7 Electro-organic Chemistry By R. LlNESt Laboratory of Organic Chemistry The Norwegian Institute of Technology The University of Trondheim N-7034 Trondheim -NTH Norway 1 Introduction In this Report the emphasis has been placed on those papers which display the utility of electrochemical methods in organic synthesis or in the investigation of organic systems. As in previous years the review is organised according to the nature of the species which is believed to participate in the initial electrode reaction. 2 Anodic Processes The Anodic Oxidation of Carboxy1ates.-The synthesis of configurationally pure disparlure (1)(the sex attractant of the gypsy moth) has been reported by Kliinenberg and Schafer,’ using the two cross-coupled Kolbe reactions outlined in Scheme 1.i. co-electrolysis +pelargonic acid ii. co-electrolysis +4-methylvaleric acid then epoxidation Scheme 1 The ‘two electron’ Kolbe reaction also offers a means of carrying out oxidative substitutions a case in point being the synthesis of 2-methoxy-l,4-dioxans from p-0x0 carboxylate acetals2 (Scheme 2). Oxidative decarboxylation of carbamoylaspartic acids or ethoxycarbonylaspara- gine has been reported by Iwasaki and co-workers’ to be a mild method for the H.Kliinenberg and H.J. Schafer Angew. Chem. Internat. Edn. 1978,17,47. D.Lelandais C.Bacquet and J. Einhorn J.C.S. Chem. Comm. 1978 194. T.Iwasaki H.Horikawa K. Matsumoto and M. Miyoshi Tetrahedron Letters 1978,4799. t Present address Chemistry Department The University Southampton SO9SNH 135 136 R.Lines FOH OMe Scheme 2 formation of uracii and its derivatives. These compounds are of current interest owing to their biological activity. The anodic decarboxylation of carboxylates normally requires high potentials (32V us. SCE) because electron transfer at the electrode is slow. A paper4 has now appeared in which electron transfer is achieved homogeneously by the use of electron-transfer reagents. Under these conditions reasonably fast reaction rates can be achieved at potentials of ca. lV for example by use of tris-(4-bromophenyl)-amine Eo= 1.3V. The reaction is also catalytic in transfer reagent (Scheme 3). In Scheme 3 contrast to the normal Kolbe reaction the alkyl radicals generated in this procedure4 undergo H-abstraction rather than radical coupling and disproportionation -a reflection of the different radical concentrations of the two methods.The Anodic Oxidation of Organic Anions.-A synthesisof azoalkanes' which avoids the use of chemical oxidants e.g. chlorine is shown in Scheme 4. Oxidation of the anions of NN'-dialkyl-sulphamides leads to a cyclic thiadiaziridine 1,l -dioxide intermediate' which cleaves to give azoalkanes in high yields (ca. 90Oh). Scheme 4 Some interesting electrochemistry stemming from the oxidation of the p-diethyl- amino-dithiobenzoate anion (2) in acetonitrile has been reported by Cauquis and Deronzier.6 The initial product is the disulphide (3) which undergoes further W. Schmidt and E. Steckhan J. Electroanalyt. Chem.Interfacial Electrochem. 1978,89 215. R.Bauer and H. Wendt Angew. Chem. Internat. Edn. 1978,17,370. G. Cauquis and A. Deronzier J.C.S. Chem. Comm. 1978,809. Elec tro-orga n ic Chemistry 137 oxidation in two le reversible steps (Scheme 5). The final product is a dicationic species containing a tetrathian ring (4). Depending on the potential (4) can be formed by either an ece or an eec process. 2e e L 7 L (312+ E2 The Anodic Oxidation of Neutral Organic Compounds.-The oxidation of cyclo- pentane and cyclohexane at Pt in fluorosulphonic acid has been re-examined by Coleman and Pletcher7 with the aim of distinguishing between the preprotonation mechanism [Ann. Reports (B) 1973 70 2991 and the elce2 (e2 >el) type process proposed by a French group.* The authors' concluded that the alkanes in the presence of KFS03 underwent a single 2e oxidation but that the role of the protonated alkanes remained unclear and must await more detailed knowledge of the fluorosulphonic acid system.Another reinvestigation concerned the effects of the presence of trace amounts of polynuclear aromatics on the o/p product ratio in the acetoxylation of anis01e.