Molecularly-imprinted catalyst

The aim of this chapter is to give a brief overview of the molecularly imprinted catalysts reported up to approximately the turn of the millennium, followed by a more concise review of the literature thereafter. In addition to catalysis, that is cases in which a reaction TSA or intermediate are used as the template, imprinted polymers capable of aiding chemical transformations will also be discussed. In these cases the reaction substrate or product are often used as the template in order to control the regio- or stereochemistry of the reaction. 

The bulk polymeric format, characterised by highly cross-linked monolithic materials, is still widely used for the preparation of enzyme mimic despite some of its evident drawbacks. This polymerisation method is well known and described in detail in the literature and has often be considered the first choice when developing molecular imprinted catalysts for new reactions. The bulk polymer section is presented in three subsections related to the main topics covered hydrolytic reactions, carbon-carbon bond forming reactions and functional groups interconversion. 

A similar approach was employed in our laboratory for the development of molecularly imprinted catalysts for the enantio-selective reduction of prochiral ketones with borane (CBS reaction) [122]. A stable, polymerisable transition state analogue of this reaction leading to the formation of one particular enantiomer product was prepared (Fig. 4.9). After polymerisation, the template molecule can... 

The application of the molecular imprinting method for the synthesis of heterogeneous catalysts has been investigated examples of molecularly imprinted catalysts are summarized in Table 22.1. In addition to acid-base catalysts with acid and/or... 

Fig. 22.2 Schematic procedures for the synthesis of molecularly imprinted catalysts (a) with organic binding sites Right triangle, Filled circle, Filled square) (e.g., NH, NH, OH, and COOH) on the wall of a polymer matrix and (b) using a metal complex coordinating a template ligand...

A ligand of a metal complex is one of the most appropriate templates for a molecular-imprinted metal-complex catalyst. Several ligands have been reported as candidates because of their analogy to transition states or reaction intermediates for target reactions [51-64], Several metal complexes with single-site Co, Cu, Zn, Ti, Ru, Rh, and Pd species have been used as active metal sites coordinated with template ligands (Table 22.1). Acrylate polymers [54, 55, 60, 63, 64] or polystyrene-divinylbenzene (DVB) polymers [51, 53, 56] are common polymer supports for molecularly imprinted catalysts. 

Meng, Z. Yamazaki, T. Sode, K., A molecularly imprinted catalyst designed by a computational approach in catalysing a transesterification process, Biosens. Bioelectron. 2004, 20, 1068-1075... 

Sode K, Ohta S, Yanai Y, Yamazaki T (2003) Construction of a molecular imprinting catalyst using target analogue template and its application for an amperometric fructosylamine sensor. Biosens Bioelectr 18(12) 1485—1490... 

Many compounds have now been used as template molecules in molecular imprinting. Basically, imprinted polymers can be used directly as separation media. Since all separation applications cannot be described here, some studies recently reported are bsted in Table 7.1. In this chapter, only selected topics, including sensor applications, signaling polymers, molecularly imprinted sorbent assays, molecularly imprinted membranes, affinity-based solid phase extraction, in situ preparation of imprinted polymers, and molecularly imprinted catalysts are discussed. For the reader requiring information on other applications, there are many review articles dealing with these, Recent review articles and books are summarized in Table 7.1. For further development of molecular imprinting techniques, newly designed functional monomers would be desirable. Various functional monomers have been reported and many applications have been conducted. These are summarized in Table 7.2. 

One of the most attractive applications would be molecularly imprinted catalysts. In principle, such catalysts could be prepared if substrate, product or transition-state analogs could be used as template molecules, since to natural catalytic antibodies are produced in a similar way. Since molecularly imprinted polymers are considered to be analogous to antibodies in that binding sites are tailor-made, catalytic antibody-like activity in imprinted polymers could also be conceived, enabling an artifi-cial catalytic antibody with the advantageous features of synthetic molecules to be produced. 

Since then, many attempts have been made to produce molecularly imprinted catalysts. However, because of the inherent strong affinity to template molecules, these almost always showed lower turnover than that of natural enzymes. A major breakthrough would be necessary for further development of molecularly imprinted catalysts. Recent investigations into this subject are summarized in Table 7.4. 

Similarly, starting from the bridged Rh-dimer precursor Rh2Cl2(CO)4, Rh-dimer molecular imprinting catalyst was attached at Si02 surface for hydrogenation of alkenes, and exposed to P(OCH3)3 to make rhodium-phosphite... 

T. Yamazaki, S. Ohta, Y. Yanai, and K. Sode, Molecular imprinting catalyst based artificial enzyme sensor for fructosylamines, Anal. Lett., 36 (2003) 75-89. 

Other topics are more or less relevant to the general context of the present contribution. For example, this would be the manufacture of molecularly imprinted catalysts (footprint catalysts) possessing an activity much higher than those described in literature until now. The manufacture of already activated catalysts (already in use in hydrotreatment or, like Raney catalysts, in hydrogenation) could also be the object of stimulating remarks. But only the corresponding specialists could express valuable opinions. 

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