8 Electro-organic Chemistry By J. H. P. UTLEY Department of Chemistry Queen Mary College Mile End Road London El 4NS 1 Introduction The choice of material in this Report is intended to highlight new developments and point but briefly at areas where consolidation has taken place. As before the chapter is organized according to the nature of the species which is believed to be involved in the initial electrode reaction. The compilation of this Report has been greatly helped by a current-awareness service provided by Ann Jarvis (Q.M.C. Library). 2 AnodicProcesses Oxidation of Neutral Organic Compounds.-Alkanes Alkenes and Alkynes. The functionalization of alkanes by anodic oxidation in strong acids such as fluorosulphonic acid was discussed in the corresponding 1973 Report (p.299). Contrary to the original report,' a suggestion has been made2 that protonation of the alkanes prior to electron transfer is not necessary. The case hinges on the observations that (i) addition of base (NaS0,F) apparently assists oxidation (E+ values become slightly less anodic) (ii) I-IF SO, and H,S04 were detected in the anolyte and (iii) the likely presence of SO (a strong Lewis acid in FS0,H) demands consideration of a chemical cleavage of alkanes to carbenium ions. However in trifluoroacetic acid solution oxidation3 of cyclohexane is assisted by the addition of a stronger acid (FS0,H). Activation towards anodic oxidation was also achieved using CF,S03H CH,SO,H and mixtures of CH,Cl and FS0,H. The product of preparative-scale electrolysis in CF,CO,H was cyclo- hexyl trifluoroacetate.A strongly acidic solution is not essential for anodic oxidation of alkanes. High anode potentials [ca.3.6 V (us. Agl Ag')] may be reached in dry acetonitrile and the usefulness of this is well demonstrated4 by the acetamidation reactions outlined in Scheme 1. The fragmentati~n~~ of branched alkanes and the position of substitution in straight-chain esters4b is consistent with the formation of J. Bertram J. P. Coleman M. Fleischmann and D. Pletcher J.C.S. Perkin If 1973 374. F. Bobilliart A. Thiebault and M. Herlem Compt. rend. 1974 278 C 1485. H. P. Fritz and T. Wiirminghausen J. Electroanalyt. Chem. Interfacial Electrochem. 1974 54 181. (a) J. Y. Becker L. L. Miller and T. Siegel J.C.S.Chem. Comm. 1974 341; (6) L. L. Miller and V. Ramachandran J. Org. Chem. 1974 39 369. 215 216 J. H. P.Utley Reagents i Pt anode 3.6 V (us. AglAg') MeCN-Et,NBF, -45 "C 2.3 F mol~; ii H,O Scheme 1 carbenium ions following electron transfer and proton loss from the hydrocarbon chain. Alkenes are more easily oxidized either directly or indirectly. For instance 3,3-dimethylbut- 1-ene is oxidized' in methanol containing either NaOMe or NaOC0,Me to a variety of products (mostly rearranged) including methoxy- lated alkylmethylcarbonates and the corresponding alcohols [e.g MeO(Me),-CCH(Me)CH,OCO,Me]. Apparently NaOC0,Me is formed in the electrolysis of NaOMe in methanol and the results have been rationalized according to an indirect mechanism which involves initial radical addition to the alkene.The direct oxidation of alkenes often results in dimerization with subsequent sub- stitution. The observation of tail-to-tail coupling is taken as evidence of initial dimerization of radical-cations. Recent examples,6 and the current mechanistic explanation are given in Scheme 2. It should be noted that the anodic dimeriza- tion of enol ethers6a in methanol provides a route to acetals of 1,4-dicarbonyl ~ dirnerize R C H=C(OEt)R2 -e R CH&( OEt)R2 -+ R2(OEt )&H( R ')CH(R ')&(OEt)R 1MeOH ere. R2COCH(R')CH(R 'FOR + R '( OEt)C(OMe)CH(R ')CH(R..')C(MeO)(Et O)R2 . PhCH=CH Ph 0 (52% current yield) Ph Reagents i graphite anode CH,Cl,-H,O-Bu,NHSO Scheme 2 compounds.The substituted tetrahydrofuran (1)is prepared' in 12 % yield by the oxidation at a graphite anode of a-methylstyrene in wet acetonitrile. The product ' R. Brettle and J. R. Sutton J.C.S. Chem. Comm. 1974 449. ' (a) D. Koch H. Schafer and E. Steckhan Chem. Ber. 1974 107 3640; (h) Angew. Chem. Internat. Edn. 1974 13 472. H. Sternerup Acta Chem. Scand. (B) 1974 28 579. Electro-organic Chemistry is unsurprising but it is noteworthy that the reaction was carried out on a molar scale (118 g) and run at 50 A using a simple capillary-gap cell.