Ligand synthesis catalyst immobilization, polymer supports

The development of polymer-immobilized ligands and catalysts for asymmetric synthesis is a rapidly growing field [1], which has great importance due mainly to the easy separation and recyclability of polymer-supported catalysts. The polymeric catalysts also represent one of the most powerful tools for green sustainable chemistry, in the sense that they can be easily recovered and reused many times. 

The most used polymer support (resin) both for organic synthesis and for immobilization of catalysts is cross-linked polystyrene. Numerous types of polystyrene resin are commercially available and can be obtained in different sizes, loading capacities (level of functionalization), and degrees of cross-linking. This makes immobilization of organometallic complexes a reasonably simple operation since the metal complex bearing a suitably functionalized ligand can be attached directly to a commercially available polymer backbone or, if necessary, built up on the support. 

Styrene-divinyl benzene resins have been used as typical support for the metal containing catalysts bound to them. The synthesis of these polymeric reagents has involved several approaches depending on the application which they are intended. The immobilization of transition metals on polymer supports involves using polymers containing ligands which can complex with the metal such that the coordination sphere of the metal remains essentially the same as in... 

Dendrimer- and polymer-supported catalysts can be employed in the ATH of sultam precursors [71, 72], including examples where the supporting material has sulfonyl groups and therefore assists reactions in water [73]. The synthesis of an amphiphilic polystyrene-type immobilized TsDPEN ligand and its application in ATH of cyclic sulfonimines has been reported (Fig. 16) [74]. 

The problem of separating the catalyst at the end of the operation can be eased in some cases by attaching the catalyst to a solid support, for instance, liquid phosphoric acid in the pores of a solid carrier for the vapor phase synthesis of cumene and the fairly wide application of enzymes that are attached (immobilized) by various means to solid polymers. Some metal ligands also are being combined with solid polymers. 

Functionalized polymers incorporating neutral, metal binding ligands such as phosphine were prepared as early as 1959 (Rabinowitz and Marcus, 1961 Issleib and Tzschach, 1959). After Merriheld introduced the concept of solid-phase synthesis, the basic idea of using polymer-immobilized transition metal complexes as catalysts burgeoned, and many more polymeric supports containing neutral donor ligands have been prepared. 

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