Evidence for the activation of unstrained carbon-carbon bonds by bare transition-metal ions M+ (M = Fe, Co) without prior C-H bond activation

Article abstract of DOI:10.1021/om00157a011

The metastable ion (MI) decompositions of RNH₂/M⁺ complexes (R = (C₂H₅)₂CHCH₂, C₂H₅C(CH₃)₂CH₂; M - Fe, Co) in the gas phase have been studied by tandem mass spectrometry with a four-sector instrument of BEBE configuration. The analyses of the MI spectra of isotopically labeled complexes uncover processes which inter alia demonstrate that the loss of C₄H₈ corresponds to a reaction in which site-specific oxidative addition of an unstrained C-C bond to the anchored transition-metal ion M⁺ takes place without prior C-H bond activation. The intramolecular methyl migration preceding the elimination of C₄H₈ is subject to a secondary kinetic isotope effect of k_H/k_D = 1.33 for M⁺ = Fe⁺ and k_H/k^D = 1.15 for M⁺ = Co⁺ per D atom. Additional processes observed correspond to the generation of molecular hydrogen, methane, ethylene and ethane. All reactions are highly specific, and mechanisms are suggested that are in keeping with the labeling data. For example, both H₂ and C₂H₄ are formed via remote functionalization involving the ω/ (ω - 1) positions of the ethyl side chain of the amines. Ethane contains an intact ethyl group, and one hydrogen is provided via specific β-hydrogen transfer which does not involve the chemically activated CH₂NH₂ group. This methylene group is also inert with regard to the reductive elimination of methane from CH₃CH₂C(CH₃)₂CHNH ₂/Co⁺. According to the labeling experiments, the intermediate from which CH₄ is lberated contains an intact CH₃ group that originates from the quaternary carbon center; the missing hydrogen atom is provided to roughly the same amount by both the second CH₃ group of C(2) and the CH₂ unit of the ethyl group. Again, the -CH₂NH₂ part does not serve as a hydrogen source for CH₄.