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Single-Handed Helical Polymer-Based Polycarboxylate with Achiral Triarylphosphine Pendants as Chiral Catalysts for
Naoaki Kamiya1, Takuma Kuroda1, Yuuya Nagata1
1Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan.
Abstract:
Helical poly(quinoxaline-2,3-diyl)s (PQX) containing chiral carboxylic acid side chains derived from natural l-lactic acid were synthesized. The newly synthesized helical polycarboxylated polymer was soluble in basic pure water by forming polycarboxylate and adopted the P-helical conformation (right-handed), whereas the M-helical conformation is predominant under weakly acidic aqueous conditions. The water-soluble PQX containing additional monomer units bearing a 2-(diarylphosphino)phenyl group was employed in the asymmetric Suzuki-Miyaura coupling reaction in pure water in the absence of organic cosolvents. Remarkable enantioselectivities up to 99% ee, which were found to be higher than the same coupling reaction in organic solvent using the corresponding lipophilic PQX, were obtained for the couplings of 2-substituted 1-halonaphthalenes. It is presumed that the hydrophobic reaction environment is provided by the hydrophobic main chain of PQX according to the Nile Red-based hydrophobicity test. Recovery of the crude product without using any trace of organic solvent has been demonstrated by the switch of solubility of PQX depending on pH.
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