Oxidation with metal oxides—VIII

Oxidation with metal oxides—VIII

Tetrahedron. Vol. 3L pp. 1171 to 117v Pergamon Press 1975. Printed in Grea~ Britain OXIDATION WITH METAL OXIDES--VIII OXIDATION OF BISPHENYLHYDRAZONE...

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Tetrahedron. Vol. 3L pp. 1171 to 117v Pergamon Press 1975. Printed in Grea~ Britain

OXIDATION WITH METAL OXIDES--VIII OXIDATION OF BISPHENYLHYDRAZONES OF 1,2-DIKETONES WITH NICKEL PEROXIDE K. S. BALACHANDRAN, I. HIRYAKKANAVARand M. V. GEORGE* Department of Chemistry, Indian Institute of Technology Kanpur, India (ReceiL.ed ill the IrK 3 October 1974;Accepted forpublication 23 December 1974)

Abstract--The nickel peroxide oxidation of bisphenylhydrazones of 1,2-diketones gives rise to a variety of products such as bisphenylazoolefins, phenylazopyrazoles and triazoles, depending on the reaction conditions. Thus, glyoxal bisphenylhydrazone in benzene at room temperature gave bisphenylazoethylene 5a, whereas methylglyoxal bisphenylhydrazone gave a mixture of bisphenylazoolefin 10a and phenylazopyrazole 14a. The oxidation of phenylglyoxal bisphenylhydrazone, on the other hand, gave a mixture of the triazole 33 and a tetraazapentalene derivative 40. Reasonable mechanisms for the formation of the various oxidation products have been suggested.

Bisphenylhydrazones of 1,2-diketones have been oxidized by a variety of reagents to give different products depending on the reaction conditions and also on the nature of the oxidizing agents. For example, glyoxal bisphenylhydrazone has been reported to give bisphenylazoethylene on oxidation with copper sulphate at room temperature.' Under reflux, however, 2 - phenyl 1,2,3 - triazole was formed in this reaction. When the oxidation was carried out using manganese dioxide, bisphenylazoethylene was the only product isolated both at room temperature and under reflux. 2 Similarly, biacetyl bisphenylhydrazone on oxidation with potassium dichromate and acetic acid gave 2,3 - bisphenylazobut - 2 - e n e Y The same product was obtained on oxidation with manganese dioxide at room temperature. Under reflux, however, the product formed was 1 - phenyl - 3 - methyl 4 - phenylazopyrazole, z Other oxidizing agents that have been successfully used in the oxidation of bisphenylhydrazones of 1,2-diketones include alkaline potassium ferricyanide 5 and a mixture of iodine and sodium ethoxide. 60sotriazoles are reported to be formed in the oxidation of sugar osazones with reagents such as copper sulphate, 7 nitrosodisulphonate, 7 chlorine, v bromine and iodine .7 Considerable controversy exists in the literature concerning the structure of bisphenylhydrazones of 1,2diketones and sugar osazones. 7's Chapman et al. 9 on the basis of detailed NMR investigations, have shown that bisphenylhydrazones of 1,2-diketones and sugar osazones exist predominantly in either a non-chelated open chain form of the anti-, anti-configuration or a cyclic, chelated form. It has been pointed out that both the open chain form and the chelated form exist in equilibrium and the relative proportions of these two forms at any time will depend upon the nature and size of the substituent groups on the starting diketone and also of the polarity of the solvent. Thus, it has been suggested that sugar osazones, benzil bisphenylhydrazone ~° and other bisphenylhydrazones which contain bulky substituent groups, exist chiefly in the chelated form, whereas glyoxal bisphenylhydrazone and biacetyl bisphenylhydrazone, which contain small substituent groups, exist predominantly in the non-chelated form. In the present investigation, we studied the oxidation of several bisphenylhydrazones of 1,2-diketones to determine whether the oxidation products bear any relation to

