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

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.