Synthesis of helical poly(phenylacetylene)s bearing cinchona alkaloid pendants and their application to asymmetric organocatalysis |
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Authors: | Garret M. Miyake Hiroki Iida Hai‐Yu Hu Zhenglin Tang Eugene Y.‐X. Chen Eiji Yashima |
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Affiliation: | 1. Department of Molecular Design and Engineering, Graduate School of Engineering, Nagoya University, Nagoya 464‐8603, Japan;2. Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523‐1872;3. Venture Business Laboratory, Nagoya University, Nagoya 464‐8603, Japan |
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Abstract: | Four novel dynamic helical poly(phenylacetylene)s bearing cinchona alkaloids as pendant groups were synthesized starting from the commercially available cinchona alkaloids, cinchonidine, cinchonine, quinine, and quinidine, by the polymerization of the corresponding phenylacetylene monomers with a rhodium catalyst. These polymers exhibited an induced circular dichroism (ICD) in the UV–visible region of the polymer backbones in solution, resulting from the preferred‐handed helical conformation induced by the optically active cinchona alkaloid pendants. In response to the solvent used, their Cotton effect patterns and intensities were significantly changed accompanied by the changes in their absorption spectra probably due to the changes in their helical conformations, such as the inversion of the helical sense or helical pitch of the polymers. When these helical polymers were used as polymeric organocatalysts for the asymmetric conjugated addition and Henry reactions, the optically active products with a modest enantiomeric excess were obtained whose enantioselectivities were comparable to those obtained with the corresponding cinchona alkaloid‐bound monomers as the catalysts. However, we observed a unique enhancement of the enantioselectivity and a reversal of the stereoselectivity for some helical polymers, suggesting the important role of the helical chirality during the asymmetric organocatalysis. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011 |
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Keywords: | catalysts chiral cinchona alkaloid helical structure polyacetylenes |
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