Uncatalyzed and BF₃-catalyzed oxa-Diels–Alder reaction of ethyl vinyl sulfide (EVS) and β-methyl-α-phenylacrolein (ACR) experimentally explored by Ishihara and coworkers is investigated by employing molecular electron density theory. Based on their report, the titled reaction resulted in the formation of two cis and trans cycloadducts with a ratio of 86:14. Conceptual density functional theory analysis indicated that EVS and ACR should act as nucleophile and electrophile, respectively. Coordination of the BF₃ LA increases significantly the electrophilicity power of ACR. Regioselectivity in the studied reaction was investigated by calculation of the Parr functions, and a good agreement was observed between both theoretical and experimental results. Furthermore, the diastereoselectivity of both catalyzed and uncatalyzed reactions was studied by using potential energy surface analysis, and a satisfactory agreement was observed with the experimental outcomes. Reduction of the activation barriers for the catalyzed reaction, relative to the uncatalyzed one, rationalized why the reaction should be performed experimentally at a very low temperature (−40 °C). Mechanistic studies indicated that in contrast to the uncatalyzed reaction, the catalyzed reaction takes place in two steps in dichloromethane and involves formation of a stable zwitterionic intermediate. QTAIM analysis indicated that the interaction of the BF₃ LA with ACR during the reaction has non-covalent character with partial covalent one. Finally, NCI analysis of TSs explained satisfactorily the preference formation of the cis cycloadduct.

Graphic abstract

中文翻译：

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BF3催化的乙基乙烯基硫醚和β-甲基-α-苯基丙烯醛的oxa-Diels-Alder反应：分子电子密度理论研究

Ishihara 及其同事通过分子电子密度理论研究了未催化和 BF ₃催化的乙基乙烯基硫化物 ( EVS ) 和β-甲基-α-苯基丙烯醛 ( ACR ) 的oxa-Diels-Alder 反应。根据他们的报告，标题反应导致形成两个比例为 86:14 的顺式和反式环加合物。概念密度泛函理论分析表明，EVS和ACR应分别充当亲核试剂和亲电试剂。BF ₃ LA 的配位显着增加了ACR。通过计算帕尔函数来研究所研究反应中的区域选择性，并且在理论和实验结果之间观察到良好的一致性。此外，通过使用势能表面分析研究了催化和未催化反应的非对映选择性，并观察到与实验结果令人满意的一致性。相对于未催化反应，催化反应的活化势垒减少，这解释了为什么该反应应该在非常低的温度 (-40 °C) 下进行实验。机理研究表明，与未催化反应相比，催化反应在二氯甲烷中分两步进行，并涉及形成稳定的两性离子中间的。QTAIM分析表明，反应过程中BF ₃ LA与ACR的相互作用具有非共价性，部分共价性。最后，TSs 的 NCI 分析令人满意地解释了顺式环加合物的偏好形成。

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