* Double-bond oxidation may also compete favourably with carboxylate oxidation in unsaturated acids. Scheme 3 displays a good exampleg of this. -e PhCH=CHCH,CH2C02-a I. cyclrzation 11. -e- MeOH 1 PhCH(OMe1LA, (> 90 %) Scheme 3 The direct oxidation of alkynes has been attempted lo using constant-current electrolysis at a graphite anode in methanol with added sodium perchlorate.Several products are obtained from phenylacetylene and the pattern can be explained in terms of initial formation of a radical-cation which is methoxylated and then further oxidized in a predictable but uncontrolled manner to produce polymethoxylated and cleaved products. For instance the most abundant products (in ca. 30 % current yield) are PhC(OMe) and HC(OMe) . Aromatic Hydrocarbons. The importance of the medium for the stabilization of cationic species is further emphasized by the Copenhagen group in the first report’ of reversible one-electron oxidation of anthracene.Reversible cyclic voltammetric behaviour is observed using tetra-n-butylammonium tetrafluoro- borate supporting electrolyte in methylene chloride that has been rigorously dried over neutral alumina or mixtures of methylene chloride trifluoroacetic acid (TFA) and trifluoroacetic anhydride (TFAn). From earlier studies of the oxidation of anthracene it is known that traces of water in solvents cause irrever- sibility owing to rapid hydroxylation. In ‘super dry’ acetonitrile (with added TFAn) two-electron irreversible oxidation of anthracene is found as usual but hydroxylation is suppressed and the nitrilium ion (2) is formed. Subsequent work-up gave the corresponding acetamide (3) in 82 % isolated yield.’’ i ’ L. Eberson K. Nyberg and H.Sternerup Chem. Scripta 1973 3 12. F. M. Banda and R. Brettle J.C.S. Perkin I 1974 1907. lo M. Katz and H. Wendt J. Electroanalyt. Chem. Interfacial Electrochem. 1974 53 465. ‘I 0. Hammerich and V. D. Parker J. Amer. Chem. Soc. 1974 96 4289. 0. Hammerich and V. D. Parker J.C.S. Chem. Comm. 1974 245. J. H. P.Utley Anodic aromatic substitution generally occurs via an ECE mechanism [cf. Ann. Reports (B),1968 65 2451. It is encouraging to see that this possibility is being considered for classical electrophilic aromatic substitution reactions particularly those involving easily oxidized aromatic compounds or reactions in which side-chain substitution occurs. A case in point is the chlorination in acetic acid of hexamethylbenzene which gives the corresponding pentamethyl- benzyl chloride.When the reaction was carried out in a tube in the cavity of an e.s.r. spectrometer the well-characterized spectrum of the hexamethyl benzene radical-cation was obtained,' which suggests that serious consideration should be given to an ECE mechanism (Scheme 4).In an important practical advan~e,'~ CH ?H2 @ ' C~OP c:,-~~ec[a] 0 \ \ Scbeme 4 Eberson and Helgee have shown that oxidation of aromatic compounds in a water-methylene chloride emulsion (with added phase-transfer agent) is a con- venient way of effecting cyanation at high current densities that are not usually reached in solvents of low conductivity. The preparative consequences of anodic oxidative coupling of methoxylated aromatic compounds which form relatively stable radical-cations have been pursued.l5 More examples of the synthesis by intramolecular coupling of morphandienones have been described. 54 In methylene chloride-trifluoro- acetic acid (2 :l),biaryls are formed 5b*cin good yield following reductive work-up of solutions of radical-cations e.g.(4) of the coupled products which are stabilized " J. K. Kochi Tetrahedron Letters 1974 4305. '' L. Eberson and B. Helgee Chem. Scripta 1974 5 47. l5 (a)A. Ronlan K. Bechgaard and V. D. Parker Acra Chem. Scand. (B) 1973 27 2375; (6) A. Ronlan and V. D. Parker J. Org. Chem. 1974 39 1014; (c)J. R. Falck L. L. Miller and F. R. Stermitz J. Amer. Chem. SOC. 1974 96 2981; (d) Tetrahedron 1974 30 93 1. Electro-organic Chemistry in the acidic solution thus precluding further oxidation.Following a systematic voltammetric and u.v.