Metalation of expanded porphyrins: a chemical trigger used to produce molecular twisting and Möbius aromaticity.-猫眼课题宝

DOI：

10.1002/anie.200704407

发表时间：

2008-01

期刊：

Angewandte Chemie

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作者：

Y. Tanaka;Shohei Saito;Shigeki Mori;N. Aratani;H. Shinokubo;N. Shibata;Y. Higuchi;Z. S. Yoon;Kil Suk Kim;S. Noh;Jong Kang Park;Dongho Kim;A. Osuka

莫比乌斯芳香性的概念，一个假设，即芳香特性将被观察到的[4 n]轮烯躺在一个扭曲的莫比乌斯带，是一个历史悠久的概念，是迷人的理论和实验的立场，[1，2]，因为它提供了一个自然的补充，众所周知的想法休克尔芳香性，管理的性质平面[4 n + 2]轮烯（图1）。虽然海尔布隆纳在1964年初首次提出基态系统[1]，后来又提出与某些过渡态[3]和反应中间体有关[4]，但真实的莫比乌斯芳烃系统被证明是难以捉摸的，直到最近埃尔热及其同事的开创性报告[5]，其中第一个莫比乌斯芳烃的例子被披露。该体系是由一个正常的平面共轭链段与一个带状共轭链段的战略性横向组合而成的。尽管需要费力的合成顺序来获得这种特定的产品，但这种大环的芳香特性不是很强，这一发现与大二面角一致（107. 78)和小的核无关化学位移（NICS）值（1.33 ± 0.33）。4ppm）。[6，7]然而，最近，根据广泛的主成分分析，莫比乌斯扭曲[16]轮烯应该比相应的非扭曲异构体更具芳香性。[8]这些研究重新点燃了对莫比乌斯芳香分子的理论兴趣，并强调了进一步合成的必要性。在这种情况下，最近的发现，二-p-benzihexaphyrin（1.1。1.1. 1.1），这是一种六吡咯“扩展卟啉”，在固态下表现出莫比乌斯结构，但在溶液中表现出动态的Hückel-Möbius拓扑相互转换，[9]是有趣的。因此，需要一个简单的，通用的方法来获得真实的莫比乌斯芳香系统具有独特的芳香特性仍然存在。莫比乌斯芳香族分子的合成本质上是困难的，因为需要适应两个相互冲突的结构目标，即扭曲的莫比乌斯拓扑结构和整体共轭的4 n π电子外围。莫比乌斯拓扑的扭曲性质需要使用大的轮生系统来减轻与分子扭曲相关的扭曲。不幸的是，这样的系统通常显示出相当大的构象灵活性，这使得“锁定”扭曲的构象变得困难。最近，共轭寡吡咯大环，所谓的扩展卟啉，已经出现作为一类新的杂轮烯变体，其显示出一系列有趣的电子和构象特征，[10-12]包括形成具有8字形几何结构的结构。[13-15]这些特征使我们认为，与更传统的轮烯相比，扩展卟啉在构建莫比乌斯型芳香体系方面可能具有明显的优势。这些包括1）整体构象灵活性，2）在某些条件下反转或“翻转”组成吡咯亚基的能力，3）通过容易释放或捕获一对吡咯NH氢原子以平衡电荷来响应双电子氧化和还原的能力，[11]和4）

The concept of Möbius aromaticity, a postulate that states that aromatic characteristics will be observed for [4n] annulenes lying in a twisted Möbius strip, is a time-honored concept that is fascinating from both the theoretical and experiment standpoints,[1, 2] since it provides a natural complement to the well-recognized idea of Hückel aromaticity that governs the properties of planar [4n+ 2] annulenes (Figure 1). Although first suggested by Heilbronner for a ground-state system early in 1964,[1] and later put forth as being relevant to certain transition states [3] and reactive intermediates,[4] real Möbius aromatic systems proved elusive until the recent seminal report by Herges and co-workers,[5] wherein the first example of a Möbius aromatic hydrocarbon was disclosed. This system was obtained by the strategic lateral combination of a normal, planar conjugated segment with a belt-shaped conjugated segment. Despite the laborious synthetic sequence required to obtain this particular product, the aromatic character of this macrocycle is not very strong, a finding consistent with the large dihedral angle (À107. 78) and small nucleus-independent chemical shift (NICS) value (À3. 4 ppm).[6, 7] Recently, however, it has been suggested on the basis of an extensive principal component analysis that Möbius twisted [16] annulenes should be more aromatic than the corresponding nontwisted isomers.[8] These studies have served to rekindle theoretical interest in Möbius aromatic molecules and underscore the need for further syntheses. In this context, the recent finding that a di-p-benzihexaphyrin (1.1. 1.1. 1.1), which is a hexapyrrolic “expanded porphyrin”, exhibits a Möbius structure in the solid state, but shows a dynamic Hückel–Möbius topological interconversion in solution,[9] is interesting. Therefore, the need for a simple, versatile approach to obtain real Möbius aromatic systems with distinct aromatic characters remains. The synthesis of Möbius aromatic molecules is made inherently difficult by the need to accommodate two conflicting structural objectives, namely a twisted Möbius topology and an overall conjugated 4n π-electron periphery. The twisted nature of the Möbius topology necessitates the use of large annulenic systems to mitigate the distortion associated with molecular twists. Unfortunately, such systems often show considerable conformational flexibility, which makes “locking in” a twisted conformation difficult. Recently, conjugated oligopyrrolic macrocycles, the socalled expanded porphyrins, have emerged as a new class of heteroannulene variants that display a range of interesting electronic and conformational characteristics,[10–12] including the formation of structures with figure-eight geometries.[13–15] These features led us to consider that, compared with more conventional annulenes, expanded porphyrins might have distinct advantages in the construction of Möbius-type aromatic systems. These include 1) an overall conformational flexibility, 2) an ability to invert, or “flip out”, the constituent pyrrolic subunits under certain conditions, 3) the capacity to respond to two-electron oxidation and reduction through the facile release or capture of a pair of pyrrolic NH hydrogen atoms to balance the charge,[11] and 4) the possibility of

Metalation of expanded porphyrins: a chemical trigger used to produce molecular twisting and Möbius aromaticity.

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