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 C6FnH6–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 SNAr-type conversion (low benzene fluorination) or as a reducing agent in a single-electron transfer/H atom abstraction sequence (high benzene fluorination).