Participation of furoxancarbonitrile oxide in [3+2] cycloaddition reaction toward C-N triple bond: a Molecular Electron Density Theory study of regioselectivity and mechanistic aspect

The [3+2] cycloaddition (32CA) reaction between furoxancarbonitrile oxide (FNO 2) and electron-deficient 2,2,2-trichloroacetonitrile (TCAN 3) in the presence of chloroform was studied within the Molecular Electron Density Theory (MEDT), at the DFT-B3LYP/6-311G(d,p) computational level. This zwitterionic-type (zw-type) 32CA reaction takes place in a highly chemo- and regioselective manner, yielding oxadiazole 4 as the sole product of the reaction, in excellent agreement with the experimental findings. The very low polar character of this zw-type 32CA reaction accounts for the high activation barrier found for this 32CA reaction. A topological analysis of the electron localization function (ELF) over some relevant points of the reaction path permits establishing that this zw-type 32CA reaction takes place along a non-concerted two-stage one-step molecular mechanism. The ELF topological analysis evidences that formation of the C1-N8 and O3-C7 single bonds take place through the sharing of the part of the electron density of the N8 nitrogen and that of the O3 lone pairs toward, respectively, the C1 and C7 pseudoradical centers created along the reaction path.