973 43 3 18. 61 D. Koch and H. Schafer Angew. Chem. 1973,85,264. Electro-organic Chemistry 30 1 Alcohols Phenols and Carbonyl Compounds-The electrochemical oxidation of saturated aliphatic alcohols is known to occur at highly positive potentials.During the oxidation of 2-methoxyethanol which contains both alcohol and ether functions in the presence of tetraethylammonium tetrafluoroborate as supporting electrolyte attack predominantly occurs on the ether rather than on the alcohol function.62 Formaldehyde bis-(2-methoxyethyl)formal,and 2-hydroxyethyl-2’-methoxyethylformal were the products. Recently there has been an increased interest in electrosynthetic reactions involving phenols. Nilsson Ronlan and Parker63 have studied the anodic hydroxylation of phenols at a lead dioxide electrode in aqueous sulphuric acid. In all cases studied the substitution occurred at the 4-position.The 4-substituted phenols gave 4-substituted 4-hydroxycylohexadienoneswhereas phenol itself was oxidized to p-benzoquinone. Lead dioxide anodes were found to be superior to carbon nickel and platinum anodes in hydroxylation reactions. The sug- gested mechanism proceeds via an electrolytic step generating surface species which chemically oxidize the phenol and are regenerated electrochemically. Phenoxonium ion is the probable intermediate. The method offers a convenient high-yield synthesis of 4-allyl-4-hydroxycyclohexa-2,5-dienones from 4-allyl- phenols. Data correlating ionization potentials electron densities and oxidation potentials for 20 phenol derivatives were also reported.64 During the investigation of anodic cleavage of benzyl ketones it was observed that tetrafluoroborate ion is not inert6’ and that high yields of fluorinated products result from its participation with reactive intermediates.The elec- trolysis of benzhydryl p-toluyl ketone in dry acetonitrile containing 0.1 M tetramethylammonium tetrafluoroborate gave 65 % yield of p-toluyl fluoride. Carboxylic Acids.-The electrochemical oxidative decarboxylation of aliphatic carboxylate anions can lead either to coupled products derived from radical reactions as in Kolbe electrosynthesis or to the so-called Hoffer-Moest products derived from the reactions of carbonium ions. It is known but not generally recognized among electro-organic chemists that the electrode material greatIy influences the nature of the products.Brennan and Brettle65 have now reported results from preparative electrolysis of triethylammonium heptanoate using different types of carbon as anodes in protic solvents. The Kolbe product dodecane was the major product at a vitreous or baked carbon or platinum anode but the major products at a graphite anode were three heptanoates(1- methylpentyl- 1-ethylbutyl- and 2-ethylbutyl-heptanoate) which arose from the rearrangements of the hexyl cation. Similarly when a platinum electrode was used the Kolbe dimer was the major product of anodic oxidation of phenyl-acetic acid phenoxyacetic acid and ethyl and methyl hydrogen succinates. At a graphite anode only a very small amount of the Kolbe product was formed. The 62 S. D. Ross J. E. Barry M. Filkenstein and E.J. Rudd J. Amer. Chem. Soc. 1973,95 2 193. ‘’ A. Nilsson A. Ronlan and V. D. Parker J.C.S. Perkin I 1973 2337. 64 A. E. Lutskii Y. 1. Beilis and V. I. Fedorchenko Zhur. ohshchei Khirn. 1973,43 101. 65 M. P. J. Brennan and R.Brettle J.C.S. Perkin I 1973 257. 302 K. Korinek and T. F. W.McKillop divergence in behaviour between a platinum and a carbon anode in the electrolysis of phenylacetic acid had been previously attributed to the presence of paramag- netic centres which bind the initially formed radical and promote the second electron transfer. This theory may now be rejected since the baked carbon electrode which is also likely to contain paramagnetic impurities gives rise to the Kolbe dimer. Marked differences have however been observed for pyrolytic graphite electrodes depending on whether the electrode surface is parallel (pyrolite face) or perpendicular (pyrolite edge) to the cleavage plane.The number of adsorption sites on the pyrolytic edge electrode is much greater and therefore the greater concentration of adsorbed radical intermediate favours dimerization. Thus some forms of carbon but not soft graphite can be used successfully in the Kolbe electrosynthesis. Whether the Kolbe reaction proceeds via free or adsorbed radical intermediates . has been the subject of much controversy in recent years. Utley et ~1 have ~~ now reported a study of anodic oxidation of 4substituted cyclohexane- and cyclohexene-carboxylates at a platinum anode in methanol. The coupling between conformationally biased cyclohexyl radicals should allow a distinction between the free and adsorbed radical intermediates.The ratio of the three coupled stereoisomers of 4,4-di-t-butylcyclohexyl (a :a; a :e; e :e) was 1 :2:1 which indicates that the recombination is in a random fashion and strongly suggests that the role of adsorption is negligible. The distribution of electrolysis products of cis-4-phenylcyclohex-2-enecarboxylatessuggested that for un-saturated carboxylates the intermediates may be adsorbed at the anode. Similar conclusions have been reached by Eberson and Ryde-Petter~on,~’ who have studied the mixed coupling of monoethyl (+)-ethylmethylmalonate and isovaleric acid in methanol. The mixed coupling product ethyl ethylisobutylmethylacetate (31)was 99.98% racemic.Since adsorbed radicals might be expected to give Me COZEt Me C0,Et \/ \/ +Me,CHCH,CO,-2i%,C C* b /\ /\ Et co,-Et CH,-CHMe (31) coupling products with at least partial retention of configuration the formation of racemic product supports the theory of free radicals. Another example of a successful use of mixed Kolbe electrolysis is the one-step synthesis of the housefly sex attractant (Z)-tricos-Pene (32).68 Anodic oxidation of the mixture of (Z)-octadec-9-enoic acid and n-heptanoic acid in methanolic sodium methoxide afforded the expected dimers. Isolated yields for a typical uncontrolled and not optimized synthesis are shown. Renaud and Sullivan69 have discussed the probable mechanism of the mixed Kolbe reaction of difluoroacetic acid and propionic acid.The mixed dimer 66 G. E. Hawkes J. H P. Utley and G. B. Yates J.C.S. Chem. Comm. 1973 305. 67 L. Eberson and G. Ryde-Petterson Acta Chem. Scand. 1973 27 1159. 68 G. W. Gribble J. K. Sanstead and J. W. Sullivan J.C.S. Chem. Comm. 1973 735. 69 R. N. Renaud and D. E. Sullivan Canad. J. Chem. 1973 51 772. 303 Electro-organic Chemistry Me(CH2),CH=CH(CH2),C02H + Me(CH2),C02H Me(CH,),,Me 20% \ + Me(CH,),CH=CH(CH,) ,Me 14% (32) + Me(CH,),CH=CH(CH,),,CH=CH(CH,),Me 7 trifluoropropane was not formed but instead 3,3,3-trifluoropropene 1,Zbis- (trifluoromethyl)ethane and 1,2,4-tris( trifluoromethy1)butane were obtained The probable mechanism could involve attack of the CF radical on ethylene formed by oxidation of propionic acid followed by further reaction with CF radical or ethylene.If electrochemical methods are to make a significant contribution to syn- thetic chemistry it is clear that a more systematic approach is needed to the study of electro-organic reactions. It is essential that electrolysis should first be investigated using controlled-potential methods before attempting to scale up or use constant-current conditions. It is therefore disappointing that most of the studies reported sn Kolbe reactions have not adopted this systematic approach especially since the necessary equipment is now readily available and reasonably cheap. In the case of mixed coupling reactions controlled- potential studies are absolutely essential. Anodic oxidation of aromatic carboxylic acids has not been investigated in great detail.Recently however the controlled-potential and controlled-current oxidation of benzoic acid in acetonitrile and propionitrile was reported?' The main product in acetonitrile was anthranilic acid and in propionitrile it was N-propionylanthranilic acid with N-propionylbenzamide also being formed (Scheme 12). Electrochemical methods can be very useful in extending the range of syn- thetic approaches available to the organic chemist. Since one usually employs mild conditions reactive and strained compounds can be prepared. The electro- synthesis of one such unstable intermediate 1,4bis(methoxycarbonyl)bicyclo-[2,2,2]octa-2,5-diene (33) was effe~ted,~' by electrolytic decarboxylation of the adduct of maleic anhydride and dimethyl cyclohexa- 1,3-diene- 1,4dicarboxylate (34).In the presence of a radical inhibitor 4-t-butylcatechol the yield of (33) was increased to 65%. Decarboxylations are only one type of important 'de- gradative reaction which may be amenable to electrochemical oxidation. Nitrogen-containing Compounds.