Journal Article

PUBDB-2025-02082

http://join2-wiki.gsi.de/foswiki/pub/Main/Artwork/join2_logo100x88.png Planarity Is Not Plain: Closed- vs Open-Shell Reactivity of a Structurally Constrained, Doubly Reduced Arylborane toward Fluorobenzenes

Buch, C. D. ; Virovets, A.Extern* ; Peresypkina, E.Extern* ; Endeward, B. ; Lerner, H.-W. ; Fantuzzi, F. ; Yamaguchi, S. ; Wagner, M. (Corresponding author)Extern*

2025
ACS Publications Washington, DC

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Please use a persistent id in citations: doi:10.1021/jacs.5c05588  doi:10.3204/PUBDB-2025-02082

Abstract: 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).

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Contributing Institute(s):

1. FS DOOR-User (FS DOOR-User)

Research Program(s):

1. 6G3 - PETRA III (DESY) (POF4-6G3) (POF4-6G3)
2. FS-Proposal: I-20230286 (I-20230286) (I-20230286)
3. FS-Proposal: I-20220865 (I-20220865) (I-20220865)

Experiment(s):

1. PETRA Beamline P24 (PETRA III)

Appears in the scientific report 2025

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