Activation of the B−F Bond by Diphenylcarbene: A Reversible 1,2-Fluorine Migration between Boron and Carbon

Experiments in low-temperature matrices reveal that triplet diphenylcarbene inserts into the very strong B−F bond of BF3in a two-step reaction. The first step is the formation of a strongly bound Lewis acid–base complex between the singlet state of diphenylcarbene and BF3. This step involves an inversion of the spin state of the carbene from triplet to singlet. The second step requires visible-light photochemical activation to induce a 1,2-F migration from boron to the adjacent carbon atom under formation of the formal insertion product of the carbene center into BF3. The 1,2-F migration is reversible under short-wavelength UV irradiation, thus leading back to the Lewis acid–base adduct.