6 Electro-organic Chemistry* By M. SAINSBURY School of Chemistry University of Bath Claverton Down Bath BA2 7AY 1 Introduction The subject of electro-organic chemistry continues to expand and it is an increasingly difficult task to summarize all those papers which best reflect trends and innovation within this general subject area. Thus it is appropriate to note that several reviews have appeared which deal with specialized topics; for example a number of surveys of the use of electrochemical reactions in synthesis both on .laboratory’.’ and on industrial scale^,^>^ have been published. Other summaries cover the synthesis of heterocycle^,^ some interconversions of p-lactam antibiotics and related structures,6 and the application of electrochemical methods to the reactions of polyenes and carotenoid~.~ For those readers unfamiliar with cyclic voltammetric techniques an introductory article by Mabbott is recommended,* and for a detailed study of reaction mechanisms by electrochemical methods the review by Parker’ is noteworthy.2 General and Mechanistic Aspects Savkant and Tessier have determined” the electrochemical electron-transfer rate constant as a function of the electrode potential for a series of simple electron- transfer processes occurring at a mercury electrode in media consisting of acetonitrile or N,N-dimethylformamide and a quaternary ammonium salt as the supporting electrolyte. In this work the reactions and the experimental conditions were selected in order to emphasize ‘outer-sphere’ processes and to minimize the effects of double-layer corrections.Convolution potential-sweep voltammetry and impedance methods were employed in order to obtain the kinetic data and it was found that the electrochemical transfer coefficient varied beyond experimental * Based on Chemical Abstracts during the period November 1982-November 1983. J. Simonet Actual. Chim. 1982 19. J. Grimshaw and D. Pletcher Electrochemistry 1983 8 171. M. M. Baizer Electrochem. Znd. [Proc. Znt. Symp. J.] 1980 (Pub. 1982) 101. J. H. Wagenknecht J. Chem. Educ. 1983 60 271. Y. Ban Yuki Gosei Kagaku Kyokai Shi 1982 40 866; Chem. Abstr. 1983 98 54254~. S. Torii H. Tanaka M. Sasoaka N. Saitoh T. Siroi and J. Nokami Bull. SOC.Chim. Belg. 1982,91,951. ’J. H. P. Utley Carotenoid Chem.Biochem. Proc. Znt. Symp. Carotenoids 6th 1981 (Pub. 1982) pp. 97-105 ed. by G. Britton and T. W. Goodwin Pergamon Oxford. G. A. Mabbott J. Chem. Educ. 1983,60 697. V. D. Parker Adv. Phys. Org. Chem. 1983 19 13. J. M. Saviant and D. Tessier Faraday Discuss. Chem. SOC. 1982 No. 74 p. 57. I23 124 M. Suinsbury error with the electrode potential. The magnitude of the variation is of the same order as that predicted by the Marcus theory of outer-sphere electron-transfer. This behaviour appears to be general in reductions of organic substrates for which charge transfer is fast and is governed mainly by solvent reorganization. Sirnonet's group have provided an interesting example of an electron-transfer reaction which occurs during the reduction of the indanone (1) in the presence of primary alkyl bromides (RBr)." The products -methyleneindanones (2) -are formed with a low overall consumption of electricity implying a chain reaction possibly following the path outlined in Scheme 1.Me Me m\ Me Me Scheme 1 Potential-pH diagrams have been constructed12 for a number of phenols and from them the relationships between chemical and electrochemical phenomena associated with the oxidation-reduction of the substrates have been analysed in an effort to provide a better understanding of these and similar reactions which involve proton