-spectroscopic investigation l6 of the stability of such radical-cations of a series of substituted biphenyls it is clear that coplanarity ofthe rings is important in determining radical-cation stability. In the same paper the slow deprotonation of methoxy-substituted 9,lO-dihydrophenanthrene radical cations (4) is both confirmed and explained. It is now almost certain that the coupling reactions described above involve the intermediacy of phenoxonium ions and recent and compelling evidence' is summarized in Scheme 5. Phenols are more easily oxidized than the corres- OMe OMe HO 0 Reagents i Pt anode -e- -H+ -e- 1.25 V (us. s.c.e.1 MeCN 2 F mol-' Scheme 5 ponding methyl ethers and in the case presented the nature of the products and the absence of intermolecular coupling argues for the phenoxonium-ion route.Heterocyclic Compounds. The methoxylation of furans by anodic oxidation in methanol has been much studied [c& Ann. Reports (B),1968,65,246].A recent investigation'* of the oxidation of 2,3,4,5-tetraphenylfuran(5) in either methanol or nitromethane shows that the intermediate radical-cation (which is sufficiently stable in nitromethane to be characterized by e.s.r. spectroscopy) undergoes substitution in the normal way (in methanol) or else (in nitromethane) cleavage to (6). The anodic oxidation of nitrogen compounds usually leads to complicated product mixtures because the substrates are often basic and nucleophilic and l6 A.Ronlan J. Coleman 0. Hammerich and V. D. Parker J. Amer. Chem. Soc. 1974 96 845. l7 U. Palmquist A. Ronlan and V. D. Parker Acta Chem. Scand. (B) 1974 28 267. M. Libert and C. Caullet Bull. SOC.chirn. France 1974 805. 220 J. H. P.Utley Reagents i MeCN-pyridine-Et,NC104; ii MeCN-Et,NCIO Scheme 6 can therefore take part in substitution in and abstraction from cationic inter- mediates. The electrochemical behaviour ' of N-aryl-2-pyrazolines is therefore smprisingly straightforward (Scheme 6). The corresponding pyrazoles are obtained in good yield by o~idation'~" in acetonitrile in the presence of pyridine whereas without the base dimers such as (7) are formed" in high yield. In the more complete report of these ~tudies"~ attention has also been paid to the electrochemistry of the dimeric products as well as to the voltammetry and coulometry associated with the reactions.The anodic trimerization of 1,3-dirnethylbarbituric acid (8) has been accom- plished2' using controlled-potential electrolysis at a pyrolytic graphite anode with acetic acid as solvent. The product formed in good yield is a new cyclic barbiturate (9). Its structure was confirmed by X-ray crystallography. 0 (9) Among recent attempts at clarifying the electrochemistry of biologically im- portant nitrogen heterocycles is an investigation2 of the electrochemistry l9 (a) F. Pragst 2. Chem. 1974 14,236; (6)F. Pragst and B. Siefke J. prakt. Chem. 1974 316 267; (c) P.Corbon G. Barbey A. Dupre and C. Caullet Bull. Soc. chim. France 1974,768. 2o S. Kato M. Poling D. van der Helm and G. Dryhurst J. Amer. Chem. Soc. 1974,96 5255. 2' C. Slifstein and M. Ariel J. Electroanalyt. Chem. interfacial Electrochem. 1973 48 447. Elec tro-organic Chemistry 22 1 1 (10) M = Me P = CH,CH,CO,H V = CH=CH H (1 1) Reagents i Au anode DMSO 0.55 V (us. s.c.e.),2 F mol-' Scheme 7 of bilirubin (10) in DMSO. A variety of voltammetric techniques have been used. Controlled-potential coulometry at +0.55 V (us. s.c.e.) reveals that 2 F mol-are consumed during oxidation. The probable reaction is given in Scheme 7 although there is little direct evidence for the formation of biliverdine (11). The smooth methoxylation of (tetrapheny1porphyrin)iron chloride follows controlled-potential oxidationz2 in methanol-methylene chloride solution.The product cation (meso-tetraphenylmethoxyisoporphyrin)iron(lIr)chloride was isolated as the hexafluorophosphate salt. Acyclic Nitrogen and Sulphur Compounds. More evidence has been produced concerning the mechanism of the oxidation of NN-dimethylbenzylanine in methanol [cf Ann. Reports (B) 1968 65 2431. Mixtures containing both the a-methoxy and N-methoxymethyl products are usually obtained and it has hitherto proved difficult to distinguish between direct oxidation and indirect oxidation involving H abstraction by an anodically produced methoxyl radical. A careful study23 reveals that in Bu,NBF,-methanol controlled-potential oxidation at platinum gives a 10 :1 ratio for the N-methoxymethyl :a-methoxy products and in these conditions cyclic voltammetry suggests that oxidation of the amine occurs before solvent discharge.In the presence of alkali the corres- ponding ratio of products is cu. 2 :1 which is the expected ratio for random hydrogen abstraction. PhCH / /\ PhCH PhCH CH,Ph (12) (60-75 % depending on R' and Rz) R' R2 Reagents i Pt anode MeCN-LiCIO,; ii Scheme 8 22 J. A. Guzinski and R. H. Felton J.C.S. Chem. Comm. 1973 715. 