the structure of the starting bisphenylhydrazones. Treatment of glyoxal bisphenylhydrazone 3a with nickel peroxide at room temperature in benzene solution gave a 93% yield of bisphenylazoethylene 5a. Pechmann 3 assigned a dihydrotetrazine structure for the oxidation product of glyoxal bisphenylhydrazone. However, subsequent studies have shown that these compounds were in fact bis-azoolefins. 4 This view was further supported by spectroscopic evidence" and also by studies of liquid crystals. ~2A probable route for the formation of 5a in the oxidation of 3a is shown in Scheme 1. In this scheme we assume that nickel peroxide abstracts a proton from glyoxal bisphenylhydrazone giving rise to a radical intermediate 4. Further loss of a proton from 4 leads to the bis-azoolefin 5. The oxidation of benzil bisphenylhydrazone 3b with nickel peroxide at room temperature, in contrast, gave a 71% yield of bisphenylazostilbene 5b and a 13% yield of 2,4,5 - triphenyl - 1,2,3 - triazole 7b. Under reflux, however, a 66% yield of 5b and a 13% yield of 7b, together with traces of azobenzene were obtained. Similarly, the oxidation of anisyl bisphenylhydrazone 3e at room temperature gave a 48% yield of bisphenylazostilbene 5c and a 21% yield of 2 - phenyl - 4,5 - dianisyl - 1,2,3 triazole 7e. The room temperature-oxidation of 4,4'dichlorobenzil bisphenylhydrazone 3d gave a 87% yield of the corresponding bisphenylazostilbene, as the only isolable product. The oxidation of acenaphthenequinone bisphenylhydrazone 3e, both at room temperature and under reflux, gave nearly quantitative yields of 1,2bisphenylazoacenaphthalene. The formation of the bisphenylazostilbenes 5b-e in these oxidations can be rationalized in terms of a free-radical process as shown in Scheme 1. The formation of triazoles in these oxidations deserves special mention. It has been recently reported that a bisazoolefin such as 5b can exist in the mesoionic form 6b (Scheme 2) and that 6b undergoes cycloadditions with different dipolarophilesJ 3 Also, it has been shown that 6b can be converted to 2,4,5 triphenyl - 1,2,3 - triazole, through the loss of phenylnitrene, both under thermal and photochemical conditions. '3''4 It is therefore, assumed that the triazoles are formed in the oxidation of the bisphenylhydrazones of 1,2-diketones through the bisphenylazoolefins. The fact that triazoles are not formed in the cases of glyoxal bisphenylhydrazone and acenaphthenequinone bis-

1171

1172

K. S. BALACHANDRANel al.

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phenylhydrazone would probably suggest that the bisphenylazoolefins formed in these cases do not exist in equilibrium with their corresponding mesoionic forms. We have also examined the oxidation of the bisphenylhydrazones of a few alkyl substituted glyoxal derivatives. Treatment of methylglyoxal bisphenylhydrazone 8a with nickel peroxide, at room temperature, gave a 90% yield of 1,2-bisphenylazopropylene 10a, as the only isolable product. Under reflux, however, the products

formed were a 3% yield of biphenyl, 45% yield of 1,2-bisphenylazopropylene and a 15% yield of a product, identified as I - pheny[ - 4 - phenylazopyrazole 14a. A probable route to the formation of products such as 10a and 14a in the oxidation of 8a is shown in Scheme 3. In this scheme we assume that 8a undergoes initial oxidation to the radical intermediate 9, which on further oxidation gives rise to the bisphenylazoolefin 10a. Another possible mode for the oxidation of 9 is through the phenylazoal-

