Immobilized Functional Active Site Models

In the second part, selected immobilized structural and spectroscopic active site models will be discussed and aspects of characterization and analytics of immobilized transition metal complexes will be exemplarily disclosed. Typical techniques include spectroscopic methods addressing the immobilized biomimetic species and determination of metal ion leaching and active site integrity, for example, by selective extraction of the intact biomimetic metal complex - the prosthetic group - from the matrix - the apoenzyme (prosthetic group extraction). The third section gives a short overview of the elementary reaction steps in the catalytic processes and their observation on solid matrixes. Selected immobilized biomimetic functional active site models will be discussed in detail in the last section. 

Abstract Enantioselection in a stoichiometric or catalytic reaction is governed by small increments of free enthalpy of activation, and such transformations are thus in principle suited to assessing dendrimer effects which result from the immobilization of molecular catalysts. Chiral dendrimer catalysts, which possess a high level of structural regularity, molecular monodispersity and well-defined catalytic sites, have been generated either by attachment of achiral complexes to chiral dendrimer structures or by immobilization of chiral catalysts to non-chiral dendrimers. As monodispersed macromolecular supports they provide ideal model systems for less regularly structured but commercially more viable supports such as hyperbranched polymers, and have been successfully employed in continuous-flow membrane reactors. The combination of an efficient control over the environment of the active sites of multi-functional catalysts and their immobilization on an insoluble macromolecular support has resulted in the synthesis of catalytic dendronized polymers. In these, the catalysts are attached in a well-defined way to the dendritic sections, thus ensuring a well-defined microenvironment which is similar to that of the soluble molecular species or at least closely related to the dendrimer catalysts themselves. 

Since he was working in an institute for organic chemistry and biochemistry, he was familiar with the work of the Israeli s Katchalski-Katzir and Patchornik, who used polymers as supports for enzyme immobilization and for performing reactions on functional polymers. It was also the time when the mechanism of chymotrypsin was elucidated. This led Wullf to contemplate the possibility of producing synthetic polymers with catalytic sites that worked in the same way as enzyme active sites. There was already some literature on using synthetic polymers as catalytic models of enzymes but these did not have specific structural binding sites as enzymes did. Clearly a new approach would be required to generate such sites with specific predetermined structure. 

The Zincke reaction has also been adapted for the solid phase. Dupas et al. prepared NADH-model precursors 58, immobilized on silica, by reaction of bound amino functions 57 with Zincke salt 8 (Scheme 8.4.19) for subsequent reduction to the 1,4-dihydropyridines with sodium dithionite. Earlier, Ise and co-workers utilized the Zincke reaction to prepare catalytic polyelectrolytes, starting from poly(4-vinylpyridine). Formation of Zincke salts at pyridine positions within the polymer was achieved by reaction with 2,4-dinitrochlorobenzene, and these sites were then functionalized with various amines. The resulting polymers showed catalytic activity in ester hydrolysis. ... 

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