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

High Resolution Image Download MS PowerPoint Slide The Ni/PPh 3 -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 (PPh 3 ) 2 Ni II (Ar)Cl as a key intermediate and to explore the indirect roles of using Zn as the reductant. The ZnCl 2 coproduct is responsible for several features, including a sequential transmetalation pathway involving ArZnCl. ZnCl 2 also abstracts halide from (PPh 3 ) 2 NiCl 2 to generate Ni II Cl(DMF) 5 + ZnCl 3 (DMF) −, and in doing so, affects the Ni II + Ni 0 ↔ 2 Ni I speciation. ZnCl 2 thus acts as an accelerator and inhibitor, resulting in mildly sigmoidal reaction profiles. When the ZnCl 2 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 (PPh 3 ) 2 Ni II (Ar)Cl but via dinuclear metathesis. The generation of ZnCl 3 (DMF) − results in mutually antagonistic effects between ZnCl 2 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/PPh 3 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.