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Oxidation with metal oxides--VIII kene intermediate 11, which can then undergo subsequent oxidative cyclization leading to the phenylazopyrazole 14a (Scheme 3). Similarly, the oxidation of biacetyl bisphenylhydrazone 8b with nickel peroxide at room temperature gave a 85% yield of 2,3 - bisphenylazobut - 2 - e n e 10b. Under reflux the products formed were biphenyl (6%) and 1 - phenyl - 3 - methyl - 4 - phenylazopyrazole 14b (52%). The oxidation of phenylmethylglyoxal bisphenylhydrazone 8c, however, gave the corresponding azopyrazole 14c both at room temperature and under reflux. In addition to the azopyrazole, a small amount of biphenyl was also isolated from the reaction, under reflux. The room temperature oxidation of benzylmethylglyoxal bisphenylhydrazone 15 with nickel peroxide gave a 50% yield of 1,5 - diphenyl - 3 - methyl - 4 phenylazopyrazole 20 and a 19% yield of a product identified as 3 - phenylazo - 3 - buten - 2 - one phenylhydrazone 17. The structure of 17 was determined on the basis of analytical and spectral data. The IR spectrum of 17 showed an absorption band at 3350 cm -~ due to an N-H group and a strong absorption band at 1600 cm-' due to a C=N group. The NMR spectrum of 17 in CDC13 showed signals at 2.17 6 (3H), 7.23 6 (17H) and 7.85 6 (lH), respectively. Of these, the sharp singlet at 2.17 6 is assigned to the methyl protons, and the multiplet centred around 7.23 fi is assigned to the aromatic protons. The NH proton appears to be hidden underneath the aromatic protons. The vinylic proton appears as a singlet at 7.85 6. Further confirmation of the structure 17 was derived from the fact that it undergoes oxidative cyclization, on treatment with nickel peroxide, to 1,4 diphenyl - 3 - methyl - 4 - phenylazopyrazole 20. The oxidation of benzylmethylglyoxal bisphenylhydrazone with nickel peroxide, under reflux gave a 14% yield of 17, a 67% yield of the pyrazole derivative 20 and a 3% yield of a product m.p. 153 ° and identified as 1 - phenyl - 3 benzoyl - 4 - phenylazopyrazole 27. Elemental analysis showed 27 to have a molecular formula of C22HI6N,~O.The IR spectrum of 27 showed a carbonyl absorption at 1665 cm -~. Further confirmation of 27 was derived by its conversion to a 2,4-dinitrophenylhydrazone derivative 27a, m.p. 242-430 and analysing for C28H2oNTO4. The formation of products such as 17, 20 and 27 in the oxidation of benzylmethylglyoxal bisphenylhydrazone may be rationalized in terms of the reaction sequences shown in Scheme 4. Oxidation of phenylglyoxal bisphenylhydrazone 28 with nickel peroxide in refluxing benzene gave a mixture of products consisting of biphenyl (20%) and 2,5 diphenyl - 1,2,3 - triazole 33 (19%). In addition, a 18% yield of a yellow compound m.p. 315 ° and analysing for CzsH20N4 was also isolated from this reaction. The structure of this compound was established as 2,3,5,6 tetraphenyl - 1,2,4,5 - tetraazapentalene 40 on the basis of analytical and spectral data. The mass spectrum of 40 showed a molecular ion peak at rn/e 412. The IR spectrum of 40 did not show any free NH group, whereas, the UV spectrum showed two intense absorption maxima at 278 nm and 386 nm. The analytical and spectral data of 40 correspond to an analogous compound, obtained from the reaction of silver phenylacetylide with pchlorobenzenediazonium chloride. ~ The formation of triazole 33 and the tetraazapentalene derivative 40 in the oxidation of phenylglyoxal bisphenylhydrazone 28 may be rationalized in terms of the route shown in Scheme 5. We assume that the initial oxidation product of 28 is the

1173

radical intermediate 29, which is further oxidized to the bisphenylazoolefin 31. The bisphenylazoolefin 31 can then be converted to the triazole 33 through the loss of phenylnitrene from the mesoionic intermediate 32. The formation of the tetraazapentalene 40, may be occurring through the pseudoallylic radical intermediate 30, which undergoes dimerization to give the intermediate 34. This dimer 34 can undergo oxidative cyclization through several possible routes to the phenylazoalkane intermediate 39. Homolytic fragmentation of 39, involving the loss of the phenylazo groups would result in the formation of 40 (Scheme 5). EXPERIMENTAl,

