Can the high reactivity of azomethine betaines in [3 + 2] cycloaddition reactions be explained using singlet-diradical character descriptors? What molecular mechanism is actually involved in these cycloadditions?
The [3 + 2] cycloaddition reaction of the simplest azomethine betaines, AZBs, as a class of three-atom-components, TACs, toward ethylene was theoretically studied at the DFT-B3LYP/6-31G(d) level. The high reactivity of AZBs in the studied reactions is reflected by the corresponding relatively low activation total electronic energies calculated at the very accurate MP4(SDTQ)/6-311++G(d,p) level. The singlet-diradical character of AZBs was estimated according to the two different descriptors based on the HF symmetry-broken approach and also by using the natural resonance theory (NRT). The high linear correlation coefficients found between the singlet-diradical character of AZBs and corresponding activation energies clearly show that the singlet-diradical character is responsible for the high reactivity of AZBs. An assessment of molecular orbital shapes in AZBs indicates that delocalization of the central nitrogen lone pair into the Pz atomic orbitals of terminal atoms accounts for the singlet-diradical character in AZBs. Our results also imply that the singlet-diradical characters are inversely proportional with the electronegativity of heteroatoms in AZBs. The ground electronic density transfer (GEDT), calculated at the transition state geometries using four different population schemes, predicts a low polar character for the investigated reactions which is in excellent agreement with the low difference in the global electrophilicity index of the reagents. Furthermore, the molecular mechanism involved in the studied reactions was characterized with the aid of ELF topological analysis supporting a non-concerted one-step mechanism; not a pericyclic concerted one as is provided in most textbooks.