Understanding the High Reactivity of the Azomethine Ylides in [3 + 2] Cycloaddition Reactions

The electronic reorganization along the [3+2] cycloaddition (32CA) reaction of the simplest azomethine ylide (AY) with ethylene has been studied using the topological analysis of the ELF at the B3LYP/6-31G(d) level of theory. This 32CA reaction, which has a negligible barrier (1.2 kcal/mol), takes place through a synchronous concerted transition state structure. The analysis of the ELF stresses that the pseudo-diradical character of the AY is responsible for the large reactivity of this species. Only the two non-bonding electrons located at the terminal carbon atoms of the AY and the two π electrons of ethylene are involved in this 32CA reaction. This cycloaddition, certainly not a pericyclic electron reorganization, may be electronically classified as [2n + 2π] process.