BARRIERS TO THERMALLY ALLOWED REACTIONS AND THE ELUSIVENESS OF NEUTRAL HOMOAROMATICITY
BARRIERS TO THERMALLY ALLOWED REACTIONS AND THE ELUSIVENESS OF NEUTRAL HOMOAROMATICITY
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DOI：
10.1021/ja00517a001
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发表时间：
1979-01-01
影响因子：
15
通讯作者：
PAQUETTE, LA
中科院分区：
文献类型：
--
作者：
HOUK, KN;GANDOUR, RW;PAQUETTE, LA
为了解释1，5，9-环十二碳三炔与三环丁烯苯快速互变的原因，本文用MINDO/3和从头算STO-3G方法研究了乙炔热允许的环三聚反应。尽管反应具有严格的允许性质，但在大量的前线轨道相互作用能够将电子密度从反应物中的键合区域转换到产物中的键合区域之前，乙炔必须发生明显的畸变。只有当孤立的分子被充分扭曲时，才能在rr系统之间稳定HOMOLUMO相互作用。非共轭系统的重叠增加了填充轨道之间的相互作用，但这使分子不稳定，而不是稳定。在某种意义上，设计用于产生中性同芳稳定化的体系反而在某种程度上是“反芳”的。芳族或同芳族相互作用的发生是因为稳定相互作用，但只是以扭曲骨架所需的能量为代价。在环状共轭体系中，骨架对几何畸变相对不敏感，因此在4 n + 2电子体系中，红外离域可以提供芳香性。
As a model for the phenomena described in the title, and, more specifically, to elucidate the factors preventing rapid interconversion of 1, 5, 9-cyclododecatriyne with triscyclobutenabenzene, the thermally allowed/^/¡-symmetrical cyclotrimeri-zation of acetylene to benzenehas been studied by MINDO/3 and ab initio STO-3G methods. In spite of the symmetry-al-lowed nature of the reaction, appreciable distortion of the acetylenes must occur before substantial frontier orbital interactions can switch electron density from bonding regions in the reactant to bonding regions in the product. Stabilizing HOMOLUMO interactions between rr systems only ensue when the isolated molecules have been substantially distorted. Overlapof nonconjugated systems increases theinteractions between filled orbitals, but this destabilizes—not stabilizes—the molecules. In a sense, the systems designed to produce neutral homoaromatic stabilization are somewhat “antiaromatic” instead. Aromatic or homoaromatic interactions occur because of stabilizing interactions, but only at the expense of the energy re-quired to distort the framework. In cyclic conjugated systems, the framework is shown to be relatively insensitiveto geo-metrical distortions so that ir delocalization can afford aromaticity in 4n+ 2 electronic systems.