Abstract
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.
Highlights
The importance of molecules that contain a large number of sp[3] centers in industries such as pharmaceuticals and agrochemicals is driving much of the reaction discovery and development in the field of nickel catalysis.[1,2] despite recent advances in our mechanistic understanding of nickel catalysis,[3,4] gaps in this understanding still remain
We have recently focused our attention on the reactions of alkyl halides with nickel(0), with the aim of developing a better understanding of these reactions and thereby underpinning future reaction discovery, development, and understanding
Our previous study[20] established that the rate of reaction between [Ni(COD)(dppf)] (1) and alkyl halides was significantly increased by the addition of free dppf ligands, as this shifted the equilibrium between 1 and
Summary
The importance of molecules that contain a large number of sp[3] centers in industries such as pharmaceuticals and agrochemicals is driving much of the reaction discovery and development in the field of nickel catalysis.[1,2] despite recent advances in our mechanistic understanding of nickel catalysis,[3,4] gaps in this understanding still remain. This is apparent for the case of the reactions of nickel(0) complexes with alkyl halides, which are quite different from the reactions of nickel(0) complexes with sp[2] organohalides.[5] The reactions of alkyl halides have a greater propensity to involve radical intermediates, and deleterious βhydride elimination presents further challenges. The size of the NHC ligand in [Ni(NHC)2] complexes determines whether [Ni(Ar)X(NHC)2] or [NiX-
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