~ Contrary to an earlier report that the presence of coronene or hexahelicene increased the o/p ratio it was foundg that the ratio in fact decreased. This decrease conforms with the generally observed isomer ratios in the presence of additives an effect attributed to co-adsorption at the anode.9 A new synthesis of indigos has been reported by a Japanese group" and is outlined in Scheme 6.The starting materials can be either N-acetyl-indolines (5)or N-acetyl- indoles (6). Acetoxylation of either (5) or (6) followed by thermolysis yields the intermediate (7) from which the indigos can be obtained by hydrolysis. The use of emulsion electrolyses with phase-transfer reagents [Ann. Reports (B) 1977,74,155] has been extended to the acyloxylation of aromatic compounds'1 and to the cyanation of stilbenes.12 For the acyloxylation reaction," good yields of aryl carboxylates were obtained from anisole with various aliphatic carboxylic acids whereas methylbenzenes gave mainly alcohols and aldehydes. This latter result was attributed" to the preferential adsorption of water at the anode thereby reversing J.P. Coleman and D. Pletcher J. Electroanalyt. Chem. Interfacial Electrochem. 1978,87 111. a S. Pitti. M. Herlem and J. Jordan Tetrahedron Letters 1976 3221. W.J. M.van Tilborg J. J. Scheele and L. Eberson Acta Chem. Scand. 1978 B32,36. lo S. Torii T. Yamanaka and H. Tanaka J. Org. Chem. 1978,43,2882. l1 L.Eberson and B. Helgte Acta Chem. Scand. 1978 B32 157. l2 L.Eberson and B. HelgCe Acta Chem. Scand. 1978 B32,313. 138 R. Lines I Ac Ac OAc (5) (6) J. X=HorBr OAc (7) Scheme 6 the usual order of nucleophilicity of water us. carboxylate. The anodic cyanation of stilbene and its derivatives’* resulted in the first examples of 1,2 addition of cyanide across the double bond. It was pointed out by the authors that this was probably due to the enhanced reactivity ofthe cyanide ion in the dichloromethane phase compared to the more commonly used solvents for cyanation such as methanol and acetonitrile.Electrolysis of aromatic compounds in trifluoroacetic acid (TFA) appears to be an excellent method for the synthesis of the corresponding hydroxy derivatives. l3 Thus chlorobenzene could be selectively mono- di- or tri-fluoroacetoxylated depend- ing on the charge passed.13 The easy hydrolysis of the trifluoroacetates together with the high anodic limit of TFA makes this reagent clearly superior to acetic acid for hydroxylation reactions. Schfnnidt and Steckhan14 have described the mild oxidative removal of the 4-methoxybenzyl ether protecting group by a homogeneous electron-transfer reaction.The electron-transfer reagent was the tris-(4-bromophenyl)amine cation radical either generated in situ electrochemically in moist acetonitrile or as the hexachloroantimonate salt. The electrochemical method has the advantage that only a catalytic quantity of the reagent is required. Chemical methods for the removal of the dithian protecting group are scarce so the efficient removal of this group by anodic cleavage in neutral conditions (MeCN-H20) demonstrated by Porter and Utley,” may well prove to be the method of choice. Two comprehensive papers have appeared which describe the anodic methoxyl- ation of phenols16 and the methoxylation of methylbenzenes and anisole deriva- tive~.~~ Phenols were found to undergo ortho and para methoxylation and dimeriza- tion16 although conditions were found which allowed the selective formation of each product.Products from the oxidation of methylbenzenes and anisole derivatives” depended on the electrolyte composition. Electrolysis at Pt in the presence of NaOMe gave predominantly nuclear methoxylation whereas at Pt or carbon anodes l3 G. Bockmair H. P. Fritz and H. Gebauer Electrochim. Actu 1978,23,21. l4 W. Schmidt and E. Steckhan Angew. Chem. Intemat. Edn. 1978,17,673. *’Q. N. Porter and J. H. P. Utley J.C.S. Chem. Comm. 1978 255. l6 A. Nilsson U. Palmquist T. Pettersson and A. Ronlh J.C.S. Perkin I 1978 696. ” A. Nilsson U. Palmquist T. Pettersson and A. Ronlkn J.C.S. Perkin I 1978 708. Electro-organic Chemistry with LiBF as supporting electrolyte side-chain oxidation became important (Scheme 7).By careful selection of the starting materials and electrolysis conditions the method may have some application to the synthesis of aromatic aldehydes and acids.6-4+$ \ \ ++\ R2 CH,OMe CH(OMe) C(OMe) R' = Me or Me0 