Task Specific Dicationic Ionic Liquids: Recyclable Reaction Media forthe Mononuclear Rearrangement of Heterocycles

Nine functionalized dicationic ionic liquids were used to study the base catalyzedmononuclear rearrangement of (Z)-phenylhydrazone of 3-benzoyl-5-phenyl-1,2,4-oxadiazole into thecorresponding triazole at 363 K. Ionic liquids were characterized by the presence of 1-(1-imidazolylmethyl)-3,5-di[1-(3′-octylimidazolylmethyl)]benzene cation, bearing on the structure a neutral imidazole unit asbasic functionality. Besides anions generally used for monocationic ionic liquids, such as [Br−], [BF4−], and[NTf2−], also dianions having a rigid aromatic spacer such as 1,4-benzenedicarboxylate, 2,6-naphthalenedicarboxylate, and 1,5- and 2,6-naphthalenedisulfonate were taken into account. Furthermore,to have information about the effect deriving from the spacer rigidity also dianions such as 1,4-butanedicarboxylate and 1,6-hexanedicarboxylate were used. The basic strength of dicationic ionic liquidswas determined using the Hammett indicator method. Data collected gave evidence that the outcome of thetarget reaction was affected by the nature of interactions operating between cation and anion of the ionicliquid used. Quite interestingly, solvent systems used had a good recyclability. Indeed, in some cases, theywere reused for four cycles without significant loss in yield.