Intramolecular Cyclization of Thiophene-Based [7]Helicenes to Quasi-[8]Circulenes

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.