Starting materials. Nickel peroxide (65 g) was prepared by the treatment of nickel sulphate (130g) with a mixture of sodium hypochlorite (6% solution. 300 ml) and sodium hydroxide (42 g) as reported. '6 Glyoxal bisphenylhydrazone, '7 m.p. 169-170% methylglyoxal bisphenylhydrazone, 'e m.p. 145% biacetyl bisphenylhydrazone, 3 m.p. 245°, phenylglyoxal bisphenylhydrazone, '9 m.p. 152% phenylmethylglyoxal bisphenylhydrazoneY m.p. 104-105°, benzylmethylglyoxal bisphenylhydrazoneY m.p. 172-73°, benzil bisphenylhydrazone)' m.p. 236°, anisil bisphenylhydrazone, 22 m.p. 197-98°, and acenaphthenequinone bisphenylhydrazone, 23 m.p. 219°, were prepared by reported procedures. 4.4'-Dichlorobenzil bisphenylhydrazone was prepared by heating a mixture of 4,4'-dichlorobenzil (1.4g, 10retool) and phenylhydrazine (1.51 g, 0.014 tool) in acetic acid (10 ml) for 3 h, on a water bath. The product (1-8g) which separated out on cooling, was filtered and recrystallised from ethanol to give 1.7 g (73%) of 3d, m.p. 192-93° (Found: C, 67.96: H, 4-07; N, 11.97. C2~,H2oN4CI~requires: C, 67.98: H, 4-36; N, 12.20%). The IR spectrum (KBr) of 3d showed an NH band at 3250 cm ' and a C=N absorption band at 1600cm-'. UV spectrum A.... (Ethanol): 246 nm (E, 41,000), 300 (18,000) and 356 (41,500). Oxidation of glyoxal bisphenylhydrazone (A) In benzene at room temperature. A mixture of glyoxal bisphenylhydrazone (2 g, 8 retool) and nickel peroxide (6 g) was stirred in benzene (150 ml) at room temperature for 3 h. Removal of the inorganic material and the solvent gave 1.85g (93%) of 1,2-bisphenylazoethylene 5a, m.p. 143-1440 (d). Recrystallisation from ethanol gave a pure product m.p. 149° (d). There was no depression of melting point, when mixed with an authentic sample of 5a. ~ (B) In refluxing benzene. Glyoxal bisphenylhydrazone (2g, 8 mmol) and 6g of nickel peroxide were refluxed in benzene (150 ml) for 4 h. Work-up of the mixture in the usual manner and chromatography over alumina employing petroleum ether (b.p. 60-80°) as eluant gave 50 mg (4%) of biphenyl, m.p. 70° (m.m.p.). Further elution of the column with benzene gave a product which on recrystallisation from ethanol gave 1.48g (75%) of 1,2bisphenylazoethylene 5a, m.p. and m.m.p. 149° (d). Oxidation of benzil bisphenylhydrazone (A) In benzene at room temperature. A mixture of benzil bisphenylhydrazone (2 g, 5.13 retool) and nickel peroxide (4 g)was stirred in benzene at room temperature for 3 h. Work-up of the mixture as in the earlier cases gave a solid which was filtered and washed with a mixture (2:1) of benzene and petroleum ether (b.p. 60-80°). Recrystallization of the solid from a mixture of benzene and petroleum ether (1:1) gave 1.2g (60%) of 1,2biphenylazostilbene 5b, m.p. and m.m.p. 179°. The mother liquor, after removal of the solid product. ,~as chromatographed over alumina. Elution with petroleum ether gave 0.2 g (13%) of 2,4,5 - triphenyl - 1.2.3 - triazole 7b. m.p. and m.m.p. 124°. Further elution of the column with the same solvent gave an additional 0.2 g (I I%) of 1,2-bisphenylazostilbene 5b, re.p, and m.m.p. 179°. (B) In refluxing benzene. Benzil bisphenylhydrazone (2g, 5.13 retool) and nickel peroxide (4g) were refluxed in benzene

1174

K . S . BALACHANDRAN et al.

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1176

K.S. BALACHANDRANet al.

