Intramolecular cyclization of thiophene-based [7]helicenes to quasi-[8]circulenes

Andrzej Rajca, Makoto Miyasaka, Shuzhang Xiao, Przemysław J. Boratyński, Maren Pink, Suchada Rajca

Research output: Contribution to journalArticle

26 Scopus citations

Abstract

(Chemical Equation Presented) 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 Br2 at about 330°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.

Original languageEnglish (US)
Pages (from-to)9105-9111
Number of pages7
JournalJournal of Organic Chemistry
Volume74
Issue number23
DOIs
StatePublished - Dec 4 2009

ASJC Scopus subject areas

  • Organic Chemistry

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