23 J. E. Barry M. Finkelstein E. A. Mayeda and S. D. Ross J. Org. Chem. 1974 39 2695. 222 J. H. P.Utley The controlled-potential oxidation of dibenzylhydrazine is a convenient means of generating the cation (12),and Cauquis and co-~orkers~~ have used the method in performing a number of interesting cycloaddition reactions (Scheme 8).Coupling of N-alkylanilines may be achieved in acidic solution provided that the N-alkyl group is bulky.25 Similarly anodic oxidation of N-ethylbis-(4-t-butylpheny1)amine gives N-ethy1-3,6-di-t-butylcarbazolein about 15 % yield.26 The intramolecular formation of sulphur-sulphur bonds may also be brought about anodically. A number of gem-polysulphides (13) undergo’ an interesting chain contraction the final product depending on the dryness of the solvent (Scheme 9). The mechanism is not certain but the suggested intermediacy of ArS /CH2\ FH2\ /CHzNu SAr -L ArS-sp’ 2ArS-SAr (1 3) (14) 1 (Nu-= nucleophile) Nu,CH + ArS-SAr (ca.70%) Reagents i Pt anode MeCN-LiClO, 2 F mol-; ii Nu-Scheme 9 the dication (14) is plausible. A sulphur cation intermediate is also implicated in the anodic production28 in 80 % yield of the sulphurane (15) from the corres- ponding diary1 disulphide dicarboxylate. The electrochemical preparation of 0 (15) sulphonium salts has been discussed [Cf. Ann. Reports (B) 1973 70 3071. Tri-phenylsulphonium perchlorate is one of the products of controlled-potential oxidation2’ of diphenyl sulphoxide in dry acetonitrileNaC10 solution in the presence of benzene. The other major product is diphenyl sulphone which sug- gests the mechanism outlined in Scheme 10. Similar electrolysis3’ of triphenyl- phosphine gives tetraphenylphosphonium perchlorate. 24 G.Cauquis B. Chabaud and M. Genies Tetrahedron Letters 1974 2389. 25 R. L. Hand and R. F. Nelson J. Amer. Chem. Sac. 1974 96 850. 26 R. Reynolds L. L. Line and R. F. Nelson J. Amer. Chem. SOC. 1974 96 1087. 27 J.-G. Gourcy G. Jeminet and J. Simonet J.C.S. Chem. Comm. 1974 634. C. S. Liao J. Q. Chambers I. Kapovits and J. Rabai J.C.S. Chem. Comm. 1974 149. 2q G. Bontempelli F. Magno G. A. Mazzocchin and R. Seeber J. Elecrroana/yt. Chem. Interfacial Electrochem. 1974 55 109. 30 G. Schiavon S. Zecchin G. Cogoni and G. Bontempelli J. Electroanalyr. Chem. Interfacial Electrochem. 1973 48 425. Electro-organic Chemistry 223 Ph,SO -3Ph:SO 0'IA Ph,i-O-SPh -P +. Ph2S + Ph2S02 ii 1-H+. -e- Ph,i c10,- Reagents i Ph,SO; ii C,H6 Scheme 10 Miscellaneous.Many examples of the oxidative cleavage of acetate from enol esters have been ~resented.~ ' The experimental procedure is simple involving constant-current electrolysis at a carbon rod anode in an undivided cell. A good example of the usefulness of this reaction is its involvement as a key step in the conversion32 of (-)-menthone into (+)-menthone (Scheme 11). ho -*b ---* Qo*c A A (-1 (+I Reagents i Carbon anode HOAc-Et,NOTs 2.5 F mol-'; ii epoxidation; iii H,NNH2-HOAc; iv Jones reagent; v H,-Pd Scheme 11 The extent of rearrangement of carbenium ions produced by oxidation of carboxylates or alkyl iodides has been compared with that of cations produced by trifluoroacetolysis of tosylates and in a comprehensive paper33 Laurent and co-workers conclude that for iodide electro-oxidation the solvent (acetonitrile) assists cation formation.This conclusion is reinforced by the ob~ervation~~ of 207; inversion in the major product [MeCONHCH(Me)C,H,,] of anodic oxidation of optically active 2-iodo-octane. The full report35 of similar oxidative cleavage of a variety of substituted adamantanes has appeared. Me CH,-H? MeCH, I 'C' /O+' MeCONHCCH,COMe Me/\CH,-C I \ 31 T. Shono Y.Matsumura and Y. Nakagawa J. Amer. Chem. Soc, 1974 96 3532. 32 T. Shono Y. Matsumura K. Hibino and S. Miyawaki Tetrahedron Letters 1974 1295. 