F430 MODEL CHEMISTRY - MECHANISTIC INVESTIGATION OF THE REDUCTION, COUPLING, AND DEHYDROHALOGENATION OF ALKYL-HALIDES BY THE NICKEL(I) OCTAETHYLISOBACTERIOCHLORIN ANION

The reactions of Ni(I)(OEiBC)- with alkyl halides and p-toluenesulfonates and with hydrogen atom and proton donors were investigated in a wide range of solvents, The ease of reduction of Ni(II)(OEiBC) by sodium amalgam and the stability of the Ni(I)(OEiBC) generated was greater in more polar solvents. Ni(I)(OEiBC)- was not obtained in solvents with weakly acidic protons like N-methylformamide. Ni(I)(OEiBC) is oxidized by weak acids like water, alcohols, and thiols to afford H-2 in a reaction with an apparent stoichiometry of 2:1 Ni(l):H2. Reaction with CH3X, X = I and OTs, afforded neutral Ni(II)(OEiBC) in quantitative yield and CH4 and C2H6. Trace quantities Of C2H4 were also observed. The mass balance of the reaction was essentially complete in THF, but the yield of volatile products was less than expected in other solvents. Reaction with C2H5X afforded hydrocarbons C2H6, C2H4, and C4H10 and trace quantities Of C4H8. The relative yields of products in both reactions was strongly dependent on the solvent. Coupling (C2H6 from CH3X and C4H10 from C2H5X) and elimination products (C2H4 from C2H5X) lincreased in more polar solvents. More coupling was observed when X = I than OTs. Isotopic labeling experiments established that the source of the fourth hydrogen in CH4 is solvent and a pool of protons that is probably residual water. Simultaneous addition of alkyl halides and weak acids led to competitive production of H-2 and alkanes. The proportion of coupled alkyl products decreased greatly in tbe presence of the weak acid. These observations and other evidence rule out free-radical processes as all but minor contributors to the mechanism and suggest that the reduction, coupling, and dehydrohalogenation of alkyl halides occur at the nickel atom. A mechanism that involves transient alkyl-Ni(OEiBC) and H-Ni(OEiBC) intermediates is proposed. It explains adequately the distributions of products and the production of the trace products and suggests that the unusual structural preferences of Ni(I) could be of functional significance. The apparent failure of Ni(I)(OEiBC) and of Ni(I)F430M to react with thioethers, including methyl coenzyme M, is discussed in light of the mechanism.