Formazans phenylazo

Bulka et al. (43) have demonstrated the electrophilic reactivity of selenazoles possessing an hydrazonc in the 2-position and nonsubstituted in the 5-position toward diazonium salt to give 5-phenylazo derivatives preferentially. For example, the main product of the coupling of 2-benzylidene hydrazino-4-phenylselenazole with diazo-o-phenetidine is the 5-(o-ethoxyphenylazo)-selenazole (Scheme 371 ruby red prisms, m.p. 206°C. yield 67"o). A formazan is obtained as by-product. (See Section III.6) (43). 

In warm mineral acid, formazans (195) rearrange to a benzotriazine (196) eliminating an amine (Scheme 29).8,26 27 334 338 339 H owever, when R is methyl, phenylazo, or oxalyl, the main product is the phenazine 197 along with a trace amount of the benzotriazine.340 The mechanism of this reaction is not well understood.341 345... 

Formazans are known to have a chelated structure which permits the equilibration of monosubstituted isomers.224-227 For example, the tautomeric pair structure of A -phenyl-A -p-bromophenylformazan (74 and 76) can be formed from either the p-bromophenylhydrazone 73 and a ben-zenediazonium salt or from a phenylhydrazone (75) and a p-bromoben-zenediazonium salt.228 The neutral D-mannose diphenylformazan229 molecule has a chelate structure in which the imino proton spans the phenylhydrazono and the phenylazo groups (Scheme 17)230... 

Treatment of l-phenylhydrazino-3-phenylazo-2-cyclohexene with a strong base also produces a blue coloration. The amphoteric character of this compound and of other formazan vinylogs has been ascribed502 to the formation of such resonance-stabilized anions as 278a,b. The absorption spectrum of this anion resembles the spectrum of the resonance-stabilized cation of 2-oxo-l,3-bis(phcnylhydrazones). indicating some amphoteric character in these compounds (Scheme 65). 

Sometimes, as in the preparation of 1,5-diphenylformazan ( formazylhydrogen ) from malonic acid,343 it is best to use only one molar equivalent of the diazonium compound, since otherwise three-fold coupling to yield (phenylazo)formazans may readily occur. 

Finally, in strongly alkaline solution formazans containing removable substituents such as COOH, COCH3, and COCOOH in position 3 can couple with a third diazonium component, giving the so-called 3-(phenylazo)form-azans for instance, l,5-diphenyl-3-(phenylazo)formazan, C6H5NH—N=C-(N=NC6H5)2, is obtained as end-product of complete coupling of acetone,344 acetaldehyde,344 351 354 pyruvic acid,344 351 acetoacetic acid,344 352 malonic acid,343 or acetonedicarboxylic acid,343 etc. 

Alditol-l-yl)-2-phenyl-5-phenylazo-l,2,3-triazoles (osotriazole forma-zans) (669) were obtained by cyclization of the corresponding osazone formazans (668) with acetic acid (60BSF350 62BSF381) (Scheme 177). 

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