Pyracylenes: Facile Synthetic Access and Continuous Face-To-Face Antiaromatic π‑Stacking

Area of Science:

• Organic Chemistry
• Materials Science
• Supramolecular Chemistry

Background:

• Pyracylene, a nonalternant hydrocarbon, is synthetically inaccessible due to its inherent antiaromaticity.
• Polycyclic antiaromatic hydrocarbons are of interest for organic materials and π-stacked assemblies.

Purpose of the Study:

• To develop a facile synthetic route to pyracylenes.
• To investigate the structural, electronic, and solid-state properties of synthesized pyracylenes.

Main Methods:

• Palladium-catalyzed ring contraction of diborinic acids for pyracylene synthesis.
• Derivatization via bromination and palladium-catalyzed cross-coupling.
• Characterization using UV-vis spectroscopy, cyclic voltammetry, DFT calculations, X-ray crystallography, and time-resolved microwave conductivity.

Main Results:

• A facile synthesis of pyracylenes was achieved.
• Three pyracylenes formed unprecedented continuous, face-to-face, antiaromatic π-stacks in the solid state.
• NICS calculations indicated reduced antiaromaticity in the stacked structures, and conductivity measurements confirmed solid-state conductivity.

Conclusions:

• The study presents a viable synthetic strategy for accessing pyracylenes.
• The discovered solid-state packing in pyracylenes offers a method to mitigate antiaromaticity.
• These findings open avenues for pyracylenes in organic materials applications.