Reactions of bromine and lead tetra-acetate with 2-(substituted hydrazino)-5-phenyl-1,3,4-oxadiazoles: routes to 3-aryl-6-phenyl-1,2,4-triazolo[3,4-b][1,3,4]oxadiazoles

Abstract

2-(Substituted hydrazino)-5-phenyl-1,3,4-oxadiazoles react with bromine in acetic acid to yield hydrazone perbromides, or hydrazonyl bromides in the presence of sodium acetate. Treatment of the hydrazonyl bromides with triethylamine in benzene or solvolysis in aqueous solvents results in cyclisation to the new 1,2,4-triazolo[3,4-b]-[1,3,4]oxadiazole ring system. The cyclisation involves the oxadiazole ring as a nucleophile which attacks the labile carbon–bromine site in the hydrazonyl bromide. In the solvolysis, this internal nucleophilic attack is in competition with external nucleophilic attack by solvent molecules, and direct solvolysis products are also obtained. A second route to the new ring system with tribromodiazabutadienes has also been developed. The oxadiazole ring of the new 1,2,4-triazolo[3,4-b][1,3,4]oxadiazoles is labile to acid and base and a preferential ring interconversion pattern of oxadiazole to triazole has been noted. In the reaction of the 2-(substituted hydrazino)-5-phenyl-1,3,4-oxadiazoles with lead tetra-acetate, the presence of the oxygen atom at the potential cyclisation site inhibits the cyclisation. The'reaction instead involves acetoxylation of the hydrazone and only gives, at most, traces of 1,2,4-triazolo[3,4-b][1,3,4]oxadiazoles.