Synthesis and Reduction Kinetics of Five Ibuprofen-Nitroxides for Ascorbic Acid and Methyl Radicals

The hybrid compounds 1-5 comprised of five nitroxides with ibuprofen were synthesized and their reduction rate for ascorbic acid (AsA) and methyl radicals were measured in comparison with 3-hydroxy-tetramethylpyrrolidine-1-oxyl (PROXYL) 6. The rate constants in reduction reaction with 200-fold excess of AsA were determined in following order: 1 (0.42±0.06), 3 (0.17±0.06), 2 (0.10±0.05), and 6 (0.09±0.02 M^-1s^-1). The remaining two sterically shielded nitroxides 4 and 5 scarcely reacted with AsA. In the reaction with the more reactive methyl radicals, produced by 200-fold excess of Fenton's reagent, the reduction rates of 2, 4, and 5 were in the following decreasing order: 2 (1.1±0.2), 4 (0.76±0.09), and 5 (0.31±0.03 M^-1s^-1).