On the antioxidant activity of ortho- and meta-substituted indolin-2-one derivatives

The antioxidant activity of ortho- and meta-substituted indolin-2-one derivatives was investigated in the gas phase and water. The reaction enthalpies of the individual steps of three antioxidant mechanisms of the studied derivatives were calculated and compared with the corresponding values of indolin-2-one. The results show that electron-withdrawing substituents increase the bond dissociation enthalpy and ionization potential, whereas electron-donating substituents increase the proton affinity. The indolin-2-one derivatives with the lowest bond dissociation enthalpy, ionization potential, and proton affinity values were identified as the compounds with high antioxidant activity. The results show that indolin-2-one derivatives with substituents in the ortho position are promising potential novel antioxidants. The results also show that the protective role of indolin-2-one derivatives occurs via hydrogen atom transfer and a sequential proton loss electron transfer mechanism in the gas phase and water, respectively. The calculated reaction enthalpies of the substituted indolin-2-ones are linearly dependent on the Hammett constants and E (HOMO) such that the latter can be utilized in the selection of suitable substituents for the synthesis of novel antioxidants based on indolin-2-one.