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 [; Dmp = 2,6-bis(2,4,6-trimethylphenyl)phenyl] with 2 equiv of 1,3-diimethyl-4,5-dimethylimidazol-2-ylidene (IMe) resulted in the formation of an iminoborane-NHC complex . Both X-ray analysis and density functional theory calculations revealed the double-bond character of the B═N bond in . Interestingly, compared with the corresponding Lewis-base-free iminoborane, 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 can be easily attacked by electrophiles. Indeed, the reaction of with trimethylsilyl triflate (MeSiOTf) afforded an NHC-stabilized borenium cation , representing a facile strategy to prepare cationic tricoordinate boron species.