Isomerization and rearrangement of (E)- and (Z)-phenylhydrazones of 3-benzoyl-5-phenyl-1,2,4-oxadiazole: evidence for a "new" type of acid-catalysis by copper (II) salts in mononuclear rearrangement of heterocycles

A kinetic investigation in methanol of the title reaction has evidenced the occurrence of two processes: the 1-E !1-Z isomerization and the rearrangement of the (Z)-isomer into the relevant 4-benzoylamino-2,5-diphenyl-1,2,3-triazole (1-Z!T). The latter reaction is in line with the ability of the (Z)-phenylhydrazones of 3-benzoyl-1,2,4-oxadiazoles to undergo the so called mononuclear rearrangement of heterocycles (MRH). The occurrence of both the examined reactions is dependent on a Lewis-acid-catalysis. The obtained results have shown the possibility of a ‘new’ type of acid-catalysis (bifunctional catalysis by Lewis salts) in the MRH. This catalysis operates through a completely different mechanism with respect to the one recently observed, and deeply investigated, in the presence of protic acids for the (Z)-phenylhydrazone of 5-amino-3-benzoyl-1,2,4-oxadiazole, in both dioxane/water and toluene, for which the catalytic process was dependent on the protonation of N(4) ring-nitrogen of the 1,2,4-oxadiazole. As a matter of fact, the copper salts seem able to interact with the >C—— N—NH—C6H5 moiety, yielding adducts which, in some cases, are prone to both isomerize and rearrange. Therefore, a similar behaviour in some manner parallel to that already observed in benzene in the presence of aliphatic amines (base-catalysis) has been evidenced.