-The oxidative cycliza tion of substituted diphenylamines has received further attention. Nelson and Berkenk~tter~~ 70 Y. Matsuda. K. Kimura C. Iwakura and H. Tamura Bull. Chem. SOC.Japan 1973 46 430. 7' C. R. Warren J. J. Bloomfield S. S. Chickos and R. A. Rouse J. Org. Chem. 1973 38 401 1. 72 P. Berkenkotter and R. F. Nelson J. Electrochem. Soc. 1973 120 346. K. Korinek and T. F. W.McKiIlop R = Me or Et IRCN I I-e j+H. 0 0f-NH-C-EtII 0 FHCOR Scheme 12 partial electrolytic ' decarboxylation C0,Me OC'O have shown that the anodic oxidation of NNN'-triphenyl-o-phenylenediamine in acetonitrile leads to 5,10-dihydro-5,10-diphenylphenazinein nearly quantita- tive yields (Scheme 13). This intramolecular cyclization is the key intermediate step in the formation of dihydrophenazines from substituted diphenylamines. An extensive study of substituted diphenylamines has shown that for efficient cyclization the para ring positions must be substituted with electron-donating neutral or weakly electron-withdrawing groups which do not undergo elimination during the Electro-organic Chemistry R 9 / R \ R R Scheme 13 reaction.Cauquis and Serve73 have measured electrochemical and spectro- scopic properties of o-phenylenediamines and studied their oxidative cyclization to dihydrophenazines in acetonitrile containing 0.1 M-Et,NClO,. They con- cluded that the basicity of the medium influenced the anodic cyclization and suggested in agreement with Nelson and Berkenk~tter,~~ that o-phenylene- diamines are possible intermediates in the oxidation of diphenylamines. Other studies on the mechanism of the oxidation of diphenylamine derivatives in aprotic solvents,74 and in various aqueous media75 have been reported. Behret76 has patented a new continuous electrochemical route to aniline from benzene and ammonia using iodobenzene as catalyst.A diaryliodonium ion is found as the product of the anodic oxidation of aromatics in acetonitrile in the presence of iodobenzene and reacts with NH to form the aromatic amine. Several authors have reported anodic oxidations of amine~~’.~~ and pyridination of Schiff bases79 in acetonitrile. The recent trend of applying electrochemistry to more complex organic systems has continued. Further progress has been made in the electrosynthesis of alkaloids by intramolecular coupling of non-phenolic tetrahydrobenzyliso- quinoline precursors. Miller Stermitz and Falck” have published a full paper ” G. Cauquis and D. Serve Tetrahedron Letters 1973 2695. 14 T. M. H. Saber G. Farsang and L. Ladanyi Microchem. J. 1973 18 66. 75 G. Farsang V. Vass L. Ladanyi and T.M. H. Saber J. Electroanalyt. Chem. Inter- facial Electrochem. 1973 43 391. 76 H. Behret G.P. 2 154 348 1973. 7’ M. P. J. Brennan and 0. R. Brown J. Appl. Electrochem. 1973 3 231. 78 H. Say0 and M. Masui J.C.S. Perkin II 1973 1640. 7q M. Masui and H. Ohmori J.C.S. Perkin II 1973 11 12. ‘O L. L. Miller F. R. Stermitz and J. R. Falck J. Arner. Chem. Soc. 1973,95 2651. K. Korinek and T. F. W. McKillop on the synthesis of morphadienones (35).The electrolysis was carried out at a platinum electrode in acetonitrile containing lithium perchlorate or tetra-methylammonium tetrafluoroborate at 0 "C and yields for a typical substituted 0R' o-benzylflavinantine (R' = R3 = Me R2 = CH,Ph) were 53% of isolated product. This compares extremely well with the available chemical routes whose yields seldom exceed 10%.A common side-reaction in biogenetic-type syntheses is the formation of C-0-C coupled products. This is completely avoided in electro-oxidative cyclization. The Pschorr synthesis which is based on the decomposition of diazonium salts can be adapted for both phenolic and non- phenolic substrates. Several steps are involved however and overall yields are usually very low. Electrochemical coupling has a very definite advantage in such situations and by modifying the conditions excellent yields have been obtained. Kotani and Tobinagas' have also reported the synthesis of morpha- dienone alkaloids. By modifying the conditions and using fluoroboric acid as supporting electrolyte an even higher yield (86%) was obtained for o-benzyl- flavinantine.The same Japanese group has describeda2 oxidative ring closure in the electrosynthesis of 5,lO-ethanophenanthridinering system. The anodic oxidation of trifluoroacetic derivatives of N-(4-methoxyphenethy1)-3,4-methyl-enedioxybenzylamine (36)in acetonitrile containing fluoroboric acid yielded (37) in 62% yield. Alkaline