33 A. Laurent E. Laurent and R. Tardivel Tetrahedron 1974 30 3423. 34 A. Laurent E. Laurent and B. Tardivel Tetrahedron 1974 30,3431. 35 V. R. Koch and L. L. Miller J.Amer. Chem. Soc. 1973,95 8631. J. H. P. Utley The oxidation of 4,4-dimethyl-2-pentanone in acetonitrile yields,36 after aqueous work-up an acetamide (16) in which the carbon skeleton is altered in a way best rationalized by a McClafferty-type rearrangement via e.g. (17).An attempt to induce a rearrangement similarly inspired by mass spectrometry of dibutylamine failed3' because of the nucleophilicity of the starting material towards the radical-cation produced at the anode. Oxidation of Organic Anions.-Carboxylates. It has been a quiet year for the Kolbe reaction. A very thorough investigation3* of the products of anodic oxidation in methanol of exo-and endo-1,7,7-trimethylnorbornane-2-carboxylic acid and of exo-and endo-2,3,3-trimethylnorbomane-Zcarboxylicacid confirms that exclusively two-electron oxidation results and that the cation (18) is a (1 8) common intermediate.Two-electron oxidation is also found39 for a-alkoxy- acetates in acetonitrile and the resulting cation reacts not with the solvent but exclusively with unreacted carboxylate to form alkoxylated esters. Full reports have appeared of the oxidation of ethylenic carboxylates4' and of phenyl- acetates4' [cf Ann. Reports (B) 1971 68,3131. At a carbon anode either one- or two-electron oxidation of y-substituted paraconic acids can be effected42 by a suitable choice of solvent (Scheme 12). Reagents i Carbon anode NaOMe-MeOH 1 F mol -I ;ii Carbon anode Et ,N-pyridine- H,O 2 F mol-I Scheme 12 Miscellaneous. The anodic reactions of the enolate of dibenzoylmethane are clearly complicated but in benzonitrile (which reduces the probability of ab- straction) oxidative dimeriza tion to tetrabenzoy lethylene is encouraged.36 J. Y. Becker L. R. Byrd and L. L. Miller J. Amer. Chem. Soc. 1974,96 4718. 37 S. Wawzonek and S. M. Heilmann J. Electrochem. Soc. 1974 121 378. 38 G. E. Cream and C. F. Pincombe Austral. J. Chem. 1974 27 589. 39 H. G. Thomas and E. Katzer Tetrahedron Letters 1974 887. 40 R. F. Garwood Naser-ud-din C. J. Scott and B. C. L. Weedon J.C.S. Perkin I 1973 27 14. 41 J. P. Coleman R.Lines J. H. P. Utley and B. C. L. Weedon J.C.S. Perkin II 1974 1064. 42 S. Torii T. Okamoto and H. Tanaka J. Org. Chem. 1974 39 2486. 43 H. W. Vandenborn and D. H. Evans J.Amer. Chem. Soc. 1974,96 4296. Electro-organic Chemistry 225 A full report has appeared of Breslow's use44 of voltammetry to assess the anti- aromaticity of cyclobutadiene by measurements of oxidation half-wave potentials for substituted quinol dianions [cJ Ann. Reports (B),1970,67,226]. 3 Cathodic Processes Reduction of Neutral Organic Compounds.-Cathodic Cleavage. Following unsuccessful attempts4' at preparing substituted bicyclo[2,2,2]propellane by reduction of 9,10-dihalogenotriptycenes,Wiberg and co-workers succeeded46 by using controlled-potential electrolysis of 1,4-dibromobicyclo[2,2,2]octane at -15 "C. Subsequent addition of chlorine gave (19) an addition product which can only come from propellane (20). Similarly the cyclopentadiene adduct of the highly strained olefin (21) has been isolated47 following electrolysis at -20 "C of l-bromo-4-chlorobicyclo[2,2,0]hexane.For acyclic vicinal di- bromides reductive cleavage to alkenes under chemically mild conditions can be highly stereo~elective.~~ For example ( +)-2,3-dibromobutane gives mostly (80:<) of the cis-but-2-ene and the mesu-dibromide gives only trans-but-2-ene.Another interesting use of halogen cleavage is the efficient synthesis49 of deuterio-organic compounds by reduction of 1 -halogeno-naphthalenes in the presence of D20. These experiments also confirm the carbanionic nature of the intermediate. Contrary to conventional wisdom it has been found that with careful control of potential benzotrifluoride may be reduced in steps and the conditions max be arranged5' to optimize formation of the CHF, CH2F and CH products.A full report of earlier work on cleavage from benzylic systems has appeared" [cJ Ann. Reports (B),1970 67 2361. When alkyl halides are reduced in DMF in the presence of carbon dioxide radical-derived products (R2Hg,ROCOC02R RC0,R) are formed at mercury,52 whereas at a graphite cathode carbanion-derived products predominate. 