Mechanism of cyclisation of N-(1,2,4-triazol-3-yl)hydrazonyl bromides to mixtures of isomeric triazolotriazoles

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Abstract
The reaction of N-(5-phenyl-1,2,4-triazol-3-yl)arenohydrazonyl bromides (2) in mixed aqueous, organic solvents may yield three products, dependent on the acidity of the medium. In strongly acidic solution, N-(5-phenyl-1,2,4-triazol-3-yl)arenohydrazides (4) are formed as major products; at pH 3–6, 3-aryl-5-phenyl-1H-s-triazolo-[3,4-c]-s-triazoles (5) were formed exclusively while in basic media the isomeric 3-aryl-6-phenyl-5H-s-triazolo-[4,3-b]-s-triazoles (6)[which are also formed by the action of lead tetra-acetate on the hydrazone (1)] predominate. Kinetic studies (in 85 : 15 dioxan–water at 25°) indicate that in neutral solution (5) is formed by intramolecular cyclisation of the neutral triazole on an azocarbonium ion centre (Hammett ρ for the variation of substituents in Ar is –1·32); when the triazole ring is protonated intermolecular attack by water [formation of (4) occurs]. The isomer (6) is formed by [1,5] dipolar cycloaddition involving the triazolyl anion as nucleophile. The isomer (6) was synthesised by an independent route. Under appropriate conditions intermolecular reactions of (2) with chloride ion, aniline, and morpholine yielded products in which bromide ion was replaced by the nucleophile.

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