Marked effect of aromatic solvent on unfolding rate of helical ethynylhelicene oligomer

Hiroki Sugiura, Yasuko Nigorikawa, Yuto Saiki, Keiichi Nakamura, Masahiko Yamaguchi

Research output: Contribution to journalArticlepeer-review

121 Citations (Scopus)

Abstract

Optically active acyclic ethynylhelicene oligomers were synthesized in high yields by a two-directional method involving Sonogashira coupling and deprotection. Their CD spectra in chloroform exhibited large differences between the oligomers with less than seven helicenes and their higher homologues, which indicated the formation of helical structures for the latter and random coil structures for the former. The helical heptamer gradually unfolded to a random coil structure in chloroform at room temperature. The unfolding rate was examined by CD in several aromatic solvents as well, and the rate constant k was found to be highly dependent on the type of aromatic substituent: k differed by seven orders of magnitude between iodobenzene and trifluoromethylbenzene. Several features of the rates are notable: The reaction rates in halobenzenes were in the order of iodobenzene > bromobenzene > chlorobenzene > benzene > fluorobenzene > m-difluorobenzene, those in alkylbenzenes were styrene > phenylacetylene > ethylbenzene > toluene > benzene, and those in heteroatom-substituted arenes were thioanisole > benzonitrile > anisole > ethyl benzoate > benzene > trifluoromethylbenzene. The log lvalues exhibited good correlation with the absolute hardness, η, of the arenes, and higher unfolding rates were observed in the soft arenes. Vapor pressure osmometry studies indicated that the helical structure of the heptamer is dimeric in benzene, fluorobenzene, and trifluoromethylbenzene, while the random coil structure of the heptamer is monomeric in chloroform and toluene. When a chloroform solution of the random coil structure was concentrated to a small volume, the helical structure could be regenerated.

Original languageEnglish
Pages (from-to)14858-14864
Number of pages7
JournalJournal of the American Chemical Society
Volume126
Issue number45
DOIs
Publication statusPublished - 2004 Nov 17

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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