Helically annelated and cross-conjugated oligothiophenes: asymmetric synthesis, resolution, and characterization of a carbon-sulfur [7]helicene.
J Am Chem Soc
; 126(46): 15211-22, 2004 Nov 24.
Article
em En
| MEDLINE
| ID: mdl-15548018
ABSTRACT
The synthesis and characterization of a novel oligothiophene, in which the thiophene rings are annelated into a [7]helicene with cross-conjugated pi-system, are described. Such [7]helicenes may be viewed as fragments of the unprecedented carbon-sulfur (C(2)S)(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 beta-positions of thiophenes and annelation between the alpha-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 pi-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 degrees 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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Coleções:
01-internacional
Base de dados:
MEDLINE
Idioma:
En
Revista:
J Am Chem Soc
Ano de publicação:
2004
Tipo de documento:
Article
País de afiliação:
Estados Unidos