R' = OMe R' = Me or Me0 R' = OMe R2= Me Scheme 7 The electrochemical properties of the sandwich compound uranocene (8)have been probed" in an attempt to test a model of its bonding arrangement. Uranocene is inert to water but removal of two electrons (predicted to be from non-bonding orbitals) would lead to a dication isoelectronic with thoracene -a highly water- sensitive compound. Contrary to expectations it was found" that oxidation of uranocene in benzonitrile gave only an unstable cation which reacted irreversibly with the parent compound to give a product of uncertain structure; possibly a dimer or cluster product cation (Scheme 8).COT = cyclo-octatetraene Scheme 8 3 Cathodic Processes The Cathodic Reduction of Organic Cations.-An interesting product has been reported" to arise from the reduction of the dihydrodiazepinium salt (9) (Scheme 9). It is thought" that the pyrrolodiazepine (10) is formed by dimerization of the initially formed radicals followed by an intramolecular displacement of ethyl- enediamine. The base is also reversibly converted by acid into the conjugated salt (11). A Japanese group" have developed a simple electrosynthesis of isoquinoline and indole alkaloids involving the electroreductive addition of alkyl halides to immonium salts.The reaction is suggested to proceed via electron transfer from the salt followed by the addition of the alkyl halide to the resulting anion. The authors2' specify the need for a lead plate in the cell during electrolysis presumably as a J. A. Butcher jun. J. Q. Chambers and R. M. Pagni J. Amer. Chem. Soc.,1978; 100,1012. D. Lloyd C. A. Vincent D. J. Walton J. P. Declerq G. Germain and M. van Meerssche J.C.S. Chem. Comm. 1978,499. 2o T. Shono K. Yoshida K. Ando Y. Usui and H. Hamaguchi TetrahedronLetters 1978,4819. 140 R. Lines H Ph (.>Ph _- (Lg H+ OH-A7 N H ClO N H .Ph Scheme 9 halogen scavenger. An example of the method applied to the synthesis of hetero-cycles containing two heteroatoms is shown in Scheme 10.OMe OMe Scheme 10 The Cathodic Reduction of Neutral Organic Compounds.-[24]Paracyclo-phanetetraene (12) is formally a derivative of [24]annulene and by analogy with the smaller 4nn electron systems would be expected to undergo stepwise reduction to the dianion. Lamm and co-workers21 have reported however that at Hg in DMF (12) is reduced in a single reversible two-electron step to the dianion. Although cyclopropanes can be synthesized by the reduction of 1,3 -dihalides extension of the method for the preparation of the higher cycloalkanes has until now been unexplored. Japanese investigators” have found that moderate yields of 3-7-ring cycloalkanes can be formed by the reductive cyclization of dimethyl K.Ankner B. Lamrn B. Thulin and 0.Wennerstrom Acra Chem. Scand. 1978,B32,155. 22 S. Satoh M. ltoh and M. Tokuda,J.C.S. Chem. Comm. 1978,481. Electro-organic Chemistry 141 2,(0 -1)-dibromo-alkanedioates (Scheme 11). The procedure represents a con- venient route to cycloalkane- 1,2-dicarboxylic acid esters. [-CHCO,Me Pt-2e MeO2CCH(CHZ),-2CHCOzMe (CHd-2 I Br Br -CHCO,Me I I (n= 3-7) Scheme 11 The cathodic addition of carbon tetrachloride to various carbonyl compounds has been reportedz3 to give the corresponding trichloromethyl carbinols in yields comparable to the phase-transfer-catalysed addition of chloroform. The same paper23 also reports the cathodic addition of ethyl trichloroacetate to cyclic ketones (Scheme 12). Scheme 12 A simple but potentially very useful electrocatalytic hydrogenation procedure for aromatic compounds has been described by Miller and Chri~tensen.‘~ The reduc- tions were carried out in 0.2M-sulphuric acid at Pt- or Rh-coated carbon cathodes in a divided cell.Phenol for example gave 92% yield of cyclohexanol. An apparent limitation of the method is that addition of organic co-solvents markedly decreases the product yield presumably by competitive adsorption at the cathode. A similar procedure for the reduction of 3-cyanopyridine to the amine has been reported by an Indian Conditions necessary for electrocatalytic aromatic nucleophilic substitution reac- tions are discussed by Pinson and SaveantZ6 and summarized in Scheme 13. The ArX+ e $ ArX (EY = electrolysis potential) ArY’-e $ ArY (Ey) ArX+Y-+ ArY +X- if EY > Ei Scheme 13 basic argumentz6 is as follows if the reduction potential E’ of the substrate (ArX) is greater than the oxidation potential E of the product radical anion (ArYT) then the oxidation of ArY; into the product occurs spontaneously and the process is catalytic.This scheme was successfully testedz6 using thiolates as nucleophiles and halogen 23 F. Karrenbrock and H. J. Schafer Tetrahedron Letters 1978 1521. 