Benzodifurantrione: A Stable Phenylogous Enol
Benzodifurantrione: A Stable Phenylogous Enol
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DOI：
10.1021/jo9022155
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发表时间：
2010-02-05
影响因子：
3.6
通讯作者：
McDouall, Joseph J. W.
中科院分区：
文献类型：
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作者：
Lawrence, Anthony J.;Hutchings, Michael G.;McDouall, Joseph J. W.
第一个例子的一个稳定的苯基烯醇，从延长酮-烯醇互变异构跨越苯环，进行了说明。烯醇已被分离出来，其结构已通过X射线晶体学证明。酮-和烯醇-互变异构体之间的平衡已经被广泛研究，并通过NMR和UV-vis光谱在溶液中定量。平衡位置与溶剂氢键受体强度（β（OH））的Kamlet-Taft溶剂化显色标度呈线性相关，并且平衡被证明是完全动态的，服从一级平衡动力学。为了试图解释为什么烯醇化发生，在什么令人惊讶的似乎是芳香族共振稳定的代价，各种结构特征已被认为是进一步探索与援助的MO计算。包含两种互变异构体的每个环的芳香性的核独立化学位移（NICS）指数计算显示，虽然中心苯环在烯醇化时失去芳香性，但α-酮-内酯环在酮-互变异构体中显示出所有意想不到的和显著的反芳香性，这绝非直观的。因此，与中心苯环相关的稳定化能量的损失在一定程度上通过去除α-酮内酯环的反芳族去稳定化而得到补偿，从而使两种结构的能量比预期的更接近。
The first example of a stable phenylogous enol, resulting from an extended keto-enol tautomerization across a benzene ring, is described. The enol has been isolated, and its structure was proven by X-ray crystallography. The equilibrium between the keto- and enol-tautomers has been extensively studied and quantified in solution by NMR and UV-vis spectroscopy. The position of equilibrium showed a linear correlation to the Kamlet-Taft solvatochromic scale for solvent H-bond acceptor strength (beta(OH)), and the equilibrium was proven to be fully dynamic, obeying first-order equilibrium kinetics. To attempt to explain why enolization occurs, at what surprisingly appears to be the expense of aromatic resonance stabilization, various structural features have been considered and explored further with the aid of MO calculations. Nucleus independent chemical shift (NICS) index of aromaticity calculations for each of the rings comprising both tautomers showed that while the central benzene ring loses aromaticity on enolization, the alpha-keto-lactone ring showed all unexpected and significant antiarornaticity in the keto-tautomer, which is by no means intuitive. The loss of stabilization energy associated with the central benzene ring is, therefore, to a certain degree compensated by removal of the antiaromatic destabilization of the alpha-keto-lactone ring rendering the two structures much closer in energy than would otherwise be expected.