The Effect of Added Ligands on the Reactions of [Ni(COD)(dppf)] with Alkyl Halides: Halide Abstraction May Be Reversible

The reactions of
dppf-nickel(0) with alkyl halides proceed via
three-coordinate nickel(0) intermediates of the form [Ni­(dppf)­(L)].
The effects of the identity of the added ligand (L) on catalyst speciation
and the rates of reactions of [Ni­(COD)­(dppf)] with alkyl halides have
been investigated using kinetic experiments and density functional
theory calculations. A series of monodentate ligands have been investigated
in attempts to identify trends in reactivity. Sterically bulky and
electron-donating ligands are found to decrease the reaction rate.
It was found that (i) the halide abstraction step is not always irreversible
and the subsequent recombination of a nickel­(I) complex with an alkyl
halide can have a significant effect on the overall rate of the reaction
and (ii) some ligands lead to very stable [Ni­(dppf)­(L)2] species. The yields of prototypical (dppf)­nickel-catalyzed Kumada
cross-coupling reactions of alkyl halides are significantly improved
by the addition of free ligands, which provides another important
variable to consider when optimizing nickel-catalyzed reactions of
alkyl halides.