6 Electro-organic Chemistry* By M. SAINSBURY School of Chemistry University of Bath Bath BA2 7AY 1 General and Mechanistic Aspects Following the example set last year this report beings with a survey of the more important recent contributions to mechanistic electrochemistry. A review has been published in which the scope mechanism and selectivity of certain electrochemical syntheses are discussed,' and a detailed analysis of the reductive cleavage of aryl halides has been undertaken that complements earlier work with related systems.' The first step after formation of the radical anion is scission of the carbon-halogen bond and the production of a neutral aryl radical. The radical then may undergo three concurrent reactions (a) hydrogen abstraction from solvent (b) electron transfer from the electrode and (c) electron transfer from the initial radical anion (Scheme 1).ArX + e $ ArXS -* Ar'+ X-Ar'+ SH -B ArH + S. Ar'+ e + Ar-Ar'+ ArXS + Ar-+ ArX X = halogen SH = solvent Scheme 1 The threefold competition has been studied with 9-anthryl- 1-naphthyl- and 4-cyanophenyl-chlorides -bromides and -iodides in dimethylsulphoxide and acetonitrile solutions and correlated with various parameters including stirring rate and cell design. In the case of 2-halonitrobenzenes at a mercury cathode in acidic 50% n-propanol-water cleavage of the carbon-halogen bond competes with reduc- tion of the nitro group when bromine is present but 2-fluoro- and 2-chloro- nitrobenzenes are reduced without dehalogenation to the corresponding anilines in good yield.3 A similar study has been undertaken with 2-hal0-5-nitrothiophenes.~ If other chlorinated heteroaromatic compounds such as 4-chloro-2,7,8-trimethylquinoline or 2-chloroquinoxaline are reduced in the presence of carbon dioxide a competition between dehalogenation and reductive carboxylation is set ' J.H. P. Utley Philos. Trans. R. SOC. London Ser. A 1981,302 (1468) 237. F. M'Halla J. Pinson and J. M. Saveant J. Am. Chem. Soc. 1980 102,4120. J. Marquez and D. Pletcher Electrochim. Acra 1981,26 1751. I. M. Sosonkin G. N. Strogov T. K. Ponomareva A. N. Domarev A. A. Glushkova and G. N. Freidlin Khim. Geterotsikf. Soedin. 1981,195(Chem. Abstr. 1981,94 147 380y). * Based on Chemical Abstracts during the period November 1980to November 1981.117 118 M. Sainsbury up and from cyclic voltammetric data and preparative scale experiments it has been estimated that when the rate constant for cleavage is <lo4 s-' carboxylation without dehalogenation is initiated.5 The electroreduction of bridgehead iodides such as iodoadamantane or iodocubane to the corresponding hydrocarbons at a mercury cathode is as expected a two-electron process,6 but the usual Haber scheme for the reduction of aromatic nitro compounds is disputed.' It is proposed that azoxybenzenes are formed by the dimerization of nitroso radical anions (Scheme 2); this conclusion backed by voltammetric evidence supports the view previously expressed by Fry.8 Some additional data concerning the reduction (and oxidation) of hydrazo and azo compounds have also been compiled.' ArN02 2e 8 ArNO % ArNO; iArfi=NAr 2H+ -Hz0 I -H2O 0-Scheme 2 It has been confirmed that the reduction of allyl- benzyl- cinnamyl- and polyenyl-phosphonium salts in aprotic solvent takes place by overall one-electron transfer to give the corresponding ylides.In cyclic voltammetric experiments peaks for the reduction of the ylides have been identified and it is proposed that the mechanism by which they are formed requires an initial two-electron transfer and scission to an allylic or benzylic carbanion