Titanium(IV)-Salan Catalyst for asymmetic Sulfoxidation with Hydrogen Peroxide

We report the synthesis and the solution and solid-phase characterization of several titanium(IV)–salan complexes. The structures of the ligands H2sal(R,R-chan), H2ovan(S,S-chan), and H6pyr(R,R-chan)4+·4Cl–·H2O were determined by single-crystal X-ray diffraction and their main features are compared and discussed. Single crystals suitable for X-ray diffraction studies were also obtained for [Ti4{sal(R,R-chan)}4(¼-O)4] and showed a tetranuclear core bearing ¼-oxo bridges between the titanium atoms and the ligand wrapped around the Ti in a fac-mer geometry. The Ti–salan complexes were tested as catalysts in the benchmark oxidation of thioanisole with H2O2 as oxidant in organic solvents. High H2O2 conversion and moderate enantioselectivities (up to 51% ee) were obtained. In an effort to perform greener catalysis the complexes were tested in selected ionic liquids with several oxidants. [Ti{5-MeO-sal(R,R-chan)}] was also covalently bound to a polystyrene matrix. tert-Butyl hydroperoxide was found to be the most suitable oxidant for use in ionic liquids (100% conversion vs. 80% with H2O2) and in general the Ti–salan complexes showed better activity in ILs at 25 °C although with reduced induced asymmetry (up to 18% ee).