Influence of reactant polarity on the course of the inverse-electron-demand Diels-Alder reaction.: A DFT study of regio- and stereoselectivity, presence of Lewis acid catalyst, and inclusion of solvent effects in the reaction between nitroethene and substituted ethenes

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.