Nickel-Catalyzed Synthesis of Strained Phenacenes

Abstract

This study reports a series of nickel-catalyzed alkyne cycloadditions on different phenylene compounds (angular [3]phenylene 1a and [4]phenylene 1b, C 3-symmetric [4]phenylene 1c). In all cases, products of single addition (2a-c) are isolated in moderate yield. Surprisingly, the second, respectively third addition reactions afford systematically the most sterically crowded and strained compounds (phenacenes). All isolated products, six of whom were characterized by X-ray crystallography, exhibit pronounced distortion owing to the 4,5-diphenylphenanthrene substructures. Interestingly, performing the cycloaddition on compounds 2a or 2c under the same reaction conditions yields less strained products (helicenes, with phenyl groups pointing to opposite sides of the aromatic core) instead of hindered 3a or 3c, respectively.