Anodic oxidation of 1-arylmethylenesemicarbazides (1-5) was
performed in methanol-tetraethylammonium perchlorate electrolyte
solution by constant-current electrolysis. As a result of anodic
oxidation, the corresponding oxadizoles (la-3a) were isolated in
good yields, ranging from 51 to 65%. The anodic oxidation represents
a novel efficient conversion of an aromatic aldehyde to an aromatic methyl ester (3b-5b) through oxidation of its semicarbazones

Ibro Tabaković

Javorka Gunić

Živko Saničanin

English

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CROATICA CHEMICA ACTACCA-1885CCACAA 62 (3) 545-549 (1989)YU ISSN 0011-1643UDe 547.497.1Original Scientific PaperElectrochemical Synthesis of Heterocyclic Compounds. Part 21.1Anodic Oxidation of l-Arylmethylenesemicarbazides in MethanolJavorka Gunić and Ibro Tabaković*FacuLty of Technology, Đuro Pucar Stari University, 78000 Banjaluka, YugoslaviaandŽivko SaničaninMedical FacuLty, Đuro Pucar Stari University, 78000 Banjaluka, YugoslaviaReceived March 9, 1989Anodic oxidation of 1-arylmethylenesemicarbazides (1-5) wasperformed in methanol-tetraethylammonium perchlorate electro-lyte solution by constant-current electrolysis. As a result of anodicoxidation, the corresponding oxadizoles (la-3a) were isolated ingood yields, ranging from 51 to 65%. The anodic oxidation repre-sents a novel efficient conversion of an aromatic aldehyde to anaromatic methyl ester (3b-5b) through oxidation of its semicarba-zones.We have shown"? in previous investigations that the anodic oxidationof compounds having an azomethine function led to different productsdepending on the structure of the starting compounds and solution conditions.Hammerich and Parker" reported anodic cyclization of 1-arylmethyelene-semicarbazides to the corresponding oxadiazoles in hizh yield by usingcanstant current electrolysis in acetic acid-sulphuric acid media. However,in our hands this reaction resulted in a mixture of at least four products(TLC) and the oxadiazoles were isolated in 10-200/0 yield. Having in mindthat anodic oxidation of hydrazones in the presence of nucleophiles gavedifferent reaction prcducts", we examined in the present research anodicoxidation of 1-arylmethylenesemicarbazides in methanol as a nucleophilicsolvent.RESULTS AND DISCUSSIONAnodic oxidations of 1-arylmethylenesemicarbazides (1-5) was typicallyperformed in a rnethanol solution containing tetraethylammonium perchlorate(0.1 M). Constant-current electrolysis was carried out until all starting 1-aryl-methylenesemicarbazide was consumed, as determined by TLC. All electro-chemical syntheses were performed in a divided cell at Pt-gauze anode andNi cathode to high yield of oxadiazoles (la-3a) and/or aromatic methylesters (3b-5b) according to Scheme 1.546oIIAr-CH=N-NH-C-NH21-5J. GUNIĆ ET AL.3b-5bScheme 1Voltametric and preparative data for the anodic oxidation of l·-aryl-methylenesemicarbazides (1-5) are given in Table 1. Cyc1ic voltammogramsshowed irreversible waves for sweep rates from 0.02 to 1.0 VS-l. The valueof the peak potential changes in amanner consistent with donor or acceptorcapability of the substituted aryl group. The current