Helically Annelated and Cross-Conjugated Oligothiophenes:  Asymmetric Synthesis, Resolution, and Characterization of a Carbon−Sulfur [7]Helicene

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.

Crystallographic Information File

• ja0462530si20040624_110836.cif (15 KB)
• ja0462530si20040624_110853.cif (25 KB)
• ja0462530si20040624_110913.cif (22 KB)
• ja0462530si20040624_110928.cif (20 KB)
• ja0462530si20040624_110946.cif (35 KB)

PDF

• ja0462530si20040827_064336.pdf (279 KB)

Electronic Supporting Information files are available without a subscription to ACS Web Editions. All files are copyrighted by the American Chemical Society. Files may be downloaded for personal use only. Users are not permitted to reproduce, republish, redistribute, or resell any Supporting Information, either in whole or in part, in either machine-readable form or any other form without permission from the American Chemical Society. For permission to reproduce this material, requesters must process their own requests via the Rightslink permission system. Information about how to use the Rightslink permission system can be found at http://pubs.acs.org/page/copyright/permissions.html. Please read only the part about Rightslink.