N-Heterocyclic Olefin Stabilized Borenium Cations

The reaction of the N-heterocyclic dicarbene (NHDC) [:C{[N(2,6-(Pr2C6H3)-C-i)](2)CHCLi(THF)}](n) with iodomethane, in a hexane/THF mixture, affords the ylidic N-heterocyclic olefin NHC=CH2 (1; NHC = :C{N(2,6-(Pr2C6H3)-C-i)CH}2) in high yield. The reaction of 1 with BBr3 in hexane gives the neutral compound NHC CH2:BBr3(2), which in THF unexpectedly converts to the borenium salt [NHC CH2:B{O-(CH2)(4)Br}(2)](+)[Br](-) (3) through ring opening of THF.