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-Ni 0 complexes and alkyl halides has been investigated using kinetic and mechanistic experiments and DFT calculations. The active species is [Ni(κ 2 -dppf)(κ 1 -dppf)], which undergoes a halide abstraction reaction with alkyl halides and rapidly captures the alkyl radical that is formed. The rates of the reactions of [Ni(COD)(dppf)] with alkyl halides and the yields of prototypical nickel-catalyzed Kumada cross-coupling reactions of alkyl halides are shown to be significantly improved by the addition of free dppf ligand.

Reactions of nickel(0) with organochlorides, organobromides, and organoiodides: mechanisms and structure/reactivity relationships

“…Radical reactions were ruled out as the addition of TEMPO had no impact on the rates of oxidative addition; however, subsequent studies have shown that TEMPO is not always a reliable radical probe in this system as it can react directly with nickel(0). 36 The activation parameters obtained were similar to those for the oxidative addition of aryl halides to [Pd(PPh 3 ) 4 ]. 37 Notably, the Hammett plot obtained by Bajo et al showed that for electron-rich substrates ρ = 1.2 whereas electron-poor substrates appeared to show an opposing trend, consistent with a change in rate-determining step.…”

“…Greaves et al explored the mechanism of the reaction of [Ni(COD)(dppf)] with (2-haloethyl)benzene substrates. 36 In contrast to the analogous reactions with aryl halides, these reactions proceeded via [Ni(dppf) 2 ], a species which exists in equilibrium (K eq = 6.8) with [Ni(COD)(dppf)] and excess dppf. The reactions of these alkyl halides with [Ni(COD)(dppf)], in the presence of dppf, were monitored by 31…”

“…The involvement of radicals in the reaction was confirmed as reaction rates dramatically increased when tert-butyl bromide or (2-bromopropyl)benzene were employed as substrates; these reactions were too fast to monitor by NMR spectroscopy. 36 There were two mechanistic possibilities: outer sphere electron transfer and halide abstraction. Outer sphere electron transfer was ruled out as a mechanistic possibility, as [Ni(dppe) 2 ], which should be capable of outer sphere electron transfer, did not undergo reaction with the (2-haloethyl)benzene substrates.…”

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

“…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.…”

“…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.…”