Synthesis of bifluorenes via cobalt halide radical coupling

Reaction of in situ generated lithium fluorenide salts with cobalt chloride results in formation of the corresponding bifluorenes. The scope of fluorenes that can be coupled include those containing substituents amenable to polymerization for application to poly(bifluorene) synthesis and alkyl substituted fluorenes for use in the preparation of bimetallic transition metal complexes. Based on experiments using a radical trap, the reaction mechanism involves reduction of cobalt and subsequent fluorenyl radical coupling. Overall, this study demonstrates a simple and effective method for preparation of hindered tertiary and quaternary C–C bonds using a readily available and inexpensive coupling agent.