Racemic resolution polymers first used

An important question in molecular imprinting has been addressed using covalent binding by two boronic acids to what extent can imprinted polymers also bind substances other than the template. For example, are racemates of other substances resolvable In the first experiments on glyceric acid esters 5 with a certain ester as template, imprinted polymers were shown to resolve a whole series of racemates even when the alcohol group in the racemate is varied (methyl, ethyl, benzyl, or 4-nitrophenyl) [39]. Aromatic amino acids were shown to behave similarly. Here, the aromatic group in the racemates can vary. A racemate resolution is possible provided that the rest of the structure remains the same [40]. 

The first successful experiments were reported by Schwab [16] Cu, Ni and Pt on quartz HI were used to dehydrogenate racemic 2-butanol 23. At low conversions, a measurable optical rotation of the reaction solution indicated that one enantiomer of 23 had reacted preferentially (eeright-handed quartz gave the opposite optical rotation it was deduced that the chiral arrangement of the crystal was indeed responsible for this kinetic resolution (for a review see [8]). Later, natural fibres like silk fibroin H5 (Akabori [21]), polysaccharides H8 (Balandin [23]) and cellulose H12 (Harada [29]) were employed as chiral carriers or as protective polymer for several metals. With the exception of Pd/silk fibroin HS, where ee s up to 66% were reported, the optical yields observed for catalysts from natural or synthetic (H8, Hll. H13) chiral supports were very low and it was later found that the results observed with HS were not reproducible [4],... 

Molecular imprinting technique was recently used to prepare highly selective tailor-made synthetic affinity media used mainly in chromatographic resolution of racemates or artiftcial antibodies [130-133]. A complex between the template molecule and the functional monomer is first formed in solution by covalent or non-covalent interactions (Figure 3.10). Subsequently, the three-dimensional architecture of these complexes is confined by polymerization with a high concentration of cross-linker. The template molecules are then extracted from the polymer leaving behind complementary sites (both in shape and functionahty) to the imprinted molecules. These sites can further rebind other print molecules. 

The significant contribution by the Nolte and Drenth group in this area was that, for the first time, they demonstrated the existence of a non-racem-ic helical structure for poly(isocyanide)s. Optical resolution using a chiral HPLC technique or asymmetric polymerization led to the isolation of optically active polymers, whose chirality was supposed to be solely due to the main chain helicity. Their effort, in conjunction with that of Novak s and Takahashi s significant contributions to asymmetric polymerization, will be discussed in the next section. Non-asymmetric and asymmetric polymerizations will be described separately in the following sections. 

The first example of chiral polymer from a disubstituted acetylene is a polyd-trimethylsilyl-l-propyne)-based polymer, poly(46), which was synthesized in moderate yields using TaCls-PhaBi (112). Poly(46) displays small optical rotations, and its molar ellipticities of the Cotton effects are up to a few hundreds. The main chain of poly(46) is, therefore, not a well-ordered helix. This is probably because of the less controlled geometrical structure (cis and trans) of the polymer backbone. However, the free-standing film of this polymer achieves an enantioselective permeation of various racemates including alcohols and amino acids. For example, the concentration-driven permeation of an aqueous solution of tryptophan by poly(46) gives 81% enantiomeric excess (ee) of the permeate at the initial stage. A characteristic of the membrane of poly(46) is its ability to enantioselectively recognize 2-butanol and 1,3-butanediol, because the direct resolution of these racemates by hplc is impossible. 

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