hydrolysis afforded (i-)-oxocrinine (38) which can be readily transformed into the Amaryllidaceae alkaloid ( -t)-crinine (39). This electrochemical approach offers a novel and attractive route to the synthesis of certain alkalbids giving selectivity high yields and adaptability to both phenolic and non-phenolic substrates. Although the synthetic and stereochemical implications of organic electrode reactions occurring at heterogeneous surfaces are widely recognized they are still only poorly understood.Pinesa3 has investigated the controlled-potential oxidation of (S)-2-acetamido-2-(3,4-dimethoxybenzyl)propionitrile at a platinum electrode in sodium acetate-acetic acid containing acetic anhydride. It was hoped to illustrate some differences between homogeneous and electrode E. Kotani and S. Tobinaga Tetrahedron Letters 1973 4759. 82 E. Kotani N. Takeuchi and S. Tobinaga J.C.S. Chem. Comm. 1973 550. 83 S. H. Pines J. Org. Chem. 1973 38 3854. Electro-organic Chemistry OMe 0' COCF / processes which coufd be attributed to the heterogeneous nature of the electrode surface. The major products were (4R,5S)-and (4R,5R)-4-cyano-5-(3,4-di-methoxyphenyl)-2,4-dimethyl-2-oxazolines(40) and (41) respectively in a 3.5 1 ratio.Similar ratios but lower yields can be achieved in the homogeneous reaction of (42) with Mn(OAc) . Me Me CN CH NHCOMe * [ArMq + I-ly-Ar + / NHCOMe NHCOMe Ar (42) - OAfiZ,,, -k Ar H Me Me (40) (41) It seems therefore that in this case the intermediates are not strongly adsorbed and the cyclization step occurs after their desorption from the electrode. Miscellaneous.-An increased use of sulphonium salts and sulphonium ylides in organic synthesis has led to studies on their electrochemical preparation. Some Japanese workers84 have now reported the synthesis of sulphonium salts 84 S. Torii Y. Matsuyama K. Kawasaki and K.Uneyano Bull. Chem. SOC.Japan 1973 46,291 I. K. Korinek and T. F. W.McKillop of the type (43) by anodic oxidation of alkyl phenyl sulphides at a platinum electrode in anhydrous acetonitrile containing lithium perchlorate as supporting electrolyte. The sulphonium perchlorates were formed in 60-70 % yield the precise yield depending on the nature of the R group. As part of their continuing investigation of the anodic oxidation of aryl sulphides Humffray and Imberger” have reported the oxidation of thianthrene in acetic acid-water mixtures (80:20) containing perchloric acid. The controlled- potential electrolysis at 1.15 V us. Ag-AgCl promises to be a useful synthetic reaction yielding 98% of the thianthrene monoxide. At higher potentials a mixture of products was formed with cis-and trans-dioxides predominating.The conventional thiocyanation of double bonds is carried out using thio- cyanogen generated in situ. Direct electrochemical generation of thiocyanogen offers the advantages of minimizing side-product formation. The utility of this method has been demonstratedB6 in the synthesis of vicinal dithiocyanates from olefins and thiocyanate salts in acidic media high yields of dithionate addition products being achieved. The detailed observation led the authors to propose a new radical mechanism for the thiocyanation as shown in Scheme 14. \/ \/ c-Y :> x+y-o;:;g* I c-x /\ /\ X = -SCN Y = -SCN or other nucleophile Scheme 14 A new procedure for alkyl coupling which might accommodate a wide variety of functional groups has been achieved by anodic oxidation of triallylboranes.The reaction was carried out in methanolic sodium hydroxide using platinum electrodes and good yields of coupled products were reported.87 85 H. E. Imberger and A. A. Humffray Electrochim. Acta 1973 18 373. a6 W. J. De Klein Electrochim. Acta 1973 18 413. T. Taguchi M. Itoh and A. Suzuki Chem. Letters 1973 7 719. Electro-organic Chemistry An electrochemical method has been developed for bonding electrically conductive adherents.88 The procedure is based on the electrochemical genera- tion of a curing agent from otherwise chemically unreactive precursor mixed with an epoxy resin and usually a supporting electrolyte sandwiched between the bonding members.Some of the advantages of electrochemical curing are obvious-no need to mix components long-term storage stability relatively rapid curing at room temperatures and ease of manipulation during fabrication. '' N. L. Weinberg G. M.Blank H. A. Aulich A. K. Hoffman andT. B. Reddy J. Appl. Electrochem. 1973 3 227.