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Electrophilic Aromatic Substitutions of Aryltrifluoroborates with Retention of the BF3– Group: Quantification of the Activating and Directing Effects of the Trifluoroborate Group

Berionni, Guillaume, Morozova, Varvara, Heininger, Maximilian, Mayer, Peter, Knochel, Paul, Mayr, Herbert
Journal of the American Chemical Society 2013 v.135 no.16 pp. 6317-6324
X-radiation, boron, zwitterions
Kinetics and mechanisms of transition-metal free reactions of furyl, thienyl and indolyl trifluoroborates with benzhydrylium (Ar₂CH⁺) and iminium (Me₂N⁺CHR) ions have been investigated. In contrast to common belief, substitutions at CH positions are often faster than ipso-substitutions of the BF₃K group, because BF₃K activates the position attached to boron by a factor of 10³–10⁴ while adjacent CH positions are activated by factors of 10⁵–10⁶. Several reactions that have previously been interpreted as ipso-substitutions actually proceed via initial substitution at a vicinal or remote CH position, followed by protodeborylation. If the proton released during electrophilic substitution at a CH position is trapped by a base, the BF₃– group can be preserved. Remote reactions of heteroaryl trifluoroborates with iminium ions provide straightforward access to novel zwitterionic ammonium or iminium trifluoroborates, which have been characterized by single-crystal X-ray analyses.