Helically annelated and cross-conjugated oligothiophenes: Asymmetric synthesis, resolution, and characterization of a carbon-sulfur [7]helicene

Andrzej Rajca, Makoto Miyasaka, Maren Pink, Hua Wang, Suchada Rajca

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144 Scopus citations

Abstract

The synthesis and characterization of a novel oligothiophene, in which the thiophene rings are annelated into a [7]helicene with cross-conjugated π-system, are described. Such [7]helicenes may be viewed as fragments of the unprecedented carbon-sulfur (C2S)n helix, possessing sulfur-rich molecular periphery. Racemic synthesis of [7]helicene is based upon iterative alternation of two steps: C-C bond homocouplings between the β-positions of thiophenes and annelation between the α-positions of thiophenes. Asymmetric synthesis is carried out using (-)-sparteine-mediated annelation of the axially chiral bis(aryllithium) with electrophilic sulfur equivalent. Alternatively, enantiomers of the [7]helicene are obtained via resolution using menthol-based chiral siloxanes. Racemic [7]helicene and four other macrocyclic products of the annelation are characterized by X-ray crystallography. One of the solvent polymorphs of the [7]helicene possesses π-stacked columns of opposite enantiomers and multiple short intermolecular contacts, including both homochiral and heterochiral short S⋯S contacts, suggesting an effective intermolecular electronic coupling in two-dimensions. The [7]helicene is configurationally stable at room temperature and racemizes at 199 °C with a half-time of about 11 h. Selected physicochemical studies (UV-vis absorption, CD, optical rotation, and cyclic voltammetry) of the [7]helicene are described.

Original languageEnglish (US)
Pages (from-to)15211-15222
Number of pages12
JournalJournal of the American Chemical Society
Volume126
Issue number46
DOIs
StatePublished - Nov 24 2004

ASJC Scopus subject areas

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

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