Host-guest interactions between beta-cyclodextrin and the (Z)-phenylhydrazone of 3-benzoyl-5-phenyl-1,2,4-oxadiazole: the first kinetic study of a ring-ring interconversion in a "confined environment".

The effect of beta-cyclodextrin (beta-CD) on the mononuclear heterocyclic rearrangement of the (Z)-phenylhydrazone of 3-benzoyl-5-phenyl-1,2,4-oxadiazole (1) in aqueous borate buffer at pH = 9.6 has been analyzed at temperatures ranging from 293.15 to 313.15 K. The trend of the absorption spectra of 1 as a function of time has been accounted for with the formation of two different 1:1 complexes between beta-CD and 1, the first, "unreactive" complex being formed faster than the "reactive" one. The occurrence of negative activation enthalpy values for the studied interconversion evidences the kinetic relevance of inclusion processes. Computational models elaborated using the MM2 molecular mechanics force field give an idea of the relative importance of the different complexes, additionally helping us to formulate a suitable reaction scheme.