研究室工作進展 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.

  背景介紹

芳香性概念及芳香性體系在自然科學中極為重要。1959年提出的同芳香性概念(Homoaromaticity) 進一步發展了芳香性概念及芳香性體系。1970年前後理論計算預測半瞬烯(Semibullvalene)和氮雜半瞬烯(2,6-Diazasemibullvalene)可能是實現中性同芳香性的最可能分子。但是，40餘年來該理論預測一直存在争議，原因是該類型分子将具有很大張力并發生極快速Cope重排，因此其合成方法、結構、以及物理和化學性質的研究極其困難。2006年，我們利用自己發展的雙锂試劑的“協同效應”（有關雙锂試劑及其“協同效應”評述詳見：Xi, Z. Acc. Chem. Res. 2010, 43, 1342），通過與銅的轉金屬反應，成功實現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，氮雜半瞬烯的合成方法、結構、以及物理和化學性質幾乎一無所知。最近（Zhang, S.; Wei, J.; Zhan, M.; Luo, Q.; Wang, C.; Zhang, W.-X.; Xi, Z. J. Am. Chem. Soc. 2012, 134, 11964），如上圖所示，我們再利用自己發展的雙锂試劑的“協同效應”，建立了高效合成該類分子的2種方法，首次測定了該類分子的活化能和單晶結構，發現了該類分子的多種化學反應類型。該項研究結果表明該類分子存在中性同芳香性，為進一步的實驗研究奠定了基礎。該工作被選為JACS Spotlights: First-time Exploration of 2,6-Diazasemibullvalene Structure and Reactivity, Herman, C. J. Am. Chem. Soc. 2012, DOI: 10.1021/ja307593d.