24 L. L. Miller and L. Christensen J. Org. Chem. 1978,43,2059. ’’ V. Krishnan K. Raghupathy and H. V. K. Udupa J.Electroanalyt. Chem. Interfacial Electrochem.. 1978 88,433. 26 J. Pinson and J.-M. Saveant J. Amer. Chem. SOC.,1978,100 1506. 142 R.Lines derivatives of benzophenone benzonitrile and naphthalene as substrates.For the cyanide ion as nucleophile Ey <Eg and a further separate oxidation was necessary to obtain the products. Reductive acylation previously restricted to activated olefins has now been extended to aryl~lefins~~ by carrying out the reductions in DMF or alkyl cyanides at low temperatures (-10 "C). The reaction is thought to proceed through reaction of the solvent with cathodically generated substrate anions. Formylation by this procedure is reported2' to be particularly successful. Reductive acylation has also been used in the carotenoid field this time for the synthesis of astaxanthin (14)28 (Scheme 14). Controlled-potential electrolysis of astacene (13) in the presence of Scheme 14 acetic anhydride gave either a 2e or 4e reduction product depending upon the potential and solvent system.The 2e product was identified as a new retro tetra-acetate while astaxanthin was obtained (10% yield) after hydrolysis of the tetra- acetate from the 4e reduction pathway. The reductive cyclization of some non-conjugated olefinic ketones to cyclic t- alcohols has been reported by Shono and co-w~rkers~~ to proceed with remarkable regio- and stereo-selectivities (Scheme 15). The reaction apparently takes place between the inner carbon atom of the double bond and the carbonyl carbon to give exclusively cis alcohols. CHR 0-Ketone C cathode -[.'d ' R'] -MeOH-OM F OH Scheme 15 Another paper by Shono and co-~orkers~~ describes the preparation of cyclo- propanes starting from a@-unsaturated carbonyl compounds (Scheme 16).The method is reported to be more selective and efficient than the corresponding Li-NH3 chemical method. *' R. Engels and H. J. Schafer Angew. Chem. Internat. Edn. 1978 17,460. ** E. A. H. Hall G. P. Moss,J. H. P. Utley and B. C. L. Weedon J.C.S. Chem. Comm. 1978,387. 29 T. Shono I. Nishiguchi H. Ohmizu and M. Mitani J. Amer. Chem. SOC.,1978,100 545. 30 T. Shono,Y. Matsumura S.Kashimura and H. Kyutoku Tetrahedron Letters 1978 1205. Electro -orga nic Che rnistr y R' -::q' Rz-R3 Pbcathode-4; R' Dry DMF AR3 MsO R4 R2 R4 R' -R4 =H or alkyl Ms =MeS02-Scheme 16 The cathodic cleavage of alkyl a-benzenesulphonyl carboxylates has been pro- posed" as an alternative to the malonic ester synthesis of carboxylic acids especially where strong acids or bases must be avoided.Preparation of the starting materials is achieved by alkylation of the sulphone by ion-pair extraction. The electrolyses are performed at Hg in DMF to give the benzenesulphinate ion and a good yield of the alkyl ester. Carbon-sulphur cleavage is also the feature of a new synthesis of olefins from &hydroxy-~ulphides,~~ The method is claimed to have wide applicability (Scheme 17). R' ko i. PhSCHILi R' OH Ptcathode4e ii.H+ R2fis,, R2 R2 Scheme 17 A different interpretation of the H-D exchange in recovered starting material from the reduction of phenacyl chloride in DMF-D20 [Ann. Reports (B),1977,74 1613 has been proposed by Merz and Th~mm.~~ The investigators found that reduction of (15; X = C1) in DMF-D20-tetraethylammonium bromide (TEABr) gave deuterium incorporation into the product (15;X =D) and also significantly into the aryl methyl groups.Recovered starting material from interrupted elec- trolysis was similarly deuteriated. Since regeneration of (15) through any inter- mediate on the electrochemical reaction path was rejected by the authors the H-D exchange phenomenon was suggested to be an example of the reactions of electro- generated bases i.e. of carbanion (16). In this system the TEA cation is thought3' to play the important role of excluding water from the electrode double layer thus preventing the immediate protonation of the carbanion by water.c1 c1 (4-MeCsH4)2C=C/ (4-MeC,jH4)2C=C-/ 'x (16) X = C1 or D(H) (15) 31 B. Lamm and K. Ankner Actu. Chern. Scund. 