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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.