functions, ip/v'/!C, wereconstant with sweep rates and showed two-electron behaviour by comparisonof the values of the current function with that obtained for tris-(4-bromo-phenyl)amine, a compound known to undergo one electron oxidaticn-". Coulo-metry at 0.7V vs. SCE of 1 in acetonitrile -0.1 M tetraethylammoniumperchlorate solution shows that the overall electrode reaction is a two-electronoxidation. The oxidation product (la) precipitated during the reaction andwas isolated by filtration in 60% yield. All other preparative oxidations werecarried out under constant-current electrolysis in methanol -0.1 M tetra-ethylammonium perchlorate solution and the results are presented in Table1. All product were verified by IR, NMR and MS spectral analysis.TABLE IEiectroomaiuticat and Preparative DataYield of productsjOfoCompound E a Current yieldjO/op Oxadiazole Methyl ester(V vs. SCE)la-3a 3b-5b1 0.72 52 712 1.16 65 753 1.30 51 38 954 1.25 80 905 1.78 55 96a Sweep rate, 0.1 Vjs, concentration of l-arylmethylenesemicarbazides, 10-3 M;CH3CN-0.l M Et4NCI04; pt electrode.l-ARYLMETHYLENESEMICARBAZIDES 547Oxidation of l-arylmethylenesemicarbazides is initiated by the rapidformation of cation A through two electron and one proton loss (Scheme 2.).~Ar-CH=N-NH-C-NH2:!.-~+ ~Ar-CH-N=N-C-NH2ACH~OH/-~9CH3 ~Ar-C=N-NH-C--NH2BlH20?iAr-C-OCH33b-~1-2e-H+H O+ I IIAr-C=N-N-C-NH2N-NAr j; ~ NH2~O~'"1a-3aScheme 2.Deprotonation of cation A should enhance cylization to oxadizole throughthe formation of the dipolar ion, as postulated by Scott and cowerkers!'.Such a mechanism was ruled out by Hammerich and Parker". lt is wellestablished that the oxidation of hydrazones in neutral, acid and basic solutionoccurs also through the formation of the cation as an intermediate and notthrough the dipolar nitrilimine.". Hammerich and Parker concluded that theintramolecular cyclization of cation A is a very rapid reaction. They offeredan alternative pathway which might be expected to favour the formationof oxadizole by nucleophilic attack of water on cation A, which is followedby cyclization. In order to test this possibility, we ran the cyclic voltammetryof 1 in the presence of activated alumina'", e. g. under super dry conditionsin CH3CN-Et4NCI04 solution, but cyclic voltammetry failed to show anyreduction peak due to the formation of A. This experiment is in line withthe conclusion that the oxadiazoles (1a-3al are formed through intramole-cular cyclization of cation A.Cation A is liable to nucleophilic attack by the methanol present, leadingto semicarbazide B, which is hydrolyzed by water, leading to loss of thesemicarbazide, which was detected by TLC, and aromatic methyl esters(3b-5b).The reaction selectivity of the anodic oxidation of l-arylmethylenesemi-carbazides in methanol, e. g. the formation of oxadiazole vs. aromatic methylesters, remains unsettled at present. It seems that the stability and/orsuitable conformation of cation A in solution might play the main role ifthe reaction occurs through intramolecular or intermolecular nucleophilicattack. Similar differences in the oxidation of l-arylmethylenesemicarbazidewithlead tetraacetate on the formation of the reaction