Stable, aromatic, and electrophilic azepinium ions: Design using quantum chemical methods

Cyclic nitrenium ions containing five‐membered and six‐membered rings are available, however, the seven‐membered cyclic nitrenium ions (azepinium ions) are rare. The chemistry of these species is related to their stability originating from the aromaticity due to 6π electrons. Very few theoretical and experimental studies have been conducted on the azepinium ions. Related clozapine and olanzapine cations (diazepinium ions) were observed during drug metabolism studies. In this work, quantum chemical analysis has been carried out to estimate the stability, aromaticity, and electrophilicity of several derivatives of azepinium ions. A few of the designed azepinium ions carry ΔES‐T values in the range of 50 kcal/mol favoring singlet state; π donating groups at the 2nd position increase the singlet‐triplet energy differences. Most of the substituents reduce the NICS(1) values compared to the parent system. Ring fusion with heterocyclic five‐membered rings generally increases the aromaticity and the stability of the azepinium ion ring systems. The electrophilicity parameters estimated in terms of HIA, FIA, and ω values indicate that it is possible to fine‐tune the chemical properties of azepinium ions with appropriate modulation. The cyclic nitrenium ions with seven membered rings are aromatic but not very stable. Their electronic, energetic properties can be fine‐tuned and the species carrying ΔES‐T ≈ 50 kcal/mol may prove to be stable.