(150 ml) for 3 h. Work-up of the mixture as in earlier cases gave a solid which was treated with a mixture of petroleum ether and benzene (2:1). The insoluble material was filtered and the mother liquor worked up separately. The solid was recrystallised from a mixture of benzene and petroleum ether (1:1) to give 1.3 g (66%) of 1,2-bisphenylazostilbene 5b, m.p. and m.m.p. 179°. The mother liquor was chromatographed on alumina. Elution with petroleum ether gave a product which on recrystallisation from petroleum ether gave 0.25 g (16%) of 2,4,5 - triphenyl - 1,2.3 triazole 7b, m.p. and m.m.p. 124°. The mother liquor showed the presence of azobenzene on TLC (silica gel). Oxidation of anisyl hisphenylhydrazone. Stirring a mixture of anisyl bisphenylhydrazone (1.5 g, 3.3 mmol) and nickel peroxide (2.5g) in benzene (125ml) for 3h at room temperature and work-up in the usual manner gave a product which was treated with a mixture of petroleum ether and benzene (1:1). The insoluble portion was recrystallised from a mixture of benzene and petroleum ether (1:1) to give 0.45g (31%) of 4,4'dimethoxybisphenylazostilbene 5c, m.p. 175° (Found: C, 74.54; H. 5.27; N, 12.60. CzsH2~N,O~ requires: C, 74.99: H, 5.36; N, 12.50%). The IR spectrum (KBr) of 5c did not show any N-H band. The UV spectrum (CHCh) of 5c showed the following absorption maxima: 272 nm (e, 28,800), 320 (18,600), 332 (13,500) and 450 (5,000). The mother liquor after the removal of the insoluble product was chromatographed over alumina. Elution with a mixture of benzene and petroleum ether (1:1) gave a solid material which was recrystallised from ethanol to give 0.25 g (21%) of 2 - phenyl 4,5 - dianisyl - 1.2.3 - triazole 7e. m.p. and m.m.p. 133°.' Further elution of the column with benzene gave a solid which on recrystallisation from a mixture of petroleum ether and benzene gave an additional yield of 0.25g (17%) of 5c, m.p. and m.m.p. 175o Oxidation of 4,4'-dichlorobenzil bisphenylhydrazone. A mixture of 4,4'-dichlorobenzil bisphenylhydrazone (1.5 g, 3.26 mmol) and nickel peroxide (3.5g)in benzene (150 ml) was stirred at room temperature for 2.5 h. Removal of the inorganic material and of the solvent gave a solid which was recrystallised from a mixture of benzene and petroleum ether (2: I) to give 1.3 g (87%) of 4,4' dichloro - bisphenylazostilbene 5d, m.p. 205° (Found: C, 68.36: H, 4.05; N, 12.26. C2~H~sN4CI~ requires: C, 68.28; H, 3.94; N, 12.26%). The IR spectrum (KBr) of 5d did not show any N-H band. The UV spectrum (CHCh) of 5d showed the following absorption maxima: 264 nm (E, 20,500), 296 (18,000), 324 (14,400), 406 (18,400) and 480 (I,700).

Oxidation of methylglyoxal bisphenylhydrc,zone (A) In benzene at room temperature. A solution of methylglyoxal bisphenylhydrazone (2 g, 8 mmol) in 150 ml of benzene and nickel peroxide (3.5 g) were stirred at room temperature for 4 h. Work-up of the mixture in the usual manner gave a product, which on recrystallisation from ethanol gave 1.8g (90%) of 1,2bisphenylazopropylene 10a, m.p. and m.m.p. 106-107°.8" (B) In refluxing benzene. A mixture of methylglyoxal bisphenylhydrazone (2 g, 8 retool) and nickel peroxide (8 g) in 150 ml of benzene was refluxed for 6 h. Work-up of the mixture gave a viscous liquid, which was chromatographed over alumina. Elution of the column with petroleum ether gave 40 mg (3%) of biphenyl, m.p. and m.m.p. 70°. Further elution of the column with petroleum ether gave 0.9 g (45%) of 1,2-bisphenylazopropylene 10a, m.p. and m.m.p. 106-107, after recrystallisation from ethanol. Subsequent elution of the column with a mixture of petroleum ether and benzene (1:1) gave 0.3g 115%) of 1 - pbenyl - 4 phenylazopyrazole 14a, m.p. and m.m.p. 126-27°,2" After recrystallisation from cyclohexane. A small amount (0.3 g, 15%) of the starting material, m.p. and m.m.p. 145° was also recovered from this reaction.