products, dependingon the donating capability of the substituents on aryl group, were observed!".548 J. GUNIĆ ET AL.Further work on the anodic oxidation of l-arylmethylenesemicarbazides inthe presence of heteroaromatic basis is in progress in our laboratory.In conclusion, one could say that the anodic oxidation of 1-arylmethylene-semicarbazides in methanol represents a useful synthetic method for the syn-thesis of oxadiazoles and novel efficient conversion of an aromatic aldehyde toan aromatic methyl ester (ArCRO --+ ArCOOCR3) through oxidation of itssemicarbazone.EXPERIMENTALThe equipment, acetonitrile and tetraethylammonium perchlorate purificationhave been described previously'', A standard three-electrode electrochemical cellwas used for all experiments with a Pt disc (2r = 3 mm) and gauze (3 X 5 cm) foranalytical experiments and large-scale electrolysis respectively.GeneraL Procedure for Oxidation of l-AryLmethyLenese1ll.icarbazides (1-5)Into the anodic compartment of the divided cell with Pt gauze (3 X 5 cm)anode and Ni cathode filled with 0.1 M solution of Et4NCI04 in methanol (100 ml)1-arylmethylenesemicarbazides (1-5) (0.3-0.5 g) was added. Constant-current elec-trolysis was carried out at 100 mA until the starting compound was consurned (TLC).The solution was evaporated to cca 10 ml and 200 ml of water was added. Theprecipitated oxadiazole (la-3a) was isolated by filtration and recrystallized fromappropriate solvent. The mother liquor was extracted with petrol ether (3 X 40 ml).The extracts were washed several times with water. After drying (MgS04), the(3b-5b) which was recrystalized from appropriate solvent.2-Amino-5-(4-N,N-dimethyLamino)phenyL-l,3,4-oxadiazoLe, lam. p. 277-279° (acetonitrile); IR (KBr) 3300, 3095, 1645, 1600, 1375, 1205, 1030,955, 815 cm"; NMR (DMSO-d6) 3.0 (s, 6H, (CH3hN); 6.75 (s, 2H, NH2); 6.95 and 7.25(AA'BB' system, 8H, Ar) ppm; MS, m/e (relative intensity) 204(100), 161(9.5), 160(20.1),149(54), 145(13.5),132(14), 118(6.5), 105(4.5), 91(4), 77(6.5).AnaL for C1oH12N40 (204.23) calc'd.: C 58.81, H 5.92, N 27.43%found: C 58.95, H 5.98, N 27.01%.2-AminQ-5-(4-methoxy)phenyL-l,2,4-oxadiazole, 2am. p. 250-251° (ethyl acetate). litY m. p. 251-253°; IR (KBr) 3300, 3105, 1650,1600, 1500, 1295, 1260, 1185, 1030, 840 cm ": NMR (CDCI3) 3.8 (s, 3H, CH30); 7.2 (s, 2H,NH2); 7.10 and 7.75 (AA'BB' system, 8H, Ar) ppm; MS, m]e (relative intensity)191(100), 162(4), 149(4.5), 148(26), 135(38), 133(71), 107(13), 105(27), 92(18), 77(23.5).2 -A mino-5-(3-methoxy-4- benzyLoxy )phenyl-l,3 ,4-oxadiazoLe, 3am. p. 214-215° (benz ene) ; IR (KBr) 3320, 3050, 2940, 1650, 1580, 1300, 1280, 1250,1225, 1150, 1085 cm! ; NMR (CDCI3) 3.55 (s, 3H, CH30); 5.2 (s, 2H, CH2), 6.9 (s, 2H,NH2); 7.1-7.6 (m, 8H, Ar) ppm; MS, tti] e (relative intensity) 297(6), 206(12), 163(4),92(10), 91(100), 77(5), 74(6), 65(16).AnaL for C16H15N303(297.31) calc'd.: C 64.64, H 5.05, N 14.48%found: C 64.55, H 5.11, N 14.27%.MethyL 3-methoxy-4-bezyLoxybenzoate, 3bm. p. 69-71° (benzene) ; IR (KBr) 2920, 1700, 1585, 1500, 1340, 1295, 1270, 1215,1180, 990, 880, 855, 765, 750, 700 cm"; NMR (CDCI3) 3.90 (s, 3H, CH30); 3.95 (s, 3H,CH30); 5.25 (s, 2H, CH2); 7.25-7.75 (m, 8H, Ar) ppm; MS, tri]e (relative intensity)272(15), 257(5), 111(5), 97(15), 91(100), 83(10), 69(9), 65(7).AnaL for C16H1604(272.28) calc'd.: C 70.58, H 5.92%found: C 70.31, H 5.72%.!