The mechanism of ionic Diels-Alder reactions. A DFT study of the oxa-Povarov reaction

The mechanism of the oxa-Povarov reaction of a cationic aryl oxonium with cyclopentene and styrene has been studied using DFT methods at the B3LYP/6-31G* level as a reaction model of ionic Diels-Alder (I-DA) reactions. Oxa-Povarov reactions are initialized by an I-DA reaction between the cationic aryl oxonium and alkenes to yield the corresponding formal [4(+) + 2] cycloadducts. The last step is a rapid loss of a proton giving chromans. While the I-DA reaction with cyclopentene takes place through a two-stage one-step mechanism, the presence of a phenyl substituent on styrene makes the mechanism of the I-DA reaction stepwise. An electron localization function (ELF) bonding analysis of selected points along the IRCs of these I-DA reactions allows the establishment of a great similarity in bond formation under both one-step and two-step mechanisms. The formation of the first C-C single bond begins in the short range of 1.95-1.90 angstrom, by the coupling of two pseudoradical centers generated at the most electrophilic center of the cationic aryl oxonium and the most nucleophilic centers of cyclopentene and styrene, resulting in the global charge transfer that takes place during I-DA reactions. The use of the recently proposed radical P-k(o) Parr functions allows the characterization of the most electrophilic centers in cationic species and the most nucleophilic centers in anionic species.