1,1-Bis(N-methylimidazole)-2-(trimethylsilyl)-1-boracyclohexa-1,4-diene Chloride: A Stable Intermediate or Tangent en Route to 1-(N-Methylimidazole)borabenzene?

The reaction of 1-chloro-2-(trimethylsilyl)-1-boracylohexa-2,5-diene (1) with N-methylimidazole (NMI) results not in the anticipated borabenzene adduct [(NMI)→BC5H5] (3) but rather the title imidazole-stabilized cyclic boronium salt [(NMI)2BC5H5SiMe3-2]Cl ([2]­Cl), which represents a rare example of a trapped intermediate or possible tangent en route to a neutral borabenzene adduct (3) which may be obtained by heating [2]Cl under vacuum.