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(κ2-dppf)(κ1-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.
Highlights
The coupling of sp[3] centers is a frontier in modern crosscoupling catalysis research[1,2] and is closely aligned to the drive in areas such as pharmaceutical industry to target sp3-rich molecules.[3,4] Due to differences in oxidative addition behavior and the occurrence of deleterious side reactions such as β-hydride elimination, couplings of sp[3] centers are typically far more challenging than the palladiumcatalyzed sp2−sp[2] couplings that have become a routine in academia and industry
Mechanistic studies of the reactions of alkyl halides with nickel are relatively few, and many studies of sp[3] couplings invoke nickel(I) intermediates.[11−15] Tilley et al have reported the oxidative addition of methyl iodide to a nickel(I) species, forming a nickel(III) complex.[16,17]
Wang et al explored the reactions of a diphosphinodithio-ligated nickel(0) complex with alkyl halides, which were followed by the addition of NaBPh4; this led to alkylnickel(II) complexes, which degenerated to hydride species via β-hydride elimination.[19]
Summary
The coupling of sp[3] centers is a frontier in modern crosscoupling catalysis research[1,2] and is closely aligned to the drive in areas such as pharmaceutical industry to target sp3-rich molecules.[3,4] Due to differences in oxidative addition behavior and the occurrence (and often the dominance) of deleterious side reactions such as β-hydride elimination, couplings of sp[3] centers are typically far more challenging than the palladiumcatalyzed sp2−sp[2] couplings that have become a routine in academia and industry. The reactions between [Ni(COD)(dppf)] (1) and an excess of each of the three alkyl halides (2-X, 15 equiv) in benzene-d6 solution were monitored over time by 31P{1H} NMR spectroscopy (Scheme 2b) These reactions were pseudo-zero order in 1 (Figure 1a) and displayed a saturation behavior in 2-Br. Surprisingly, these reactions were pseudo-zero order in 1 (Figure 1a) and displayed a saturation behavior in 2-Br This is synthesis (dppf = 1,1′-bis(diphenylphosphino)ferrocene).[30] This is known to be the catalyst that is formed in reactions where [Ni(COD)2] and dppf are combined in situ.[30] Our key finding in this study is that the active Ni0 species must bear two dppf ligands, even though most pre-catalysts with bidentate phosphine ligands have a 1:1 ligand-to-nickel ratio
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