1978 B32 193. 32 T. Shono Y. Matsumura S. Kashimura and H. Kyutoku Tetrahedron Letfers 1978 2807. 33 A. Merz and G. Thumm Tetrahedron Letters 1978 679. 144 I?. Lines An X-ray crystallographic e~amination~~ of the product from the electro- hydrocyclodimerization reaction of dimethyl benzene-l,2-diacrylate has revealed that the originally proposed structure is incorrect [Ann.Reports (B),1976,73,148] and is in fact the tetracyclic compound (17). I CH,R (17) R=COzMe 4 Miscellaneous Chemically Modified Electrodes.-An important aspect of chemically modified electrodes is their potential catalytic applications and in this connection it would be useful to compare their characteristics with the corresponding homogeneous systems.With this aim Andrieux and Sa~eant~~ have now presented an analysis of the cyclic voltammetric response of catalytic electrodes thus enabling the catalysis kinetics to be extracted from experimental data. Another important aspect of modified electrodes is the ability to predict the redox potentials of the immobilized species. To this end twenty three redox couples have been both in solution and bound on an electrode surface. The results showed that the bound redox species were thermodynamically predictable i.e. the immobilization had only a slight effect on the formal potentials. In this connection it has been pointed that second harmonic a.c.voltammetry is a much more sensitive technique than cyclic voltammetry for the measurement of redox potentials and surface characteristics of electrode-bonded species. Following last year's report on the favourable properties of sulphur polynitride (SN) for chemical modification [Ann. Reports (B) 1977 74 1631 a paper has appeared3' in which pretreatment of the perpendicular fibre ends of (SN),by a Ru"' complex resulted in an electrode catalytic for the &/I-couple. Interestingly pretreatment of the parallel fibre ends resulted in no catalytic The attachment of redox groups to electrode surfaces generally requires a linking agent. Such agents must be both chemically and electrochemically inert and be reasonably small so that electron transfer from the base electrode material is not impeded.Until recently the most common linking agents were silane derivatives but cyanuric chloride (C1CN)3appears to be a good alternati~e.~~ The compound 34 J. Anderson L. Eberson and C. Svensson Actu Chem. Scand. 1978 B32,234. 35 C. P. Andrieux and J.-M. Saveant J. Electroanalyt. Chem. Interfacial Electrochem. 1978 93 163. 36 J. R. Lenhard R. Rocklin H. Abruna K. Willman K. Kuo R. Nowak and R. W. Murray J. Amer. Chem. SOC.,1978,100,5213. 37 A. F. Diaz and K. K. Kanazawa J. Electroanalyt. Chem. Interfacial Electrochem. 1978,86 441. 38 A. N. Voulgaropoulos R. J. Nowak W. Kutner and H. B. Mark jun.,J.C.S. Chem. Comm. 1978,244. 39 A. M. Yacynych and T. Kuwama Analyt. Chem. 1978,50,640.Electro-organic Chemistry 145 binds to the surface hydroxy-groups (of graphite or Sn02 electrodes) to give stable ether linkages while the chloride groups allow the attachment of a wide variety of redox reagents. Electrode surfaces can also be modified by coating with an organic polymer. These 'polymer electrodes' are simple to prepare are tough and have some interesting properties; e.g. Miller and van de Mark4' have shown that the reduction of FeC12 although reversible at Pt is irreversible at a polyester-coated electrode. The same have also demonstrated that a poly(4-nitrostyrene)-coated electrode is catalytic for the reduction of oxygen. A point that has been made by several investigators is that charge transfer to a solution species via an electroactive polymer electrode only takes place at potentials where the polymer is charged.Thus Merz and Bard4' have suggested that for polyvinylferrocene electrodes electron transfer at potentials where the polymer is not reduced takes place through holes or channels in the coating. Miscellaneous Processes.