10.1002/anie.202002595
Angewandte Chemie International Edition
COMMUNICATION
isolated yields of the overall sequence. Conditions: 4 (0.22 mmol), Aryl1Li
(0.22 mmol), HCl (0.24 mmol), Aryl2Li (0.20 mmol), 1 (0.24 mmol) in MeCN
(2.0 mL).
processes for the oxidative ligand coupling in tetraarylborates are
unprecedented.
Based on our observations and earlier reports,[13,22] we propose
the following mechanism for the ligand cross-coupling in
tetraarylborates (Scheme 5). One-electron oxidation of borate 4
by Bobbitt’s salt occurs most likely selectively at the most
electron-rich aryl moiety while the two dummy substituents remain
untouched. The thus generated intermediate I undergoes an
intramolecular 1,2-aryl shift[23] to afford cyclohexadienyl radical II.
Migration of the more electron-rich aryl group is favored over
migration of the two trifluoromethylated phenyl groups.[24] As
supported by crossover experiments of Hirao et al., the ligand
coupling likely proceeds intramolecularly.[14] Intermediate II could
further react via two slightly different pathways. One-electron
oxidation of II to cation III by 4-AcNH-TEMPO, followed by
nucleophilic attack of the thus generated 4-AcNH-TEMPO− at the
boron atom affords biaryl 5 along with the byproduct V.
Alternatively, radical/radical cross-coupling of II with 4-AcNH-
TEMPO, steered by the persistent radical effect,[25] leads to
intermediate IV that can then heterolyze to III.
Acknowledgements
We thank the European Research Council ERC (Advanced Grant
agreement No. 692640) and the Fonds der Chemischen Industrie
(fellowship to C.G.) for supporting this work. We thank Dr. Kazuhiro
Okamoto (Kyoto University) for conducting initial experiments.
Keywords: oxidative coupling • biaryls • tetraarylborates •
transition-metal-free • oxoammonium salt
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Scheme 5. Proposed mechanism for the oxidative cross-coupling of
tetraarylborates with Bobbitt’s salt.
In summary, we have reported a novel transition-metal-free
oxidative ligand coupling in tetraarylborates for the synthesis of
various biaryls using an oxoammonium salt as an inexpensive
and mild oxidant. This strategy could also be applied to selective
cross-couplings for the preparation of unsymmetric biaryls.
Tetraarylborates bearing different aryl groups were formed in-situ
and the cross-selectivity was controlled by installation of
unreactive trifluoromethylated phenyl groups as dummy ligands.
In addition to the standard method that uses a stoichiometric
amount of Bobbitt’s salt 1, ligand coupling in tetraarylborate salts
could also be achieved using oxidant 1 as a catalyst in
combination with NO2 as cocatalyst and O2 as the terminal oxidant.
For reactive borate salts, coupling worked even in the absence of
1 with NO2 as the sole catalyst. Transition-metal-free, catalytic
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