transfer. In the case of arylsulphonyl halides the half-wave potentials of the corresponding chlorides and fluorides differ by >1 V the fluorides being the more negative.I3 The effect of para-substituents is small for the chlorides but large for the fluorides.In agreement with the values of the half-wave potentials arylsulphonyl chlorides are considerably more chemically active than the corresponding fluorides. A number of cyclic voltammetric studies have been conducted this year. Thus the behaviour of 2-alkylthiocycloalkanoneson anodic oxidation has been examined J. Delaunauy M.-A. Orliac-Le Moing and J. Sirnonet J. Chem. SOC.,Chem. Commun. 1983 820. 12 S. I. Bailey I. M. Ritchie and F. R. Hewgill J. Chern. SOC.Perkin Trans. 2 1983 645. 13 L. Horner and R. E. Schmitt Phosphorus Sulphur 1982 13 189. Electro-organic Chemistry 125 in order to assess the possible uses of these compounds in ~ynthesis,'~ and the cathodic reactions of a series of azo-compounds have been similarly ana1y~ed.I~ Substituent effects in the electrochemical oxidation of arylmethyl anions (lithium salts) have also been considered16 and a body of data has been accumulated which provides information about electronic distributions and geometrical preferences in such molecules.The direct electrochemical oxidation of aromatic hydrocarbons at a lead oxide anion has been investigated using the method of electrode kinetic^.'^ The results obtained were then analysed to predict conditions under which the reactions could be employed in large-scale syntheses of aromatic alcohols and aldehydes from arenes. The Kolbe reaction still attracts interest and the kinetics of the dimerization process and its side reactions have been measured for a series of alkali-metal alkanoates.18 Dimer formation is a second-order reaction whereas by-products are formed by first-order processes.Aprotic conditions for the Kolbe reaction have also been studied and the basis for preparative-scale reactions of this type has been established." Trifluoromethyl radicals are generated by the electrolysis of sodium trifluoroace- tate in the presence of activated dienes (3). These react to yield cyclic structures (4)and (5) through addition and the coupling of the biradicals so formed.20 In F3c\ C0,Et / CH=CHCO,Et CH2-CHC0,Et CH-CH (~72~ (A2)"/ / 1 (C\"/ 2)n \ CH=CHCO,Et CH-CHC0,Et /CH-CH\ I F3c C02Et CF3 (3) (4) (5) the case of simple alkenes trifluoromethyl radicals normally give the corresponding 1,2-bis(trifluoromethyl)alkanes but it has been shown that when such alkenes bear an isopropenyl group only one trifluoromethyl group is added.21 For example isopropenyl acetate (6) gives only the ketone (7) when electrolysed in an aqueous medium containing sodium trifluoroacetate (see Scheme 2).H3C CF3CH2 CF3CH (6) (7) Scheme 2 14 H. D. Cajon and H. Viertler An. Simp. Bras. Electroquim. Electroanaf. 3rd 1982,1 197;-Chem. Abstr. 1983,98 80419e. 15 A. J. Bellamy 1. S. MacKirdy and C. E. Niven J. Chem. SOC.Perkin Trans. 2 1983 183. 16 S. Bank A. Schepartz P. Giammatteo and J. Zubieta J. Org. Chem. 1983 48 3458. 17 J. A. Harrison and J. M. Mayne Electrochim.Acfa 1983 28 1223. 18 K. Kase N. Sato and T. Sekine Denki Kagaku Oyobi Kogyo Butsuri Kagaku 1982 50 914; Chem. Abstr. 1983 98 125182r. 19 R. Engels C. J. Smit and W. J. M. Van Tilborg Angew. Chern. Znt. Ed. Engl 1983 22 494. 20 R. N. Renaud C. J. Stephens and D. Berube Can. J. Chem. 1982,60 1687. 21 N. Muller J. Org. Chem. 1983 48 1370. 