that then abstracts a proton from a second molecule of the phosphonium salt (Scheme 3). However on the longer time-span of preparative electrolysis competitive cleavage of the phosphonium salts may occur giving radical intermediates which then dimerize." ~ R'CH2;R 2e RICH; R1CH26R R'CH=PR -PR3 -R'M~ Scheme 3 Stereochemical control in electrode reactions has been reviewed," as have the effects of inorganic and organic mediators.'2 A very interesting reductive coupling reaction of aryl carbonyl compounds has been ob~erved,'~ the course of which is directed by the nature of their complexes with p-cyclodextrin (p-CD).Aceto-phenone for example forms a 1:1 complex with the sugar wherein the aryl ring of the substrate is held within the apolar cavity of the p-CD torus. On reduction in dimethylformamide solution at -1.34 V a novel coupling reaction occurs that gives rise to two products (1)and (2) and it is proposed that in order to form these P.Fuchs U. Hess H. H. Hess and H. Lund Actu Chem. Scund. Ser. B 1981 B35 185. R. S. Abeywickrema and E. W. Della J. Org. Chem. 1981,46 2352. ' L. J. T. Janssen and E. Barendrecht Electrochim. Actu 1981 26 1831. A. J. Fry,'Synthetic Organic Electrochemistry' Harper and Row New York 1972 p. 225. G. A. Elenien N. Ismail J. Reiser and K.Wallenfels Liebigs Ann. Chem. 1981 1598. lo V. L. Pardini L. Roullier J. H. P. Utley and A. Weber J. Chem. Soc. Perkin Trans.2 1981 1520. T. Nonaka Kuguku Kogyo 1981,32,401 (Chem.Abstr. 1981,95,60 720f). l2 T. Shono and M. Yoshihiro Kuguku (Kyoto) 1981 36 635 (Chem. Abstr. 1981 95 186 176b 186 178d). l3 C. Z. Smith and J. H. P.Utley J. Chem. Soc. Chem. Commun. 1981,492. Electro-organic Chemistry PhCOMe (complexed?) 1 /\ *H (cavity?) (-Jp(==-J+o Me (I2-ro:OMe OH OH 4 COMe (1) (2) Scheme 4 compounds hydrogen abstraction occurs inside the host complex possibly as shown in Scheme 4.One-electron reduction of maleimide (M) in aqueous solution gives the dihy- drodimer (MH), but different reaction pathways occur for low M) and high (>lop4M) maleimide concentrations and it is concluded that substrate-substrate hydrogen bonding becomes important at When this concentration is exceeded the mechanism of reduction is consistent with the steps shown in Scheme 5.14 (i) 2M(aq) $ Ma * * M (as) (ii) M * -* M (as) + e $ M * -M7 (aq) (iii) 2(M * * MS) (aq) + M * * * MZ-* M (aq) (rate determining) (iv) M * -MZ-.-* M (aq) + 2H20 + M --(MH)2 --M (as) + 2HO- (v) Ma -. (MH),. . -M(aq) $ (MH)*(s) + 2M(aq) Scheme 5 The product distribution of the dimers (4),(3,and (6) formed by the reduction of the 2-cyclohexenone (3) is a function of the water content of the solvent. In l4 R. G. Barradas S. L. Gust and J. D. Porter Tetrahedron Lett. 1981 22,4579. 120 M. Sainsbury (51 (6) acetonitrile for example the presence of water favours (6) but reduces the relative amounts of the other two products (4) and (5).15 The mechanistic course of the electrodimerization of methyl cinnamate also differs in the presence or absence of water. A higher-order mechanism operates when water is excluded and the data obtained in this case are interpreted in terms of a radical-substrate coupling process.16 The mechanism of the anodic oxidation of dialkyl alicyclic and diaryl dithioacetals to disulphides has been studied.l7 For diaryl dithioacetals the reaction proceeds by C-S bond cleavage followed by dimerization whereas for the aliphatic compounds a dicationic intermediate is proposed which scavenges available nucleophiles. An analysis of the anodic behaviour of 10-methylphenothiazine (HPNMe) in the presence of nucleophiles (R-)suggests