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Remarkable Enhancement of the Enantioselectivity of an Organocatalyzed Asymmetric Henry Reaction Assisted by Helical Poly(phenylacetylene)s Bearing Cinchona Alkaloid Pendants via an Amide Linkage
journal contribution
posted on 2016-02-22, 05:26 authored by Zhenglin Tang, Hiroki Iida, Hai-Yu Hu, Eiji YashimaA series of novel helical poly(phenylacetylene)s bearing
amino-functionalized
cinchona alkaloid pendant groups connecting to the phenyl rings through
an amide linkage were prepared by the polymerization of the corresponding
phenylacetylenes using a rhodium catalyst. All of the polymers formed
a preferred-handed helical conformation biased by the optically active
pendants, resulting in the induced circular dichroism in their π-conjugated
polymer backbone regions. The optically active helical polymers efficiently
catalyzed the asymmetric Henry reaction of benzaldehydes with nitromethane,
giving optically active products up to 94% enantiomeric excess (ee)
when the poly(phenylacetylene) bearing an amino-functionalized quinine
pendant group was used as the polymeric organocatalyst; the enantioselectivity
was remarkably higher than those catalyzed by the corresponding nonhelical
poly(phenylacetylene) (18% ee) and the monomer (28% ee).