Electrophilic C-H Borylation of Aza[5]helicenes Leading to Bowl-Shaped Quasi-[7]Circulenes with Switchable Dynamics

The intrinsic relationship between helicenes and circulenes is of fundamental interest and importance in molecular engineering. Herein, electrophilic borylation of phenanthroline-derived aza[5]helicenes is presented, resulting in the incorporation of a boryl unit into two termini of helicenes to afford quasi-[7]circulenes. Their bowl-shaped structures were determined by X-ray diffraction. UV-vis absorption and fluorescence spectroscopy, as well as electrochemical measurements and DFT calculations, gave insight into their electronic properties. Variable-temperature NMR studies and DFT calculations revealed bowl-to-bowl inversion at room temperature and bowl-to-helix equilibria at elevated temperature, highlighting the important role of B <- N bond strength in tuning their dynamic properties.

Automated summary

The intrinsic relationship between helicenes and circulenes is of fundamental interest and importance in molecular engineering. In this study, electrophilic borylation of phenanthroline-derived aza[5]helicenes was performed, resulting in the incorporation of a boryl unit into two termini of helicenes to form quasi-[7]circulenes. Their bowl-shaped structures were determined by X-ray diffraction. UV-vis absorption and fluorescence spectroscopy, electrochemical measurements, and DFT calculations provided insights into their electronic properties. Variable-temperature NMR studies and DFT calculations revealed the bowl-to-bowl inversion at room temperature and bowl-to-helix equilibria at elevated temperature, highlighting the important role of B <- N bond strength in tuning their dynamic properties.