Synthesis and Properties of Twisted and Helical Azulene Oligomers and Azulene‐Based Polycyclic Hydrocarbons

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 C(60) 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.