CB11Me11 Boronium Ylides: Carba-closo-dodecaboranes with a Naked Boron Vertex

Abstract

In pentane solution, 2 equiv of the icosahedral CB₁₁Me₁₂^• radical cleaves the Si−Si bond of hexaalkyldisilanes by boron to silicon methyl transfer with formation of 2 equiv of methyltrialkylsilanes. The loss of a methyl radical converts the CB₁₁Me₁₂^• radical into an internally charge-compensated “boronium ylide” CB₁₁Me₁₁ with a naked vertex, which can be formally viewed as a deprotonated hypercloso carborane. It has been isolated as an air-sensitive solid, stable only below ∼ −60 °C. The naked vertex appears to be in position 12 since the material reacts instantaneously with alcohols and ethers to form the 12-alkoxy anions 12-CB₁₁Me₁₁OR. It reacts with many other nucleophiles to yield more complex mixtures containing similar products. DFT calculations for the four CB₁₁Me₁₁ isomers give closed-shell ground-state electronic structures. For the isomer with naked vertex 12, a DFT computational search failed to reveal any skeletal dimers, apparently due to excessive methyl−methyl repulsions, and only a cyclic dimer bound through weak interactions of one of the 7-methyl hydrogen atoms on each cage with the empty exocyclic orbitals on B12 of the other cage was found. Natural hybrid orbital populations suggest that the three possible isomers of monomeric boronium ylides are close to true singlet ylides, with triplet states ∼50 kcal/mol higher. The calculated electronic structure of the carbonium ylide is close to a singlet carbene, with a triplet state ∼16 kcal/mol higher. An attempted preparation of Me₃C⁺CB₁₁Me₁₂^- yielded neopentane and products consistent with a sequential loss of methyl groups from the carborane cage with a transient formation of similar boronium ylides. Probable mechanisms of these methyl transfer reactions are considered, and the possibly quite general role of “ylide” structures in Lewis acid induced substitution reactions on the boron vertices of carboranes and boranes is noted.