Single-Handed Helical Polymer-Based Polycarboxylate with Achiral Triarylphosphine Pendants as Chiral Catalysts for Asymmetric Cross-Coupling Reactions in Pure Water

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 polymer-based polycarboxylic acid was soluble in basic pure water by forming poly-carboxylate 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 reac-tion 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.