研究室工作進展 Jan. 21st, 2016
Aromatic Dicupra[10]annulenes
Junnian Wei, Yongliang Zhang, Yue Chi, Liang Liu, Wen-Xiong Zhang, and Zhenfeng Xi*
J. Am. Chem. Soc. 2016, 138, 60−63.
BACKGROUND: The chemistry of metal-containing aromatics (metalla-aromatics) has long been a fundamental and fascinating topic. However, the synthesis of macrocyclic metalla-aromatics with different metals still remains a great challenge. On the other hand, annulenes, which usually have a general formula C2nH2n, are classic cyclic conjugated systems. However, the [10]annulene having 10 π-electrons is non-aromatic, due to the steric hindrance of the two internal hydrogens. To release the steric hindrance of [10]annulene, we considered to replace the two internal C-H fragments with transition metals. Thus, if each of the two metals could offer one electron to form delocalized π-bonds, then the dimetalla[10]annulene could be aromatic. We have been working on the synthetic applications of multiply substituted 1,4-dilithio 1,3-butadienes (dilithio reagents for short). Recently, we found that appropriate dilithio reagents could react with low-valent transition metals, offering aromatic dilithio-metalloles (Wei, J.; Zhang, W.-X.; Xi, Z. Angew. Chem., Int. Ed. 2015, 54, 5999; Wei, J.; Zhang, Y.; Zhang, W.-X.; Xi, Z. Angew. Chem., Int. Ed. 2015, 54, 9986.), in which the dilithio reagents behaved as non-innocent ligands. Inspired by the novel behavior of dilithio reagents, we envisioned that macrocyclic metalla-aromatics could be obtained by annulating two or more dilithio reagents with suitable transition metals. This work reports the synthesis, characterization, and theoretic study of macrocyclic metalla-aromatics, dicupra[10]annulenes.
ABSTRACT: Metal-containing aromatic systems (metalla-aromatics) are unique and important both experimentally and theoretically. Among metalla-aromatics, 6-membered metallabenzenes and metallabenzynes have attracted much attention in recent years. However reports on their superior homologues are rare. In this work, the first series of aromatic dicupra[10]annulenes were isolated from the reaction of dilithio reagents and copper salts. Single-crystal X-ray structural analysis revealed dicupra[10]annulenes with averaged bond lengths. 7Li NMR spectra and theoretical calculations revealed a considerable aromatic character. XPS data suggested that the oxidation state of Cu atom in dicupra[10]annulenes was more likely to be Cu(I), indicating that the dilithio moieties in dicupra[10]annulenes were participated as non-innocent ligands. This work demonstrates a novel approach to construct macrocyclic metalla-aromatics.
亮點介紹
利用雙锂試劑的氧化還原活性合成芳香性金屬雜環
在已知的氧化還原反應中，碳負離子如常見的有機锂試劑可能作為還原劑而失去電子，但是不作為氧化劑進一步得到電子。本課題組魏俊年同學利用我們自己發展的雙锂試劑及其協同效應，與低價過渡金屬配合物如Ni(cod)2和[RhCl(cod)]2反應，合成了相應的芳香性金屬雜環戊二烯衍生物（for Ni，see: Angew. Chem. Int. Ed. 2015, 54, 5999-6002; for Rh, see: Angew. Chem. Int. Ed. 2015, 54, 9986-9990.）。在該反應中，雙锂試劑的丁二烯基雙碳負離子共轭體系的反鍵軌道與低價過渡金屬的d軌道中的一對電子産生“協同效應”，使富電荷的丁二烯骨架繼續獲得電子，成功構建了具有芳香性的金屬雜環戊二烯衍生物。該研究打破了傳統所認為的碳負離子不能夠繼續獲得電子的認知，為共轭體系碳負離子化學的進一步發展和應用提供了一個全新的思路。單晶結構和譜學表征包括锂譜結合理論計算證實了所生成的金屬雜環戊二烯衍生物具有顯著的芳香性。X-射線光電子能譜顯示過渡金屬的氧化态顯著上升（Ni近似+2價，Rh近似+3價）。由于雙锂試劑中碳負離子與丁二烯骨架的協同效應，其LUMO軌道能級相對于1,3-丁二烯顯著降低，故一些過渡金屬的d軌道電子可離域至雙锂試劑的π*軌道，完成芳香性金屬雜環的構建。雙锂試劑在該反應中可視為一類特殊的non-innocent ligand或者redox-active ligand，發生了氧化還原反應。
在以上工作的基礎上，魏俊年同學和他的小夥伴們秉承本實驗室“協同效應”理念和“深新信”原則，繼續深入探索了該類反應的理論意義、普适性和應用。本工作“芳香性雙銅雜輪烯”為最新研究成果之一。
芳香性概念及芳香性體系在自然界和自然科學中極為重要。雖然芳香性的定義一直存在争議，沒有定論，但是芳香性的概念、理論和新的芳香體系一直層出不窮。【推薦科普中文閱讀：芳香性概念的新發展，嚴兢、宋寅、彭德高、李珉，《大學化學》，2007， 22（1），33-40.】芳香性金屬有機化合物化學具有重要意義。近年來我國廈門大學夏海平教授和香港科技大學賈國成教授在芳香性金屬雜環研究領域取得了一系列開創性成果。但是，由于該類化合物的合成方法有限，已知的該類化合物無論是結構多樣性和金屬多樣性都很有限。芳香性金屬有機化合物的合成方法和後續的結構、成鍵模式、和性質研究仍然存在巨大挑戰。另一個方面，輪烯（一般通式：C2nH2n）是一大類具有共轭結構的環狀有機化合物。其中，最有代表性的是環丁二烯（4π，反芳香性）和苯環（6π，芳香性）。而[10]輪烯（C10H10）雖然具有共轭的10個π電子，但是不具有芳香性。結構決定性質。這是由于兩個反式雙鍵碳上的氫原子處于環内，它們相互排斥導緻環被扭曲以緻環不在同一平面上。我們考慮，如果這兩個CH基團能夠分别被金屬原子所取代，并且這兩個金屬原子的軌道還可以參與其它有機共轭部分的離域化，就有可能在避免相互排斥作用的同時，實現大環芳香性金屬雜環的合成，即芳香性雙金屬雜輪烯。受到我們前期雙锂試劑與低價金屬新穎反應性的啟發，我們研究了易于生成鍊狀結構的有機銅化合物的合成，得到了芳香性雙銅雜輪烯。通過單晶結構，譜學表征和理論計算證實了該類化合物具有顯著的芳香性。XPS顯示Cu近似+1價。
該系列工作也是本課題組雙金屬有機合成試劑化學的新展開。