2,6-Diazasemibullvalenes: Synthesis, Structural Characterization, Reaction Chemistry, and Theoretical Analysis

A series of 2,6-diazasemibullvalenes (NSBVs) were synthesized
and
isolated from the reaction of 1,4-dilithio-1,3-dienes with nitriles
via oxidant-induced C–N bond formation. For the first time,
the activation barrier and an X-ray crystal structure of a substituted
2,6-diazasemibullvalene were determined. All NSBVs show extremely
rapid aza-Cope rearrangement in solution, but the rapid aza-Cope rearrangement
is “frozen” in the solid state, as shown by solid-state
NMR measurements and X-ray single-crystal structural analysis. Insertion
of unsaturated compounds or a low-valent metal center into the NSBV
C–N bond gave diverse and interesting ring-expansion products.
Theoretical analysis showed that the localized structure is predominant
and that the homoaromatic delocalized structure exists as a minor
component in the equilibrium.