Septulene: The Heptagonal Homologue of Kekulene
Septulene: The Heptagonal Homologue of Kekulene
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DOI：
10.1002/anie.201203266
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发表时间：
2012-01-01
影响因子：
16.6
通讯作者：
King, Benjamin T.
中科院分区：
文献类型：
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作者：
Kumar, Bharat;Viboh, Ruth L.;King, Benjamin T.
Staab和Diederich将环芳烃定义为包含形成具有向内指向的C13 H键的大环的稠合苯环。[1]他们对典型的环芳烃kekulene的合成和表征[1-4]回答了一个关于芳烃和芳香性性质的长期存在的基本问题：π电子是否如Pauling假设的那样在整个系统中移动[5]，或者它们是否如McWheeny预测的那样保持在环中[6]，并由Clar进行了唯象描述。[7]2的内部质子的化学位移给出了答案。鲍林的模型表明，克库勒烯2应该表现为同心轮烯（图1右下），内部质子应该被强烈屏蔽，就像[18]轮烯一样。[8]定域模型，如克拉尔的芳香六偶体所体现的，表明环电流来自定域环（图1右上），并且内部质子应该被去屏蔽，就像在苯中一样。实验表明，当d> 7 ppm时，内层质子被去屏蔽并发生共振.这种去屏蔽现象表明，电子不能在整个分子中自由移动，而是局限于单个环中，就像苯中一样。我们报道了Kekulene的七边表兄弟septulene（1）的合成和性质，发现它的性质加强了上述结论，并提出了一些新的问题，1的性质与2的性质惊人地相似，尽管它们的Kekulene结构（图1底部）根本不同。这种差异是由于内环和外环中的碳原子数都是奇数，这使得在Kekulket结构中需要一个径向双键。也就是说，1是不可替代的。1和2之间的显着相似性，尽管它们从根本上不同的Kekul结构，消除了这样的概念，即考虑一些Kekul结构提供了对凝聚芳烃化学的深入了解。[9]以前唯一已知的环芳烃是2及其六氮杂类似物。[10]一些关于1的计算[11]和综合研究[12]出现在论文中，但除此之外，1似乎没有引起注意。已知环芳烃的稀缺性被其命名方案的丰富性所抵消。目前的方案要么假设不包括1的石墨烯晶格，要么无法传达分子的结构。我们同意Staab和Diederich的说法，即“根据IUPAC命名规则，（2）作为多环系统的命名导致了一个非常复杂的名称，它没有给出任何关于分子结构和对称性的直接信息”。[1]我们提出以下解决方案：为了处理环芳烃的一般情况，我们采用Agranat等人开发的corannulene命名法，[13]其中kekulene（2）的名称是
Staab and Diederich defined cycloarenes as comprising annelated benzene rings that form a macrocycle with inward-pointing CÀH bonds.[1] Their synthesis and characterization of kekulene, the prototypical cycloarene,[1–4] answered a long-standing and fundamental question about arenes and the nature of aromaticity: do π electrons move throughout the entire system, as hypothesized by Pauling,[5] or do they remain localized in rings, as predicted by McWheeny [6] and described phenomenologically by Clar.[7] The chemical shift of the inner protons of 2 gave the answer. Pauling s model suggested that kekulene 2 should behave as concentric annulenes (Scheme1 bottom right), and that the inner protons ought to be strongly shielded, as in [18] annulene.[8] The localized model, as embodied by Clar s aromatic sextets, suggests that the ring currents arise from localized rings (Figure 1 top right), and that the inner protons should be deshielded, as in benzene. Experiments showed that the inner protons are deshielded and resonate at d> 7 ppm. This deshielding demonstrates that electrons do not move freely about the entire molecule, but are instead localized into individual rings, just as in benzene. We report the synthesis and properties of kekulene s seven-sided cousin septulene (1), and find that its properties reinforce the conclusions above and open some new questions.The properties of 1 are strikingly similar to those of 2, even though their KekulØ structures (Scheme 1 bottom) are fundamentally different. This difference arises from the odd number of C atoms in both the inner and outer annuli of 1, which necessitates a radial double bond in the KekulØ structure. It also follows that 1 is non-alternant. The remarkable similarity between 1 and 2, despite their fundamentally different KekulØ structures, dispels the notion that a consideration of a few KekulØ structures provides much insight into the chemistry of condensed arenes.[9] The only previously known cycloarenes were 2 and its hexaaza analogue.[10] Some computational [11] and synthetic studies [12] towards 1 have appeared in dissertations, but otherwise 1 appears to have escaped attention. The scarcity of known cycloarenes is offset by the abundance of schemes for their nomenclature. The current schemes either assume a graphene lattice, which excludes 1, or fail to convey the structure of the molecule. We agree with the assertion by Staab and Diederich that,“The naming of (2) as a polycyclic system according to the IUPAC rules on nomenclature leads to an extraordinarily complicated name which does not give any direct information about the structure and symmetry of the molecule”.[1] We propose the following solution: To handle the general case of cycloarenes, we adopt the corannulene nomenclature developed by Agranat et al.,[13] in which the name for kekulene (2) is