Oxidation of biacetyl bisphenylhydrazone (A) In benzene at room temperature. Treatment of biacetyl bisphenylhydrazone (2 g. 7 retool) with nickel peroxide (6g)in benzene (150 ml) for 4 h at room temperature and work-up of the

mixture in the usual manner gave a product which after recrystallisation from ethanol gave 1.7g (85%) of 2,3 - bisphenylazobut - 2 - ene 10h, m.p. and m.m.p. 159° (d).9 (B) In refluxing benzene. Biacetyl bisphenylhydrazone (3g, 10.5 retool) and 9 g of nickel peroxide in 150 ml of benzene were refluxed for 5 h. Work-up of the mixture in the usual manner and chromatography over alumina, using petroleum ether as eluant gave 0.1 g (6%) of biphenyl, m.p. and m.m.p. 70°. Further elution of the column with a mixture of petroleum ether and benzene (1: 1) gave 1.7 g of a crude product, which after recrystallisation from cyclohexane gave 1.52 g (52%) of 1 - phenyl - 3 - methyl - 4 phenylazopyrazole 14b, m.p. 127° (Found: C, 72-80; H, 5.50; N, 21.30. MW 262 (mass spectrometry). C,~H,4N4 requires: C, 73.20; H, 5.30; N, 21.30%; MW 262). The IR spectrum (KBr) of 14h did not show any N-H band. UV spectrum Am,x(ethanol): 220 nm (e, 13,400), 340 (17,800) and 430 (1,150). The NMR spectrum of 14b in CDCL showed chemical shifts at 2.6 8 (3 H) due to methyl protons, 7.6 8 (10 H) due to phenyl protons and 8.3 S (1 H) due to the pyrazolyl proton.

Oxidation of phenylglyoxal bisphenylhydrazone Phenylglyoxal bisphenylhydrazone (3g, 9mmol) and nickel peroxide (12 g) were refluxed in benzene (175 ml) for 4 h. Removal of the inorganic material and of the solvent gave a viscous liquid which was chromatographed over alumina. Elution with petroleum ether gave 0.3 g (20%) of biphenyl, m.p. and m.m.p. 70°. Further elution of the column with a mixture of petroleum ether and benzene (3:1) gave a semi-solid product, which on recrystallisation from petroIeum ether gave 0.4g (19%) of 2,5 diphenyl - 12,3 - triazole 33, m.p. 52° (lit25 m.p. 52°). Further elution of the column with benzene gave a viscous mass, which after repeated chromatography and recrystallisation from a mixture of benzene and petroleum ether gave 0.35 g (18%) of 2,3,5,6 - tetraphenyl - 1,2,4,5 - tetraazapentalene 40, m.p. 315° (Found: C, 81.20: H, 4.70; N, 13.30; MW 412 (mass spectrometry). C,~H_,oN4 requires: C, 81.50: H, 4.80; N, 13.50%: MW 412). The IR spectrum (KBr) of 40 did not show any N-H band. The UV spectrum (Ethanol) of 40 showed the following absorption maxima: 278 nm (e, 31,200) and 386 (22,300).

Oxidation of phenylmethylglyoxal bisphenylhydrazone (A) In benzene at room temperature. A mixture of phenylmethylglyoxal bisphenylhydrazone (0.8 g, 2.4 mmol), and nickel peroxide (2.4 g) was stirred in benzene (125 ml) for 4 h at room temperature. Removal of the inorganic material and of the solvent gave a viscous material which was chromatographed over alumina. Elution with a mixture of petroleum ether and benzene (1:1) gave 0.6 g (76%) of 1,3 - diphenyl - 4 - phenylazopyrazole 14c. m.p. 113° (lit~ m.p. 113°), after recrystallisation from cyclohexane. (B) In refluxing benzene. Phenylmethylglyoxal bisphenylhydrazone (3 g, 9 mmol) and nickel peroxide (12 g) were retluxed in benzene (175ml) for 4h. Work-up of the mixture as in the previous cases gave a viscous liquid which was chromatographed on alumina. Elution with petroleum ether gave 0.3 g (21%) of biphenyl, m.p. and m.m.p. 70°. Further elution of the column with a mixture of petroleum ether and benzene (1:1) gave 2 g of a material, which on recrystallisation from cyclohexane gave 1.7 g (58%) of 1,3 - diphenyl - 4 - phenylazopyrazole 14c, m.p. and m.m.p. 113°,

