Mutual Activation of Two Radical Trapping Agents: Unusual “Win–Win Synergy” of Resveratrol and TEMPO during Scavenging of dpph(•) Radical in Methanol

[Image: see text] 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(•).