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

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 C₆F_nH_6-n by the doubly reduced, structurally constrained DBA [1]^2- in THF (n: 1,3,4,5,6). Compound 1 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 M₂[1] were synthesized in excellent yields by stirring neutral 1 with alkali metals M in THF (M: Li, Na, K); comproportionation of Li₂[1] with 1 generates the blue radical salt Li[1], characterized by EPR spectroscopy and X-ray diffraction. While Li₂[1] is inert toward C₆FH₅ up to 120 °C, it reacts with 1,3,5-C₆F₃H₃ at 100 °C to yield a B(sp²)/B(sp³) adduct with a difluorophenyl ligand (Li[2]). Treatment of Li₂[1] with 1 eq. of C₆F₅H or C₆F₆ induces selective monohydrodefluorination, occurring in parallel with the formation of a unique B(sp²)/B(sp³) tetrahydrofuran-2-yl adduct (Li[3]). The three isomers of C₆F₄H₂ 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 Li₂[1] acts either as a B-centered nucleophile in an S_NAr-type conversion (low benzene fluorination) or as a reducing agent in a single-electron transfer/H atom abstraction sequence (high benzene fluorination).