Kinetics and Mechanism of PPh₃/Ni-Catalyzed, Zn-Mediated, Aryl Chloride Homocoupling: Antagonistic Effects of ZnCl₂/Cl^–

The Ni/PPh₃-catalyzed homocoupling of aryl chlorides in DMF using Zn as the stochiometric reducing agent is one of a general class of Ni-catalyzed processes, where the mechanism has been a matter of long-standing debate. This study re-evaluates prior conclusions and insights. NMR spectroscopy is used to identify [(PPh₃)₂Ni^II(Ar)Cl] as a key intermediate and to explore the indirect roles of using Zn as the reductant. The [ZnCl₂] coproduct is responsible for several features, including a sequential transmetalation pathway involving [ArZnCl]. [ZnCl₂] also abstracts halide from [(PPh₃)₂NiCl₂] to generate [Ni^IICl(DMF)₅]⁺[ZnCl₃(DMF)]^-, and in doing so, affects the Ni^II + Ni⁰ ↔ 2 Ni^I speciation. [ZnCl₂] thus acts as an accelerator and inhibitor, resulting in mildly sigmoidal reaction profiles. When the [ZnCl₂] concentration becomes too high or the phosphine ligand concentration too low, catalysis stalls. Turnover is restored by the addition of further phosphine ligand, or chloride ion. In the presence of an exogenous chloride ion, turnover is rapid, again proceeding via [(PPh₃)₂Ni^II(Ar)Cl] but via dinuclear metathesis. The generation of [ZnCl₃(DMF)]^- results in mutually antagonistic effects between [ZnCl₂] and [Cl]^- such that turnover proceeds via one mechanism or the other, depending on which species is in excess. The intermediacy of [ArZnCl] suggests a solution to the long-standing anomaly that many other reductants were found to be much less effective than Zn in inducing turnover of Ni/PPh₃ catalyzed aryl chloride homocoupling in DMF. The use of DMAc as a solvent in place of DMF inhibits stalling through the steric inhibition of mixed metalate generation.