126 M. Sainsbury Nelsen and his associates in a series of paper^^^-^^ have shown that the radical cations derived from various bridged heterocycles such as azaoxabicyclo-octenes (8) and azabicyclo-octenes (9) are long lived -a fact associated with the implementa- tion of Bredt's rule. Oxidation of 2-phenylnorbornene (10) leads to the dehy- drodimer (ll),and an analysis of the anodic behaviour of the substrate by cyclic voltammetry shows a novel curve-crossing which is claimed to be the first experi- mental observation of this kind.2s An ECE mechanism with a significant redox cross-reaction is proposed to explain this undsual result.Positional selectivity and isotope effects have been studied for the chemical and electrochemical oxidations of polyalkylbenzenes.26 Si,milar results were obtained for those reactions occurring at the anode and for those using cerium(1rr) ammonium nitrate as oxidant. Both techniques generate radical cations which then deprotonate to give benzylic radicals in the selectivity-determining step. However in oxidations promoted by cobalt(II1) acetate no correlations with the electrochemical method were found and here it is proposed thaf the mechanism involves hydrogen atom transfer as the key step.A number of diverse unsaturated compounds have been subjected to electroreduc- tion at a mercury pool cathode in acetonitrile solution containing methyl chlorofor- mate.27 These structures include activated alkenes ketones aromatic imines nitro- compounds and nitrogen heterocycles. From the products isolated and from the voltammetric data obtained possible reaction mechanisms are advanced for the reduction of each type of compound. Most follow predictable paths as for example in the formation of 9-methoxycarbonylfluoren-9-yl methyl carbonate ( 13) from the reduction of fluorenone (12) (see Scheme 3). R' RZ ClCOMe R' R' R2 CIC0,Me R' RZ RTOR~ 5 'f + -0 OC0,Me OC0,Me OC0,Me 22 S.F. Nelsen D. J. Steffek G. T. Cunkle and P. M. Gannett J. Am. Chem. SOC.,1982 104 6641. 23 S. F. Nelsen and J. A. Thompson-Colon J. Org. Chem. 1983 48 3364. 24 S. F. Nelsen G. T. Cunkle D. H. Evans and T. Clark J. Am. Chem. Soc. 1983 105 5928. 25 M. A. Fox and R. Akaba J. Am. Chem. SOC.,1983,105 3460. 26 E. Baciocchi L. Eberson and C. Rol J. Org. Chem. 1982 47 5106. 27 J. Armand C. Bellec L. Boulares and J. Pinson J. Org. Chem. 1983 48 2847. Electro-organic Chemistry 127 On reduction l-alkyl-2,4,6-trisubstituted pyridinium salts (14) afford v-radicals which are stable on the time-scale of cyclic voltammetry but the radicals from the corresponding 1-benzyl- and 1-allyl-compounds are not and undergo cleavage of the C-N bond at rates which are dependent on the size of the 2,6-~ubstituents.~~ A cyclic voltammetric study of the pyrazolidinones (15) shows that here the chemical reaction step after formation of the radical cation is deprotonation at position-2." In the absence of added base and at substrate concentrations >2 mM it is the parent compound which scavenges the proton.The lifetimes of the radical cations are relatively insensitive to the nature of the substituents at C-4 and C-5 but are increased markedly by substitution at N-2. R3 Ph\ N-N /R' R2A N+ R4-R4qo R3 R2 R' = alkyl; R2 R3,R4= alkyl or phenyl R' R2,R3,R4= H alkyl or phenyl (14) (15) It has been argued that the N-radical cation (16) of laudanosine through a form of homocon jugation promotes aryl-aryl coupling which eventually leads to 0-methylflavinantine (19).This radical cation has now been synthesized by non- electrochemical means but in dilute solution instead of undergoing the coupling reaction (16) -+ (19) it decomposes by eliminating the benzyl group at C-1 thus producing the 3,4-dihydroisoquinolinium salt ( 17).30An alternative mechanism for the