Nickel Bromide Catalyzed Ligand-Free and Activator-less Suzuki Coupling Reactions

Readily available and inexpensive bromide salt based on earth-abundant nickel has been used to catalyze the Suzuki coupling of a variety of aryl halides (ca. 37 examples) in high yields under ambient conditions with good functional group tolerance. The reactivity of various aryl halides follows the corresponding C-X (X=I, Br and Cl) bond strength in the order Ar-I>Ar-Br>Ar-Cl. EPR analysis, mercury-drop experiments and reactions with radical scavenger such as TEMPO point to the involvement of well-defined molecular species that operate via a Ni(0)/Ni(II) catalytic cycle where oxidative addition of the aryl halide to Ni(0) is likely to be the rate-determining-step (RDS). For the first time, valuable new polycyclic aromatic hydrocarbons (PAHs) with exciting photophysical properties have been synthesized (ca. 23 examples) via a Ni(II) catalyzed multi-fold Suzuki coupling.