Planarity Is Not Plain: Closed- vs Open-Shell Reactivity of a Structurally Constrained, Doubly Reduced Arylborane toward Fluorobenzenes.

The ability to activate small molecules is imparted to 9,10-dihydro-9,10-diboraanthracenes (DBAs) through the injection of two electrons. We report on the activation of fluorobenzenes CFH by the doubly reduced, structurally constrained DBA [] in THF (: 1,3,4,5,6). Compound is a 9,10-diphenyl DBA, forced into planarity by methylene bridges between the phenyl substituents and the DBA core. This rigidity results in enhanced stability under ambient conditions and an elevated planar-to-pyramidal reorganization energy upon boron tetracoordination, unlocking new reactivity. The dianion salts M[] were synthesized in excellent yields by stirring neutral with alkali metals M in THF (M: Li, Na, K); comproportionation of Li[] with generates the blue radical salt Li[], characterized by EPR spectroscopy and X-ray diffraction. While Li[] is inert toward CFH up to 120 °C, it reacts with 1,3,5-CFH at 100 °C to yield a B(sp)/B(sp) adduct with a difluorophenyl ligand (Li[]). Treatment of Li[] with 1 eq. of CFH or CF induces selective monohydrodefluorination, occurring in parallel with the formation of a unique B(sp)/B(sp) tetrahydrofuran-2-yl adduct (Li[]). The three isomers of CFH represent intermediate cases, where the competition between trifluorophenyl- and tetrahydrofuran-2-yl-adduct formation is governed by the relative positions of the F substituents and the nature of the countercation (M: Li, K). Through experimental and quantum-chemical studies, we unveil the underlying reaction mechanisms and show that Li[] acts either as a B-centered nucleophile in an SAr-type conversion (low benzene fluorination) or as a reducing agent in a single-electron transfer/H atom abstraction sequence (high benzene fluorination).