Synthesis Of Zirconium Poly (Styrene-phenylvinylphosphonate)-phosphate Immobilized Schiff Base Dioxomolybdenum(Ⅵ) Complexes As Heterogeneous Catalysts For Epoxidation Of Unfunctionalized Olefins

Epoxidation of unfunctionalized olefins in organic synthesis has always attracted the interest of chemists in catalytic field. Many Schiff base metal complexes have been applied in this process so far. While the Schiff base dioxomolybdenum(Ⅵ) complexes are the most promising candidates for the epoxidation of unfunctionalized olefins by organic hydroperoxides in homogeneous systems. These catalysts usually display higher catalytic activity, selectivity and mild reaction conditions for the catalytic reactions. However, the separation and recycling of the expensive catalysts, as well as leaching of the active metal into the solvent and the insufficient stability of the catalysts are still problematic in homogeneous phase. Another disadvantage in homogeneous system is that the formation ofμ-oxo dimmers and other polymeric species will lead to irreversible catalyst deactivation. To solve this problem, active homogeneous catalysts were immobilized onto organic polymer and inorganic materials to attain immobilized catalyst. Therefore, in recent years, many groups have devoted to utilizing solid support in the heterogenization of some transition metal catalysts in an attempt to tailor-made catalysts for desired performance. Thus, the synthesis of various organic-inorganic hybrid mesoporous materials emerges as a potential tool to anchor various homogenous metal salts/complexes and it is interesting to probe the stability of such materials during various modification processes.Taking into account aforementioned factors, our continuing interest in the field of the support led us to regard the organic-inorganic hybrid material as a potential tool. A series of tridentate and bidentate Schiff base dioxomolybdenum(Ⅵ) complexes have been immobilized onto modified synergistic organic-inorganic hybrid support chloromethyl-ZPS-PVPA [zirconium poly (styrene-phenylvinylphosphonate)-phosphate] to prepare a new type of heterogeneous catalyst. The as-synthesized catalysts exhibited good to excellent catalytic efficiency in epoxidation of unfunctionalized olefins, using tert-butyl hydroperoxide (TBHP) as oxidant and 1.2-dichloroethane as reaction media. The effect of various oxidants, solvent nature and reaction temperature on the activity and selectivity were also studied. Recycling experiments further demonstrated that efficient catalysts were highly reusable without noticeable loss of activity even after repeated use for ten times in the epoxidation reactions.Surprisingly, up to 99% ee values were obtained whenα-methylstyrene was chosen as substrate in this oxidant system with these heterogeneous catalysts. Excellent enantiomeric excess was obtained for the epoxidation of a-methylstyrene and indene in this oxidant system, which don't involved any costly chiral ligand or additives, while the catalysts only effectively promote the process of catalytic reactions. The above mentioned principles and extensible designs, which will have the potential to open a new avenue and definitely exert a profound influence on the practical catalytic asymmetric epoxidation as well as in industry of achiral immobilized Schiff base dioxomolybdenum(Ⅵ) catalyst. Further studies on this approach are underway in our laboratory.