Copper Powder and Pd(II) Salts Triggered One-Pot Aromatic Halide Homocoupling via a Radical Pathway

(sp²)C-(sp²)C bond formation is one of the most utilitarian techniques in target synthesis and material and pharmaceutical production. Biaryls usually emerge with the coupling of aryl halides or pseudohalides and require the prepreparation of an organometallic reagent, which results in low efficiency and atomic economy. The classic Ullmann reactions could be adopted to directly synthesize biaryls from aromatic halides. However, the requirement of extremely high temperatures limits the usage of the methodology in manufacturing. At the same time, the mechanism triggers a wide debate between classic redox and redox reactions involving radicals. In this work, a bimetallic system was demonstrated, referring to stoichiometric copper powder in the presence of a catalytic amount of Pd(OAc)₂, which contributed to delivering various symmetric/asymmetric (sp²)C-(sp²)C species. It has been proposed that the coupling process might be promoted via radicals produced by redox between Cu(0) and Pd(IV) species in the heating system. Increasing examples demonstrated the good tolerance of this method for aryl bromide among functional groups.