Noncatalyzed Intramolecular B-N and B-O Cross-Coupling of "Inert" Carboranes Lead to the Formation of an Unusual Oxoborane, via Reversible Cluster C-B Bond Scission.

Polyhalogenated -12-vertex carborane anions are thought to be inert species incapable of participating in direct B- substitution reactions. Here, we show that this is not true and that such species can be easily coaxed into intramolecular cross-coupling cyclizations without the need for a catalyst. When cage C-tethered O and N-heteroallylic anions are generated, a variety of cyclized products can be formed in high yield under mild conditions. Additionally, we show that even C-tethered neutral nucleophiles, such as the pyridine moiety, undergo facile B- substitution chemistry and these reactions are not dependent on the countercation. Serendipitously, we also found that when these cyclizations are attempted with acetamide derivatives, an unprecedented cluster C-B bond scission reaction occurs, producing an unprecedented oxoborane stabilized by multicentered bonding. Amazingly this molecule can be protonated, leading to reformation of the C-B bond and cluster reorganization, and this process is reversible.