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首页> 外文期刊>Chemistry: A European journal >Substitution of hydrogen by deuterium changes the regioselectivity of ethylbenzene hydroxylation by an oxo-iron-porphyrin catalyst
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Substitution of hydrogen by deuterium changes the regioselectivity of ethylbenzene hydroxylation by an oxo-iron-porphyrin catalyst

机译：氘置换氢会改变氧-铁-卟啉催化剂对乙苯羟基化的区域选择性

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摘要

Heme oxo-iron complexes are powerful oxygenation catalysts of environmentally benign hydroxylation processes. We have performed density functional theoretic calculations on a model system, that is, an oxo-iron-porphyrin (Por) complex [(Fe=O)Cl(Por)], and studied its reactivity toward a realistic substrate, namely, ethylbenzene. The calculations showed that the dominant reaction process in the gas phase is benzyl hydroxylation leading to 1-phenylethanol, with an energetic barrier of 9.1 kcal mol(-1), while the competing para-phenyl hydroxylation has a barrier 3.0 kcal mol(-1) higher in energy. This benzyl hydroxylation barrier is the lowest C-H hydroxylation barrier we have obtained so far for oxo-iron-porphyrin complexes. Due to electronic differences between the intermediates in the phenyl and benzyl hydroxylation processes, the phenyl hydroxylation process is considerably stabilised over the benzyl hydroxylation mechanism in environments with a large dielectric constant. In addition, we calculated kinetic isotope effects of the substitution of one or more hydrogen atoms of ethylbenzene by deuterium atoms and studied its effect on the reaction barriers. Thus, in a medium with a large dielectric constant, a regioselectivity change occurs between [H-10]ethylbenzene and [D-10]ethylbenzene whereby the deuterated species gives phenol products whereas the hydrogenated species gives mainly 1-phenylethanol products. This remarkable metabolic switching was analysed and found to occur due to 1) differences in strength between a C-H versus a C-D bond and 2) stabilisation of cationic intermediates in a medium with a large dielectric constant. We have compared our calculations with experimental work on synthetic oxo-iron-porphyrin catalysts as well as with enzyme-reactivity studies.
机译：血红素氧-铁络合物是对环境有益的羟基化过程的强力氧化催化剂。我们已经在模型系统，即氧-铁-卟啉（Por）络合物[（Fe = O）Cl（Por）]上进行了密度泛函理论计算，并研究了其对实际底物乙苯的反应性。计算表明，气相中的主要反应过程是苄基羟基化反应生成1-苯基乙醇，能量屏障为9.1 kcal mol（-1），而竞争性对苯基羟基化反应的屏障为3.0 kcal mol（-1） ）能量更高。迄今为止，对于苄氧基铁铁卟啉配合物而言，这种苄基羟基化阻挡层是最低的C-H羟基化阻挡层。由于苯基和苄基羟基化过程中中间体之间的电子差异，在介电常数较大的环境中，相比于苄基羟基化机理，苯羟基化过程具有相当大的稳定性。此外，我们计算了氘原子取代乙苯的一个或多个氢原子的动力学同位素效应，并研究了其对反应势垒的影响。因此，在介电常数大的介质中，在[H-10]乙苯和[D-10]乙苯之间发生区域选择性变化，由此氘代物质产生苯酚产物，而氢化物质主要产生1-苯乙醇产物。分析这种显着的新陈代谢转换，是由于1）C-H键与C-D键之间的强度差异以及2）介电常数大的介质中阳离子中间体的稳定性而发生的。我们将我们的计算结果与合成氧-铁-卟啉催化剂的实验工作以及酶反应性研究进行了比较。

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