formation of 0-methylflavinantine is now proposed namely one involving homogeneous electron-transfer from the aromatic m-system of a neutral laudanosine molecule to the radical cation (16). The newly formed rr-radical cation (18) then forms 0-methylflavinantine by a conventional aryl-aryl coupling process. Clearly such a reaction will be concentration dependent and may well be favoured near to the electrode surface (see Scheme 4).A study has been made of the anodic oxidation of a-methyldopamine (20a) a-methylnoradrenaline (20b) and dopamine (20c) at a carbon-paste electrode in 1M perchloric acid and McIlvaine buffers of varying pH and at temperatures ranging from 15 to 30 "C in attempts to simulate the formation of natural melanins.31 Cyclic voltammetric analyses show that each catecholamine undergoes an ECC sequence reaction which begins with a two-electron oxidation to a quinone (21) and then involves deprotonation to the free amines (22). These products cyclize rapidly to 5,6-dihydroxyindolines (23). Further oxidation then gives the aminochromes (24) which in the case of the dopamines (20a) and (20c) rearrange to electrochemically detectable 5,6-dihydroxyindoles (25).No peak for 5,6-dihydroxy-2-methylindole can be discerned probably because loss of water from the precursor (24b) yields 28 J. Grimshaw S. Moore N. Thompson and J. T. Grimshaw J. Chem SOC.,Chem. Commun. 1983,783. 29 A. J. Bellamy D. I. Innes and P. J. Hillson J. Chem. SOC.,Perkin Trans. 2 1983 179. 30 M. Hutchins M. Sainsbury and D. I. C. Scopes J. Chem. SOC.,Perkin Trans. 1 1983 2059. 31 T. E. Young and B. W. Babbitt J. Org. Chern 1983 48 562. 128 M. Sainsbury Me0 Me0 -+ =?Me M e O T M e Me0 (17) Me0 M eO OMe OMe A OMe Me0 Me0 _.._ NMe Me0 Scheme 4 (25 a and c) (24 oxidation 11 [H(aT$R] +melanoid pigments (26 a+> a series R' = R3= H; R2 = Me b series R' = H; R2 = Me; R3= OH c series R' = R2 = R3= H Scheme 5 Electro-organic Chemistry 129 the transient iminoquinone (26b).In all cases on further oxidation melanin-like pigments are ultimately produced. Anodic oxidation of benzophenone hydrazones (27) gives several products dcpending upon the electrolysis conditions used.32 At a platinum anode in acetonitrile containing lithium perchlorate azines (28) are the major products but in sodium methoxide however methanol dimethylacetals (29) are formed. If the electrode is changed to one made of graphite arylmethyl methyl ethers (30) and diarylmethanes (3 1) result. It seems likely that diaryldiazomethanes or their equivalents are intermediates in these last reactions and that in the undivided cell diarylmethanes form through cathodic reduction (see Scheme 6).Ar Ar Ar OMe Ar Ar >N-N< X >OMe ) Ar Ar Ar OMe Ar Ar (28) (29) (30) (31) Scheme 6 3 Cathodic Processes The electrochemical behaviour of a series of a,w-dibromo-esters (32) has been studied in an attempt to optimize the intramolecular cyclization reaction (32) -* (33).33 Although a yield as high as 90% is claimed for the conversion of ethyl a,y-dibromobutyrate (32; n = 1) into ethyl cyclopropanecarboxylate (33; n = l) reactions with other dibromo-esters are much less productive and often mono-debromination is found to be a competitive process to cyclization. Two-electron reductions of dibromocyclopropanes (34)afford mixtures of cis and trans mono-brominated products (35) and (36).34When the substituent groups are R' = H or alkyl and R2 = phenyl only a low stereoselectivity is noted but when R2is a carboxyl or alkoxycarbonyl function the cis form of the products predominates.Possibly in such cases the carbonyl group of the substrate is best orientated away from the negatively charged electrode surface thus inducing the observed stereochemical preference. Conformation plays an important role in determining the product ratios in electrochemical reductions of vicinal dibromides and it has been dem~nstrated~~ that there is a strong preference for reductions to occur in substrates which allow 32 T. Chiba M. Okimoto H. Nagai and Y. Takata J. Org. Chem. 1983,48 2968. 