π-Extended 1,1′-Binaphthyl with Fused Pentagons
Xiaoying Zhang, Huimin Ma, Qi Yin, Fenghua Bai, Yoshifumi Hashikawa, Chaolumen Chaolumen
Abstract
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 S E Ar-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 | g CD | = 7.3 × 10 –4 at 669 nm. Due to the low flipping barrier of the nor[5]helicene (Δ G ⧧ calc = +7.2 kcal/mol at 298 K), the chiroptical character of the π-extended 1,1′-binaphthyl originates solely from the chiral axis (Δ G ⧧ calc = +20.2 kcal/mol), as supported experimentally (Δ G ⧧ 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.