Unexpected Nickel Complex Speciation Unlocks Alternative Pathways for the Reactions of Alkyl Halides with dppf-Nickel(0)

Abstract

The mechanism of the reactions between dppf-Ni0 complexes and alkyl halides has been investigated using kinetic and mechanistic experiments and DFT calculations. The active species is [Ni(dppf)2], which undergoes a halide abstraction reaction with alkyl halides and rapidly captures the alkyl radical that is formed. The yields in prototypical nickel-catalysed Kumada cross-coupling reactions are shown to be improved by the addition of free phosphine ligands<br>

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

“…DFT calculations carried out during our earlier study 18 supported the proposal that the reaction occurs via formation of a three-coordinate nickel(0) complex, halide abstraction to form nickel(I) plus a radical, and recombination of these species to form a nickel(II) complex. The nickel(II) complex is then proposed to undergo β-hydride elimination followed by comproportionation to form [NiX(dppf)] (3), styrene, and hydrogen.…”

“…A series of reactions were carried out with different concentrations of triphenylphosphine, confirming that the reaction is first order in added triphenylphosphine (Figure 1(a) and (b)); our previous work noted that the reaction was first order in dppf when this was the added ligand. 18 The use of diphenylphosphinoferrocene (FcPPh2) as an additive led to a higher rate of reaction than the corresponding experiment with dppf. This can be rationalized by considering the requirement for (bidentate) dppf to dissociate one phosphine atom from the nickel center to enable the reaction to occur; the binding of a second FcPPh2 ligand does not benefit from the chelate effect.…”

“…Our previous study 18 established that the rate of reaction between [Ni(COD)(dppf)] (1) and alkyl halides was significantly increased by the addition of free dppf ligand, as this shifted the equilibrium between 1 and [Ni(dppf)2] towards the latter species. For this study, a selection of monodentate group 15 ligands was assembled with a diverse range of steric and electronic properties, and where the Lewis basic atom was nitrogen, phosphorus, arsenic, or antimony.…”

“…All experiments were pseudo-first order in 1, and the 31 P NMR spectra confirmed that dppf remained bound to the nickel center throughout, with no free dppf ligand detected (δP = -17 ppm). Data were collected at one or two of three temperatures (263 K, 273 K, or 293 K) depending on how fast the reaction proceeded; the results for reactions with fifteen monodentate ligands, along with previous data for the reaction with added dppf, 18 are recorded in Scheme 2. Pseudo-first order constants are listed in order of largest to smallest.…”

“…We recently published a detailed study of the reactions of the model complex [Ni(COD)(dppf)] 17 (1) with alkyl halides. 18 The experimental and computational evidence that was gathered supported a mechanism in which [Ni(COD)(dppf)] was in equilibrium with [Ni(dppf)2] (2) (with the additional dppf being a trace impurity in 1), and that [Ni(κ 2 -dppf)(κ 1 -dppf)] performed a halide abstraction step to produce [Ni(X)(κ 2 -dppf)(κ 1 -dppf)] plus an alkyl radical (Scheme 1(a)); subsequent dppf dissociation and the recombination of the alkyl radical and nickel(I) complex yielded the formal oxidative addition product [Ni(X)(R)(dppf)], which underwent rapid β-hydride elimination. The final products were [Ni(X)(dppf)] (3) and alkene, with no alkane product observed.…”