π‑Extended 1,1′-Binaphthyl with Fused Pentagons

1,1′-Biaryls are a class of axially chiral compounds,
while
π-extension with nonbenzenoid rings has been less known. Herein,
we report a π-extended 1,1′-binaphthyl containing a nor[5]helicene
skeleton. The synthesis was performed via an intermolecular SEAr-type reaction of 5,11-di(naphthalen-2-yl)tetracene. The
resolved enantiomers exhibited mirror images in their circular dichroism
spectra, with a dissymmetry factor of |gCD| = 7.3 × 10–4 at 669 nm. Due to the low flipping
barrier of the nor[5]helicene (ΔG⧧calc = +7.2 kcal/mol at 298 K), the chiroptical character
of the π-extended 1,1′-binaphthyl originates solely from
the chiral axis (ΔG⧧calc = +20.2 kcal/mol), as supported experimentally (ΔG⧧exp = +17.3 ± 0.52 kcal/mol).
This axially chiral nanocarbon was found to be racemized even at 0
°C, with a half-life of 6.63 ± 0.53 h.