-The electrochemical oxidation of titanium in a 1 :1 :1 mixture of methanol acetonitrile and acetic acid has been found4j to be a con- venient source of Ti"'. The electrogenerated reagent can be used to reduce organic compounds in the same way as aqueous Tic& and gives similar yields and reaction rates. An interesting synthesis of carboxylic acid has resulted from the elec- trolysis of trialkylboranes in the presence of ap-unsaturated esters (Scheme 18).Pt. Undivided cell R3B +CH,=CCO,Et MeCN-TEABr 'RCH2CHCO2Et I I R' R' Scheme 18 The mechanism is thought to involve the products of both electrodes in a similar fashion to that reported for the synthesis of nitroalkanes from organoboranes [Ann. Reports (B),1976 73 1381. A novel application of strong acid ion-exchange resins has been in trapping electrogenerated intermediate^.^' Nitrilium ions generated by the oxidation of hydrocarbons in acetonitrile were trapped on a sulphonic acid resin. Filtration of the resin followed by treatment with aqueous NaOH liberated the amide products and regenerated the resin. Higher yields of amides are reported45 for this method compared to oxidation in the absence of resin. The first example of a synthetic compound displaying the sequential transfer of two electrons at the same potential has been claimed46 for the binuclear Cu" complex (18).In DMF (18) underwent reversible electron transfer at -0.47V (SCE) with a peak separation of 42mV in agreement with the Polcyn and Shain theory for reactions involving sequential transfer of electrons. 40 L. L. Miller and M. R. van de Mark J. Electroanalyt. Chem. Interfacial Electrochem. 1978,88 437. 41 M. R. van de Mark and L. L. Miller J. Amer. Chem. Soc. 1978,100,3223. 42 A. Merz and A. J. Bard J. Amer. Chem. Soc. 1978,100,3222. 43 0. Christofis,J. J. Habeeb R. S. Steevensz and D. G. Tuck Canad. J. Chem. 1978,56,2269. 44 Y. Takahashi K. Yuasa M. Tokuda M. Itoh and A. Suzuki Bull.Chem. Soc. Japan 1978,51,339 45 A. Bewick J. M. Mellor and B. S. Pons J.C.S. Chem. Comm. 1978,738. 46 D. E. Fenton R. R. Schroeder and R. L. Lintvedt J. Amer. Chem. SOC.,1978,100 1931. 146 R. Lines (18) Recent interest in the area of radical ion chemistry has centred on the anomalous reactivities of aromatic cation radicals with various nucleophiles. A theory which prompted some discussion4’ was put forward by Eberson and co-w~rkers,~~ based on the Dewar-Zimmerman (D-Z) rules. A paper has now appeared by Eberson and Nyberg4’ in which cation radical reactivity vs. nucleophiles is re-examined in the light of thermochemical calculations. The conclusion49 is that the previous arguments based on the D-Z rules are open for discussion in that the reactivity is mainly determined by the relative oxidation potentials of the parent substrate and nucleo- phile respectively.The question ‘do ece mechanisms occur under conditions where they could be characterized by electrochemical kinetic techniques?’ has been raised by Amatore and Saveant.” The authors considered the ece route (i) and the disproportionation mechanism (ii) (Scheme 19) where C is more easily oxidized (or reduced) than the A*e $ B A*e =k B 1. B+C C*e * D 11. B*C B+C $ A+D Scheme 19 starting material. It was concludedSo that measurement times <0.1 ps would be required to characterize the ece process and therefore ece mechanisms of this type do not occur in conditions where they could be directly characterized by electrochemical techniques.A potentially valuable source of data on electrode reactions is to carry out the reactions in a calorimeter. The feasibility of this approach has been explored51 using the reduction of the trichloroacetate ion in aqueous solution. To give meaningful data the chosen reaction must have unit current efficiency and no side reactions. The results of the AH calculation on the trichloroacetate ion reduction indicated5’ that although further refinement of the apparatus was required the method showed promise where alternative forms of calorimetry would be difficult. 47 M. J. Shine B. K. Bandlish and M. T. Stephenson Tetrahedron Letters 1978 733. 48 L Eberson Z. Blum B. HelgCe and K. Nyberg Tetrahedron Letters 1978,731. 49 L. Eberson and K.Nyberg Acta Chem. Scand. 1978 B32,235. c.Amatore and J.-M. Saveant J. Electroanalyt. Chem. Interfacial Electrochem. 1978,86 227. 51 p. J. Turner and H. 0.Pritchard Canad. J. Chem. 1978,56,1415.