33 C. Giornini A. Inesi and E.Zeuli J. Chem. Res. 1983 280. 34 R. Hazard S. Jaouannet and A. Tallec Electrochim. Actu 1983 28 1095. '' K. M. O'Connell and D. H. Evans J. Am. Chem. SOC.,1983,105,1473. 130 M. Sainsbury conformations with an antiperiplanar disposition of bromine atoms. Thus conforma- tional interconversions prior to electron transfer are observed for trans-l ,2-dibromo-(37) and 1,l -dimethyl-trans-3,4-dibromo-cyclohexanes (38) and also in the cases of other similar substrates. BrCH2(CH2),CH( Br)CO,Et &H2- (CH,) -kHCO,Et (32) (33) R:-Br (34) (35,cis) (36 trans) (j:,,=Br (37) Further examples of the electrochemical synthesis of catalytic species capable of initiating coupling reactions have appeared this year. As an illustration the reduction of nickel(I1) bromide yields a form of nickel which causes ethene to couple with aryl halides giving 1,l-diarylethanes as the principal products.36 Electrochemical generation of organonickel-phosphine complexes similarly allows a new catalytic route to substituted styrenes via the coupling of aromatic halides and alkenes (see Scheme 7).37 ArX + RCH=CH 2 ArCH=CHR + HX Scheme 7 Primary and secondary alcohols and a,w-diols may be converted into carboxylic acids ketones and dicarboxylic acids respectively by heterogeneous oxidation with nickel(o) hydroxide electrochemically generated at a nickel(I1) ele~trode.~' In this way butan-1-01 in aqueous 1 M sodium hydroxide at 70 "C gives butanoic acid in 85% yield and 1,6-dihydroxyhexane at 25 "C affords adipic acid in 84% yield.Electrochemically generated superoxide ion has been used as a base to deprotonate secondary nitro alkane^.^^ The anions thus formed are then oxidized by molecular oxygen bubbled through the cathode compartment of the electrolysis cell thereby giving rise to the corresponding ketones. Productivity is good and for example the 36 Y. Rollin G. Meyer M. Troupel and J.-F. Fauvarque Tetrahedron Lett. 1982 23 3573. 37 Y. Rollin G. Meyer M. Troupel J.-F. Fauvarque and J. Perichon f. Chem. SOC.,Chem. Commun. 1983,793. 38 J. Kaulen and H.-J. Schafer Tetrahedron 1982 38 3299. 39 W. T. Monte M. M. Baizer and R. D. Little J. Org. Chem. 1983 48 803. Electro-organic Chemistry nitroacetal (39) is converted into the ketone (40) in 71% yield.Using the same reagent ethyl cyanoacetate (41) is transformed directly (a) into ethyl glyoxylate (42) and (b) by a 'one-pot' two-step sequence into ethyl oxomalonate (43)."' (39) NCCH,CO,Et OHCC0,Et O=C(CO,Et) (41) (42) (43) The anion (45) generated by electrochemical reduction of 2-pyrrolidone (44) in dimethylformamide solution containing tetraethylammonium toluene-4-sulphonate is sufficiently basic to promote the formation of trichloromethyl alcohols (47) from ketones (46) and chloroform4' (see Scheme.8). H (44) (45) (45)+ CHCl -+ CCI3 + (44) R'COR + Cc13 (46) (47) Scheme 8 Reduction of the bicyclic enone (48)affords the tetracyclodecanone (52).42 Cyclic voltammetric analysis indicates that the reduction requires the formation of the radical anion (49)? which then undergoes intramolecular hydrodimerization-aldol condensation leading via the isomeric radical anion (50) to the dianion (51) and thence to the final product (see Scheme 9).This compound is also produced when the starting material is reduced with magnesium in the presence of trimethylsilyl chloride and a catalytic amount of titanium(1v) chloride in HMPT solution. Cathodic reduction of fluorenes (53; R' = H or Br R2 = CN) or (53; R1= H R2 = C02Et) in dimethylformamide solution gives dianions which are efficient in converting phosphonium salts into the corresponding ylide~.