Formal insertion of o-carborynes into ferrocenyl C-H bonds: A simple access to o-carboranylferrocenes

Insertion of o-carborynes (1,2-dehydro-o-carboranes) into ferrocenyl C-H bonds has been described, providing a convenient methodology for the preparation of functionalized ferrocenyl o-carboranes. Reaction of the carboryne precursors 1-I-2-Li-9,12-Me ₂-1,2-C ₂B ₁₀H ₈ or 1-I-2-Li-3-R-1,2-C ₂B ₁₀H ₉ (R = H, Ph, Me, Cl, Br, and I) with 2.5 equiv of ferrocene at 80 °C in cyclohexane afforded [1-(η ⁵-C ₅H ₄)(9,12-Me ₂-1,2-C ₂B ₁₀H ₉)]Fe(η ⁵-C ₅H ₅) and [1-(η ⁵-C ₅H ₄)(3-R-1,2-C ₂B ₁₀H ₁₀)] Fe(η ⁵-C ₅H ₅) (R = H, Ph, Me, Cl, Br, and I) in 19-44% isolated yields. On the other hand, in addition to the desired C-H bond insertion products [1-(η ⁵-C ₅H ₄)(3-X-1, 2-C ₂B ₁₀H ₁₀)]Fe(η ⁵-C ₅H ₅), bis(ferrocenyl)-o-carboranes 1,2- [(η ⁵-C ₅H ₅)Fe(η ⁵-C ₅H ₄)] ₂-3-X-1,2-C ₂B ₁₀H ₉ were isolated from the reactions of 1-I-2-Li-3-X-1,2-C ₂B ₁₀H ₉ (X = Cl, Br, and I) in 12%, 15%, and 8% yields, respectively. Treatment of 1,2-dilithio-o-carborane or 1-lithio-2-methyl-o-carborane with ferrocenium hexafluorophosphate in cyclohexane also generated [1-(η ⁵-C ₅H ₄)(1,2-C ₂B ₁₀H ₁₁)] Fe(η ⁵-C ₅H ₅) and [1-(η ⁵- C ₅H ₄)(2-Me-1,2-C ₂B ₁₀H ₁₀)]Fe(η ⁵-C ₅H ₅) in 17% and 31% isolated yields, respectively. On the basis of these observations, it is suggested that the single-electron oxidation of ferrocene to the electrophilic ferrocenium cation by o-carborynes is crucial for the insertion reaction. Accordingly, a reaction mechanism is proposed.