Mechanism of C-C coupling reactions of aromatic halides, promoted by Ni(COD)2 in the presence of 2,2'-bipyridine and PPh3, to give biaryls

Article abstract of DOI:10.1016/0022-328X(92)83232-7

Reactions of aryl halides, such as phenyl bromide, p-methylphenyl bromide, p-acetylphenyl bromide, p-methoxyphenyl bromide, phenyl chloride, and phenyl iodide, with Ni(COD)2 (COD = 1,5-cyclooctadiene) in the presence of 2,2'-bipyridine (BPY) or PPh3 as the auxiliary ligand give the corresponding biaryl as the coupling product at 50-70 deg C in DMF.Reaction of phenyl bromide with Ni(COD)2-BPY in benzene at room temperature gives NiBr(Ph)(BPY) (1).Heating a DMF solution of a mixture of 1 and phenyl bromide at 60 deg C gives biphenyl, while 1 dissolved in DMF liberates biphenyl at room temperature.Reaction of a large excess amount of phenyl bromide with the Ni(COD)2 * BPY system obeys second-order kinetics to the nickel complex.Concentration of phenyl bromide in the range 0.2-7.2 mol dm^-3 influences the reaction rate to little extent.Temperature dependence of the rate constants gives the kinetic parameters of ΔG^<*>₃₃₃ = 90 kJ mol^-1, ΔH^<*>₃₃₃ = 60 kJ mol^-1 and ΔS^<*>₃₃₃ = -90 J mol^-1 K^-1, respectively.The reactivity of phenyl halide increases in the order, PhCl less than PhBr less than PhI.Substitution of the para hydrogen of phenyl bromide with an electron-withdrawing group causes lowering of the rate constant, whereas introduction of an electron-donating group like OCH3 enhances the reaction.The Hammett plot of the reaction shows linar correlation between the ? value of the substituent and the logarithm of the second-order rate constants giving the ρ value of -2.4.Based on the results, the mechanism of the coupling reaction of PhBr is proposed to involve disproportionation of the intermediate NiBr(Ph)(BPY) as the rate determining step to give NiPh2(BPY) which is responsible for the reductive elimination of biphenyl under the reaction conditions.