Click-Like Reactions with the Inert HCB₁₁Cl₁₁^– Anion Lead to Carborane-Fused Heterocycles with Unusual Aromatic Character

The chlorinated carba-closo-dodecaborate anion HCB₁₁Cl₁₁^– is an exceptionally stable molecule and has previously been reported to be substitutionally inert at the B–Cl vertices. We present here the discovery of base induced cycloaddition reactions between this carborane anion and organic azides that leads to selective C and B functionalization of the cluster. A single crystal X-ray diffraction study reveals bond lengths in the heterocyclic portion of the ring that are shortened, which suggests electronic delocalization. Molecular orbital analysis of the ensuing heterocycles reveals that two of the bonding orbitals of these systems resemble two of the doubly occupied π-MOs of a simple 5-membered Hückel-aromatic, even though they are entangled in the carborane skeleton. Nucleus independent chemical shift analysis indicates that both the carborane cluster portion of the molecule and the carborane fused heterocyclic region display aromatic character. Computational methods indicate that the reaction likely follows a stepwise addition cyclization pathway.