| Research Abstract |
Helical polymers have aroused wide interest from synthetic, structural, and functional viewpoints. In the present dy, we report the induced circular dichroism (ICD) based on the helix formation of achiral poly ((4-dihydroxyborophenyl) acetylene) (poly-1) and poly ((4-disopropylaminomethylphenyl) acetylene) (poly-2) by the complexation with optically active molecules, for instance, chiral amines, sugars, and chiral acids, respectively. The split-type and magnitude of the Cotton effects can be used as a novel probe for determining the chirality of amines, sugars, or acids.
Poly-1 and poly-2 were prepared through the stereospecific polymerization with [Rh (nbd) Cl] 2. They exhibited an ICD in the UV-visible region upon complexation with chiral molecules, probably due to the predominantly one-handed helical conformation of the polymers. For instance, poly-1 showed an ICD with various kinds of chiral molecules such as diols, amino alcohols, hydroxycarboxylic acids, dicarboxylic acids, diamin
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es, carbohydrates, and steroids in aqueous solution depending on the configuration and stereochemistry of the molecules. Poly-2 also showed similar ICDs upon complexation with optically active carboxylic and alpha-hydroxy acids.
We also found a novel helix-helix transition of optically active polyphenylacetylene (poly-3) bearing chiral aminoalcohol residues induced by the complexation with chiral acids. Poly-3 is an optically active polymer and shows an ICD in the UV-visible region. However, the binding of chiral acids, for instance, (R) -mandelic acid and (R) -1-phenylpropionic acid to poly-3, results in a dramatic change in the ICD of poly-3 in solution owing to a x-helix transition, while the CD spectrum of poly-3 hardly changed even in the presence of excess (S) -mandelic acid and (S) -1-phenylpropionic acid. Therefore, the changes in the ICD of poly-3 can be used as a probe for chiral recognition of the acids. Although such a conformational transition has been observed in polypeptides, polynucleotides, and polymethacrylates, their helix-sense is governed by external achiral factors including pH,temperature, light, and salt concentration. This may the first example of a helix-helix transition induced by external chiral factor through diastereomeric acid-base interactions.
Poly-3 was used as an optically active catalyst for asymmetric alkylation. Enantiomeric excess of 1-phenylethanol obtained by the reaction of benzaldehyde with diethylzinc in the presence of catalytic amount of poly-3 was 30 - 49 %. Less
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