Applications of Helical Polymers

Potential applications of optically active helical polymers that mimic the structures of enzymes, involve enantioselective catalysis and adsorbents [1, 2, 5]. The one-handed helical polymethacrylates prepared by the helix-sense selective polymerization of TrMA and its analog 5 (Section 11.2.1) exhibit excellent chiral recognition abilities when coated on a macroporous silica gel and used as chiral stationary phases (CSPs) in HPLC [121, 122]. These packing materials can resolve a wide range of racemic compounds including chiral drugs and stereochemically

Fig. 11.20 Chromatogram for the resolution of 2,20-dihydroxy-1,10-binaphthyl on a PTrMA (1) column and the structures of stereochemical^ interesting compounds resolved on 1.

interesting molecules, and are commercialized [1]. The typical chromatogram for the separation of 2,20-dihydroxy-1,10-binaphthyl and some chiral molecules resolved on PTrMA are shown in Fig. 11.20 [5, 121, 122]. A stereoregular helical poly(phenylacetylene) (31) can also be used as a CSP for HPLC, which resolved several enantiomers including Troger's base and stilbene oxide [57]. However, a stereoirregular poly(phenylacetylene) with an identical chemical structure as 28, prepared by a different synthetic route showed poor chiral recognition, clearly indicating that the one-handed helical conformation induced by a stereoregular polymer backbone with chiral pendant groups is indispensable for effective chiral recognition. Other helical polyacetylenes such as 27 have been used as enantiose-lective permeable membranes for separating amino acids and chiral alcohols [123].

Reggelin et al. took advantage of the versatility of helical polymethacrylates developed by Okamoto and reported the first successful catalytic asymmetric C-C bond forming reaction using the helical polymers as a chiral polymeric ligand. The polymethacrylates were prepared by the helix-sense selective anionic polymerization or copolymerization with TrMA [1], producing an isotactic, fully one-handed helical polymer and copolymer with a large optical rotation. Complexed with palladium, the resulting monodentate (87) [124] and bidentate (88) [125] palladium catalysts promoted the asymmetric allylic alkylation reaction (Scheme 11.8) resulting in the substitution product with ca. 30 and 40-60% ee, respectively. Reggelin et al. further applied this strategy to a dynamic helical polyisocya-nate. The copolymer composed of a chiral isocyanate and an achiral isocyanate bearing a phosphine pendant (60:40, mol/mol) (89) [125], although its helical sense excess was not perfect, showed a low, but apparent catalytic enantioselective

Scheme 11.8

activity in an asymmetric hydrogenation reaction when complexed with a rhodium catalyst, thus producing a hydrogenated product with 14.5% ee. These static and dynamic helical polymers lacking any other elements of chirality except for helicity are a promising new class of ligands for asymmetric catalysis.

A large number of other chiral polymeric ligands have been synthesized from chiral small molecules such as 1,10-bi-2-naphthol (BINOL) and 2,20-bis(diphenylphosphino)-1,10-binaphthyl (BINAP). Some of them may have a helical structure and serve as ligands in various enantioselective transformations [126].

As previously described, the most important and unique feature of dynamic helical polymers is a remarkable amplification of chirality, which may be utilized to construct a novel helical polymer with the desired pendant group in a one-handed helical array along the polymer backbone. In fact, the copolymerization of an achiral phenylacetylene bearing a fullerene pendant with a small amount of an optically active phenylacetylene yielded a helical copolymer with an excess of one helical-sense in which the pendant C6o groups adopt a predominant screw-sense along the polymer backbone (Scheme 11.9) [127], because the copolymer exhibited an ICD in the achiral fullerene chromophore region as well as in the polymer backbone region. In a complementary approach, an enantiomerically pure cati-onic C60-bisadduct (90) induced a predominantly one-handed helix in a dynamically racemic poly(phenylacetylene) (37b) with the opposite negative charges in DMSO-water mixtures through noncovalent bonding interactions, which further results in a helical array of the C60-bisadducts with a predominant screw-sense along the polymer chain [128].

Optically active helical polymers often show chiral LC phases due to their rigid rodlike backbones. Such liquid crystalline helical polymers combined with a specific property of inversion of the helicity regulated by external stimuli will offer switchable chiral materials suitable for data storage, optical devices and use in other fields involving chiral nanotechnology [129].

Scheme 11.9
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