Distal substituents effect of 4-oxo-TEMPO derivatives bearing a benzyl group at the 2-position on the reduction resistance toward ascorbate.

Nitroxides, which have unpaired electrons, find diverse applications owing to their characteristic redox and radical chemistries. To fine-tune the properties of nitroxide compounds for various applications, we investigated the effect of distant substituents on their reactivity. We synthesized 4-oxo-2,2,6,6-tetramethylpiperidine-N-oxyl (4-oxo-TEMPO) derivatives with electron-donating or electron-withdrawing groups at the para position of the benzyl group attached to the 2-position. The reactivities of these compounds were evaluated by measuring their second-order reaction rate constants with ascorbate using ESR spectroscopy. Density functional theory (DFT) calculations for each compound revealed a correlation between the charge of the N-O moiety and the second-order reaction rate constant. Notably, even substituents positioned far from the nitroxide center significantly affected the reductive reactivity with ascorbate. These findings suggest that both proximal and distal structural modifications can be leveraged to fine-tune nitroxide properties, providing a basis for the rational design of nitroxides with tailored reactivities.