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.

本研究通过1,4-二锂代-1,3-二烯烃与腈类的反应，经氧化剂诱导的C-N键形成路径，合成并分离得到一系列2,6-二氮杂半布瓦烯（2,6-diazasemibullvalenes，NSBVs）。本工作首次测定了取代型2,6-二氮杂半布瓦烯的活化能垒与X射线晶体结构。所有NSBVs在溶液中均表现出极快的氮杂-Cope重排（aza-Cope rearrangement）活性，但该快速重排过程在固态下被“冻结”，该结论通过固态核磁共振（solid-state NMR）测试与X射线单晶结构分析得以验证。将不饱和化合物或低价金属中心插入NSBV的C-N键中，可得到多样且具有研究价值的扩环产物。理论分析表明，该类化合物的定域结构为优势构型，同芳香性离域结构仅在平衡体系中以次要组分形式存在。