Mononuclear heterocyclic rearrangements. Effect of the structure of the side chain on the reactivity. Part 2. Rearrangement of some N-(5-phenyl-1,2,4-oxadiazol-3-yl)-N′-arylformamidines into 1-aryl-3-benzoylamino-l,2,4-triazoles in dioxane–water at various pS+

Abstract

In the framework of studies concerning the effect of the structure of the side chain on the mechanism and the reactivity in mononuclear heterocyclic rearrangements, the reactivity of some N-(5-phenyl-1,2,4-oxadiazol-3-yl)-N′-arylformamidines 3 in dioxane–water in the pS⁺-range 6.0–15.0 was studied and compared with that of arylhydrazones of 3-benzoyl-5-phenyl-1,2,4-oxadiazole 7 and of 3-arylureido-5-phenyl-1,2,4-oxadiazoles 5. A plot of the logarithms of the rate constants versus pS⁺showed the occurrence of an uncatalysed (pS⁺-independent) and of a catalysed (pS⁺-dependent and then pS⁺-independent) range. Kinetic data pointed out that the base catalysis is specific, in accord with the relatively high acidity of the formamidines used. The substituent effects on the reactivity were compared with those observed in other mononuclear heterocyclic rearrangements.