Synthesis of Complex Boron–Nitrogen Heterocycles Comprising Borylated Triazenes and Tetrazenes Under Mild Conditions

The
reactions of organic azides with diaryl­(dihalo)­diboranes(4)
were explored, resulting in the observation of a number of surprising
reactivity patterns. The reaction of phenyl azide with 1,2-diaryl-1,2-dihalodiboranes(4)
resulted in the formation of five-membered rings comprising diboryl-triazenes
with retention of the boron–boron bond, while the reaction
of the peculiar 1,1-di­(9-anthryl)-2,2-difluorodiborane(4) with phenyl
azide yielded a six-membered ring bearing a diboryl-triazene, whereby
the B–B bond was ruptured by the insertion of an arylnitrene-like
reactive intermediate. Both types of heterocycles feature unprecedented
connectivity patterns and are very rare examples of boryl triazenes
beyond the more common 1,2,3-triazolatoboranes. They are also the
product of a unique type of aryl migration from a boron center to
the phenyl azide γ-nitrogen center. Lastly, the substitution
of 1,2-diaryl-1,2-dihalodiboranes(4) with azide groups, using trimethylsilyl
azide as the transfer reagent, yielded boryl-tetrazaboroles and diboryldiazadiboretidines
(as side-products), invoking the intermediacy of the first N-boryl-substituted
iminoboranes, which are BN isosteres of monoborylated alkynes. The
synthetic results are complemented with mechanistic proposals derived
from quantum-chemical calculations.