title: Solvent-dependent switch of helical main-chain chirality in sergeants-and-soldiers-type poly(quinoxaline-2,3-diyl)s: effect of the position and structures of the "sergeant" chiral units on the screw-sense induction.
abstract: Poly(quinoxaline-2,3-diyl) copolymers bearing various "sergeant" chiral units with common "soldier" achiral units have been synthesized to investigate the efficiency of screw-sense induction and its dependence on the nature of the solvents. Optically active 2-alkoxymethyl side chains located at the 6- and 7-positions of the quinoxaline ring induced a single-handed helical conformation more efficiently than 3-methylpentyl or 2-methylbutoxy chiral side chains. Among the 2-alkoxymethyl side chains, those bearing higher 2-alkoxy groups induced a single-handed screw sense more efficiently. For instance, a monomer unit bearing (R)-2-octyloxymethyl groups stabilized the P-helix by 1.01 kJ/mol, whereas the monomer bearing (S)-2-butoxymethyl groups stabilized the M-helix by 0.59 kJ/mol. The effect of the position of the sergeant units in the polymer main chain on the screw-sense induction was also investigated using copolymers in which the positions of the sergeant units were carefully controlled by their synthesis via living polymerization. Chiral units placed sparsely could induce single-handed helical structure efficiently. Chiral units bearing 2-alkoxymethyl, 3-methylpentyl, and 2-methylbutoxy groups showed solvent-dependent helix inversion in CHCl3 and 1,1,2-trichloroethane. No helix inversion was observed in those solvents with chiral units bearing 2-butoxy or (2-methylbutoxy)methyl side chains. The 40-mer of the (R)-2-octyloxymethyl units showed P-helical structures in THF, t-BuOMe, and c-C5H11OMe, toluene, pyridine, Et3N, 1-BuOH, CHCl3, CH2Cl2, 1,4-dichlorobutane, 1,1,-dichloroethane, and 1,1,1-trichloroethane, whereas M-helical structures were induced in 1-BuCN, 1-PrCN, 1,2-dichloroethane, 1,3-dichloropropane, and 2-BuOH.
authors.affiliation: Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 606-8501, Japan.
authors.forename: Yuuya
authors.initials: Y
authors.lastname: Nagata
authors.forename: Tetsuya
authors.initials: T
authors.lastname: Yamada
authors.forename: Takumi
authors.initials: T
authors.lastname: Adachi
authors.forename: Yuto
authors.initials: Y
authors.lastname: Akai
authors.forename: Takeshi
authors.initials: T
authors.lastname: Yamamoto
authors.forename: Michinori
authors.initials: M
authors.lastname: Suginome
mesh_terms: D015394:Molecular Structure; D011108:Polymers / Q000138:chemical synthesis / Q000737:chemistry*; D011810:Quinoxalines / Q000737:chemistry*; D012997:Solvents / Q000737:chemistry
chemical_list: D011108:Polymers; D011810:Quinoxalines; D012997:Solvents