Carbon–carbon bond cleavages in agostic cations formed by protonation of cobalt–diene complexes

Abstract

The fluxional, agostic, norborn-5-en-2-ylcobalt(III) cations [Co(η-C₅R₅)(2 : 5,6-η-C₇H₉)]⁺(R = H,Me) undergo C–C bond cleavage at –20 °C to give 4-vinylcyclopentenylcobalt(III) complexes [Co(η-C₅R₅)(1-3 : 6,7-η-CH₂CH[graphic omitted]H₂)]⁺[R = H (4)(structurally characterized by X-ray crystallography), Me (7)], which react with LiAIH₄ or LiBHEt₃ to give neutral 4-vinylcyclopentenecobalt(I) complexes [Co(η-C₅R₅)(1,2 : 6,7-η-CH₂CH[graphic omitted]H₂)][R = H (8), Me (9a)]; (9a) is protonated at 0 °C to give the 4-ethylcyclopentenyl complex [Co(η-C₅Me₅){1-3-η-C₅H₆(endo-Et-4)}]⁺(11), which is stabilized by an agostic interaction between cobalt and the endo-hydrogen–carbon bond of the 5-position, and readily undergoes conrotatory ring-opening to afford [Co(η-C₅Me₅){η-C₅H₆(syn-Et-5)}]⁺(12) containing an acyclic 5-ethylpentadienyl ligand, as shown by X-ray crystallography.