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

The decay of dpph(center dot) in absolute ethanol at 25 degrees 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 pi-stacked complex, formed between one curcumin anion (in the case of 1, 3 and 5 the enolate anion) and the picryl moiety of dpph(center dot), 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(alpha) for the forward reaction of 2, 5 and 1 with dpph(center dot) are in fact similar to 0.7.5 x 10(3) and 1.8 x 10(4) M-1 s(-1), respectively, in a clear dependence on the number of phenolic OHs.