Crystalline Neutral Allenic Diborene.

A zwitterionic boraalkenyl boronium 3 was synthesized by reduction of cyclic (alkyl)(amino)carbene (cAAC) and trimethylphosphine (PMe3 )-coordinated tetrabromodiborane 2 with KC8 in the presence of PMe3 . Further reduction of 3 led to the formation of neutral allenic diborene 4. X-ray diffraction and computational studies revealed that 4 features the cumulated C=B and B=B double bonds. The reaction of 4 with four isonitrile molecules afforded a heterocycle 5 with the B2 C3 five-membered ring, via a complete scission of the B=B bond of 4.