that attack at the aminomethyl function is initiated by iminium salt formation (by an ECE process) and then a simple cation-anion interaction whereas substitution in the carbocyclic -+ ring involves a radical (R') radical-cation (HPNMe) coupling. In the latter case the radical is generated by reaction of the nucleophile with the radical cation derived by initial electron transfer from the heterocyclic system (Scheme 6)." HP&=CH~ 5HPNCH~R t HPNMe 3HPNMe HPNMe = a:n Me Scheme 6 lS P.Tissot J. P. Surbeck F. 0.Guelacar and P. Margaretta Helu. Chim. Acta 1981,64,1570. l6 V.D.Parker Acta. Chem. Scand. Ser. B 1981,B35,149. J. Gourcy P. Martigny J. Simonet and G. Jeminet Tetrahedron 1981,37 1495. l8 G.Bidan and M. Geniks Nouu. J. Chim. 1981,5,117. Electro-organic Chemistry The cation radical (7) and ultimately the dication (8) are formed when oc dianisidine is oxidized in a thin-layer cuvette both species being detected by spectrophotometric methods.19 However the dication (10) is in equilibrium with the cation radical (9)generated from the anodic oxidation of 1,4-diaminobenzene.The rate of protonation and the kinetic stability of the cation radical have been studied by e.s.r. techniques.*’ L (9) The product distribution and the mechanistic course of the anodic oxidation of 1,2- 1,3- 1,4- 1,5- 1,6- 1,7- 2,3- 2,6- and 2,7-dimethoxynaphthalenesin MeOH-KOH have been studied by Swenton’s groupz1 as a preliminary to a synthesis of daunomycinonq and it has been demonstrated that the key step in the oxidation is the reaction of methoxyl radicals with the aromatic radical cation by a sequence deemed to be of the EEC,C type (see also reference 36). 2 Anodic Processes Carbon-carbon coupling reactions continue to attract the interest of organic chemists; thus the Kolbe synthesis of alkanes with multiple quaternary carbon atoms has been studied.These couplings proceeded normally although some rearrangements and elimination products were noted.22 The oxidation of p -oxocar-boxylate cyclic acetals (11) in methanol does not afford C-C coupled products; instead 2-methoxy-l,4-dioxacycloalkanesare formed possibly through a ring expansion of the intermediate cations (12) followed by neutralization with methoxide (Scheme 7).23 Liquid products from the electrolysis of sodium butanoate in methanol containing methylsodium carbonate at a graphite anode include propyl- and l-methylethyl- methylcarbonates whereas sodium 2,2-dimethylpropanoate at a platinum anode affords 1,l-dimethylethylmethylcarbonate.Typical secondary products derived from propene or 2-methylpropene are also formed.24 However the anodic oxidative decarboxylation of the endo,cis half ester (13) in methanol containing 0.1 mol. l9 M. Otto J. Stach R. Kirmse and G. Werner Z. Chem. 1981 21,296. 2o J. W. Albery R. G. Compton and I. S. Kerr J. Chem. SOC.,Perkin Trans. 2 1981 825. *’ M. G. Dolson and J. S. Swenton J. Am. Chem. SOC.,1981,103 2361; M. G. Dolson B. L. Chenard and J. S. Swenton ibid. p. 5263. 22 N. Rabjohn and G. W. Flasch jun.,J. Org. Chem. 1981,46,4082. 23 D. Lelandais C. Bacquet and J. Einhorn Tetrahedron 1981 37 3131. 24 R. Brettle M. A. Khan and J. D. Rowbottom J. Chem. SOC.,Perkin Trans. 1,1981 2927. 122 M. Sainsbury MeO' R2 Scheme 7 equiv.of sodium methoxide causes exclusive oxygen-assisted Wagner-Meerwein rearrangement to the hemiacetal (14).