44 R. Breslow D. R. Murayama S. I. Murahashi and R. Grubbs J. Amer. Chem. SOC. 1973,95 6688. 45 A. Bohm J. Kalo Ch. Yarnitsky and D. Ginsburg Tetrahedron 1974 30 217; G. Mark1 and A. Mayr Tetrahedron Letters 1974 1817. 46 K. B. Wiberg G. A. Epling and M. Jason J. Amer. Chem. SOC.,1974 96 912. '' J. Casanova and H. R.Rogers J. Org. Chem. 1974 39 3803. 48 J. Casanova and H. R. Rogers J. Org. Chem. 1974 39 2408. 49 R. Renaud Cunud. J. Chem. 1974 52 376. 50 H. Lund and N. J. Jensen Acta Chem. Scand. (B) 1974 28 263. '' J. P. Coleman Naser-ud-din H. G. Gilde J. H. P. Utley B. C. L. Weedon and L. Eberson J.C.S. Perkin II 1973 1903. 52 J. H. Wagenknecht J. Electroanulyt. Chem. Interfacial Electrochem. 1974 52 489. 226 J. H. P. Utley It now seems clear53 that an earlier report [cf. Ann. Reports (B) 1968 65 2581 of substantial inversion of configuration in the electrochemical reduction of 2-chloro-2-phenylpropionicacid is incorrect. Similarly the carbon-sulphur bond in ethyl 2-phenylmercaptopropionateis cleaved5 with little or no stereo- selectivity. The reductive cleavage of sulphones continues to attract attention although recent experiments are aimed at clarifying mechanistic detail.54 Cathodic cleavage in aprotic solvents of arylsulphonyl chlorides can however lead to a number of useful products. In particular yields of ArSO,SAr ArSSAr and ArSH may be optimized55 by control of the amount of electricity used. Electrolysis has also been used to cleave disulphide bridges in immunoglobulins56 and efficiently to remove benzylidine and benzoyl shields from carbohydrates. 57 Cathodic Substitution. A probably very important polarographic study'* of homogeneous electron exchange (Scheme 13) offers the hope that molecules -A 2A' A' + BX S A + BX' BXL S B' + X-Then e.g. B' +A' S AB-B' +A' $ B-+A Scheme 13 (e.g.BX) may be reduced in the presence of a more easily reduced compound (A) at a potential significantly below that normally required. The requirement is that A should form a relatively stable radical-anion and that decomposition of BXL be rapid. In a polarogram the rate of reduction of BX by A-is reflected in a catalytic increase of the height of the reduction wave for A and this is found to be dependent on the difference between the half-wave potentials of A and BX. Examples of A are aromatic hydrocarbons aromatic ketones esters and nitro- compounds and examples of BX are halides esters alcohols sulphonamides and azides. If for cleavage and substitution reactions the preparative consequen- ces of this idea are realized an electrochemical equivalent of photosensitization will result.An intriguing example of 'electrochemistry without current' has been provided by Saveant and co-~orkers~~ (Scheme 14). The key result is that in wet acetoni- 53 C. M. Fischer and R. Erickson J. Org. Chem. 1973 38 4236. 54 (a)J. A. Cox and C. L. Ozment J. Electroanalyt. Chem. Interfacial Electrochem. 1974 51 75; (b) B. Lamm and J. Simonet Acra Chem. Scand. (B) 1974 28 147. G. Jeminet J. Simonet and J. G. Gourcy Bull. SOC.chim. France 1974 1102. 56 C. Rivat M. Fontaine C. Ropartz and C. Caullet European J. Immunol. 1973 3 537. 57 V. G. Mairanovskii N. F. Loginova A. M. Ponomarev and A. Y. Veinberg Elektro-khimiya 1974 10 172. 58 H. Lund M.-A. Michel and J. Simonet Acta Chem. Scand. (B) 1974,28 900. 59 J.Pinson and J.-M. Saveant J.C.S. Chem. Comm. 1974 933. 227 Electro-organic Chemistry PhCOC6 H,Br PhC(O)C,H,Br PhCOC6H4' + PhS-Ph k(0)C 6 H,s Ph Scheme 14 trile the product (22) of nucleophilic substitution is formed in 80 % yield after the passage of only 0.2 F mol-Cyclic voltammetry confirms that at potentials required for the reduction of p-bromobenzophenone the radical-anion of (22) is oxidizable. The cathodic cleavage of disulphides6' in the presence of methyl chloride or acetic anhydride gives high (80-90%) yields of the corresponding RSMe and RSCOMe compounds. The generality of the method of synthesis of sulphones which involves alkylation of the sulphur dioxide radical-anion has been exten- ded6 to include the preparation of unsymmetrical sulphones [cf.Ann. Reports (B) 1973 