Streamlined Synthesis of Polycyclic Conjugated Hydrocarbons Containing Cyclobutadienoids via C–H Activated Annulation and Aromatization

The
juxtaposition of fused cyclobutadienoid (CBD) with benzenoid
creates intriguing alternating antiaromatic and aromatic conjugation.
Synthetic accessibility of such molecules, however, has been challenging
and limited in scope. We report a modular and streamlined synthetic
strategy to access a large variety of polycyclic conjugated hydrocarbons
with fused CBD. Synthesis was achieved through efficient palladium-catalyzed
C–H activated annulation between abundant aryl bromides and
oxanorbornenes, followed by aromatization under acidic conditions.
The influence of four-membered ring was examined using spectroscopy,
crystallography, and computation. This strategy will facilitate exploration
on the chemical, structural, and electronic properties of such conjugated
systems containing CBD.