Mononuclear heterocyclic rearrangements. Part 13. Substituent effects on the rearrangement of some Z-arylhydrazones of 3-benzoyl-5-phenyl-1,2,4-oxadiazole to 2-aryl-4-benzoylamino-5-phenyl-1,2,3-triazoles in benzene, dioxane, ethyl acetate, acetonitrile, and methanol

Abstract

The effects of substituents in the aryl fragment of the Z-arylhydrazones of 3-benzoyl-5-phenyl-1,2,4-oxadiazole (1) on the rate of the title reaction have been studied at 313.15 K. The results indicate a changeover of mechanism with changing substituent: in fact in benzene, dioxane, and methanol non-linear concave-upward Hammett plots were observed. In ethyl acetate and acetonitrile also, a tendency to an inversion in the substituent effects is present, but it is less pronounced. A comparison of reactivity in the various solvents of the p-nitrophenylhydrazone with that of the unsubstituted compound shows, reactivity ratios which strongly increase on going from benzene to ethyl acetate, through dioxane, according to the polar nature of the transition state for the mononuclear heterocyclic rearrangement.