Single-Handed Helical Polymer-Based Polycarboxylate with Achiral Triarylphosphine Pendants as Chiral Catalysts for Asymmetric Cross-Coupling Reactions in Pure Water
Naoaki Kamiya, Takuma Kuroda, Yuuya Nagata, Takeshi Yamamoto, Michinori Suginome
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.