Abstraction of Trifluoroborane from Tetrafluoroborate: Li$^+$-Assisted Borylation of Nucleophilic Carbenes

Abstract

The small, lipophilic tetrafluoroborate anion is ubiquitous in the imidazol(in)ium and iminium precursors to N-heterocyclic carbenes (NHCs), including cyclic (alkyl)(amino)carbenes (CAACs). Its symmetrical distribution of charge over four highly electronegative fluorine atoms confers high stability, attenuating basicity while shielding the boron center. Here we report that this inertness does not withstand the nucleophilicity of free carbenes, when combined with a polarizing lithium cation. Reaction of CAAC ⋅ HBF4 salts with LiN(SiMe3)2, a protocol widely used to liberate free CAACs, generates zwitterionic CAAC-BF3 adducts within minutes at 80 °C, or hours at room temperature, even for bulky N-diisopropylphenyl (DIPP) CAACs. Imidazol(in)ium tetrafluoroborates likewise form NHC-BF3 adducts in excellent yields.