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.