-ARYLMETHYLENESEMICARBAZIDES 549Methyl 3-methoxy-4-ethoxybenzoate, 4bm.p. 78-79° (methanol); IR (KBr) 2930, 1700, 1590, 1505, 1435, 1340, 1290, 1260,1220, 1180, 1130, 1030, 875, 825, 780, 765 cm' NMR (CDC1s) 1.5 (t, 3H, CH3); 3.9 (s, 3H,CHsO); 3.95 (s, 3H, CHsO); 4.17 (q, 2H, CH2) 6.75-7.75 (m, 3H, Ar) ppm; MS, mle(relative intensity) 210(78), 183(6), 182(55), 179(14), 167(9), 151(100), 123(8), 79(2).Ana!. for CllH1404 (210.22)calc'd.: C 62.85; H 6.71010found: C 62.53, H 6.48010.Methyl 4-nitrobenzoate, 5bm. p. 86-88° (methanol) ; liU5 m.p. 96°; IR (KBr) 17l0, 1600, 1520, 1435, 1350,1275, 1105, 830 cm"; NMR (CDClg) 4.0 (s, 3H, CH30); 8.2 and 8.65 (AA'BB' system,4H, Ar); MS, mle (relative intensity) 181(53), 166(5), 164(8), 150(100), 135(8), 120(19),104(21), 92(16), 76(21).AcknowLedgement. - This publication was supported by the U.S.-YugoslavJoint Board for Scientific and Technological Cooperation under Grant No. 546.REFERENCES1. J. G u n i ć, 1. Ta b ako v i Ć, and Ž. S a n i Č a n i n, Electrochem. Acta sub-mitted for publication.2. 1. Ta b ako v i Ć, M. Trk o v n i k, and D. Gal i j a Š, J. Etectroaruii. Chem.86 (1979) 241.3. 1. Ta b ako v i Ć, M. Trk o v n i k, and Z. G r u j ić, J. Chem. Soc. Perkin II,(1979) 166.4. 1. Ta b ako v i ć and S. C r 1jen a k, Heterocydes 16 (1981) 699.5. M. Bat u š i Ć, 1. T a b ako v i ć, and S. C r 1jen a k, Croat. Chem. Acta 54(1981) 397.6. M. La ć a n, V. Rog i Ć, 1. Ta b ako v i Ć, D. Gal i j a š, and T. S o lom u n,Electrochim Acta 28 (1983) 199.7. S. C r 1jen a k, 1. Ta b ako v i Ć, D. Jer e m i Ć, and 1. Ga o n, Acta Chem.Scand. B37 (1983) 527.8. O. Ham m eri c h and V. D. Par k e r, J. Chem. Soc. Perkin L (1972) 17l8.9. E. G u n i ć and 1. Ta b ako v i Ć, J. Org. Chem. 53 (1988) 5081.10. R. F. Ne 1s o n and R. N. Ada m s, J. Am. Chem. Soc. 90 (1968) 3925.11. F. L. S c o o t, T. M. La m b e, and R. N. B u tle r. Tetrahedron Lett. (1971)2669.12. O. Ham rh eri c h and V. D. Par k e r, Electrochim. Acta 18 (1973) 537.13. P. K n i t tel and J. War ken t i n, Can. J. Chem. 50 (1972) 4066.14. T. M. La m b e, R. N. B u tle r, and F. L. S c o o t, Chem. and lnd. (1971) 996.15. R. C. W e a s t (57th edition) Handbook of Chemistry and Physics, The Chemi-cal Rubber Co., Ohio (1976-77).SAŽETAKElektrokemijska sinteza heterocikličkih spojeva. Dio 21. Anodna oksidacija1-arilmetilensemikarbazida u metan 01uJ. Gunić, t. Tabaković i Ž. SaničaninAnodna oksidacija 1-arilmetilensemikarbazida (1-5) izvršena je u elektrolit-skoj otopini metanol-tetraetilamonijev perhlorat pri konstantnoj gustini struje. Kaorezultat anodne oksidacije izolirani su odgovarajući oksadiazoli (la-3a) u dobromiskorištenju od 51 do 65010.Anodna oksidacija predstavlja novu i efikasnu konver-ziju aromatskih aldehida u aromatske metilestere (3b-5b) oksidacijom njihovih semi-karbazona.

Electrochemical Synthesis of Heterocyclic Compounds. Part 21.1 Anodic Oxidation of 1-Arylmethylenesemicarbazides in Methanol

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