~~ The electrode poten- tials required are low enough to allow the generation of the bases in the presence of several phosphonium salts and of aldehydes so that the method provides a 40 M.Sugawara and M. M. Baizer TetrahedronLett. 1983,.24 2223. 41 T. Shono S. Kashimura K. Ishizaki and 0.Ishige Chem. Lett. 1983 1311. 42 P. Margaretha and P. Tissot Helv. Chim. Acta 1982 65 1949. 43 R. Mehta V. L. Pardini and J. H. P. Utley J. Chem. SOC.,Perkin Trans. 1 1982 2921. 132 M. Sainsbury Scheme 9 convenient way of carrying out the Wittig reaction under very mild conditions. It is interesting that the stereoselectivity of the process depends upon the nature of the counter cation. Lithium cation for instance gives predominantly cis-isomers. Cathodic reduction of nitroalkenes (54) formed from aldehydes and nitroalkanes in a divided cell containing methanol and 20%aqueous sulphuric acid affords mainly oximes (55),together with small amounts of acetals (56)and ketones (57).44 Since the subsequent transformations of the oximes and the acetals into the ketones are readily achieved by hydrolysis this constitutes a very convenient three-step synthesis of longer-chain ketones from aldehydes.R1 R' R' R2CH=&-N02 A. R2 (54) NC 1,I,3,3-Tetraphenylallene (58) undergoes reduction at a mercury pool cathode to give the alkenes (60) although if carbon dioxide is introduced into the catholyte mixture the acid (61)is formed.45 In the presence of dimethylformamide the aldehyde (64) is produced and it is proposed that the first two products arise from the unrearranged anion (59),whereas in the case of the aldehyde it seems likely that 44 T.Shono H. Hamaguchi H. Mikami H. Nogusa and S. Kashimura J. Org. Chern. 1983 48 2103. 45 G. Schlegel and H.-J. Schafer Chern. Ber. 1983 116 960. Electro-organic Chemistry isomerization of this intermediate initially to the anion (62) and thence to the species (63) is necessary before reaction with the electrophile occurs (see Scheme 10). Ph )=C<PhPh ph4PhPh Ph (58) (60) Ph Scheme 10 4 Anodic Processes Electrochemical methoxylation is a very convenient procedure which has application in the synthesis of heterocycles and acetals as well as in the a-functionalization of amides and carbamates. Thus the derivatives of L-ornithine and L-lysine (65; n = 0) and (65; n = 1) may be cyclized to the corresponding optically pure a’,P’-unsatur-ated a-amino-acids (67;n = 0) and (67; n = 1) through electrolysis in the presence of methanol followed by acid treatment and elimination of methanol from the intermediate products (66)46(see Scheme 11).( * )-Lupidine (72) and its epimer (73) are prepared similarly by a sequence of reactions wherein the lactam (68) is methoxylated anodically to give the ether (69). This is converted into the iminium salt (70) or its equivalent by treatment with a Lewis this species then undergoes intramolecular cyclization to the diester 46 T. Shono Y. Matsumura and K. Inoue J. Chem. SOC.,Chem. Commun. 1983 1169. 47 M. Okita T. Wakamatsu and Y. Ban Hererocycles 1983 20 401. 