*' MeOCO. CO H (13) A review has been published concerning the electro-oxidative behaviour of pyrrole indole and carbazole as well as their substituted and partially reduced derivatives,26 and the preparations of pyridoxine maltol cyclotene and tropinone derivatives through the anodic oxidation of furans have been The complex mechanisms involved in aryl-aryl coupling reactions have again been demonstrated,28 and it is stated that aryl-aryl coupling of the bridged ether deriva- tive (15) of (&)-retidine affords the proethyrinadienone (16) and the mor-phinadienone ( 17).29 '' T. Imagawa S. Sugita T. Akiyama and M. Kawanisi Tetrahedron Lett.1981 22 2569. 26 J. M. Bobbitt C. L. Kulkarni and J. P. Willis Heterocycles 1981 15,495. " T. Shono and Y. Matsumura Kagaku (Kyoto) 1981,36,426 (Chem. Abstr. 1981,95,61 871t). '' B. Aalstad A. Ronlan and V. D. Parker Acta Chem. Scand. Ser. B 1981 B35 247. 29 M. Murase and S. Tobinaga Heterocycles 1981 15 1219. Electro-organic Chemistry 123 In related studies it has been observed that there is often a strong preference to form products resulting from six- rather than five-membered transition Thus the first of these two products is unusual and it is also surprising that it does not undergo a dienone-phenol rearrangement. In an extension of earlier studies it has now been shown that the electrochemical oxidation of 2-propenylphenols in methanol solution using an undivided cell yields a variety of products including astone- carpanone- arylpropanoid- and aus- trobailignan-like compounds (Scheme 8).31 Meoq HO I Me 3 Me + OMe OMe Scheme 8 Another example of C-C coupling is provided by the oxidation of NN-dimethyl- mesidine (18),which is assumed to give rise to a radical cation which deprotonates to a neutral radical.This may either then dimerize or be further oxidized to an iminium cation which is trapped by nucleophiles such as diethylphosphonate when these are present in the electrolysis medium (Scheme 9).32 Recent illustrations of carbon-nitrogen coupling reactions include the preparation of the dehydrotetramer (19)of acridine through oxidative electrolysis of the parent heterocycle at +1.45 V.Analysis of this product shows that it is in equilibrium with the N-acridylacridinium radical cation (20) and it is proposed that the formation of the tetramer from acridine takes place by an ECEC mechanistic pathway.33 30 M. Powell and M. Sainsbury Tetrahedron Lett. 1981,22 4751. 31 M. Iguchi A. Nishiyama H. Eto and S. Yamamura Chem. Lett. 1981,939. 32 G.Bidan M. Genies and R. Renaud Electrochim. Acta 1981,26,275. 33 K. Yasukouchi I. Taniguchi H. Yamaguchi and K. Arakawa J. Electroanal. Chem. Interfacial Electrochem. 1981,121,231. 124 M. Sainsbury .Me 1 Me Me J Me Scheme 9 2c10; (19) Several useful synthetic procedures also rely on the fact that methylene groups a to a nitrogen atom can be anodically oxidized and the product iminium species then reacted with a nucleophile.For example the lactone (22) an intermediate en route to alkaloids of the eburnamoine type is obtained by oxidation-cyclization of the substituted piperidineacetic acid (21).34Similarly a-methoxylated carbamates (24) are prepared by the oxidation of carbamates (23) in methanol Et &coZH -2e,-2Hi aTAo N I I C0,Me C0,Me (21) (22) R' / R'-N-CH,R' -2e -2H+ R2-N-cH __7 I I\ CO,R~ MeOH CO,R~OMe (23) (24) 34 K. Irie and Y. Ban Heterocycles 1981 15 201. 