Singly and doubly twisted [36]annulenes: synthesis and calculations

Area of Science:

• Organic Chemistry
• Supramolecular Chemistry
• Computational Chemistry

Background:

• Cyclophanes are macrocyclic organic compounds with unique structural and electronic properties.
• Annulenes, cyclic hydrocarbons with alternating single and double bonds, are fundamental in aromaticity studies.
• Investigating twisted annulene peripheries in cyclophanes can reveal novel topological and electronic behaviors.

Purpose of the Study:

• To synthesize a novel cyclophane featuring a [36]annulene periphery.
• To characterize the conformational diversity of the synthesized cyclophane.
• To investigate the aromaticity and electronic properties of different topological conformations.

Main Methods:

• Four-step synthesis sequence culminating in McMurry cyclization.
• X-ray structure analysis of crystallized samples to determine conformations.
• Density Functional Theory (DFT) calculations and Nuclear Magnetic Resonance (NMR) spectroscopy to probe electronic properties.

Main Results:

• Successful synthesis of a cyclophane with a [36]annulene periphery comprising four anthrylene and two phenylene units.
• Identification of four distinct conformations in the solid state via X-ray crystallography.
• Two conformations displayed double-twist (Hückel topology), while two exhibited single-twist (Möbius topology).
• DFT calculations predicted a triple-twist Möbius conformation.
• NMR spectroscopy and DFT calculations did not support aromaticity in Möbius structures or antiaromaticity in Hückel structures.

Conclusions:

• The synthesized cyclophane exhibits significant conformational flexibility.
• The study challenges conventional understanding of aromaticity in twisted polyannulene systems.
• Topological effects in macrocycles do not necessarily correlate with predicted aromatic or antiaromatic character based on simple Hückel or Möbius rules.