Applications in Catalysis

Enantioselective catalysis is one of the most effective processes for synthesizing target chiral molecules that are further used for the production of useful compounds such as medicines and pesticides. Numerous homogeneous chiral metal complex catalysts have been developed to date, and various transformations have been achieved with excellent enantioselectivities. Despite the great performance of these catalysts, difficulties associated with their separation from products and their reuse remain problematic, especially when metal complex catalysts consisting of precious and/or toxic metals are used. In this context, the use of heterogeneous catalysts to overcome these drawbacks is preferred for industrial-scale synthesis. One of the typical strategies that has been used to construct heterogeneous metal catalysts for asymmetric reactions is immobilization of chiral ligands on solid supports followed by introduction of metal salts to form “immobilized chiral metal complexes” (Scheme 8.1). This simple strategy is applicable to a wide range of catalyst systems; however, complicated preparation of monomeric ligands, introduction of linker parts to ligands, and complete complexation of metal complexes are ineluctable problems, and the catalytic activity and enantioselectivity of these immobilized catalysts are lower than those of the corresponding homogeneous metal complexes. Moreover, because the active catalytic species in the immobilized catalysts are basically the same as the original homogeneous catalysts, stability issues associated with the metal complexes themselves cannot generally be overcome, and thus, a decrease in their activity and enantioselectivity is problematic because of catalyst deactivation. For these reasons, only a few examples of chiral metal complex catalysts have been applied to an industrial-scale synthesis; in particular, the application of these catalysts to asymmetric C—C bond-forming reactions has been limited. Therefore, the development of more active, stable, and reusable catalysts is required.