π-Extended 1,1'-Binaphthyl with Fused Pentagons

Area of Science:

• Organic Chemistry
• Materials Science
• Chirality Studies

Background:

• 1,1'-Biaryls are known axially chiral compounds.
• π-extension with nonbenzenoid rings in such systems is less explored.
• Helicene derivatives offer unique structural and electronic properties.

Purpose of the Study:

• To synthesize and characterize a novel π-extended 1,1'-binaphthyl incorporating a nor[5]helicene skeleton.
• To investigate the origin of chiroptical properties in this extended system.
• To determine the thermal stability and racemization kinetics of the synthesized compound.

Main Methods:

• Intermolecular SEAr-type reaction of 5,11-di(naphthalen-2-yl)tetracene.
• Resolution of enantiomers.
• Circular dichroism (CD) spectroscopy.
• Calculation of energy barriers for flipping and experimental determination of racemization kinetics.

Main Results:

• Successful synthesis of a π-extended 1,1'-binaphthyl with a nor[5]helicene core.
• Resolved enantiomers showed distinct CD spectra with a dissymmetry factor of |gCD| = 7.3 × 10⁻⁴.
• Chiroptical properties were attributed solely to the chiral axis, not the nor[5]helicene.
• Low flipping barrier for nor[5]helicene (ΔG‡calc = +7.2 kcal/mol) vs. chiral axis (ΔG‡calc = +20.2 kcal/mol, ΔG‡exp = +17.3 ± 0.52 kcal/mol).
• Rapid racemization observed even at 0 °C with a half-life of 6.63 ± 0.53 h.

Conclusions:

• The synthesized molecule represents a new class of axially chiral nanocarbons with extended π-systems.
• The study elucidates the dominant contribution of axial chirality over helical chirality in this specific structure.
• The rapid racemization highlights the dynamic nature and potential applications in chiroptical switching or sensing.