Unveiling the intramolecular [3+2] cycloaddition reactions of C,N-disubstituted nitrones from the molecular electron density theory perspective

The zwitterionic (zw-) type intramolecular [3 + 2] cycloaddition (IM32CA) reactions of C,N-disubstituted nitrones bearing acyclic and cyclic alkene functionalities have been studied from the molecular electron density theory (MEDT) perspective. The nitrones under study were classified as zwitterionic species from the topological analysis of the electron localization function (ELF). These IM32CA reactions show non-polar character and follow a non-concerted one step mechanism with activation free energies of 22.8-35.2 kcal mol(-1) in toluene. When the N-butenyl system is attached to the nitrogen atom of the nitrone, the IM32CA reaction presents total regioselectivity with the terminal alkene carbon involved in C-C bond formation in complete agreement with the experimental findings. Interestingly, the activation free energy shows a sharp increase of 6.1 kcal mol(-1) on dimethyl substitution of the terminal carbon owing to the considerable increase in the non-covalent strongly repulsive interaction revealed by the reduced density gradient scatter map of the NCI plot. This agrees well with the experiments showing no reaction of the corresponding nitrone. For the nitrones with a cyclic alkene functionality, the activation energies are decreased by 5.1-6.2 kcal mol(-1) compared to the acyclic one, due to the relatively lower energy cost demanded for the similar bonding changes along the reaction path.