Synthesis, Structure, and Reactivity of a Cylinder-Shaped Cyclo[12]orthophenylene[6]ethynylene: Toward the Synthesis of Zigzag Carbon Nanobelts

Benzannulated cyclacenes (BCs) have
been proposed as stable zigzag
carbon nanobelts. Density functional theory (DFT) calculations revealed
a closed-shell ground state for [12]­BC, whereas an open-shell ground
state was suggested for [12]­cyclacene. The calculated strain energy
and frontier molecular orbital energies of [12]­BC also implied high
stability. An unstrained macrocycle 1, consisting of
orthophenylene and ethynylene moieties, was designed as a potential
precursor for [12]­BC and synthesized by sequential Suzuki–Miyaura
cross-coupling of diphenylacetylene derivatives. While the conversion
of 1 into [12]­BC is still under investigation, an unexpected
rearrangement of the triene moieties in 1, affording
a tribenzo­[f,k,m]­tetraphene structure, was discovered during the screening of reaction
conditions. An attempt was made to rationalize this result by proposing
a plausible reaction mechanism that proceeds via intermediates containing
cyclobutadiene or Dewar benzene moieties. The proposed mechanism is
partially supported by DFT calculations.