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-Me2-1,2-C2B10H8 or 1-I-2-Li-3-R-1,2-C2B10H9 (R = H, Ph, Me, Cl, Br, and I) with 2.5 equiv of ferrocene at 80 °C in cyclohexane afforded [1-(η5-C5H4)­(9,12-Me2-1,2-C2B10H9)]­Fe­(η5-C5H5) and [1-(η5-C5H4)­(3-R-1,2-C2B10H10)]­Fe­(η5-C5H5) (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-(η5-C5H4)­(3-X-1,2-C2B10H10)]­Fe­(η5-C5H5), bis­(ferrocenyl)-o-carboranes 1,2-[(η5-C5H5)­Fe­(η5-C5H4)]2-3-X-1,2-C2B10H9 were isolated from the reactions of 1-I-2-Li-3-X-1,2-C2B10H9 (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-(η5-C5H4)­(1,2-C2B10H11)]­Fe­(η5-C5H5) and [1-(η5-C5H4)­(2-Me-1,2-C2B10H10)]­Fe­(η5-C5H5) 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.