Catalytic olefin epoxidation with cationic molybdenum(VI) cis-dioxo complexes and ionic liquids

The complexes [MoO2Cl(HC(bim)3)]Y (Y = Cl (1), BF4 (2) and PF6 (3)) have been prepared by reaction of MoO2Cl2(THF)2 (for 1) or [MoO2Cl(THF)3]Y (for 2 and 3) with the tridentate ligand HC(bim)3 = tris(benzimidazolyl)methane, and characterized by IR and Raman spectroscopy, and 1H NMR. The turnover frequencies for the epoxidation of cis-cyclooctene at 55 °C with tert-butyl hydroperoxide (TBHP, in decane) as the oxidant and complexes 1–3 as catalysts are in the range of 70–200 mol molMo−1 h−1. 1,2-Epoxycyclooctane is always the only reaction product for reaction times up to 24 h. With the aim of facilitating the recyclability of the complexes, the ionic liquids (ILs) [BMIM]Y and [BMPy]Y (BMIM = 1-n-butyl-3-methylimidazolium, BMPy = 1-n-butyl-3-methylpyridinium; Y = BF4 or PF6) were applied as ionic solvents. The catalytic performance for cyclooctene epoxidation depends strongly on the catalyst solubility in the IL. Of the 12 catalyst/IL mixtures examined, the systems 1/[BMIM]PF6 and 1/[BMPy]PF6 exhibit the most favorable reaction rates allied with good recyclability. The 1/[BMIM]PF6 system was further applied using different oxidants (aqueous TBHP, aqueous H2O2 and urea–hydrogen peroxide adduct) and olefins (norbornene, cyclohexene, styrene, α-pinene).

The complexes mentioned in the title can be used as quite active catalysts for the oxidation of olefins to epoxides using tert-butyl hydroperoxide as the oxidant. No diol formation was observed with the substrates cis-cyclooctene, cyclohexene and norbornene. Hexafluorophosphate ionic liquids were used successfully with [MoO2Cl(HC(bim)3)]Cl to allow catalyst/solvent recycling and reuse.Figure optionsDownload as PowerPoint slide