The [3+2] cycloaddition (32CA) reactions of the nitrile ylide (NY) with ethylene and with dicyanoethylene (DCE) have been studied using the Molecular Electron Density Theory through DFT calculations at the MPWB1K/6-31G(d) level. The analysis of the electronic structure of NY indicates that it presents a carbenoid structure with an sp2 lone pair at the carbon atom. While the 32CA reaction with ethylene presents a low activation energy, 6.1 kcal mol-1, the transition state structure associated with the 32CA reaction of NY with DCE is located 7.5 kcal mol-1 below the reagents, the reaction being completely regioselective. The topological analysis of the Electron Localisation Function (ELF) along the reaction path permits to establish a new model for the C-C bond formation characterised by the donation of the electron density of an sp2 carbon lone pair to the most electrophilic carbon atom of an electron-deficient ethylene. The carbenoid character of NY allows introducing a new type of 32CA reaction, carbenoid type (cb-type).
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