The Reaction of Sulfenic Acids with Peroxyl Radicals: Insights into the Radical-Trapping Antioxidant Activity of Plant-Derived Thiosulfinates

Sulfenic acids play a prominent role in biology as key participants in cellular signaling relating to redox homeostasis, in the formation of protein-disulfide linkages, and as the central players in the fascinating organosulfur chemistry of the Allium species (e.g., garlic). Despite their relevance, direct measurements of their reaction kinetics have proven difficult owing to their high reactivity. Herein, we describe the results of hydrocarbon autoxidations inhibited by the persistent 9-triptycenesulfenic acid, which yields a second order rate constant of 3.0 x 106?M-1?s-1 for its reaction with peroxyl radicals in PhCl at 30 degrees C. This rate constant drops 19-fold in CH3CN, and is subject to a significant primary deuterium kinetic isotope effect, kH/kD=6.1, supporting a formal H-atom transfer (HAT) mechanism. Analogous autoxidations inhibited by the Allium-derived (S)-benzyl phenylmethanethiosulfinate and a corresponding deuterium-labeled derivative unequivocally demonstrate the role of sulfenic acids in the radical-trapping antioxidant activity of thiosulfinates, through the rate-determining Cope elimination of phenylmethanesulfenic acid (kH/kD4.5) and its subsequent formal HAT reaction with peroxyl radicals (kH/kD3.5). The rate constant that we derived from these experiments for the reaction of phenylmethanesulfenic acid with peroxyl radicals was 2.8 x 107?M-1?s-1; a value 10-fold larger than that we measured for the reaction of 9-triptycenesulfenic acid with peroxyl radicals. We propose that whereas phenylmethanesulfenic acid can adopt the optimal syn geometry for a 5-centre proton-coupled electron-transfer reaction with a peroxyl radical, the 9-triptycenesulfenic is too sterically hindered, and undergoes the reaction instead through the less-energetically favorable anti geometry, which is reminiscent of a conventional HAT.