Mononuclear heterocyclic rearrangements. Effect of the structure of the side chain on the reactivity. Part 1. Rearrangement of some 3-arylureines of 5-pheny-1,2,4-oxadiazole into 1-aryl-3-benzoylamino-1,2,4-triazolin-5-ones in acetonitrile, benzene, and dioxane–water

Abstract

The effect of the structure of the side chain both on the mechanism and the reactivity in mononuclear heterocyclic rearrangements has been studied by comparing the base-catalysed rearrangement of some 3-arylureines of 5-phenyl-1,2,4-oxadiazole with that of some arylhydrazones of 3-benzoyl-5-phenyl-1,2,4-oxadiazole. In acetonitrile and in benzene, in the presence of amines, on changing from the side chain CNN (arylhydrazones) to the side chain NCN (arylureines), a strong decrease of the reactivity has been observed (rate ratios ca. 10³), but the reaction mechanism is the same for the two series of compounds. In contrast, in dioxane–water, the reactivity variation is much less (rate ratios ca. 25 in the pS⁺-dependent range) and a change of mechanism is observed. For arylureines and for arylhydrazones, specific- and general-base catalysis respectively has been shown. This is in keeping with the high acidity of arylureines, which in the presence of a strong base can be converted into the corresponding anions and then rearrange to the 1,2,4-triazolin-5-ones.