Intermolecular Cyclization between Carboranylphosphines and Electron-Deficient Alkynes

An
efficient and facile method to synthesize zwitterionic nido-carborane-fused five-membered P-heterocycles was developed.
Intermolecular cyclization between carboranylphosphines and electron-deficient
alkynes occurred at 50 °C in THF, affording the products with
up to 82% yield. For unsymmetrical alkynes, the more electrical-deficient
site is selectively connected to phosphorus in the products. Structures
of three products have been confirmed by X-ray diffraction analysis
of single crystals. The key step of the reaction is believed to be
deprotonation of acidic C–H of carborane by the alkenyl anion
derived from addition of carboranylphosphines to alkynes.