Convergent results from experimental and theoretical DFT studies of the intramolecular rearrangement of Z-hydrazones of 3-acyl-1,2,4-oxadiazoles

A combined kinetic and theoretical study of the monocyclic rearrangements of heterocycles (MRH) has been carried out. The interconversion of the Z-hydrazone of 3-benzoyl-5-phenyl-1,2,4-oxadiazole into the corresponding triazole has been experimentally investigated in dioxane/water in the pS(+) range 5.5(5)-13.9. The uncatalyzed region has been examined at the DFT level using a model system formed by the Z-hydrazone of 3-formyl-1,2,4-oxadiazole and one or two water molecules. The environmental effect of the solvent has been emulated using a continuum model (COSMO) approach. The kinetic data suggest a concerted process where the magnitude of the activation barrier is determined by the interplay of two opposite factors, that is, the nucleophilicity of the nitrogen atom and the acidity of the nitrogen-bonded protons. The computations indicate the existence of two multistep reaction pathways. When the solvent environment is taken into account, the preferred path, which involves two water molecules acting as a base, becomes a concerted highly asynchronous path, where the nucleophilic attack and the proton transfer occur not simultaneously but in the same kinetic step.