Oxidation of benzylmethylglyoxal bisphenylhydrazone (A) In benzene at room temperature. A mixture of benzylmethylgJyoxal bisphenylhydrazone (0.8 g, 2.3 mmol) and nickel peroxide (2 g) was stirred in benzene (125 ml) at room temperature for 2 h. Work-up of the mixture in the usual manner gave a red viscous liquid which was chromatographed on alumina. Elution of the column with petroleum ether and benzene mixture (2:1) gave a product, which was recrystallised from petroleum ether to give 0.15g (19%) of 4 - phenyl - 3 - phenylazo - 3 - buten - 2 - one phenylhydrazone 17, m.p. 124--25° (Found: C, 77.76; H, 5.85; N, 16.15; MW 340 (mass spectrometry). C:2H2oN4 requires: C, 77.65; H, 5.88; N, 16.47%: MW 340). The IR spectrum (KBr) of 17 showed absorption bands at 3350 (N-H), 1600 (C=N) and 1500

Oxidation with metal oxides--VIII (C=C) cm '. The UV spectrum of 17 in ethanol was characterized by the following absorption maxima: 240nm (e, 14,600), 280 (20,000), 348 (33,400) and a shoulder at 366 (23,900). The NMR spectrum of 17 in CDC13 showed a singlet at 2.17 6 (3H) due to methyl protons, a multiplet centred around 7.23 6 (15H) due to phenyl protons, and a singlet at 7,85 8 (1H) due to -CH=C proton. The N-H proton appeared to be merged with the aromatic protons. Further elution of the column with the same solvent mixture gave a product, which on recrystallisation from ethanol gave 0.4 g (50%) of 1,5 - diphenyl - 3 - methyl - 4 - phenylazopyrazole 20, m.p. 136 ° (lit 2v m.p. 136°).

(B) In re~uxing benzene. A solution of benzylmethylglyoxal bisphenylhydrazone (1.3 g, 3.8 retool) in 175 ml of benzene was refluxed with nickel peroxide (3.5 g) for 2.5 h. Work-up of the mixture as in the earlier cases gave a red, viscous liquid which was chromatographed on alumina. Elution with petroleum ether gave 20 mg (3%) of biphenyl, m.p. and m.m.p. 70°. Further elution of the column with a mixture of petroleum ether and benzene (2: 1) gave 0.2 g of a red product, which on recrystallisation from petroleum ether gave 0.19g (14%) of 4 - phenyl - 3 - phenylazo - 3 -buten - 2 one pbenylhydrazone 17, m.p. and m.m.p. 124-25 °. Subsequent elution with the same solvent mixture gave 0.85 g (67%) of 1,5 diphenyl - 3 - methyl - 4 - phenylazopyrazole 20, m.p. and m.m.p. 136 °.

Elution of the column with benzene gave an yellow solid, which on recrystallisation from ethanol gave 40 mg (3%) of 1 - phenyl - 3 - benzoyl - 4 - phenyl - azopyrazole 27, m.p. I62-163 °. (Found: C, 75.11; H, 4.70: N, I5.56; MW 352 (mass spectrometry). C_,_,H.~N,O requires: C, 75.0; H, 4.54; N, 15.91%; MW 352). The IR spectrum (KBr) of 27 showed the C=O absorption at 1665 cm '. UV spectrum ;~m,~ (ethanol): 256 nm (e, 21,700), 284 (17,200), 342 (13,400) and 424 (1,050). Oxidation of 4 - phenyl - 3 - phenylazo - 3 -buten - 2 - one phenylhydrazone 10 with nickel peroxide A mixture of 10 (25 rag, 0.073 mmol) and nickel peroxide (50 mg) was stirred in benzene (25 ml) for 3 h at room temperature. Work-up in the usual manner gave a product which on recrystallisation from ethanol gave 18 mg (72%) of 1,5 - diphenyl 3 - methyl - 4 - phenylazopyrazole 20, m.p. and m.m.p. 136°. Acknowledgements--The authors thank Mr. A. H. Siddiqui for his help in microanalysis. Two of the authors (K.S.B. and I.H.) are thankful to the authorities of the Indian Institute of Technology for the award of Senior Research Assistantships.

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