Department of Chemistry, Kwame Nkrumah University of Science and Technology, Kumasi, Ghana.
Department of Chemistry, Kwame Nkrumah University of Science and Technology, Kumasi, Ghana.
J Mol Graph Model. 2018 May;81:1-13. doi: 10.1016/j.jmgm.2018.02.003. Epub 2018 Feb 13.
The mechanistic aspects of cycloaddition reactions of 1,2-cyclohexadiene with olefins and nitrones have been investigated with DFT calculations. The results show that the cycloaddition reactions of 1,2-cyclohexadiene with olefins do not go through a concerted pathway (one-step mechanism) but rather a stepwise one involving the formation of a biradical intermediate which then closes to form final cycloadduct. Electron-withdrawing substituents on the 1,2-cyclohexadiene decrease the activation barrier of the biradical-forming step but increase the barrier of the product-forming step and product stability, while electron-donating substituents on the 1,2-cyclohexadiene increase the barriers for both the biradical-forming step and the product-forming step but decrease the product stability. In the reaction of 1,2-cyclohexadiene with nitrones, the four pathways investigated have activation barriers within 1 kcal/mol of one another, the lowest being 10.45 kcal/mol and the highest 11.04 kcal/mol, indicating that these reactions are very unselective. Electron-withdrawing groups on the nitrone increase the stability of the resulting products whereas electron-donating group on the nitrone decrease the stability of the resulting products. The [3 + 2] cycloadduct proceeds to the formation of a more stable formal [5 + 2] cycloadduct if a phenyl substituent is present on the nitrogen of the nitrone.
用 DFT 计算研究了 1,2-环己二烯与烯烃和亚硝酮的环加成反应的机理。结果表明，1,2-环己二烯与烯烃的环加成反应不是通过协同途径（一步机制）进行的，而是通过形成双自由基中间体的分步途径进行的，然后该中间体闭环形成最终的环加成产物。1,2-环己二烯上的吸电子取代基降低了双自由基形成步骤的活化势垒，但增加了产物形成步骤和产物稳定性的势垒，而 1,2-环己二烯上的供电子取代基增加了双自由基形成步骤和产物形成步骤的势垒，但降低了产物稳定性。在 1,2-环己二烯与亚硝酮的反应中，研究了四条途径，它们的活化势垒彼此相差在 1 kcal/mol 以内，最低为 10.45 kcal/mol，最高为 11.04 kcal/mol，表明这些反应非常无选择性。亚硝酮上的吸电子基团增加了所得产物的稳定性，而亚硝酮上的供电子基团降低了所得产物的稳定性。如果亚硝酮上的氮上存在苯基取代基，则[3+2]环加成物会进一步形成更稳定的形式[5+2]环加成物。
