Catalysis With Molecularly Imprinted Polymers

The cavity is first imprinted with the end product of the reaction, and a precursor is then embedded into the cavity so that a reaction can convert it into the product. The first experiments were carried out by the research groups of Shea [92] and Neckers [93], who performed cycloadditions to obtain cyclopropanedicarboxylic and cyclobutanedicarbox-ylic acids. The latter compounds were obtained with remarkable regio- and diastereose-lectivity. 

The first asymmetric syntheses in the chiral cavity were achieved in our research group [27, 52, 53]. A cavity was made with an l-DOPA methyl ester. After removal of the template, glycine was embedded in the cavity, deprotonated, and alkylated. So far, the highest enantiomeric excesses (36% e.e.) on using imprinted polymers have been with the amino acids formed in this way. This excess is purely a result of the shape of the asymmetric cavity. 

In some very remarkable experiments, Bystrom et al. [77] were recently able to demonstrate high regio- and stereoselectivity in reactions inside the imprinted cavity. The steroid 12 was copolymerized as the template monomer, and removed by reduction. The hydroxyl group in the polymer newly formed from the carboxyl group was converted into an active hydride by LiAlH4. With the help of this polymer, androstan-3,I7-dione was reduced to the alcohol exclusively in position 17, whereas in solution or with a polymer with statistically distributed hydride groups it is reduced exclusively in position 3. 

Imprinting should also be an excellent method to prepare active sites of enzyme analogues. It has already been reported that antibodies prepared against the transition state 

Better results were obtained for the catalysis of the dehydrofluorination of 4-fluoro-4-(4-nitrophenyl)butanone by Shea et al. [94] and Mosbach et al [147]. Shea used benzyl-malonic acid as the template to position two polymerizable amines in a definite arrangement in the cavity. After removal of the template, dehydrofluorination was enhanced by this catalyst by a factor of 8.6. 

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Visnjevski A et al (2005) Catalysis of a Diels-Alder cycloaddition with differently fabricated molecularly imprinted polymers. Cat Commun 6 601-606... 

However, also in this case enantio-selectivities never exceeded the values obtained with the oxazaborolidine in solution, probably because of diffusional limitations within the polymer support, which enhanced the contribution of the non-selective, direct borane reduction of the ketone. In spite of the rather low imprinting effects obtained in these initial attempts, we feel that this approach still represents a most interesting application of molecularly imprinted polymers in catalysis and deserves further attention in the near future. 

Reactions inside imprinted cavities are another interesting area. Of great importance to the field is catalysis with imprinted polymers and imprinted silicas. For a broader application of molecularly imprinted polymers further improvement of the method will be necessary. The following problems are in the forefront of investigation today ... 

These results show MIPs are capable of catalytic turnover, as well as substrate selectivity and rate enhancement for the catalysis of C-C bond formation. Furthermore, Matsui and coworkers presented a thorough evaluation of the fabricated materials, which allowed the authors to state with a high degree of confidence the mode of operation of these molecularly imprinted polymers. 

Polborn and Severin [23] recently reported ruthenium- and rhodium-based TSAs for the transfer hydrogenation reaction. These complexes were used as catalyst precursors in combination with molecular imprinting techniques. Phosphinato complexes were prepared as analogs for the ketone-associated complex. They demonstrated that the results obtained in catalysis were better in terms of selectivity and activity when these TSAs were imprinted in the polymer. This shows that organometallic complexes can indeed serve as stable TSAs (Figure 4.9). 

One particular asset of structured self-assemblies is their ability to create nano- to microsized domains, snch as cavities, that could be exploited for chemical synthesis and catalysis. Many kinds of organized self-assemblies have been proved to act as efficient nanoreactors, and several chapters of this book discnss some of them such as small discrete supramolecular vessels (Chapter Reactivity In Nanoscale Vessels, Supramolecular Reactivity), dendrimers (Chapter Supramolecular Dendrlmer Chemistry, Soft Matter), or protein cages and virus capsids (Chapter Viruses as Self-Assembled Templates, Self-Processes). In this chapter, we focus on larger and softer self-assembled structures such as micelles, vesicles, liquid crystals (LCs), or gels, which are made of surfactants, block copolymers, or amphiphilic peptides. In addition, only the systems that present a high kinetic lability (i.e., dynamic) of their aggregated building blocks are considered more static objects such as most of polymersomes and molecularly imprinted polymers are discussed elsewhere (Chapters Assembly of Block Copolymers and Molecularly Imprinted Polymers, Soft Matter, respectively). Finally, for each of these dynamic systems, we describe their functional properties with respect to their potential for the promotion and catalysis of molecular and biomolecu-lar transformations, polymerization, self-replication, metal colloid formation, and mineralization processes. 

Molecular imprinting has recently attracted considerable attention as an approach to the preparation of polymers containing recognition sites with predetermined selectivity. The history and specifics of the imprinting technique pioneered by Wulff in the 1970s have been detailed in brilliant review article [40]. These materials, if successfully prepared, are expected to find applications in numerous areas such as the resolution of racemates, chromatography, substrate selective catalysis, and the production of "artificial antibodies". Imprinted monoliths have also recently received... 

Alexander, C. Davidson, L. Hayes, W., Imprinted polymers artificial molecular recognition materials with applications in synthesis and catalysis, Tetrahedron. 2003, 59, 2025-2057... 

In homogeneous catalysis using chiral diamine 18 complexed with Rh, the acetophenone was reduced quantitatively with 55% ee, in 7 days. In the case of polymerized complex 36a, acetophenone reduction leads to 33% ee and with its templated analog 43% ee. With 36b, an increase of about 20% ee is observed between polymerized and templated ligand. These increases in ee were ascribed to a favourable molecular imprinting effect of the PM, ereating chiral pockets within the polymer network. 

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