Coordination and Ring Expansion of 1,2-Dipolar Molecules with 9‑Phenyl-9-borafluorene

The reactivity of 9-phenyl-9-borafluorene with a series of 1,2-dipolar organic molecules is investigated, revealing that either adducts or seven-membered heterocycles are generated. A nitrile, imine, and isonitrile formed the corresponding coordination complexes that showed no evidence of conversion to ring expanded products. An aldehyde, ketone, and ketene inserted the CO moiety into the endocyclic B–C bond resulting in BOC5 boracycles. Isocyanates inserted either CN or CO depending on the polarization of the substrate with the former being the only substrate to generate a BNC5 ring system. The results demonstrate the potential of 9-borafluorenes as effective reagents to generate seven-membered boracycles with a biphenyl backbone.