Silica-bound homogenous catalysts as recoverable and reusable catalysts in organic synthesis

Very frequently the most costly components in a chemical reaction are not the starting materials or the reaction products, but the catalyst. In addition to the advantages from the economic point of view, recovery and reuse of the catalyst is equally important in order to avoid wastes, so improving the,Greenness of the process. There is a current tendency to transform homogeneous into heterogeneous catalysis, that will even be accelerated in the near future. Starting from a successful homogeneous catalyst, one a general methodology allowing its recovery and reuse is to immobilize a suitable derivative of the active catalyst on an insoluble solid support. When the catalyst does not deactivate and is sufficiently stable under the reaction conditions, the ultimate immobilization methodology is to attach covalently the catalytically active species to the support. In this contribution, after introducing some general principles describing the fundamentals of the covalent anchoring, the emphasis is placed more on giving an overview of the most important types of covalently anchored catalysts, including Bronsted and Lewis acids, covalently anchored bases and hydrogenation complexes. Hot topics such as silica-bound organocatalysts and the application of periodic mesoporous organosilicas as heterogeneous catalysts is also covered. 1 Introduction 2 Homogeneous versus Heterogeneous Catalysis 3 Covalent Immobilization as the Definite Methodology for Converting a Homogeneous Catalyst into a Heterogeneous Catalyst 4 Silanol Groups and Covalent Binding 5 Alternative Synthetic Procedures for Covalent Anchoring to Silica 6 Sulfonic Groups Covalently Anchored to Silicas as Solid Brbnsted Acids 7 Perfluoroalanesulfonic Groups Covalently Anchored to Silicas as Solid Bronsted Superacids 8 Solid Lewis Acids Derived from Covalently Attached Silicas 9 Hybrid Basic Catalysts 10 Silica-Bound Enantioselective Hydrogenation Catalysts 11 Periodic Mesoporous Silicas as C-C Coupling Catalysts 12 Design of Hybrid Organic-Inorganic Catalysts for Cascade Reactions 13 Silica-Bound Organocatalysts 14 Silica-Bound Heterogeneous Catalysts for Enantioselective Cyanosilylation of Aldehydes 15 Concluding Remarks and Future Prospects