Insights into the electrochemical radical reaction of polycyclic aromatic amines from Clar’s aromatic π-sextet rule and NICS criteria

Abstract

This article takes the representative ternary and quaternary polycyclic aromatic amines (PAAs) isomers such as phenanthamine and pyreneamine as examples, and uses the Clar’s aromatic π-sextet rule combined with nucleus independent chemical shift (NICS) criteria and structural and electronic indices (such as HOMA, MCBO and MCI), to study their structure–reactivity relationship in electrochemical radical reactions based on the experimental results. By analyzing and discussing the changes in aromaticity and bond length of various ring systems in the structures of the substrates, radical intermediates, and products, as well as radical stability, we have gained a deeper understanding of the reaction mechanism and regioselectivity for construction of large π-conjugated frameworks of nitrogen fused helicenes and functional polymers through electrochemical tandem reactions of PAAs. In general, resonant structures containing more Clar’s aromatic π-sextet in the structure have higher stability, which determines the reaction pathway and types of the product. In this study, Clar’s rule, NICS criteria, HOMA, MCBO and MCI mutually supported and validated each other well, demonstrated that the combination of Clar’s rule and NICS criteria may be a beneficial tool for analyzing and predicting the reactivity of PAH derivatives and the structure of complex polycyclic aromatic products, moreover, expanding their application to non-planar heteroatom doped helicenes beyond PAH compounds.