Mononuclear rearrangement of heterocycles in ionic liquids catalyzed by copper(II) salts

The reactivity of E- and Z-phenylhydrazones of 3-benzoyl-5-phenyl-1,2,4-oxadiazole in the presence of CuCl2 and Cu(ClO4)2·6H2O has been studied in four imidazolium ionic liquids [bmim][X] (X=BF4−, PF6−, SbF6− and CF3SO3−). The reaction may follow different mechanistic patterns, depending on the nature of the ionic liquid anion, accounting for both qualitative and kinetic data. In the presence of CuCl2, two processes take place at the same time, i.e., the E⇆Z isomerization and the rearrangement of Z-isomer into the relevant 4-benzoylamino-2,5-diphenyl-1,2,3-triazole. In contrast, in the presence of Cu(ClO4)2·6H2O, the rearrangement occurs only in solution of [bmim][BF4] and [bmim][CF3SO3]. Collected data show that the effect of the ionic liquid on the isomerization and the rearrangement is different. In particular, a higher cross-linking degree seems to favour the triazole, but disfavour the E⇆Z isomerization.