MECHANISM OF OXIDATIVE ADDITION - REACTION OF NICKEL(0) COMPLEXES WITH AROMATIC HALIDES

The oxidative addition of aryl halides to triethylphosphinenickel(0) complexes affords rrara-arylnickel(II) halides A, together with paramagnetic nickel(I) halides B as side products. The relative yields, A/B, are strongly dependent on the halide (I < Br < Cl), as well as the nuclear substituents and the solvent polarity. The second-order rate constants (log kobsd) for various meta-and para-substituted iodobenzenes are linearly related to those of the corresponding bromo-and chloroarenes. However, the absence of a direct correlation between the reactivities of various aryl halides and the distribution between nickel(I,II) products demands that the rate-limiting activation process precedes, and is separate from, the product-forming step(s). Evidence for the paramagnetic ion pair [Ni(I)ArX-.] as the common intermediate which is partitioned between A and B is presented, and discussed in the light of electrochemical measurements of the one-electron oxidation of nickel(0) complexes and the reduction of aryl halides. According to the mechanistic Scheme III, the rate-limiting electron transfer from the nickel(0) donor to the aryl halide acceptor produces an ion pair which is subject to two competitive modes of decay, viz., (1) collapse to oxidative adduct and (2) fragmentation of the ArX-moiety, followed by diffusion of aryl radicals (observed by ESR spectroscopy) out of the solvent cage. The importance of electrostatic effects in the collapse of the ion pair to oxidative adduct is shown by the high sensitivity of A to the presence of charged nuclear substituents such as Me3N+-and -O2C-groups. The aryl-halo-gen bond strength is the most important factor in the spontaneous fragmentation of the anion radical of the aromatic halide as determined from lifetimes obtained from electrochemical studies. Finally, the coordinatively unsaturated Ni(PEt3)3 is the kinetically active species and responsible for the inverse phosphine dependence on the rate of reaction. It is included in the pre-equilibrium formation of a π complex [(Et3P)3NiArX] as a possible precursor to electron transfer. Scheme III for oxidative addition is discussed in relation to other mechanisms involving either a concerted or a radical-chain process. © 1979, American Chemical Society. All rights reserved.