Effects of metal ions distinguishing between one-step hydrogen- and electron-transfer mechanisms for the radical-scavenging reaction of (+)-catechin

A kinetic study of a hydrogen-transfer reaction from (+)-catechin (1) to galvinoxyl radical (G^·) has been performed using UV-vis spectroscopy in the presence of Mg(ClO₄)₂ in deaerated acetonitrile (MeCN). The rate constants of hydrogen transfer from 1 to (G^·) determined from the decay of the absorbance at 428 nm due to (G^·) increase significantly with an increase in the concentration of Mg²⁺. The kinetics of hydrogen transfer from 1 to cumylperoxyl radical has also been examined in propionitrile (EtCN) at low temperature with use of ESR. The decay rate of cumylperoxyl radical in the presence of 1 was also accelerated by the presence of scandium triflate [Sc(OTf)₃ (OTf = OSO₂CF₃)]. These results indicate that the hydrogen-transfer reaction of (+)-catechin proceeds via electron transfer from 1 to oxyl radicals followed by proton transfer rather than via a one-step hydrogen atom transfer. The coordination of metal ions to the one-electron reduced anions may stabilize the product, resulting in the acceleration of electron transfer.