35 T. Shono Y. Matsumara and K. Tsubata Tetrahedron Lett. 1981,22,2411,3249; J. Am. Chem. Soc. 1981,103 1172. Electro-organic Chemistry 125 Interest in the methoxylation and acetoxylation of aromatic ring systems has been sustained throughout the year and several examples of quinone monoacetals (27) have been prepared by the oxidation of 2-(4-methoxyaryloxy)ethanols(25) in 1Oh methanolic potassium hydroxide followed by selective monohydrolysis of the initial products (26).36 Similarly anodic nuclear monoacetoxylation of some alkylated aromatic hydrocarbons ( p-xylene isodurene mesitylene and durene) can be achieved with high selectivity with respect to acetylation in the side-chain by carrying out the reaction in the presence of palladium on carbon in a non-divided cell.In this case any products resulting from benzylic oxidation are cleaved by hydrogen formed at the cathode thereby continuously regenerating the sub~trate.~' 2-Acetoxyfuran (28) is readily prepared by the anodic oxidation of furan in the presence of sodium acetate and acetic acid in a~etonitrile,~' and chromone deriva- tives (29) undergo electrohalomethoxylation when oxidized in aqueous methanol containing potassium chloride or bromide to give 2-methoxy-3-halogeno- chromanones (30) .39 ' I' '0 qR' R2 -Kt,"' 0 0' 0 (29) (X = ClorBr) (30) Enol ethers (31) are oxidized at a glassy carbon anode in acetonitrile solution to give dimers and oligomers but if nucleophiles are present the anode reactions are more selective.In the reaction shown in Scheme 10 for example the nature of the products (32) and (33) suggests that after the transfer of tvlo electrons from the substrate an epoxonium species may form which can ring-open in two ways (Scheme lo)."" A regiospecific generation of vicinal fluoroamides from alkenes occurs when the latter are oxidized in acetonitrile solution containing tetraethylammonium tetrafluoride (Et4N+H3F4-).Difluoro compounds aie formed as by-products. The 36 M. G. Dolson and J. S. Swenton J. Org. Chem. 1981,46 177. 37 L.Eberson and E. Oberrausch Acta Chem. Scand. Ser. B 1981,B35,193. 38 T.Shono Y. Matsumura and S. Yamame Tetrahedron Lett. 1981,22 3269. 39 M.Yamauchi S. Katayama Y. Nakashita and T.Watanabe Synthesis 1981 33. 40 M.A.Le Moing G. Le Guillanton and J. Simonet Electrochim. Acra 1981,26 139. 126 M. Sainsbury R3 R2 -2e R3 H-NC OR' Ho-NC -NC COR2 R' R)I1<Rz3R3COCOR2 NC OH OR'OH (33) Scheme 10 alkene reacts by direct one-electron transfer followed by combination of the cation radical with tetrafluoride ion; further oxidation leads to a carbonium species which then adds acetonitrile (Scheme 11).41 A similar reaction takes place with cyclo- alkenes but it is now claimed to be both regiospecific and stereo~elective.~~ Scheme 11 The anodic oxidation of some aldehyde hydrazones (34) in the presence of pyridine leads to s-triazolo[4,3-a]pyridinium salts (37) and here it is proposed that nitrilimines (35)and dihydropyridines (36)are intermediates in the reaction^.^^ Ar' -2e Ar'NHN=CHAr2 -2~+-Ar'%-N=6-Ar2 -(34) (35) Ar2 41 A.Bensadat G. Bodennec E. Laurent and R. Turdivel,Nouo. J. Chim. 1981 5 127. 42 In ref.41 p. 397. 43 I. TabakoviC and S. Crljenak Heterocycles 1981,16 699. 127 Elec tro -orgaItic Chemistry HN-NH -2e N=N R R (38) (39) A convenient preparation of 1,2,4-triazoline-3,5-diones (39) is achieved through the oxidation of urazoles (38);however in an undivided cell simultaneous cycloaddi- tion may take place if a suitable diene is added.44 a-Thiolated aldehydes (41)are valuable synthons and many routes to them have been developed; however a very attractive alternative approach requires the anodic oxidation of vinyl sulphides (40) in aqueous acetonitrile Mechanistically the reaction probably proceeds through the intermediacy of an episulphonium ion (cf.reference 40). PhCH=CHSR -2e* -2H+ p PhCH(SR)CHO H2O (40) (41) Isoprenoids are chlorinated electrochemically in high yields by an ene-type reaction,46 and the anodic fluorination of benz[a]anthracene in acetonitrile solution containing tetramethylammonium trifluoride (Me4N+H2F3-) gives a mixture of 7-and 12-monofluoro- and 7,12-difluoro-ben~[a]anthra~ene.