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 (kMeOH = 192 M–1 s–1) is greatly accelerated in the presence of stable nitroxyl radical TEMPO• (kmixMeOH = 1.4 × 103 M–1 s–1). This synergistic effect is surprising because TEMPO• alone reacts with dpph• relatively slowly (kS = 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•