Is the pericyclic transition structure of aza-Diels-Alder reaction aromatic?

The notion of aromaticity is an elusive concept, typically delineated as an electron delocalization pattern within a cyclic structure, and is characterized by unusual stability, reactivity, and other properties. Recently, the aromatic concept has been extended to inorganic molecules and saturated systems. In our work, the so-called pericyclic transition state structures (TSs) involved in aza-Diels-Alder reactions are investigated. Interestingly, geometries and energy barriers evidence that these reactions are highly energetic and are extremely asynchronous. Additionally the localized orbital locator, ELF and QTAIM topological analyses have proven that at least one pair of atoms is not bound at the TS configuration. Moreover, HOMA and PDI indexes show that these TSs where aromaticity is not important. Therefore, these results provoke us to rule out the aromatic character of these so-called pericyclic TSs structures.