(Chemical Equation Presented) Intramolecular cyclization in a series of thiophene-based dibromohelicenes (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 circulene devoid of an atom linkage, is referred to as quasi-circulene. Vacuum pyrolysis of 4 gives insoluble, unidentified products, while 5 and 6 yield the corresponding quasi-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-circulenes for all three helicenes, though only 4 gives a good isolated yield. Tributyltin hydride-mediated radical cyclization of 4-6 provides quasi-circulenes in excellent yields, and it is practically insensitive to the helix structure. Experimental and calculated UV-vis absorption spectra for quasi-circulenes and circulenes are reported. The results suggest that the lack of atom linkage in quasi-circulene does not significantly affect properties and conformation, compared to those for the corresponding circulenes.
ASJC Scopus subject areas
- Organic Chemistry