Research News, Aug. 12th, 2012
2,6-Diazasemibullvalenes: Synthesis, Structural Characterization, Reaction Chemistry and Theoretical Analysis
2,6-Diazasemibullvalene (NSBV) and its all-carbon analog semibullvalene (SBV) have long been of fundamental interest both theoretically and experimentally, because of their unique strained ring systems, their intramolecular skeletal rearrangements, as well as their rapid degenerate Cope rearrangement and predicted existence of homoaromatic delocalized structure. Synthesis and structural study of these highly strained ring systems have been a great challenge in organic chemistry. Reaction chemistry investigation and synthetic applications of their non-classical bonding have long been attractive.
In this work, a series of 2,6-diazasemibullvalenes (NSBVs) were synthesized and isolated from the reaction of 1,4-dilithio-1,3-dienes and nitriles via oxidant-induced C-N bond formation. For the first time, the activation barrier and the structure of NSBV were determined. All NSBVs show extremely rapid aza-Cope rearrangement in solution. However, the rearrangement is “frozen” in the solid state as demonstrated by the solid-state NMR measurement and X-ray single-crystal structural analysis. Insertion reaction of unsaturated compounds and a low-valent metal center into the C-N bond of NSBVs generated diverse and interesting ring-expansion products. Theoretical analysis shows that the localized structure is the predominant form, and the homoaromatic delocalized structure exists as a minor component in the equilibrium.