Palladium-Catalyzed Dimerization of Bis(2-biphenylyl)acetylene toward Sterically Hindered Acephenanthrylene.

The reaction of 2,2''-(1,2-ethynediyl)bis-1,1'-biphenyl under homogeneous (heterogeneous) catalysis with Pd/C/TMS-Cl results in a dimerization and the additional formation of five- and six-membered rings. The steric demand of the acetylene prevents a [2 + 2 + 2] cycloaddition, but paves the way toward an alternative reaction. By the formation of an unexpected planar acephenanthrylene, the system avoids steric stain. The synthetic scope and requirements are explored, and a reaction mechanism is proposed.