化学
取代基
周环反应
区域选择性
过渡状态
路易斯酸
反应性(心理学)
路易斯酸催化
立体选择性
药物化学
亲核细胞
溶剂效应
计算化学
光化学
催化作用
有机化学
溶剂
医学
替代医学
病理
作者
Luís R. Domingo,Manuel Arnó,Juán Andrés
摘要
The molecular mechanisms for the inverse-electron-demand Diels−Alder reactions between nitroethene and three substituted ethenes (propene, methyl vinyl ether, and dimethylvinylamine) to give the corresponding nitroso cycloadducts have been characterized with density functional theory methods using the B3LYP/6-31G* calculational level. On the basis of stability arguments and molecular orbital analysis relative rates, regioselectivity, and stereoselectivity, the presence of Lewis acid catalyst modeled by the BH3 system and the inclusion of solvent effects as a function of the nature of substituent in the dienophile fragment are analyzed and discussed. The ortho attack mode presents transition structures more stable than the meta one. For the former, reactivity, endo selectivity, and asynchronicity are enhanced with the increase of the electron-releasing character of the substituent on dienophile fragment. The reaction between nitroethene and propene has dissymmetric concerted transition structures associated with a pericyclic process, while the reaction between nitroethene and dimethylvinylamine takes place along an asynchronous transition structure corresponding to a nucleophilic attack to nitroethene, with concomitant ring closure and without participation of zwitterionic intermediates. For the most unfavorable meta attack modes, the reactions have synchronous mechanisms that are not sensible to the substitution on the dienophile system. For the ortho channels, the inclusion of Lewis acid catalyst and solvent effects contributes to the charge-transfer process from the substituted ethenes to nitroethene and rate acceleration, as well as a significant increase of the endo stereoselectivity.
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