研究室工作进展 Aug. 12th, 2012

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

Shaoguang Zhang, Junnian Wei, Ming Zhan, Qian Luo, Chao Wang,

Wen-Xiong Zhang, and Zhenfeng Xi*

J. Am. Chem. Soc. 2012, 134, 11964-11967.

(JACS Spotlight: J. Am. Chem. Soc. 2012, DOI: 10.1021/ja307593d)

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.

  背景介绍

芳香性概念及芳香性体系在自然科学中极为重要。1959年提出的同芳香性概念(Homoaromaticity) 进一步发展了芳香性概念及芳香性体系。1970年前后理论计算预测半瞬烯(Semibullvalene)和氮杂半瞬烯(2,6-Diazasemibullvalene)可能是实现中性同芳香性的最可能分子。但是，40余年来该理论预测一直存在争议，原因是该类型分子将具有很大张力并发生极快速Cope重排，因此其合成方法、结构、以及物理和化学性质的研究极其困难。2006年，我们利用自己发展的双锂试剂的“协同效应”（有关双锂试剂及其“协同效应”评述详见：Xi, Z. Acc. Chem. Res. 2010, 43, 1342），通过与铜的转金属反应，成功实现了八取代半瞬烯的高效合成（Wang, C.; Yuan, J.; Li, G.; Wang, Z.; Zhang, S.; Xi, Z. J. Am. Chem. Soc. 2006, 128, 4564）。但是，氮杂半瞬烯的合成方法、结构、以及物理和化学性质几乎一无所知。最近（Zhang, S.; Wei, J.; Zhan, M.; Luo, Q.; Wang, C.; Zhang, W.-X.; Xi, Z. J. Am. Chem. Soc. 2012, 134, 11964），如上图所示，我们再利用自己发展的双锂试剂的“协同效应”，建立了高效合成该类分子的2种方法，首次测定了该类分子的活化能和单晶结构，发现了该类分子的多种化学反应类型。该项研究结果表明该类分子存在中性同芳香性，为进一步的实验研究奠定了基础。该工作被选为JACS Spotlights: First-time Exploration of 2,6-Diazasemibullvalene Structure and Reactivity, Herman, C. J. Am. Chem. Soc. 2012, DOI: 10.1021/ja307593d.