Imprinted Polymers: Versatile New Tools in Synthesis

Abstract

Abstract:The use of molecularly imprinted polymers in synthetic organic chemistry is reviewed. These materials are prepared in the presence of a template for which they carry a functional and stereochemical "memory" and can be likened to artificial antibodies or enzymes. Their unique properties have been exploited by the use of imprinted polymers as stereo-and regio-selective solid supports in condensation reactions and hydride reduction, as protecting groups in the acylation of polyols and as catalysts to enhance the rates of reactions as diverse a Diels-Alder reaction, an Aldol condensation, β-elimination, the benzisoxazole isomerization, transesterification and ester hydrolyses.Key words: molecular imprinting, imprinted polymer, selectivity, chemical modification, synthesis IntroductionThe technique of molecular imprinting, in which a crosslinked polymer is assembled around a template, utilizing its interaction with functional monomers to create a recognition site, complementary in both shape and functionality to the original occupant, has become well established and many research groups are currently active in this field. [1][2][3][4][5][6][7][8][9][10][11] A schematic diagram of the imprinting process is shown in Figure 1.There have now been a number of examples illustrating the spectacular performance of imprinted polymers in a range of applications from chiral chromatographic separations 12-14 to the binding of ligands with a specificity rivaling that of antibodies. [15][16] However, it is becoming clear that the versatility of the imprinting methodology can be exploited to generate polymers for uses beyond those involving simply the recognition or separation of small organic molecules. In particular, researchers have begun to explore the use of imprinted polymers in synthesis, with a view to creating novel and robust reactive supports, as "protecting groups" and catalysts. In this short review we focus on the above applications, and outline some recent work which has taken the field of imprinted polymers into the realms of organic, inorganic, and biomimetic chemistry. "Microreactors" for chiral synthesisThe use of the imprinted recognition sites as potential chiral cavities for enantioselective synthesis was first described by the research groups of Shea 17 and Neckers, 18 and synthetic transformations including enantioselective protonation-deprotonation 19 and selective hydrolysis 20 have also been carried out in imprinted sites. However, perhaps the most elegant early work in this area was due to Wulff and co-workers, who targeted their initial studies to C-C bond formation in a biomimetic synthesis of aamino acids from glycine. 21 The model system set out by Wulff involved alkylation of a polymer-bound glycine, present as an ester enolate. The synthetic scheme on which this is based is shown in Figure 2, in which the glycine-Schiff base (1) is deprotonated to the enolate (2) amenable to alkylation via appropriate routes (alkyl halide, aldol condensation, Michael addition) to give the substituted amino...

Molecular imprinting science and technology: a survey of the literature for the years up to and including 2003

“…[1398][1399][1400][1401][1402] This is because imprinted materials offer synthetic chemists the chance to carry out selective reactions within highly defined stereochemical environments that can be designed specifically for almost any desired chemical task. Furthermore, the inherent stability of these highly cross-linked polymers makes imprinted systems especially interesting for industrial applications, where temperature and solvent regimes are often such that biological catalysts are rapidly denatured or destroyed.…”