Kinetics of curcumin oxidation by 2,2-diphenyl-1-picrylhydrazyl (DPPH˙): an interesting case of separated coupled proton–electron transfer

Abstract

The decay of dpph˙ in absolute ethanol at 25 °C and in the presence of curcumin (1), 4-methylcurcumin (3), 4,4-dimethylcurcumin (4) or curcumin 4′-methyl ether (5) follows bi-exponential kinetics. These unusual reaction kinetics are compatible with a two-step process in which an intermediate accumulates in a reversible first step followed by an irreversible process. As in other similar cases (Foti et al., Org. Lett., 2011, 13, 4826–4829), we have hypothesised that the intermediate is a π-stacked complex, formed between one curcumin anion (in the case of 1, 3 and 5 the enolate anion) and the picryl moiety of dpph˙, in which an intra-complex electron transfer from the (enolate) anion takes place. By comparing the kinetics of curcumin 4′,4′′-dimethyl ether (2) (no phenolic OH), (5) (one phenolic OH) and (1) (two phenolic OHs), we have deduced that the electron transfer process must be accompanied by a simultaneous proton transfer from the phenolic OHs to the bulk solvent (separated coupled proton–electron transfer). The rate constants k_α for the forward reaction of 2, 5 and 1 with dpph˙ are in fact ∼0, 7.5 × 10³ and 1.8 × 10⁴ M⁻¹ s⁻¹, respectively, in a clear dependence on the number of phenolic OHs.