On the rearrangement in dioxane/water of (Z)-arylhydrazones of 5-amino-3-benzoyl-1,2,4-oxadiazole into (2-aryl-5-phenyl-2H-1,2,3-triazol-4-yl)ureas: substituent effects on the different reaction pathways

We have recently evidenced an interesting differential behavior in the reactivity in dioxane/water between the (Z)-2,4-dinitrophenylhydrazone (1a) and the (Z)-phenylhydrazone (1b) of 5-amino-3-benzoyl-1,2,4-oxadiazole. The former rearranges into the relevant triazole 2a only at pS+ > 4.5 while undergoing hydrolysis at high proton concentration (pS+ < 3.5); on the contrary, the latter rearranges into 2b in the whole pS+ range examined (0.1 < or = pS+ < or = 14.9). Thus, for a deeper understanding of these differences we have now collected kinetic data on the rearrangement in dioxane/water of a series of 3- or 4-substituted (Z)-phenylhydrazones (1c-l) of 5-amino-3-benzoyl-1,2,4-oxadiazole in a wide range of proton concentrations (pS+ 0.1-12.3) with the aim of gaining information about the effect of the substituent on the course of the reaction. All of the (Z)-arylhydrazones studied rearrange via three different reaction routes (specific-acid-catalyzed, uncatalyzed, and general-base-catalyzed), and the relevant results have been examined by means of free energy relationships.