Studies of Immobilized Catalysts - Introduction

For immobiUzed catalysts, a (chiral) Ugand or a preassembled metal complex, which has previously proven to be efficient in nonsupported transformations, is used as starting material. Covalent attachment of catalysts, either via copolymerization or via covalent attachment (anchoring), is the most often used immobilization method, which can also be applied to synthesize soluble immobilized catalysts. 

Noncovalent attachment of the catalytic systems can be realized by the adsorption of the catalyst to the support, by electrostatic interactions between the Ugand and the support, or by entrapment in porous soUd supports. Alternatively, the 

Independent of the procedure, covalent binding always requires modification of the ligand structure by introducing a linker and a spacer between the catalyst and the chemical support. 

The linker provides a functional group that allows the formation of a chemical bond between the ligand and the support material, whereas the spacer serves to ensure accessibility of the bonded catalyst to possible substrates. It therefore represents a crucial part in any immobilization, as it greatly influences the activity, selectivity, and lifetime of any given immobilized catalyst. 

There are two principal strategies for the selection of an appropriate linker for a given set of catalyst and support material. One is to choose a long linker to avoid interference of the catalyst with the support material the other is to use a short linker in order to generate additional interactions with the solid support material. 

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Part V Studies of Immobilized Catalysts - Introduction 403 Table 1 Enantioselective sulfoxidation of benzylphenylsulfide. 

Applications to Phase-transfer Methods.—Dehmlow has published a review on advances in phase-transfer catalysis (PTC) which discusses the introduction of crown ethers into this area. The full details are now available of a study of alkyl-substituted azamacrobicyclic polyethers (78a) as PT catalysts. When the alkyl chains are C14—C20, such molecules are very efficient catalysts in both liquid-liquid and solid-liquid phase-transfer modes, which contrasts with the lower catalytic ability of the less organophilic unsubstituted cryptand (78b). Crown ethers immobilized on polymeric supports have been demonstrated to possess increased PTC activity in 5n reactions, up to that of the non-immobilized systems, when the connection to the polymer involves long spacer chains [e.g. (79)]. 

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