Twisted or helical shape 1,2-position-connected terazulene and quaterazulene were synthesized. Variable temperature NMR measurements and theoretical calculations of the quaterazulene suggest that the syn-type structure with terminal azulene overlap is more stable. Planar or bent shape fused terazulenes were also synthesized. Nucleus-independent chemical shift (NICS) calculations carried out for the central seven-membered ring of the bent shape fused terazulene showed a positive value, suggesting anti-aromatic properties.

Abstract

The construction of 1,2-position-connected azulene oligomers was achieved. In the crystal packing structure of the terazulene, two molecules of (R _a)- and (S _a)-configurations formed a pair. Variable temperature NMR measurements and theoretical calculations of the quaterazulene suggest that the helical and syn-type structure with terminal azulene overlap is more stable. Two kinds of fused terazulenes (1,2′′-closed and 1,8′′-closed) were also synthesized by intramolecular Pd-catalyzed C−H/C−Br arylation of the terazulene moieties. X-ray structure analysis of 1,2′′-closed terazulene revealed a planar structure, while an analysis of 1,8′′-closed terazulene performed on a C₆₀ co-crystal revealed a curved structure forming a 1 : 1 complex covering the co-crystal. Nucleus-independent chemical shift (NICS) calculations carried out for the central seven-membered ring of 1,8′′-closed terazulene showed a positive value, suggesting anti-aromatic properties.