Mcm-41 Immobilized On The Preparation Of Heterogeneous Chiral Catalysts In The Synthesis Of Chiral Applications

MCM-41 molecular sieve is a mesoporous materials with high surface area, unique and adjustable mesopores. A heterogeneous catalyst for asymmetric alkylation addition of diethylzinc to aldehyde was prepared by immobilizing proline derivative ligand on the surface of siliceous MCM-41. Prior to catalyst preparation, siliceous MCM-41 was pretreated to modify the surface of its inner pore walls. It is shown that ion exchange of MCM-41 with HNO3 solution of different concentrations (0.1-10 M) leads to improvements in both crystalinity and surface area, and that the ion exchange with 0.1 M HNOs solution seems to be the most efficient. Silylation and passivation of the ion-exchanged MCM-41 were conducted in order to introduce quantitively the grafting function groups. Two routes of grafting chiral ligand were adopted in the preparation, i.e. homogeneous synthesis followed by immobilization and solid-phase synthesis (ship in the bottle strategy, i.e., in situ assembling) . The element analyses of the prepared catalysts indicate that the chiral ligands have been immobilized on the surface of the support successfully by both routes. Solid-phase synthesis route yields less immobilized quantity (a[ligand]=0.314 mmol - g-1) than the route of homogeneous synthesis followed by immobilization (a[ligand]=0.527 mmol - g-1) . However, solid-phase synthesis route proves to be a much more convenient method in the synthesis of organic-inorganic hybrid catalysts. The prepared catalysts were evaluated by catalyzing asymmetric alkylation addition of diethylzinc to aldehyde so as to form new carbon-carbonbond. The heterogeneous catalysts gave a lower enantioselectivity (ca.10%e.e and 2.9% e.e.) than its homogeneous counterparts. It is proposed that both the quantity of grafted chiral ligand and the ratio of uncovered surface may have great effects on the enantioselectivity. It is expected that the solid-phase synthesis route may find wide applications in the preparation of heterogeneous catalysts used in bulky-molecule-involved reactions.