Metal catalysts, polymer-anchored

The most frequently used organic supports are polystyrene and styrene-divinylbenzene copolymer beads with functional groups such as diphenylpho.sphine covalently bonded. The polymer-anchored catalyst complex can then be obtained, for example, by displacement of a ligand already co-ordinated to a soluble metal complex (Cornils and Herrmann, 1996) ... 

The alternative strategy for heterogenization has been pursued by Blechert and co-workers, for a polymer-supported olefin metathesis catalyst. A polymer-anchored carbene precursor was prepared by coupling an alkoxide to a cross-linked polystyrene Merrifield-type resin. Subsequently, the desired polymer-bound carbene complex was formed by thermolytically induced elimination of ferf-butanol while heating the precursor resin in the presence of the desired transition metal fragment (Scheme 8.30). 

The catalytic epoxidation proceeds via the formation of peroxytungstic acid. Similarly, other metal catalysts are effective in the H2O2 oxidation. Aqueous conditions are not appropriate for epoxidations since epoxides are prone to undergo acid-catalyzed hydrolysis36. Polymer-anchored catalysts are conveniently separated from the reaction mixture after catalyzed H2O2 epoxidations (equation 8)9. 

The determination of structure and bonding of polymer anchored catalysts is another area where the insolubility of the materials often precludes solution spectroscopic studies and one is limited to techniques that can be applied to irregular solids (57). In addition, combining oxygen plasma etching and surface analysis allows investigation of the depth of penetration of the metal into the polymer and allows detection of components that require concentration to allow detection. 

In other reactions, particularly where strongly complexing reactants, e. g., carbon monoxide, are involved, leaching of the immobilized metal center may take place. Generally, the parameters to be considered in a polymer-anchored metal complex catalyst are of a manifold nature. It is still an unsolved problem and an incompatible situation that, on the one hand, a leaching process should be avoided while, on the other hand, sufficient activity and the selectivity necessary for industrial applications are to be maintained. As a consequence it has become... 

The more often practiced routes to polymer-anchored complex catalysts include the displacement of a ligand already coordinated to a soluble metal complex by a polymer-bonded ligand [38] (eq. (5)), or the splitting of a weakly bridged dimeric metal complex [34] (eq. (6)). 

The use of polymer-supported metal catalysts for the hydrogenation of thiophe-nie substrates has recently been extended to Ru and Rh complexes anchored to silica via hydrogen bonding [23, 24]. 

The selectivity of metal catalysts improves in some reactions with alloying for example the alumina-supported Pd-Cu catalyst hydrogenales butadiene to 1-butene with 99% selectivity, i.e. the isomerization is less than lOli. The explanation is that hydrogen adsorption decreased on the Cu-containing catalysts . Similarly, better selectivities were observed with a polymer anchored Pd, or a Pd-Co catalyst in the gas-phase hydrogenation of butadiene and cyclopentadiene in a hollow-liber reactor 2 in the liquid-phase hydrogenation of 1,5-hexadiene with Pd-Ag catalyst. ... 

Transition metals are used as catalysts for various reactions. These same transition metals can be anchored to an organic resin. Resins, as used here, are defined as functional organic synthetic polymers. The transition metal can be introduced into the polymer matrix in two different ways. The transition metal can be included as part of the functional monomer prior to polymerization. This approach presents... 

Transition metal complexes, zeolites, biomimetic catelysts have been widely used for various oxidation reactions of industrial and environmental importance [1-3]. However, few heterogenized polymeric catalysts have also been applied for such purpose. Mild condition oxidation catalyzed by polymer anchored complexes is attractive because of reusability and selectivity of such catalysts. Earlier we have reported synthesis of cobalt and ruthenium-glycine complex catalysts and their application in olefin hydrogenation [4-5]. In present study, we report synthesis of the palladium-glycine complex on the surface of the styrene-divinylbenzene copolymer by sequential attachment of glycine and metal ions and investigation of oxidation of toluene to benzaldehyde which has been widely used as fine chemicals as well as an intermidiate in dyes and drugs. 

The use of asymmetric catalysts in chiral syntheses is taking on increasing importance. Asymmetric ligands or asymmetric metal complexes used in these transformations are quite expensive and need to be efficiently separated from reaction mixtures and recycled. Scheme 16 shows the preparation of a polymer-anchored dibenzophosphole-DIOP platinum-tin catalysts system. The asymmetric ligand places the Pt-SnClj system in a chiral environment. This catalyst has given the highest enantiometric excesses ever observed in catalytic hydroformylation. The initially achieved 70-83% e.e. values were improved to >95% by the use of triethylorthoformate (TEOF) as the solvent. ... 

While the overall physical properties of the polymers have received rather cursory treatment, more attention has been devoted to the characterization of attached ligands and the coordination of the supported metal complexes. It is clear in this respect that polymer-anchored catalysts are more easily characterized than are conventional heterogeneous catalysts. Interpretations of catalyst performance rely upon both analysis of the polymer catalyst and a thorough knowledge of the homogeneous situation. 

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