134 M. Sainsbury ,,OH-Y (CH2)" Me0/r>&+(J C0,Me C02Me I 1 C0,Me C0,Me (66) (67) Scheme 11 (71) which through a reductive sequence eventually gives an epimeric mixture of the alkaloids (see Scheme 12).Me0,C C0,Me Me0,C C0,Me *qfJ QNJ TiCI b 0 0 1 (73) (72) Scheme 12 The products of anodic methoxylation of ergolines (74; R' = R2 = H) previously thought to be the 2-methoxylated derivatives (74; R1 = H R2 = OMe) are in fact l-hydromethylergolines (74; R' = CH20H R2 = H).48 However when the 1-position is blocked anodic cyanalation does yield 2-cyanoergolines (74; R' = Me R2 = CN).49 In the case of the p-lactam (75; R = H) anodic methoxylation gives the ether (75; R = OMe). The yield 90% is remarkably high bearing in mind that there are two potential sites for meth~xylation.~' Swenton" and PedlerS2 have 48 B.Danieli G. Fiori G. Lesma and G. Palmisano Tetrahedron Lett. 1983 24 819. 49 K. Seifert S. Hartling and S. Johne Tetrahedron Lett. 1983 24 2841 T. Shono Y. Matsumura and K. Inoue J. Org. Chem. 1983 48 1389. 51 J. S. Swenton Synthesis 1983 74. 52 G. M. Elgy W. B. Jennings and A. E. Pedler J. Chem. Soc. Perkin Trans. 1 1983 1255. Electro-organic Chemistry H2NOC8 H Me02CN R R2 0 ’?Me ‘N CO,Me R’ (74) (75) published further details concerning the electrochemical synthesis of quinone bis- and mono-acetals from aromatic precursors and it has been shown that annulenes behave similarly. Thus oxidation of 2-methoxy- 1,&methano[ 1Olannulene (76) in methanolic 1% potassium hydroxide solution gives the 1,4-addition product (77) which on deprotection and base treatment affords the hydroxy compound (78)53 (see Scheme 13).-2e (i)HCl ,&Me @ / KOH-MeOH \ (ii) Et,N \ H OMe OMe (76) (77) (78) Scheme 13 Electrochemical methoxylation of 3-furoic acid yields a mixture of isomeric 2,4,5-trimethoxylated tetrahydrofurans (79).54 Bond breaking reactions are also possible and during the oxidation of the cyclohexanone (80) in the presence of methanol cleavage of the C-1 and C-2 bonds occurs to give the methyl ester (81) later used in a synthesis of methyl trans-~hrysanthemate.~~ A new method of 1,4-transdisposition of a carbonyl group has been developed in which the key step is the anodic methoxylation of a dienol acetate (82).56 The product (83) is then reduced to the corresponding allylic alcohol (84) and this is esterified with toluene-4-sulphonyl chloride.Solvolysis of the ester (85) in acetone ’’ R. Neidlein and G. Hartz Synthesis 1983 463. 54 1. Stibor J. Srogl M. Janda N. Piricova and K. Vlazny Collect. Czech. Chem. Commun. 1982,47,3261. 55 S. Torii T. Inokueki and R. Oi J. Org. Chem. 1983 48 1944. 56 T. Shono and S. Kashimura J. Org. Chem. 1983 48 1939. 136 M. Sainsbury containing 10% water gives the 1,4-transposed carbonyl product (86) in good yield (see Scheme 14). NaBH 1 (86) (85) Scheme 14 Anodic methoxylation of carbamates (88) prepared from primary amines (87) gives the a-methoxycarbamates (89) as transient intermediates which are converted into the acetals (90) by the acid generated in situ at the anode during the electrolysis step.57 Yields are good for aliphatic substrates and the route is probably the simplest yet devised for the interconversion of amines to carbonyl compounds (see Scheme 15).R' CIC0,Me ' R' FNHCOzMe -2e R2 R2 (87) (88) (89) MeoH I"' R1 H+ R' OMe R2>o H,O R ~ ~ O M ~ (90) Scheme 15 Secondary aliphatic nitro-compounds (91) may be converted into the correspond- ing ketones (92) by oxidation in methanol or ethanol solution containing sodium formate or sodium acetate respectively. Yields range from 40 to 90% depending upon the nature of the substituents.s8 No mechanism for the process is suggested but it is likely that nitro-esters are intermediates which hydrolyse and lose nitrous acid on aqueous work-up (see Scheme 16).R2 NaOCOR3 ' R' R' OCOR~ R' (91) (92) Scheme 16 57 T. Shono Y. Matsumura and S. Kashimura J. Org. Chem. 1983 48 3338. 