Understanding the carbenoid-type reactivity of nitrile ylides in [3+2] cycloaddition reactions towards electron-deficient ethylenes: a molecular electron density theory study

The [3+2] Cycloaddition (32CA) reaction of nitrile ylide (NY) 10 with electron-deficient ethylene 11 has been studied within the molecular electron density theory through DFT calculations at the MPWB1K/6-31G(d) computational level. A structural analysis of NY 10 indicates that this three-atom component has a carbenoid structure, allowing its participation in carbenoid-type (cb-type) 32CA reactions. This 32CA reaction takes place through a one-step mechanism with very low activation energy, 2.3 kcal mol⁻¹. In gas phase, this 32CA reaction is not stereoselective and has low regioselectivity. Inclusion of solvent effects does not modify the activation energy, but increases the meta regioselectivity in clear agreement with the experimental outcomes. Electron localisation function topological analysis for the formation of the two C–C single bonds along the four competitive channels associated with this 32CA reaction makes it possible to characterise two dissimilar mechanisms. Along the more favourable meta regioisomeric channels, the 32CA reaction takes place through a two-stage one-step mechanism, while along the ortho regioisomeric channels it takes place via a synchronous C–C bond formation process.