Modern Strategies in Supramolecular Catalysis

This chapter summarizes the main achievements in the area of supramolecular catalysis in the past decade. Supramolecular chemistry emerged 40 years ago. The initial focus was host guest chemistry, and one target application was the use of such interactions to bring catalyst and substrate together. Examples in the first part of this chapter illustrate how rates of reactions, selectivities, regioselectivities, and enantioselectivities may change through assemblies designed as models for enzymes. In the beginning, natural host molecules such as cyclodextrins and modified cyclodextrins received most attention, but later a plethora of synthetic hosts were developed. More recently, the construction of host molecules was facilitated enormously by the introduction of supramolecular "tools"; according to this principle, large entities are constructed by bringing together smaller building blocks via non-covalent forces, such as hydrogen bonding, ionic bonding, metal-ligand coordination bonding, fluorophilic interactions, etc. A large number of host molecules were reported in the past decade, and most of them do not function merely as hosts but instead are containers that can host more than one molecule and have catalytic functions incorporated. A variety of names are used for these entities, such as capsule, cavitand, nanoreactor, nanocontainer, cage molecule, and receptor molecule. Large changes in selectivities and rates of catalytic reactions relative to those of bare catalytic sites have been reported. The second part of this chapter deals with the supramolecular construction of ligands or entire catalyst assemblies. This modular construction has enabled the synthesis of large catalyst libraries, which are useful for catalyst optimization and catalyst screening. In this way, new catalysts were developed, and new ways to control rates and selectivities of catalytic reactions were recognized. Biomacromolecules (and modified variants) have been used, particularly as sources of chirality in catalytic transformations, via supramolecular interactions with homogeneous catalysts. The last part of the chapter shows that supramolecular interactions can be used successfully for the immobilization of homogeneous catalysts. By its nature, the bonding is reversible, and the developments have led to a new reactor configuration for use of homogeneous catalysts, termed reverse-flow adsorption.