Understanding the Reactivity of C-Cyclopropyl-N-Methylnitrone Participating in [3+2] Cycloaddition Reactions Towards Styrene with a Molecular Electron Density Theory Perspective

The [3+2] cycloaddition (32CA) reactions of C-cyclopropyl-N-methylnitrone 1 with styrene 2 have been studied within molecular electron density theory (MEDT) at the B3LYP/6-311++G(d,p) level of theory. These zwitterionic type 32CA reactions occur through a one-step mechanism. The 32CA reactions undergo four stereoand regioisomeric reaction paths to form four different products, 3, 4, 5 and 6. Analysis of the conceptual density functional theory (CDFT) indices predict the global electronic flux from the strong nucleophilic nitrone 1 to the styrene 2. These 32CA reactions are endergonic with reactions Gibbs free energies between 2.83 and 7.39 kcal.mol in the gas phase. The 32CA reaction leading to the formation of cycloadduct 3 presents the lowest activation enthalpy than the other paths due to a slightly increase in polar character evident from the global electron density transfer (GEDT) at the transition states and along the reaction path. The bonding evolution theory (BET) study suggests that these 32CA reactions occur through the coupling of pseudoradical centers and the formation of new C-C and C-O covalent bonds has not been started in the transition states.