Solvent Effect on the Sergeants-and-Soldiers Effect Leading to Bidirectional Induction of Single-Handed Helical Sense of Poly(quinoxaline-2,3-diyl)s Copolymers in Aromatic Solvents.

Abstract

Random poly(quinoxaline-2,3-diyl) copolymers, containing chiral (S)-3-octyloxymethyl and achiral propoxymethyl side chain units, experience an abnormal sergeants-and-soldiers effect, that is, they adopt, depending on the chiral monomer mole fraction, either P- or M-helical conformations in anisole (PhOCH3) and benzotrifluoride (PhCF3). In benzene (PhH) and toluene (PhCH3), these copolymers exclusively adopt an M-helical conformation, regardless of the chiral monomer mole fraction. For a co-300mer, with a 40% mole fraction of chiral units, the selective induction of an M-helix (>99%) was observed in PhH, while in PhCF3, a P-helical conformation was induced selectively (>99%). This helix inversion of the polymer backbone is thus able to control the chirality of a chiral polymer ligand in aromatic solvents. The incorporation of a small amount of coordinating PPh2 groups into the copolymer resulted in a chiral macromolecular ligand, which allowed the enantioselective synthesis of both enantiomeric products in an asymmetric Suzuki-Miyaura coupling reaction (R-product: 91% ee in PhH; S-product: 95% ee in PhCF3) from a single catalyst.