Aromaticity and conjugation
Aromaticity and conjugation
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DOI：
10.1021/ja00444a022
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发表时间：
1977
影响因子：
15
通讯作者：
M. Randic
中科院分区：
文献类型：
--
作者：
M. Randic
介绍了一种测定共轭烃芳香性的新方法。它基于共轭电路的概念，共轭电路最近被认为是表征共轭系统的基本结构元素。理论物理。列托人。， 38, 68（1976）。研究了共轭烃的凯库勒结构，列举了CC双键和单键交替的电路。只有（An + 2）共轭电路的体系被定义为芳族体系。只有4n共轭电路的系统被认为是反芳族的，即由于T电子的失实而不稳定。最后，具有（An + 2）和An共轭电路的系统被归类为中间体，显示出部分芳香性质。该方法将著名的Hiickel （An + 2）规则（仅对单环结构严格有效）推广到多环系统。对几种备选方案进行了简要比较，并说明了它们的局限性。众所周知，试图描述芳香性所涉及的困难简单的Hiickel （An + 2）规则，只对单环系统有效，2和Piatt的周长模型，3，试图将规则扩展到多环系统，仍然经常用于更一般的情况，没有适当的理由，尽管公认的缺陷。这可能表明了对（an + 2） 7r电子作用的意义的直观理解。我们在这里提出了一种芳香性的方法，其中（an + 2） tr电子的作用也占主导地位。然而，事实证明，关键因素不是tr电子的数量，而是它们在共轭电路中的耦合，这是由系统的凯库勒结构推导出来的。该方法将著名的Hiickel （An + 2）规则（仅对单环共轭多烯严格有效）推广到多环结构。希克尔规则被认为是有机化学中最成功的理论预测之一，它的价值广受好评，因此，这里提出的环共轭ir-电子系统的分类方法似乎相当有趣，主要是因为它也包括了多环系统。这项工作中提出的芳香性方法的基础是共轭电路的概念多环结构包含各种电路，单个凯库勒结构赋予电路中的键一个单键或双键的特征。具有CC单键和双键交替的电路称为共轭电路。它们必须是偶数，并且是（An + 2）或An类型。共轭电路的概念在化学中并不陌生；然而，人们还没有认识到它们是一个重要的结构元素。分析包括所有独特共轭电路的枚举；因此这里我们有一个典型的图理论方案。我们用azupyrene来说明这种方法，其两个凯库勒结构分解如图1所示，其余两个结构-方案1
A new approach to aromaticity of conjugated hydrocarbons is described. It is based on the concept of conjugated cir cuits, which has been recently recognized as an essential structural element for characterization of conjugated systems (Chem. Phys. Lett., 38, 68 (1976)). Kekule structures of a conjugated hydrocarbon are examined and circuits with an alternation of CC double and single bonds enumerated. Systems having only (An + 2) conjugated circuits are defined as aromatic. Systems having only 4n conjugated circuits are considered antiaromatic, i.e., destabilized by the derealization of T electrons. Finally, systems having both (An + 2) and An conjugated circuits are classified as intermediate, showing partial aromatic nature. The approach represents a logical generalization of the famous Hiickel (An + 2) rule, valid rigorously only for monocyclic struc tures, to polycyclic systems. A brief comparison with several alternative schemes is given and their limitations illustrated. Difficulties involved in attempts to characterize aromatic ity are well known.1 The simple Hiickel (An + 2) rule, valid only for monocyclic systems,2 and Piatt's perimeter model,3 an attempt to extend the rule to polycyclic systems, remain frequently used for more general situations without a proper justification and despite recognized deficiencies. This perhaps indicates an intuitive appreciation of the significance of the (An + 2) 7r-electron role. We present here an approach to aroma ticity in which also a role of (An + 2) tr electrons is dominant. However, it turns out that not the number of tr electrons is the critical factor, but their coupling in conjugated circuits as derived from the Kekule structures of the system. The ap proach has lead to a logical generalization of the famous Hiickel (An + 2) rule, valid rigorously only for monocyclic conjugated polyenes, to polycyclic structures. In view of the acclaimed value of the Hiickel rule, believed to have been one of the most successful theoretical predictions made in organic chemistry,4 the approach of a classification of cyclic conjugated ir-electron systems developed here seems to be rather inter esting mainly because it encloses the polycyclic systems too. The basis for the approach to aromaticity suggested in this work is the concept of conjugated circuits.5 A polycyclic structure contains various circuits, and an individual Kekule structure assigns a single or a double bond character to bonds in a circuit. Circuits which have an alternation of the CC single and double bonds are called conjugated circuits. They neces sarily are even, and are either of a (An + 2) or An type. The notion of conjugate circuits is not so unfamiliar in chemistry; however, it has not been realized that they represent an im portant structural element. The analysis consists of the enu meration of all distinctive conjugated circuits; hence here we have a typical graph theoretical scheme. We illustrate the approach with azupyrene, two Kekule structures of which are decomposed as shown in Scheme I. The remaining two struc- Scheme I