~~ 3 Cathodic Processes The regiospecific addition of substituted ally1 halides to a,@-unsaturated esters has been reported.48 Ally1 chloride for example reacts with methyl crotonate to give 3,4,4-trimethyl-5-hexenoate(42) as a single product when the two substrates are reduced at a platinum cathode.Similarly a,P -unsaturated carbonyl compounds under aprotic conditions and in the presence of alkylating agents afford dimers through coupling at the y-position and alkylation at the oxygen atom.1,2,3-Triphenylpropenone (43),for example when reduced together with dimethylsul- phate gives a mixture of two products (44)and (45).49 Ph Ph Ph (i) 2e 2H+ Ph Ph (ii) Me2S04 (43) Me0 Ph Ph (44) (45) 44 H. Wamhoff and G. Kunz Angew. Chem. Int. Ed. Engl. 1981 20 797. A. Matsumoto K. Suda and C. Yijima J. Chem. SOC.,Chem. Commun. 1981 263. 46 S. Torii K. Uneyama T. Nakai and T. Yasuda Tetrahedron Lett. 1981,22 2291. 47 R. F. O’Malley H. A. Mariani D. R. Butler and D. M. Jerina J. Org. Chem. 1981 46 2816. 48 S. Satoh H. Suginome and M. Tokuda TetrahedronLett. 1981,22 1895. 49 T. Troll W. Elbe and G. W. Ollmann in ref. 48 p. 2961. 128 M. Sainsbury Vicinal di-oxalates (46) undergo fragmentation and elimination on cathodic reduction providing a useful preparative route to alkenes.When applied to mono- oxalates this technique affords a means of selectively cleaving a single hydroxyl function in a dihydroxylated substrate (Scheme 12).50 R' I:') ~O~OC0,Et H)==(:2 (whenX = ErOCOC02) + or XR (46) (whenX = OH) HO R2 Scheme 12 An efficient reduction of alkenes is achieved both in polar and non-polar media in cells divided by a cationic membrane with electrocatalytic metal electrodes deposited on either side (the so-called spe method).51 In more conventional equip- ment and in weakly acidic solution the reduction of ketones of the type PhCOCH(R1)S02R2occurs by C-S bond cleavage giving a-ethylenic ketones. However in more strongly acid solution pinacols are formed.52 1,2-Bis(alkylthio)- 1,2-diphenylethenes (48) are formed by the reduction of dithiobenzoate esters (47) followed by the addition of an alkyl iodide,53 and a convenient synthesis of 3-methyl- lH-phenalen-1-one (50) is provided by the cathodic reduction of 1,8-diacetyl-naphthalene (49).54 S-SR RI I -2PhCS2R -% 2Ph-C:I __* 2Ph-C:-%PhC(SR)=C(SR)Ph I I (47) SR SR (48) -Mao [HI \/ \/ (49) (50) 1,2-Diphenylthi-iren dioxide (51) when reduced electrochemically yields 1,2- diphenylethene and the sulphinate (52) resulting from scission of one or both carbon-sulphur bonds (Scheme 13).55 On the other hand a,a'-dibromodibenzyl- sulphide -sulphoxide and -sulphone give stilbene as a major product together with other products which suggests that in these reactions the crucial step is cyclization to a sulphur-containing three-membered ring (53) which then ring- opens with extrusion of the sulphur atom or one of its oxides (Scheme 14).56 50 D.W. Sopher and J. H. P. Utley J. Chem. SOC., Chem. Commun. 1981,134. Z. Ogurni K. Nishio and S. Yoshizawa Electrochim. Acta 1981 26 1779. 52 R.Kossai and J. Simonet in ref. 51,,p.189. 53 G.Adwidjaja L. Kistenbriigger and J. Voss J. Chem. Res. (S),1981 88. " B. M.Davis P. H. Gore K. A. K. Lott E. L. Short and H. G. Hakim J. Chem. SOC., Perkin Trans. 2,1981 58. 55 A. J. Fry K. Ankner and V. K. Handa J. Chem. SOC.,Chem. C&nmun. 1981 120. 56 A. J. Fry K. Ankner and V. K. Handa Tetrahedron Left. 1981,22 1791. Electro-organic Chemistry VPh dso2-=bH5 S Ph Ph SO,- (5 1) 02 /-SO2 i-.