Manganese(iii) acetate-mediated activation of C–H bonds of weak CH-acids; addition of o-carborane, its derivatives, and some other CH-acids to [60]-fullerene

Abstract

o-Carborane, 9-I-o-carborane, 1-Me-o-carborane, and several other CH-acids, 9H-fluorene, 2-Br-9H-fluorene, and trimethylsylylacetylene, have been shown to react with C₆₀ affording their monoadducts with fullerene, the reaction being mediated by Mn(OAc)₃·2H₂O. In the case of o-carborane, when the molar ratio of C₆₀ : o-carborane : Mn(OAc)₃·2H₂O was 1 : 21 : 20, polyaddition occurred to furnish adducts bearing between one and six o-C₂HB₁₀H₁₀ groups. A distinguishing characteristic of the carboranyl derivatives of C₆₀ obtained appeared to be that the carboranyl moieties were connected to the fullerenyl one by their carbon atom. No such fullerene derivatives have been known so far. Based on the results previously obtained for the phosphonylation of fullerenes, an oxidative-ion-transfer (OIT) mechanism was suggested for the reactions. The mechanism involves the replacement of an acetate group in Mn(OAc)₃ with the corresponding R of a CH-acid (RH), oxidation of C₆₀ by Mn(OAc)₂R with the formation of a pair [C₆₀˙⁺, Mn(OAc)₂R˙⁻] followed by the transfer of R⁻ to C₆₀˙⁺ to furnish the radical RC₆₀˙. The radical abstracts a hydrogen atom from another molecule of RH or from the solvent. The polyaddition occurs in an analogous way. This mechanism found support in this study owing to the fact that the reactions of both o-carborane and 9H-fluorene did not proceed with perfluorobenzophenone as a substrate, because it was unable to undergo oxidation. C₇₀ reacted with o-carborane in a similar way to give the corresponding monoadduct. No reaction of C₆₀ with m-carborane was observed and this was explained by its insufficient CH-acidity.