Hexamethylbenzene elimination enables the generation of transient, sterically unhindered multiply bonded boron species

We present a method for the generation of boron-containing unsaturated small molecules <i>via</i> hexamethylbenzene elimination. The fragmentation precursors are obtained through bond insertion into phenyl boranorbornadiene (PhB(C₆Me₆), 1). Compound 1 undergoes 1,1-insertion with 2,6-xylyl isocyanide, affording a boron-doped bicyclo[2.2.2]octa-2,5-diene 2. Heating 2 in toluene results in the formation of a base-stabilized boraketenimine PhB(CNxyl)₂ (<i>i.e.</i>, borylene diisocyanide) as an intermediate <i>via</i> retro-Diels-Alder reaction. Surprisingly, PhB(CNxyl)₂ dimerizes to give a boron-doped 6-membered ring (PhB)₂C₄(CNxyl)₆4. The reaction of 1 with trimethylamine <i>N</i>-oxide and phenyl azide yields triphenyl boroxine and a BN₄ ring, respectively, implying the involvement of transient oxoborane (PhB[triple bond, length as m-dash]O) and iminoborane intermediates (PhB[triple bond, length as m-dash]NPh), respectively. Furthermore, boranorbornadiene also undergoes 2,3-insertion with mesityl isocyanate (MesNCO), affording a fused 6/5-membered heterocycle 11. This insertion profile is analogous to the insertion of phenyl azide into 1.