Protonation-Triggered Conformational Changes to Mobius Aromatic [32]Heptaphyrins(1.1.1.1.1.1.1)
Protonation-Triggered Conformational Changes to Mobius Aromatic [32]Heptaphyrins(1.1.1.1.1.1.1)
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DOI：
10.1002/anie.200804457
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发表时间：
2008-01-01
影响因子：
16.6
通讯作者：
Osuka, Atsuhiro
中科院分区：
文献类型：
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作者：
Saito, Shohei;Shin, Jae-Yoon;Osuka, Atsuhiro
近年来，由于其独特的性质，如广泛和灵活的π共轭，[2]多金属配位，[3]多样的氧化态，[4]阴离子结合，[5]和大的双光子吸收截面，扩展卟啉的化学[1]引起了相当大的兴趣。[6]最近的一个感兴趣的话题是扩大卟啉形成莫比乌斯芳香分子的能力。[7]莫比乌斯芳香性的概念最早由Heilbronner在1964年提出，它预言了[4 n]轮烯位于单面不可定向莫比乌斯带上时的电子离域芳香回路。[8]这一概念因其简单美观而受到理论和实验的广泛挑战。[9]Herges及其同事在2003年报道了第一个稳定的中性莫比乌斯芳香分子的合成[10]，其中采用了正常共轭体系和带状共轭体系的巧妙组合来实现分子扭曲，然而，报道的[16]莫比乌斯拓扑结构的轮烯在环电流和核独立化学位移方面表现出相对较弱的芳香性（NICS）值。[11]最近，Latos-Grazynski及其同事报道了二对苯并[28]六卟啉的Hückel和Möbius构象之间的温度依赖性拓扑变化（1.1. 1.1. 1.1）。[7a]不久之后，我们的研究小组证明，一系列meso-arylsubstituted膨胀卟啉的容易金属化导致构象刚性，扭曲的结构具有明显的莫比乌斯芳香性。[7b我们还发现游离碱[28]六卟啉的莫比乌斯构象之间存在快速平衡（1.1. 1.1. 1.1），这是冻结在173 K的NMR时标。[7e]尽管有这些发现，莫比乌斯芳香族化合物仍然是罕见的，这是非常可取的，以开发一个更一般的路线，以这些分子。在此，我们报告的构象meso-芳基取代的[32] heptaphyrins（1.1。1.1. 1.1. 1)取决于meso-芳基取代基，溶剂，温度和质子化，其中质子化过程提供了一种通用的和可逆的手段来实现具有不同芳香性的扭曲Möbius结构。meso-五氟苯基取代的[32]七卟啉1在非极性溶剂如己烷、甲苯和CH 2Cl 2中呈现8字形构象，如通过1H NMR和UV/维斯光谱和X-射线衍射分析所证实的。[12]晶体从戊烷溶液中生长，X射线结构如图1a所示。大环共轭很好地保留了吡咯和中位碳单元之间的二面角小于268。这种结构与一个32π电子电路相一致，该电路可以被认为是具有C2对称性的扭曲双侧（可定向）休克尔拓扑。事实上，1在[D8]甲苯中的1H NMR光谱通过分别在δ= 11.27和8.50 ppm处记录位于大环共轭内部的Ha和Hb的信号而显示出中等的顺向环电流（图2a）。而其余的吡咯β质子处于δ= 5.11- 6.42ppm的屏蔽区。1在非极性溶剂中的吸收光谱显示出相当宽的谱带，在低能区没有谱带，这是与反芳族特征一致的特征（图3a）。[6，13]因此，在所述中心处的NICS值是
In recent years, the chemistry of expanded porphyrins [1] has attracted considerable interest because of their unique properties such as extensive and flexible π conjugation,[2] multimetal coordination,[3] versatile oxidation states,[4] anion binding,[5] and large two-photon absorption cross section.[6] A recent topic of interest is the ability of expanded porphyrins to form Möbius aromatic molecules.[7] The concept of Möbius aromaticity, which was first proposed by Heilbronner in 1964, predicts an electronically delocalized aromatic circuit for a [4n] annulene when lying on a single-sided, nonorientable Möbius strip.[8] This concept has been extensively challenged from both the theoretical and experimental viewpoints, as it is simple and aesthetically pleasing.[9] The first synthesis of a stable, neutral Möbius aromatic molecule was reported by Herges and co-workers in 2003,[10] in which the smart combination of a normal conjugated system and a belt-shaped conjugated system were employed to implement a molecular twist, however, the reported [16] annulene of Möbius topology exhibited relatively weak aromaticity with respect to its ring current and nuclear-independent chemical shift (NICS) value.[11] Quite recently, Latos-Grazynski and co-workers reported the temperature-dependent topology change between Hückel and Möbius conformations for di-p-benzi [28] hexaphyrin (1.1. 1.1. 1.1).[7a] Soon after, our research group demonstrated that facile metalations of a series of meso-arylsubstituted expanded porphyrins led to conformationally rigid, twisted structures with distinct Möbius aromaticity.[7b, d] We also found that there is a fast equilibrium among Möbius conformers of free-base [28] hexaphyrins (1.1. 1.1. 1.1), which is frozen on the NMR time scale at 173 K.[7e] Despite these findings, Möbius aromatic compounds are still rare and it is highly desirable to develop a more general route towards these molecules. Herein, we report that conformations of meso-aryl-substituted [32] heptaphyrins (1.1. 1.1. 1.1. 1) are dependent upon meso-aryl substituents, solvents, temperature, and protonation, among which the protonation process provides a general and reversible means to realize a twisted Möbius structure with distinct aromaticity. meso-Pentafluorophenyl-substituted [32] heptaphyrin 1 takes a figure-eight conformation in nonpolar solvents such as hexane, toluene, and CH2Cl2 as confirmed by 1H NMR and UV/Vis spectroscopic, and X-ray diffraction analysis.[12] Crystals were grown from a pentane solution and the X-ray structure is shown in Figure 1a. The macrocyclic conjugation is nicely preserved with the dihedral angles between the pyrrole and meso carbon units being smaller than 268. This structure is consistent with a 32π-electron circuit within what can be considered a twisted double-sided (orientable) Hückel topology with C2 symmetry. In fact, the 1H NMR spectrum of 1 in [D8] toluene exhibits a moderate paratropic ring current by recording signals for Ha and Hb located inside the macrocyclic conjugation at δ= 11.27 and 8.50 ppm, respectively (Figure2a. Meanwhile, the rest of the pyrrolic βprotons are in a shielded region of δ= 5.11–6.42 ppm). The absorption spectra of 1 in nonpolar solvents exhibit rather broad bands and without bands in a low-energy region, which is a feature consistent with antiaromatic character (Figure 3 a).[6, 13] Accordingly, a NICS value at the center of the