Quantitative evaluation of H-donating abilities of C(sp3)-H bonds of nitrogen-containing heterocycles in hydrogen atom transfer reaction

Nitrogen-containing heterocycles are an important class of antioxidants, and their reactivity and selectivity in hydrogen atom reactions have attracted significant interest from chemists. In this work, the kinetics of hydrogen atom transfer reactions from C(sp3)- H bonds of 28 nitrogen-containing heterocycles, oxygen-containing heterocycles, alicyclic amines and cycloalkanes, which were denoted as XH, to the CumOc radical, were investigated. The characteristic physical parameter of the substrate, i.e., the thermo-kinetic parameter Delta Gso(XH), was determined using the kinetic equation [Delta Gs XH/Y = DGso(XH) + Delta Gso(Y)] to quantitatively evaluate the H-donating ability of XH. The effects of the substrate structure, substituent attached to the nitrogen atom, and ring size on the H-donating ability were discussed carefully. By comparing the H-donating abilities of cycloalkanes, alicyclic amines and nitrogen/oxygen-containing heterocycles, the influence of the introduction of N, O, or carbonyl groups in the carbon ring on the H-donating ability of C(sp3)-H bond was determined. The electronic, steric and stereo-electronic effects of the groups were also discussed. Herein, we not only quantitatively determined the H-donating ability of the substrate, but also provided ideas for the synthesis of new antioxidants.

Automated summary

In this study, the kinetics of hydrogen atom transfer reactions from C(sp3)-H bonds of various nitrogen-containing and oxygen-containing heterocycles, alicyclic amines, and cycloalkanes to the CumOc radicals were investigated. The H-donating ability of the substrates was quantitatively evaluated using the thermo-kinetic parameter ΔGs0(XH), and the effects of substrate structure, nitrogen atom substituents, and ring size on H-donating ability were carefully discussed. The influence of N, O, or carbonyl groups on the H-donating ability of C(sp3)-H bonds and the electronic, steric, and stereo-electronic effects of the groups were also analyzed. The results not only provide quantitative information on H-donating ability but also suggest ideas for the synthesis of new antioxidants.