70 2951. Amino-acids may be prepared62 in good yield by the con- trolled-potential reductive alkylation in DMF of Schiff's bases. The potential is selected on the basis that the halide is the first-reduced species so the reaction presumably goes via carbanionic (two-electron) or radical (one-electron) addition to the carbon-nitrogen double bond. Cathodic Addition and Oligornevization. The considerable importance of ion- pairing in electro-organic reactions is becoming clear. A simple illu~tration~~ is that a-trifluoroacetophenone gives reversible cyclic voltammetric behaviour in acetonitrile-Et4NC1O4 and its radical-anion may be characterized by e.s.r. spectroscopy. However in the presence of LiClO, the reduction potential becomes more anodic irreversible voltammetric behaviour is observed and preparative-scale electrolysis leads to the pinacol(35 % isolated yield).Similarly dimerization of dimethyl fumarate radical-ions is enhanced6 by conditions which favour ion-pairing and a simple view is that ion-pairing effectively causes charge dispersal which by reducing coulombic repulsion favours radical combination. In a careful in~estigation,~~ the many organic products of reduction of carbon dioxide in an aprotic solvent have been separated by high-pressure liquid chromatography. The optimism engendered by earlier studies [cf:Ann. Reports (B) 1969 66,2351 has not been restored but undoubtedly malic succinic and tartaric acids are among the products if some water is present.In dry DMF at a 6o P. E. Iversen and H. Lund Acta Chem. Scand. (B) 1974 28 827. 61 D. Knittel and B. Kastening Ber. Bunsengesellschaft phys. Chem. 1973 77 833. 62 T. Iwasaki and K. Harada J.C.S. Chem. Comm. 1974 338. 63 C. P. Andrieux and J.;M. Saveant Bull. SOC. chim. France 1973 2090. 64 M. D. Ryan and D. H. Evans J. Electrochem. SOC. 1974 121 881. 65 U. Kaiser and E. Heitz Ber. Bunsengesellschaft phys. Chem. 1973 77 818. J. H. P.Utley 2e 2cs * 2cs,-+ -sc-cs-& s=c-c=s II I1 I I ss ss 'C' -s/\ s-1 -s2-Reagents i CS2;ii 2MeI Scheme 15 mercury cathode up to 90% current yield of oxalic acid is achievable. Carbon disulphide is trimerized66 following electroreduction in DMF solution.The product of addition of methyl iodide to the electrolysed solution was (23) and the reaction presumably proceeds according to Scheme 15. Radical-anions formed in aprotic solvents by reduction of activated olefins will give nucleophilic addition to carbon dioxide. Useful examples of this reaction are discussed in two papers from the Monsanto Conditions may be adjusted to give carboxylation or carboxylation with dimerization. For instance methyl acrylate is reduced in acetonitrile saturated with carbon dioxide to yield,67" after methylation with methyl iodide the triester (24); dimethyl maleate may similarly be reduced at a lower potential to yield the carboxylated dimer (25). C02Me I MeO,CCH,CH(CO,Me) (MeO,C),CHCHCHCH(CO,Me) Cathodic Hydrogenution.A simple undivided cell has been described" that overcomes for reductions where the products are not easily oxidized and which require a proton source the problem of the basicity of the cathode compartment increasing during electrolysis. In essence it employs a mercury pool cathode and a platinized platinum anode over which hydrogen is bubbled. The anode re- 66 S. Wawzonek and S. M. Heilmann J. Org. Chrm. 1974 39 51 I. 67 (a)D. A. Tyssee and M. M. Baizer. J. Org. Chem. 1974 39 2819; (b) ibid.. p. 2823. " J.-M. Saveant and S. K. Binh J. Electroanalyt. Chem. Interfacial Electrochem. 1974. 50 417. Electro-organic Chemistry action is therefore production of protons at a rate which must balance the demand at the cathode. A miniature cell is described in the report of an in~estigation~~ of electrocatalytic hydrogenation of unsaturated steroids.Depending mainly on temperature isolated carbon-carbon double bonds may be reduced selectively (Scheme 16). OH ?H (70 ",") Me0& \ (96Yo) Reagents i Pd cathode EtOH-H,SO, reflux; ii Pd cathode EtOH-H,SO, room temperature Scheme 16 The importance of ion-pairing has been referred to (p.227). The stereoehemis- try of reduction of exocyclic double bonds is seemingly controlled by ion-pairing ; e.g. in ethanol the presence of zinc or magnesium perchlorate directs" the reduction of 4-t-butylcyclohexanone exclusively to the less stable axial alcohol. Under conditions of solvent less favourable to ion-pairing the equatorial alcohol is the major product.These results may be rationalized according to Scheme 17 in which the rate of protonation is assumed to be greater than the rate of inversion of the carbanionic conformers. 