Metal catalyzed oxidations with H2O2 in ionic liquids

Some transition metals in their d0 oxidation state are capable to activate hydrogen peroxide, via formation of peroxo complexes, in diverse oxidation reactions, with several advantages in terms of wastes and costs. In order to render these systems even more interesting from the sustainability point of view, it was tried to carry out them in alternative media to commonly used organic solvents, such as ionic liquids. In particular vanadium(V) and molybdenum(VI) catalyzed oxybromination of multiple bonds was studied in biphasic conditions (H2O/hydrophobic solvent) , changing reaction media from dichlorometane to ionic liquids, and remarkable increase both in yield and selectivity towards bromohydrins were observed. Beside a synthetic strategy for ionic liquid themselves, very preliminary results for molybdenum(VI) catalyzed alkene epoxidation and for vanadium(V) catalyzed benzene hydroxylation in ionic liquids are also presented, even though synthetically disappointing. Moreover, for the first time, heteronuclear NMR experiments, in particular 51V, have been performed with the aim to elucidate the nature of vanadates and peroxovanadates in hydrophilic ionic liquids. The data so far reported clearly indicate that ionic liquids have a strong influence on the solution chemistry of vanadate in water both for the aggregates formation (with and without H2O2) and for the rate of peroxide consumption catalyzed by vanadium. In particular in BF4- containing ionic liquids formation of vanadium-fluoride adducts can be suggested, whose NMR signal are not yet reported in literature. The obtained results are also discussed in the light of reactivity data of peroxovanadates in ionic liquids.