Immobilization techniques site isolation

Early successes in use of polymer supp s as immobilizing media led to c timistic predictions about the future of the synthetic technique of "site isolation on" a polymer supi rt. Failures of the site isolation method soon were discovered. Dieckmann cyclizadon of polymer-bound diesteis gave acceptable yields of six-membeied fi-keto esters, but valiant attempts to synthesize the nine-membeied fi-keto ester failed (Equation 1) (3.41. (Zero yield was found also in attempted high dilution Dieckmann cycl-ization to the nine membered ring in solution ( ).) The IMeckmann cyclization results led to a pessimistic review (g) of the potential of site isolation synAesis in polymer gds, which has discouraged further research. 

The above example outlines a general problem in immobilized molecular catalysts - multiple types of sites are often produced. To this end, we are developing techniques to prepare well-defined immobilized organometallic catalysts on silica supports with isolated catalytic sites (7). Our new strategy is demonstrated by creation of isolated titanium complexes on a mesoporous silica support. These new materials are characterized in detail and their catalytic properties in test reactions (polymerization of ethylene) indicate improved catalytic performance over supported catalysts prepared via conventional means (8). The generality of this catalyst design approach is discussed and additional immobilized metal complex catalysts are considered. 

When combined with the isolation and reactivity studies of the patterned aminosilica (7), the increased activity of the patterned catalysts provide further evidence that the patterning technique developed allows for the synthesis of aminosilicas which behave like isolated, single-site materials (although a true single site nature has not been proven). As the olefin polymerization catalysts supported by the patterned materials show a marked improvement over those materials supported on traditional aminosilicas, these patterned materials should be able to improve supported small molecular catalysis as well. Future improvements in catalysis with immobilized molecular active sites could be realized if this methodology is adopted to prepare new catalysts with isolated, well-defined, single-site active centers. 

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