Catalyst in a Box: Zeolite-supported Enantioselective Catalysis

INTRODUCTION Enantioselective Catalysis Beginning with the first homogeneous asymmetric metal-catalyzed reaction performed by Noyori—a cyclopropanation—the area of enantioselective catalysis in organic chemistry has become a major topic of discussion and research. Homogeneous, asymmetry-inducing catalysts have been developed for a wide range of reactions, especially notable are those developed by Knowles, Noyori and Sharpless, for which they jointly received the 2001 Nobel Prize in Chemistry. Although these reactions can be highly stereoselective, there are disadvantages associated with homogeneous catalysis: namely (1) expense of the catalyst, which is often lost upon work-up of the reaction, (2) the difficult separation of the organic ligands and metal catalyst from the resulting products, and (3) the inability of homogeneous catalysis to provide high stereoselectivity for all substrates and reaction types. The first two of these difficulties can be avoided by immobilizing the chiral catalyst on a solid support. This support allows for easy separation of the solid catalyst and product by simple filtration, and in optimal conditions the supported catalyst can be reused multiple times without significant loss in activity or enantioselectivity, thereby decreasing cost. A number of different solid supports have been applied for the formation of heterogeneous chiral catalysts. Of these, the synthetic and bio-polymers, solid metal, amorphous silica or alumina, and zeolite-based catalysts have received the most attention. Zeolites are especially promising in their application as heterogeneous, chiral catalysts or catalyst carriers due to their ordered structure and high porosity. This structure allows for ready incorporation and immobilization of the chiral catalyst onto the surface of the zeolitic pores, and not only enhances the surface area available for catalyst integration but also creates a highly structured and confined cavity which can positively affect stereoselectivity. Zeolites Zeolites are traditionally defined as microporous, crystalline, aluminosilicate compounds of the general formula M2/nO•Al2O3•ySiO2•wH2O, where M is an alkali or alkali earth metal, n is the charge of the cation, y correlates the ratio of Al2O3 to SiO2, and w is the number of water molecules trapped in the porous space of the zeolite. The ordered structure of the zeolite is composed of staggered AlO4 ̄and SiO4 tetrahedra, which are linked by shared oxygens, with each aluminum tetrahedron contributing a single negative charge to the lattice. These negative charges can be offset by a number of organic or inorganic cations, including protons, oxidized metals, and ammonium ions. In the last three decades,