A role of ionic liquid as an activator for efficient olefinepoxidation catalyzed by polyoxometalate

Abstract

Ionic liquid (IL), such as 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF₆]), as an activator for the epoxidation of a wide variety of olefins with H₂O₂ catalyzed by Keggin polyoxometalate [bmim]₃PW₁₂O₄₀ has been demonstrated. Use of IL results both in significant enhancement of TOF and selectivity for olefin epoxidation compared to that of traditional organic solvents, e.g. 289 times TOF and 1.3 times selectivity as found in CH₂Cl₂ for the epoxidation of cis-cyclooctene. The kinetic ³¹P NMR spectra uncover that the active peroxotungstate [PO₄{W(O)(O)₂}₄]³⁻ (Venturello complex), contributing to the epoxidation, can be readily generated in the presence of [bmim][PF₆]. Furthermore, the counterion such as PF₆⁻ and [(CF₃SO₂)₂N]⁻ is evidenced to play a crucial role in the epoxidation of olefin. BF₄⁻ as counterion, instead of PF₆⁻, is found to reduce TOF and selectivity for cis-cyclooctene epoxidation dramatically from 289 to less than 1 and from 99% to 12%, respectively.