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.

Graphical abstractA 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 full-size imageDownload 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.