methyl-3-4-dihydroxybenzoate and 1-1-diphenyl-2-picrylhydrazyl

This work evaluates the reduction kinetics of the antiradical probe 2,2-diphenyl-1-picrylhydrazyl (DPPH (*)) in methanol and acetonitrile by the antiradical activity of protocatechuic acid (3,4-dihydroxybenzoic acid, 1) and protocatechuic acid methyl ester ( 2). The reduction kinetics of DPPH (*) in both solvents by the antiradical activity of the p-catechol group in 2 is regular, that is, coincide with the proposed standard kinetic model for the reduction kinetics of DPPH (*) by the antiradical activity of an isolated p-catechol group. Therefore, the antiradical activity of 2 experimentally exhibits two rate-two stoichiometric constants in acetonitrile and three rate--three stoichiometric constants in methanol. In contrast, the reduction kinetics of DPPH (*) in both solvents by the antiradical activity of the p-catechol group in 1 is perturbed, that is, deviate from the proposed standard kinetic model. The deviations arise from the presence of the reactive carboxylic acid function which, in methanol, induces an additional reversible side reaction and, in acetonitrile, turns an irreversible reaction reversible, thus modifying the otherwise regular reduction kinetics of DPPH (*) by the antiradical activity of the p-catechol group in 1. On the other hand, the approximated theoretical kinetic equation that applies for those p-catechol groups whose reduction kinetics is regular and that experimentally exhibit three rate--three stoichiometric constants has been derived and used for fitting.

Topics: Acetonitriles; Biphenyl Compounds; Free Radical Scavengers; Hydroxybenzoates; Kinetics; Methanol; Models, Chemical; Molecular Structure; Oxidation-Reduction; Picrates

A novel oxidative dimer was isolated as a major product from a reaction mixture of methyl protocatechuate and DPPH radical in methanol. Its unusual benzobicyclo[3.2.1]octane structure was elucidated by extensive spectral analysis. This result suggests that the regeneration of catechol structures by the nucleophilic addition of an alcohol molecule on o-quinones and subsequent dimerization is one of the key reactions in the high radical-scavenging activity of protocatechuic esters in an alcoholic solvent.

Topics: Biphenyl Compounds; Catechols; Dimerization; Free Radical Scavengers; Hydrazines; Hydroxybenzoates; Molecular Structure; Oxidants; Picrates