The oxidative cyclization of formazans to tetrazolium salts

Abstract

Bromine oxidizes 1,3,5-triarylformazans (1) quantitatively to the corresponding 2,3,5-triaryltetrazolium salts (2) in methanol or aqueous acetic acid. The reaction is ionic and is markedly aided by electron donation in the aryl rings. In methanol containing 0·1M-sodium bromide (at 25°) substituents in the C-aryl ring give a ρ value of –1·40, while those at N-1 and -5 give ρ–1·10 and –2·42. A mechanism of oxidation is proposed involving electrophilic attack by bromine at carbon and formation of a covalent intermediate, followed by unimolecular C–Br bond cleavage to yield the tetrazolium salt.