Reactions of the Nickel(I) Octaethylisobacteriochlorin Anion with Alkyl Halides

Article abstract of DOI:10.1021/ja00224a026

Reactions of Ni^I(OEiBC)^- with alkyl halides were investigated in bulk solution and by electrochemical means.With the exception of factor F₄₃₀, the Ni hydrocorphinoid prosthetic group of methyl coenzyme M reductase, Ni(OEiBC) is the only tetrapyrrole to date that has been reduced to an isolable nickel(I) complex.Ni^I(OEiBC)^- reacts with CH3I in a 2:1 stoichiometry to afford neutral Ni^II(OEiBC) in quantitative yield and CH4 and I^- in lesser yields.The nickel(I) complex also reacts with methyl p-toluenesulfonate to afford CH4.Ni(OEiBC) mediates electrocatalytic reductions of alkyl halides and of methyl p-toluenesulfonate.Evidence is presented for a transient alkyl-Ni^III(OEiBC) intermediate, which is reducible at potentials positive of the Ni^II/Ni^I couple.The current/potential curves of the electrocatalytic reductions were analyzed to obtain relative rate constants for a series of alkyl halides.The reactivity trends I > Br > tosyl >/= Cl and CH3 > n-C4H9 > sec-C4H9 > t-C4H9 are consistent with a nucleophilic, S_n2-like mechanism.Estimates of the second-order rate constant for reaction of Ni^I(OEiBC)^- and CH3I suggest that the nucleophilicity of the nickel(I) complex is comparable to that of the "supernucleophile" vitamin B_12s.