- PhCH=CHPh $ Phe=CPh H Ph Ph SO,-H (52) Scheme 13 Ph Ph Ph BryxyB' ycX\rBr 2e -Br-X -A +PhCH=CHPh -Br-Ph Ph Ph Ph Ph Ph Ph' (X=S SO or SO2) (53) Scheme 14 The rate constant for heterogenous electron transfer from a platinum electrode to aromatic disulphides is low but that from aromatic anion radicals is higher.This fact can be used in the 'assisted reduction' of disulphides thus the anion radicals are generated at potentials more negative than the normal standard reduction potential of the disulphide but less negative than the observed reduction potential (Scheme 15).57 A+e $ A7 A' + ArSSAr + A + [ArSSAr]' [ArSSAr]' + ArS' + ArS-A' + ArS' -+ A + ArS-Scheme 15 When but-2-yne is reduced at a platinum electrode in sulphuric acid solution the main products are butane and E-and 2-but-2-enes with an overall total current efficiency of -100%; however whereas the alkenes are formed by the simple addition of two hydrogen atoms the production of butane is more complex and several routes to the compound are possible.58 Electrochemical reductive techniques are often used in the synthesis or modification of heterocyclic systems; some examples are included below.Thus isoxazoles (56)can be synthesized by the electrochemical reduction of nitroethylenic ketones (54) in acidic media (pH -l),possibly through the intermediacy of a 1,3-diketone monoxime (55),59and a convenient synthesis of 1,4-benzothiazin-l,1-dioxides (58)involves the electrochemical reduction of the 2-nitrophenyl sulphones (57).60Similarly the controlled potential reduction of y-nitroketones (59) at a " J.Simonet M. Carriou and H. Lund Liebigs Ann. Chem. 1981,1665. H. Kita and H. Nakajima J. Chem. Soc. Faraduy Trans. 1 1981,77,2105. " C.Bellec P. Maitte J. Armand and C. Viele Can. J. Chem. 1981,59,527. 6o C.P.Maschmeier H. Tanneburg and M. Matschiner 2. Chem. 1981,21,219. 130 M. Sainsbury 1 OH (57) (58) (59) 2Hf 2e 1 mercury electrode in aqueous organic media allows the sequential formation of 1-pyrroline-1 -oxides (60) pyrrolines (61) and pyrrolidines (62).61 The electrochemical reduction of 3-acetyl-1 -benzylpyridinium chloride (63) in an aqueous buffered solution gives the corresponding 1,6-dih~dropyridine,~~ but the reduction of 4-aminopyrimidine (64) under similar conditions is accompanied by isomerization ring-opening and/or deamination of the primary reduction prod- ucts and secondary chemical Similarly the l-benzyloxy-l,2,3,4-tetra-hydroisoquinoline (65) useful as an intermediate en route to the alkaloid cularine (67) can be synthesized by the electrochemical reduction of the iminium salt (66) in the presence of the appropriate benzyl bromide (Scheme 16).64 In conclusion three important reviews have been published which are concerned with (a) the industrialization of electrochemical reaction^,^' (b) electro-organic syntheses,66 and (c) the electrochemistry of that interesting group -the viologen~.~' " M.Carriou R. Hazard M. Jubault and A. Tallec Tetrahedron Lett. 1981 22 3961. F. M. Moracci S. Tortorella B. Di Rienzo and I. Carelli Synth. Commun. 1981 11,329. 63 B. Czochralska and P. J. Elving Electrochim. Acta 1981 26 1755. 64 T. Shono T. Miyamoto M. Mizakarni and H. Hamaguchi Tetrahedron Lett. 1981 22 2385. 65 R. E. W. Jansson Trans. R. SOC. London Ser. A 1981 302 285. 66 K. Koester and H. Wendt Compr. Treatise Electrochem. 1981 2 251. 67 C. L. Bird and A. T. Kuhn Chem. Soc. Rev. 1981 10,49. Elec tro -organic Chemistry 131 (64) J H2NCH=N-CH=CHCH(OH)NHZ H Me0 W M e ] + Me0oBf Me0 w ! M e A I OCH,Ph x-OCH,Ph J OMe (65) Me0 QMe M e 0 7 Me steps *--PhCH20 1 Me0QBr Me0 OMe OMe (67) Scheme 16