4 '0 Scheme 17 K. Junghaus Chem. Ber. 1974 107 3191. 'O R. J. Holman and J. H. P. Utley Tetrahedron Lerrers 1974 1553. J. H. P. Utley Miscellaneous. Breslow and co-~orkers'~ have measured the half-wave potentials for a number of annulenediones e.g. (26) p-benzoquinone (28) and dibenzoyl- diacetylene (29) in the hope that the sum of the reduction potentials for the 0 0 Ill iii I I 0 i Ill Ill PhCOC-CC-CCOPh (29) two waves (El + E2) which reflects the ease of attainment of the aromatic dianions will also indicate the degree of aromatic stabilization.The electrostatic effect on the addition of the second electron must vary however so interpretation of the results is not straightforward. However a comparison of (El + E2) for(26)and (29)shows that the annulenedione dianions e.g.(27),have considerable aromatic character. Redaction of Organic Cations.-Tropylium and triphen ylcyclopropen yl salts may be reduced cathodically and dimerization usually results. For instance tropylium tetrafluoroborate gives,72 at a mercury cathode in 50% acetic acid- acetonitrile a quantitative yield of 7,7'-bis(cycloheptatrieny1). Dimers are formed by reduction at the potentials of either the first or second voltammetric wave and the persistence of dimer formation following two-electron reduction in acidic solution is rationalized according to Scheme 18.Support for this mechanism R+ 5 R. > R-BH'CI0,-7 RH 1 lR+ 9% R BH+ = guanidinium ion (at 1st wave) (at 2nd wave) Scheme 18 comes from the use of a new proton donor guanidinium perchlorate (301 which can be expected to penetrate into the electrical double-layer and make the proto- nation step competitive. With this proton donor (0.4moll-in acetonitrile) the " R. Breslow D. Murayama R. Drury and F. Sondheimer J. Amer. Chem. SOC. 1974 96 249. 72 R. Breslow and R. F. Drury J. Amer. Chem. SOC.,1974,96,4702. Electro-organic Chemistry 231 + (H,N),C=NH,CIO,-(30) monomer product cycloheptatriene is obtained in 30% yield. Similar results are obtained for the reduction of triphenylcyclopropenyl cations.The behaviour of quinolizinium ions often resembles that of the pyridinium ion and the analogy may be extended to electroreduction. The quinolizinium perchlorate (3 1)is reduced to (32) and (33) in aqueous solvents and to a red dimer in dry DMF.73The proportions of(32) and (33) vary greatly with pH and cathode (31) X = C0,Me (32) (33) potential. Enammonium salts such as (34) are also readily reduced,74 with ring-opening to the corresponding 2-substituted cyclohexylamines and cyclo- hexanones. Again the ratio of the products is greatly dependent on pH. The highly conductive charge-transfer complexes that are formed between derivatives of tetrathiofulvalene and tetracyano-p-quinodimethane provoke interest in methods of preparation of the required sulphur compound.The cathodic dimerization of sulphonium salts is therefore of interest and in par- MeS (see Scheme 15) lii 1 Reagents i Et,OBF,-CH,CI,; ii Pt cathode MeCN 1 Fmol-'; iii CCI, 100°C Scheme 19 73 S. Kato Y. Tanaka and J. Nakaya Denki Kagatu Oyobi Kogyo Butsuri Kuguku 1974 42. 223 (Chem. Abs. 1974,81 135 919). '' P. E. Iversen and J. 0.Madsen Tetrahedron 1974 30 3477. 232 J. H. P.Utley ticular the elegant method outlined in Scheme 19 is n~teworthy.~~ Yields in excess of 75 % were achieved for each step. 4 General Of the books and reviews that have appeared attention is drawn to a which concentrates on practical aspects ofthe subject and to a long authoritative and comprehensive review which deals with the applications of electrochemistry to carbohydrate chemistry.77 ” (a)P.R. Moses and J. Q. Chambers J. Electroanalyt. Chem. Interfacial Electrochem. 1974 49 105; (6) J. Amer. Chem. SOC.,1974 96 945. 76 M. R. Rifi and F. H. Covitz ‘Introduction to Organic Electrochemistry’ Marcel Dekker New York 1974. ” M. Fedoronko Ado. Carbohydrate Chem. 1974 29 107.