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首页> 外文期刊>Chemistry: A European journal >From a theoretical concept to biochemical reactions: Strain-induced bond localization (SIBL) in oxidation of vitamin E
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From a theoretical concept to biochemical reactions: Strain-induced bond localization (SIBL) in oxidation of vitamin E

机译：从理论概念到生化反应：维生素E氧化过程中的应变诱导键定位（SIBL）

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摘要

The regioselectivity of the oxidation of a-tocopherol (the main component of vitamin E) to an orthoquinone methide (oQM) has been explained in the literature mostly by the ill-defined term "Mills-Nixon effect". In this paper we describe the preparation of eleven alpha-tocopherol derivatives, different from each other by the sum of annulation angles, but keeping the electronic factors unchanged. These compounds underwent Ag2O oxidation, forming two isomeric oQMs that were trapped by vinylmethyl ether. It was found that the isomeric product ratio changes smoothly as a function of the annulation angles, not abruptly from one regioisomer to the other on going from five- to six- and seven-membered rings, as predicted by the Mills-Nixon effect. The relative amounts of the products were determined at four different temperatures, and assuming that the product ratio represents the relative rates ratio, the relative enthalpy of activations could be obtained. Theoretically (at B3LYP/6-31G* theoretical level) four different intermediates were considered. Each of these underwent angular angles deformations to model the two regioisomers. At each deformation angle the energy difference between the two intermediates models was correlated to the experimental data for each of the four intermediates. It was found that the angle-deformed lithium (6-methyl-2-benzylium)phenolate correlated best (R>0.994) to the experimental data. This study confirms that the regioselectivity of the two isomeric oQMs in the oxidation of alpha-tocopherol and related compounds is simply a function of angular strain, best explained by the SIBL (strain-induced bond localization) model. In addition, this study provides evidence that the highest energy intermediate in the oxidation of vitamin E is a phenolate-benzyl cation.
机译：α-生育酚（维生素E的主要成分）氧化为邻醌甲基化物（oQM）的区域选择性在文献中主要通过定义不明确的术语“米尔斯-尼克森效应”进行了解释。在本文中，我们描述了11种α-生育酚衍生物的制备方法，它们的相变角之和互不相同，但保持电子因子不变。这些化合物经过Ag2O氧化，形成两个异构的oQM，它们被乙烯基甲基醚捕获。已经发现，异构体产物比率随环角的变化而平稳地变化，而不是如由米尔斯-尼克森效应所预测的那样，在从五元环到六元环和七元环时，一个区域异构体到另一个区域异构体突然变化。在四个不同的温度下确定产物的相对量，并且假设产物比率代表相对速率比率，则可以获得活化的相对焓。从理论上（在B3LYP / 6-31G *理论水平上）考虑了四种不同的中间体。这些中的每一个都经历了角形变以对两个区域异构体建模。在每个变形角处，两个中间体模型之间的能量差与四个中间体中每个中间体的实验数据相关。发现角变形的（6-甲基-2-苄基锂）酚酸锂与实验数据最佳相关（R＞ 0.994）。这项研究证实，两种异构体oQM在α-生育酚和相关化合物氧化中的区域选择性仅仅是角应变的函数，最好用SIBL（应变诱导键定位）模型来解释。此外，这项研究提供了证据，证明维生素E氧化过程中最高的能量中间体是酚盐-苄基阳离子。

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