TY - JOUR
T1 - Helically annelated and cross-conjugated oligothiophenes
T2 - Asymmetric synthesis, resolution, and characterization of a carbon-sulfur [7]helicene
AU - Rajca, Andrzej
AU - Miyasaka, Makoto
AU - Pink, Maren
AU - Wang, Hua
AU - Rajca, Suchada
PY - 2004/11/24
Y1 - 2004/11/24
N2 - 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.
AB - 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.
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U2 - 10.1021/ja0462530
DO - 10.1021/ja0462530
M3 - Article
C2 - 15548018
AN - SCOPUS:9344251690
VL - 126
SP - 15211
EP - 15222
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
SN - 0002-7863
IS - 46
ER -