Planarity Is Not Plain: Closed- vs Open-Shell Reactivity of a Structurally Constrained, Doubly Reduced Arylborane toward Fluorobenzenes
Christoph D. Buch, A.V. Virovets, Eugenia V. Peresypkina, Burkhard Endeward, Hans‐Wolfram Lerner, Felipe Fantuzzi, Shigehiro Yamaguchi, Matthias Wagner
Abstract
High Resolution Image Download MS PowerPoint Slide 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 C 6 F n H 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 M 2 [ 1 ] were synthesized in excellent yields by stirring neutral 1 with alkali metals M in THF (M: Li, Na, K); comproportionation of Li 2 [ 1 ] with 1 generates the blue radical salt Li[ 1 ], characterized by EPR spectroscopy and X-ray diffraction. While Li 2 [ 1 ] is inert toward C 6 FH 5 up to 120 °C, it reacts with 1,3,5-C 6 F 3 H 3 at 100 °C to yield a B(sp 2 )/B(sp 3 ) adduct with a difluorophenyl ligand (Li[ 2 ]). Treatment of Li 2 [ 1 ] with 1 eq. of C 6 F 5 H or C 6 F 6 induces selective monohydrodefluorination, occurring in parallel with the formation of a unique B(sp 2 )/B(sp 3 ) tetrahydrofuran-2-yl adduct (Li[ 3 ]). The three isomers of C 6 F 4 H 2 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 2 [ 1 ] acts either as a B-centered nucleophile in an S N Ar-type conversion (low benzene fluorination) or as a reducing agent in a single-electron transfer/H atom abstraction sequence (high benzene fluorination).