Insights into the mechanism and stereoselectivity of the [3+2] cycloaddition reaction between N-methyl-C-(4-hydroxylphenyl) nitrone and maleic anhydride with a molecular electron density theory perspective

The [3+2] cycloaddition (32CA) reaction of N-methyl-C-(4-hydroxylphenyl) nitrone 1 and maleic anhydride 2 has been investigated using molecular electron density theory (MEDT) at the MPWB95/6-311++G(d,p) computational level. This 32CA reaction undergoes two stereo- and stereoisomeric reaction paths to form two different products 3 and 4. An electron localization function (ELF) study predicts that the N-methyl-C-(4-hydroxylphenyl) nitrone 1 has a zwitterionic character and it takes place through a one-step mechanism, with activation enthalpies in between 17.48 and 23.41 kJ mol−1 in the gas phase. The CDFT indices are used to forecast the global electron density flux from the strong nucleophilic N-methyl-C-(4-hydroxylphenyl) nitrone 1 to the electrophilic maleic anhydride 2. These exergonic 32CA reactions have negative Gibbs free energy along the endo and exo stereochemical routes. The endo stereochemical process is favored over the exo stereochemical pathway due to the increased thermodynamic stability of the cycloadduct. Bonding evolution theory (BET) predictions for the endo and exo routes indicate a one-step process with early transition states, which is consistent with the ELF topological investigation at the transition states.