58 J. Nokarni T. Sonoda and S. Wakabayashi Synthesis 1983 763. Electro-organic Chemistry A useful application of the Kolbe reaction is to be found in the synthesis of 1,4-diketones (94) through the anodic oxidation of the P-acetal acids (93)59 (see Scheme 17). r (93) R2 R' 0 R 3 w R 3 0 R' R2 Scheme 17 Last year Simonet and his associates described an electron-transfer chain reaction for the conversion of epoxides (95) into ketones (96) (Annu. Rep. Progr.Chern. Sect. B. 1982 79 109). In a complementary study Torii now reports a practical route to ketones which is promoted by the electrochemical generation of catalytic amounts of acid.60 Aprotic solvents such as dichloromethane are normally employed so that the acid is produced from small amounts of water present in the medium. Should acetone be used as solvent acetonides (97) are formed directly. R2 R1 (95) (96) (97) a-Fluoroketones (99) have been prepared by the anodic oxidation of enol-acetates (98) in the presence of triethylammonium fluoride and hydrofluoric acid.61 The mechanism of the reaction probably follows the course shown in Scheme 18. R3 R2 R3 R2 )yt AcO R' AcO R' AcO R'. (98) R3 RZ -e R3 R2 )+R' -Ac' -)-tR1 OF AcO F (99) Scheme 18 59 J.Einhorn J. L. Soulier C. Bacquet and D. Lelandais Can. J. Chern. 1983 61,584. 60 K. Uneyama A. Isimura K. Fujii and S. Torii Tetrahedron Lett. 1983 24 2857. 61 E. Laurent R. Tardivel and H. Thiebaut Tetrahedron Left. 1983 24 903. 138 M. Sainsbury A mild method for the deprotection of thioacetals is provided by the potential- controlled oxidation of such compounds at platinum or vitreous-carbon electrodes in aqueous acetonitrile solution. Applied to the phosphonium nitrates (100) this allows the synthesis of p-branched &-unsaturated ketones and aldehydes ( 101)62 (see Scheme 19). Platinum electrodes coated first with a silylated polypyrrole film and then with poly-( L-valine) have been used in the asymmetric oxidation of phenyl cyclohexyl sulphide (102) to phenyl cyclohexyl sulphoxide ( 103).63The optical yield is 54%.Electrochemical oxidation of the commercially available nitroxyl from 2,2,6,6-tetramethylpiperidine gives the oxoammonium ion (104) which effects the oxidation of primary alcohols to aldehydes at a potential ( -+ 0.4V) well below that normally required thus avoiding over-oxidation to the corresponding carboxylic acids.64 Catechol on electrochemical- oxidation in the presence of nucleophiles such as 4-hydroxycoumarin ( 105) and indane- 1,3-dione ( 106) affords the coumestan deriva- tives ( 107) and ( 108) re~pectively.~~ OH Me y+ Me 0 fyJoa *: ' 00 0 i107) (108) 62 H. J. Cristau B. Chabaud and C. Niangoran J. Org.Chem. 1983 48 1527. 63 T. Komori and T. Nonaka J. Am. Chem. SOC.,1983,105 5690. 64 M. F. Semmelhack C. S. Chou and D. S. Cortes J. Am. Chem. SOC.,1983 105 4492. 65 I. Tabakovik Z. Grujik and Z. BejtoviC J. Heferocycl. Chem. 1983 20 635. Electro-organic Chemistry The anodic oxidation of a series of benzyl-and phenethyl-tetrahydroisoquinolines has been studied ; these afford tetracyclic products through six-membered ring forming reactions. Thus the substrates (109) and (1 10) yield compounds (1 11) and OMe OMe Me0 (111) Me0 (112) (1 12) respectively.66 In the case of the isochromanone (113) the anodic product is the y-lactone ( 1 15) not the 8-lactone (1 14) as previously supposed (see Scheme (20). OMe OMe I II Me0 Me0 OMe (115) P.Bird M.Powell M. Sainsbury and b. I. C. Scopes J. Chem. SOC.,Perkin Trans. 1 1983 2053.