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.