Reactions of the Ni(0) Compound Ni(PPh₃)₄ with Unactivated Alkyl Halides: Oxidative Addition Reactions Involving Radical Processes and Nickel(I) Intermediates

Abstract

Reactions of the nickel(0) compound NiL4 (L = PPh3) with alkyl halides RX involve initial inner-sphere halogen atom abstraction from the alkyl halides to form alkyl radicals R· and halonickel­(I) metalloradical species NiX­(PPh3)2,3. The radical pairs then undergo combination within the solvent cage to give the square planar nickel­(II) compounds NiRX­(PPh3)2. Radical intermediacy is demonstrated persuasively by observations that the relative rates vary in the orders tert-butyl > sec-butyl > n-butyl and RI > RBr > RCl, while density functional theory calculations indicate that the radical mechanism provides a lower energy pathway than do alternative, more conventional pathways. The product of the reaction of Ni­(PPh3)4 with methyl iodide, NiMeI­(PPh3)2, decomposes in solution to ethane and NiI­(PPh3)2,3, but when RX = EtI, n-BuI, sec-BuI, tert-BuI, the alkyl-nickel products undergo rapid β-hydrogen elimination to give the hydride NiHI­(PPh3)2 plus the corresponding alkene(s). Reactions also occur in which a portion of the alkyl radicals diffuses from the solvent cage and abstracts hydrogen from NiHI­(PPh3)2 to form alkanes RH and Ni­(I) species NiI­(PPh3)2. As a result, NiHI­(PPh3)2 is invariably a minor product while the major products are alkanes RH, alkenes R–H, and NiI­(PPh3)2. Hydride NiHI­(PPh3)2 is found to decompose to H2 and NiI­(PPh3)2 but is stable at low temperatures where it exhibits unusual NMR behavior because of exchange involving free PPh3 and the bis- and trisphosphine species, NiHI­(PPh3)2 and NiHI­(PPh3)3. Present in all of the reactions are paramagnetic, substitution-labile Ni­(I) metalloradical species. As a result, resonances of PPh3, ethylene, and the smaller iodoalkenes are generally broad and shifted because of exchange between free and coordinated ligands.

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

“…Kehoe et al used variable temperature NMR spectroscopy to elucidate the mechanism of oxidative addition of various alkyl halides to [Ni(PPh 3 ) 4 ]. 60 The 31 P NMR spectrum of [Ni(PPh 3 ) 4 ] at room temperature contains a single broad resonance (δ P = 24 ppm). When cooled to 200 K, the spectrum contains two resonances (δ P = 8, 25 ppm) which are attributed to free PPh 3 and [Ni(PPh 3 ) 3 ], respectively, confirming that the dominant species in the reaction mixture is [Ni(PPh 3 ) 3 ]; K eq for this dissociation is >10 6 mol L −1 .…”

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

“…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 Baird et al have studied the reactions of alkyl halides with [Ni 0 (PPh 3 ) 4 ], which occurred via halide abstraction and which generate mixtures of alkane and alkene products, with the latter arising from β-hydride elimination; 18 studies were typically carried out by combining Ni 0 and alkyl halide at low temperatures and carefully increasing the temperature and observing NMR spectral changes. Wang et al explored the reactions of a diphosphinodithio-ligated nickel(0) complex with alkyl halides, which were followed by the addition of NaBPh 4 ; this led to alkylnickel(II) complexes, which degenerated to hydride species via β-hydride elimination.…”

Steric effects determine the mechanisms of reactions between bis(N-heterocyclic carbene)-nickel(0) complexes and aryl halides

“…13 Baird and Budzelaar have shown that halide abstraction from alkyl halides by [Ni(PPh3)4] can account for experimental observations with this system. 14 Nickel-NHC complexes have been applied widely in catalysis. 15 Table S1).…”