Ionic Liquids as Micellar Agents in Perrhenate-catalysed Olefin Epoxidation

The perrhenate-catalysed epoxidation of cyclooctene with aqueous H₂O₂ was performed in micellar media using imidazolium-based ionic liquids (ILs) as surfactants. The catalytic performance of perrhenate was greatly increased in micellar media compared to IL used as neat solvent which allows for efficient use both of catalyst and IL. The choice of the surfactant alkyl chain length (C₆ to C₁₂) as well as the counterion (BF₄ ⁻, TfO⁻, Cl⁻) is crucial for achieving high reaction rates. The positive effect of the addition of surfactants relies on formation of micelles. Best results are obtained for BF₄ ⁻ anion with long alkyl chain imidazolium cations, outperforming results for previously reported perrhenate ILs, while the use of Cl⁻ anion leads only to similar reactivity. Cross-over experiments revealed that the catalytically active micellar species forms in situ and that the BF₄ ⁻ anion promotes the reaction beyond a mere change in ionic strength. The use of Cl⁻ anion as surfactant counterion bears potential as a novel approach for rapid screening of potentially catalytically active anions, which is exemplarily shown for tungstate anion.