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 BF3 in 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.