Epoxidation of olefins using a dichlorodioxomolybdenum(VI)-pyridylimine complex as catalyst

The ligand N-(n-propyl)-2-pyridylmethanimine (pyim) and an immobilized analogue of this ligand (pyim-MTS) were prepared by the condensation reaction of 2-pyridinecarboxaldehyde with either n-propylamine or 3-aminopropyl groups covalently attached to a micelle-templated silica (MTS). Free and immobilized dioxomolybdenum(VI) complexes of the type MoO2Cl2(L1) (L1 = pyim (1), pyim-MTS) were then prepared by treatment of the organic ligand or ligand-silica with the solvent adduct MoO2Cl2(THF)2. MoO2Cl2(pyim) (1) is a highly active catalyst for the epoxidation of olefins (cyclooctene (Cy), cyclododecene, 1-octene, trans-2-octene, R-(+)-limonene) at 55 °C using tert-butylhydroperoxide (TBHP) as the oxidant under solvent-free conditions, giving the corresponding epoxides as the only reaction products. A turnover frequency of 1855 mol molMo−1 h−1 was measured for the epoxidation of Cy, and the epoxide (CyO) was formed quantitatively within 4.5 h. The MTS-supported complex was less active, and exhibited temperature-dependent leaching of active species. As an alternative approach to facilitating catalyst recycling, complex 1 was investigated with the ionic liquid (IL) 1-butyl-3-methylimidazolium tetrafluoroborate as solvent. The 1/IL phase could be reused, giving ca. 95% CyO yield in each run.

A novel dioxomolybdenum(VI) complex has been prepared that exhibits high catalytic activity and selectivity for the epoxidation of olefins. Two approaches were investigated to immobilize the complex: (a) Anchoring on a chemically modified micelle-templated silica, and (b) use of an ionic liquid as solvent in a biphasic system.Figure optionsDownload as PowerPoint slideHighlights
► A new complex of the type MoO2Cl2(L) bearing a pyridylimine ligand is described.
► The complex is a very efficient homogenous catalyst for the epoxidation of olefins.
► Under mild, solvent-free conditions, epoxides are formed as the only products.