Antiaromaticity-Promoted Radical Stability in α-Methyl Heterocyclics

Aromaticity is a fundamental and important concept in chemistry, and usually, the enhancement of aromaticity brings additional thermodynamic stability to a compound. Moreover, since radicals can act as intermediates in chemical reactions, they have attracted considerable attention from both experimental and theoretical chemists for a long time. However, it remains unclear whether there is a relationship between the thermodynamic stability of cyclic planar radicals and their aromaticity. In this work, using various aromaticity indices including anisotropy of the induced current density analysis and nucleus-independent chemical shifts against the radical stabilization energy, we systematically investigated the relationship between aromaticity and the thermodynamic stability of alpha-methyl heterocyclics. Density functional theory calculations suggest that the stronger the antiaromaticity of the original form heterocyclics, the higher the thermodynamic stability of the corresponding radicals, which is in sharp contrast to the general knowledge that aromaticity brings compounds' thermodynamic stabilities. The principal interacting spin orbital analysis shows that the stronger the pi-bond formed between the heterocyclics and the alpha-methyl carbon, the more spin density the radicals tend to be distributed on the heterocyclics. Thus, the strong pi-bonding is one of the factors for improving the thermodynamic stability of radicals.

Automated summary

Aromaticity is an important concept in chemistry, and enhancing it can bring thermodynamic stability to compounds. This study found that the stronger the antiaromaticity of heterocyclics, the higher the thermodynamic stability of the corresponding radicals, which is contrary to the general understanding of aromaticity enhancing stability. Strong pi-bonding between heterocyclics and alpha-methyl carbon was identified as a factor for improving the thermodynamic stability of radicals.