Unveiling Intramolecular [3+2] Cycloaddition Reactions Leading to Functionalized Heterocycles in the Light of Molecular Electron Density Theory.

The relative reactivity and cis/trans selectivity of the intramolecular [3+2] cycloaddition (IM32CA) reactions of nitrile oxide (NO), azide (AZ), nitrile sulfide (NS) and nitrile ylide (NY), leading to functionalized heterocycles are studied within the Molecular Electron Density Theory. The kinetically controlled IM32CA reactions are predicted to be cis stereospecific, while the reaction feasibility follows the order NY > NS > NO > AZ with the respective activation Gibbs free energies of 13.7, 17.8, 21.1 and 27.3 kcal·mol-1 in benzene at 353 K. The decreased activation energy of NY could be correlated with its carbenoid character, relative to the zwitterionic one predicted for the other species. The AZ reaction showed relatively higher activation parameter, and accordingly higher than that of the NO one, in conformity with the experimental outcomes. Topological analysis of the electronic structure of the transition state structure shows that the formation of the new single bonds has not yet started on any of them according to a non-concerted mechanism. Analysis of the kinetic parameters of the intermolecular [3+2] cycloaddition reactions of NOs and AZs shows that the low activation entropies associated with the intramolecular processes are the factor responsible for the feasibility of these non-polar zw-type IM32CA reactions.