Theoretical Study of the Aromatic Properties and Reactivity of Carbon Atoms in a Group of Five-Membered Heterocycles Containing N-Oxide Bonds

In this study, we evaluated the structural and magnetic properties of a group of five-membered single-ring heterocyclic compounds containing N+–O– bonds. We used quantitative criteria to describe their aromaticity, including magnetic parameters like nuclear independent chemical shift, structural harmonic oscillator for aromaticity, and aromatic stability energy, as well as magnetic susceptibility and the HOMO–LUMO gap. For this study, all compounds were optimized using the B3LYP method and G**+311-6 basis set, and various aromaticity parameters were calculated at this level. It was found that the presence of the N‑oxide group decreased the diatropic flow within the ring, leading to a reduction in the compounds' aromaticity. Specifically, the N+–O– dipole enhanced electron polarization within the structure, resulting in decreased electron density within the ring. Then, with the assistance of molecular orbital analysis, the impact of the N-oxide group on the reactivity of these compounds was explored. It was noted that the N-oxide group alters the distribution of electron density on the carbon atoms of the ring, consequently enhancing the reactivity of C–H bonds with higher electron density to facilitate direct arylation reactions. Increasing the number of nitrogen atoms in the ring also increases the energy and electron density of these compounds.