Helicenes are polycyclic aromatic hydrocarbons (PAHs) consisting of ortho-fused aromatic rings which form a helical structure. Circulenes are macrocycles formed from fused aromatic rings. Aromatic compounds can be functionalized via the direct borylation of C–H bonds, but the regioselectivity can be difficult to control in the presence of multiple similar C(sp²)–H bonds.

Xiaolei Zhang, Agnieszka Nowak-Król, Todd B. Marder, University of Würzburg, Germany, Simon J. Garden, Universidade Federal do Rio de Janeiro, Brazil, and colleagues have performed electrophilic borylations of phenanthroline-derived aza[5]helicenes to give quasi-[7]circulenes (pictured). The team synthesized different benzo[4,5]imidazo[1,2-a][1,10]phenathrolines, i.e., aza[5]helicenes, from 2-chlorophenanthroline and anilines via aminations followed by cyclizations. The aza[5]helicenes were then reacted with BBr₃ in the presence of i-Pr₂NEt to obtain BBr₂-functionalized intermediates, which were treated, for example, with NEt₃ and pinacol to obtain the desired borylated derivatives.

The products were obtained in yields up to 87 %. The team found that several of the synthesized compounds have bowl-shaped structures with a central seven-membered ring containing an N→B bond (example pictured above on the right).  The “bowls” have depths of 1.62–1.73 Å. The bowl-shaped quasi-[7]circulenes showed a temperature-dependent dynamic behavior. At room temperature, the team observed bowl-to-bowl inversion. At elevated temperatures, the bowl shape is in a dynamic equilibrium with its open, helical form. According to the researchers, the developed boryl-insertion strategy could be a useful alternative to direct C–C bond formations for the synthesis of quasi-circulenes.

• Electrophilic C–H Borylation of Aza[5]helicenes Leading to Bowl-Shaped Quasi-[7]Circulenes with Switchable Dynamics,
  Xiaolei Zhang, Florian Rauch, Jan Niedens, Ramon B. da Silva, Alexandra Friedrich, Agnieszka Nowak-Król, Simon J. Garden, Todd B. Marder,
  J. Am. Chem. Soc. 2022.
  https://doi.org/10.1021/jacs.2c10865