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

Abstract

The reactions of dppf-nickel(0) with alkyl halides proceed via three-coordinate nickel(0) intermediates of the form [Ni(dppf)(L)]. The effect of the identity of the added ligand (L) on catalyst speciation and the rates of reactions of [Ni(COD)(dppf)] with alkyl halides has been investigated using kinetic experiments and DFT 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<br>[Ni(dppf)(L) 2 ] species. The yields of prototypical (dppf)nickel-catalysed Kumada cross-coupling reactions of alkyl halides are significantly improved by the addition of free ligand, which provides another important variable to consider when optimizing nickel-catalysed reactions of alkyl halides.