Intramolecular cyclization of thiophene-based [7]helicenes to quasi-[8]circulenes.
J Org Chem
; 74(23): 9105-11, 2009 Dec 04.
Article
em En
| MEDLINE
| ID: mdl-19886651
ABSTRACT
Intramolecular cyclization in a series of thiophene-based dibromo[7]helicenes (4-6) with different helix structures is investigated by vacuum pyrolysis, tin- and palladium-mediated C-C bond forming reactions. The product with the cyclic structure of the annelated aromatic rings, which resembles [8]circulene devoid of an atom linkage, is referred to as quasi-[8]circulene. Vacuum pyrolysis of 4 gives insoluble, unidentified products, while 5 and 6 yield the corresponding quasi-[8]circulenes under similar conditions. Thermogravimetry (TG) and differential scanning calorimetry (DSC) analyses for 4 indicate complex reaction pathways, while those for 5 and 6 show a single process corresponding to a loss of 1 equiv of Br(2) at about 330 degrees C. Pd-mediated reductive cyclization provides quasi-[8]circulenes for all three [7]helicenes, though only 4 gives a good isolated yield. Tributyltin hydride-mediated radical cyclization of 4-6 provides quasi-[8]circulenes in excellent yields, and it is practically insensitive to the helix structure. Experimental and calculated UV-vis absorption spectra for quasi-[8]circulenes and [8]circulenes are reported. The results suggest that the lack of atom linkage in quasi-[8]circulene does not significantly affect properties and conformation, compared to those for the corresponding [8]circulenes.
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1
Coleções:
01-internacional
Base de dados:
MEDLINE
Idioma:
En
Revista:
J Org Chem
Ano de publicação:
2009
Tipo de documento:
Article
País de afiliação:
Estados Unidos