Mutual Activation of Two Radical Trapping Agents: Unusual "Win-Win Synergy" of Resveratrol and TEMPO during Scavenging of dpph • Radical in Methanol.

The reaction of the 2,2'-diphenyl-1-picrylhydrazyl radical (dpph • ) with resveratrol in methanol ( k MeOH = 192 M -1 s -1 ) is greatly accelerated in the presence of stable nitroxyl radical TEMPO • ( k mix MeOH = 1.4 × 10 3 M -1 s -1 ). This synergistic effect is surprising because TEMPO • alone reacts with dpph • relatively slowly ( k S = 31 M -1 s -1 in methanol and 0.03 M -1 s -1 in nonpolar ethyl acetate). We propose a putative mechanism in which a mutual activation occurs within the acid-base pair TEMPO • /RSV to the resveratrol (RSV) anion and TEMPOH •+ radical cation, both being extremely fast scavengers of the dpph • radical. The fast initial reaction is followed by a much slower but continuous decay of dpph • because a nitroxyl radical is recovered from the TEMPOnium cation, which is reduced directly by RSV/RSV - to TEMPO • or recovered indirectly via a reaction with methanol, producing TEMPOH subsequently oxidized by dpph • to TEMPO • .