Understanding the Bond Formation in Hetero-Diels-Alder Reactions. An ELF Analysis of the Reaction of Nitroethylene with Dimethylvinylamine

The bonding evolution in hetero-Diels-Alder (HDA) reactions has been studied by an ELF analysis of the electron reorganization along the HDA reaction between nitroethylene 6 and dimethylvinylamine (DMVA) 9 at the B3LYP/6-31G* level. This cycloaddition takes place along a two-stages one-step mechanism. In the first stage of the reaction, the C1-C6 bond is formed by coupling of two pseudoradical centers positioned at the most electrophilic carbon of nitroethylene 6 and the most nucleophilic center of DMVA 9. In the second stage, the formation of the second O4-C7 bond takes place between a pseudoradical center positioned at the C7 carbon of DMVA and some electron-density provided by the lone pairs of the O4 oxygen. This behavior is in complete agreement with the analyses of the local electrophilicity and nucleophilicity indices, and the spin density of the radical anion of nitroethylene 6 and of the radical cation of DMVA 9. Finally, a relationship between the polar character of the reaction and the regioselectivity is established.