在碱性和酸性条件下通过 SN2 反应进行环氧化物开环的区域选择性,European Journal of Organic Chemistry

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在碱性和酸性条件下通过 SN2 反应进行环氧化物开环的区域选择性
European Journal of Organic Chemistry ( IF 2.5 ) Pub Date : 2020-06-23 , DOI: 10.1002/ejoc.202000590
Thomas Hansen _{1,

2} , Pascal Vermeeren ₁ , Anissa Haim ₁ , Maarten J. H. van Dorp ₁ , Jeroen D. C. Codée ₂ , F. Matthias Bickelhaupt _{1,

3} , Trevor A. Hamlin ₁

Affiliation

我们在 OLYP/TZ2P 上使用密度泛函理论对模型非对称环氧化物 2,2-二甲基环氧乙烷在碱性和酸性条件下的开环反应进行了量子化学分析。我们的组合活化应变和 Kohn-Sham 分子轨道分析方法首次揭示了控制这些化学反应区域选择性的物理因素的相互作用。碱性条件下的开环发生在亲核试剂 (OH-) 和环氧化物之间的强相互作用中，并且相互作用受空间（泡利）排斥控制。后者优先将攻击导向空间上较少阻碍的 C-beta。在酸性条件下，亲核试剂 (H2O) 和环氧化物之间的相互作用很弱，现在，区域选择性受活化应变的控制。环氧化物的质子化会导致较弱的 (CH3)(2)C-α-O 键的伸长，并有效地使底物预变形以在空间位阻更大的一侧进行攻击，从而降低整体应变能的不稳定。我们的定量分析显着建立在这些开环反应的区域选择性背后被广泛接受的基本原理上，并为理解这些必不可少的教科书反应提供了具体的框架。

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Regioselectivity of Epoxide Ring-Openings via SN2 Reactions Under Basic and Acidic Conditions

We have quantum chemically analyzed the ring-opening reaction of the model non-symmetrical epoxide 2,2-dimethyloxirane under basic and acidic conditions using density functional theory at OLYP/TZ2P. For the first time, our combined activation strain and Kohn-Sham molecular orbital analysis approach have revealed the interplay of physical factors that control the regioselectivity of these chemical reactions. Ring-opening under basic conditions occurs in a regime of strong interaction between the nucleophile (OH-) and the epoxide and the interaction is governed by the steric (Pauli) repulsion. The latter steers the attack preferentially towards the sterically less encumbered C-beta. Under acidic conditions, the interaction between the nucleophile (H2O) and the epoxide is weak and, now, the regioselectivity is governed by the activation strain. Protonation of the epoxide induces elongation of the weaker (CH3)(2)C-alpha-O bond, and effectively predistorts the substrate for the attack at the sterically more hindered side, which goes with a less destabilizing overall strain energy. Our quantitative analysis significantly builds on the widely accepted rationales behind the regioselectivity of these ring-opening reactions and provide a concrete framework for understanding these indispensable textbook reactions.

更新日期：2020-06-23

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