Base-catalyzed intramolecular C-H annulation reactions of o-carboranes containing nitriles, alkynes, and alkenes.

The C-H bonds of carboranes display acidity and can be functionalized under stoichiometric amounts of strong bases. However, the harsh reaction conditions lead to poor functional-group compatibility and cumbersome reaction handling. Herein, we achieved intramolecular C-H bond addition of o-carboranes to nitriles, unactivated alkynes, and alkenes using a catalytic alkoxide-mediated protocol for nitriles and a silylamide-promoted protocol for alkynes and alkenes at room temperature. Through this method, we synthesized a series of carborane-fused five- and six-membered compounds. Further deboronation afforded the corresponding five-membered carbocyclic fused nido-carboranes from arylnitrile-substituted o-carboranes, whereas those containing benzeneacetonitrile groups isomerize to six-membered fused carborane enamines. This protocol also enables intramolecular C-H annulation reactions with nonpolar alkynes and alkenes, leading to highly selective formation of five-membered E-olefinic carboranes and five-membered alkane-fused carboranes, respectively. Theoretical calculations reveal that the new fused five- and six-membered carbocycles exhibit small NICS values and thus cannot be considered weakly aromatic, indicating that leveraging the synergistic effects of electronic coupling and properties between the 3D carborane and the 2D polycyclic ring yields unique physicochemical properties. Moreover, the novel nido-carborane structures exhibit a distinct pH probe and a highly efficient yellow emitter, with a solid-state luminous efficiency of 42.5% obtained using this method.