New Approaches to N-Heterocyclic-Carbene-Coordinated Iminoborane and Borenium Species

The synthesis, characterization, and reactivity of an iminoborane-N-heterocyclic carbene (NHC) adduct were described. The reaction of DmpNHB(OEt)Br [1; Dmp = 2,6-bis(2,4,6-trimethylphenyl)phenyl] with 2 equiv of 1,3-diimethyl-4,5-dimethylimidazol-2-ylidene (IMe4) resulted in the formation of an iminoborane-NHC complex 2. Both X-ray analysis and density functional theory calculations revealed the double-bond character of the B=N bond in 2. Interestingly, compared with the corresponding Lewis-base-free iminoborane, 2 features a nitrogen atom with increased electron density, which could be attributed to coordination of the NHC. Similar to the isoelectronic species imine, this nitrogen center in 2 can be easily attacked by electrophiles. Indeed, the reaction of 2 with trimethylsilyl triflate (Me3SiOTf) afforded an NHC-stabilized borenium cation 3, representing a facile strategy to prepare cationic tricoordinate boron species.