UNVEILING THE EXCLUSIVE STEREO AND SITE SELECTIVITY IN [3+2] CYCLOADDITION REACTIONS OF A TRICYCLIC STRAINED ALKENE WITH NITRILE OXIDES FROM THE MOLECULAR ELECTRON DENSITY THEORY PERSPECTIVE

Abstract

The [3+2] cycloaddition reactions of formonitrile oxide and benzonitrile oxide with a tricyclic strained alkene bearing norbornene and cyclohexene double bonds have been studied from the molecular electron density theory perspective at the MPWB1K/6-311G(d,p) computational level. Electron localization function shows the absence of pseudoradical and carbenoid centers, classifying formonitrile and benzonitrile oxides as zwitterionic three-atom components, consistent with the high activation free energies of 26.0 and 28.5 kcal·mol^–1, respectively, in their cycloaddition reaction with the strained alkene in CH₂Cl₂. These reactions follow a one-step mechanism under kinetic control and present total site selectivity, as the addition of formonitrile and benzonitrile oxides to the norbornene double bond is energetically preferred by 4.9 and 8.0 kcal·mol^–1, respectively, over the cyclohexene double bond in agreement with the experiments, and complete exo-stereoselective control is predicted. The minimal global electron density transfer predicts nonpolar character, while the electron localization function topological analysis implies that the activation energy is related only to the formation of non-bonding electron density at N2 nitrogen and pseudoradical center at C3 atom of the nitrile oxides. The total electron densities less than 0.1 e and positive Laplacian of electron density at the forming C–C and C–O bond critical points of the early transition states indicate noncovalent interactions which were characterized by visualization of the AIM-RDG isosurfaces.