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

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 10(3)-10(4) while adjacent CH positions are activated by factors of 103 10(6). 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.