Characterization and Fate of Hydrogen-Bonded Free-RadicalIntermediates and Their Coupling Products from the Hydrogen Atom Transfer Agent 18-Naphthalenediol

摘要

1,8-Naphthalenediol (dihydroxynaphthalene, 1,8-DHN) has been shown to be a potent hydrogen atom transfer (HAT) antioxidant compound because of the strong stabilization of the resulting free radical by intramolecular hydrogen bonding. However, the properties, reactivity, and fate of the 1,8-DHN phenoxyl radical have remained so far uncharted. Herein, we report an integrated experimental and computational characterization of the early intermediates and dimer products that arise by the oxidation of 1,8-DHN. Laser flash photolysis (LFP) studies of HAT from 1,8-DHN to the cumyloxyl and aminoxyl radicals showed the generation of a transient species absorbing at 350, 400, and >600 nm attributable to the 1,8-DHN phenoxyl radical. Peroxidase/H2O2 oxidation of 1,8-DHN was found to proceed via an intense blue intermediate (λmax 654 nm) preceding precipitation of a black melanin-like polymer. By halting the reaction in the early stages, three main dimers featuring 2,2′-, 2,4′-, and 4,4′-bondings could be isolated and characterized in pure form. Density functional theory calculations supported the generation of the 1,8-DHN phenoxylradical and its subsequent coupling via the 2- and 4-positions givingextended quinone dimers with intense transitions in the visible range,consistent with UV–vis and LFP data. Overall, these resultsallowed to elucidate the mechanism of oxidative polymerization of1,8-DHN of possible relevance to melanogenesis in fungi and otherprocesses of environmental and astrochemical relevance.