Aryne-based synthesis of cyclobutadiene-containing oligoacenes and related extended biphenylene derivatives

A novel aryne derived from a π-extended biphenylene, 2,3-didehydrobenzo[b]biphenylene, has been developed. The participation of this new aryne building block in [4+2] and palladium-catalyzed [2+2+2] cycloaddition reactions has been effectively applied to the synthesis of a variety of polycyclic conjugated hydrocarbons (PCHs) with appealing structures which combine (aromatic) benzene and (antiaromatic) cyclobutadiene (CBD) rings. Among them, a family of unsubstituted (or barely substituted) CBD-oligoacenes has been accessed by iterative Diels-Alder reactions of the new aryne wit furans and/or isobenzofurans, followed by deoxygenative aromatization of the resulting epoxy-derivatives. The experimental and computational study of the newly synthesized PCHs suggests an important degree of electron delocalization along the polycyclic skeleton, more pronounced in the linearly fused derivatives. Interestingly, the computed ACID plots reveal clockwise current density vectors at the peripheral bonds, originating from the sigma contributions of the antiaromatic cyclobutadiene rings.