Importance of π-Stacking Interactions in the Hydrogen Atom Transfer Reactions from Activated Phenols to Short-Lived <i>N</i>-Oxyl Radicals

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/H 2 O 2 oxidation of 1,8-DHN was found to proceed via an intense blue intermediate (λ max 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 phenoxyl radical and its subsequent coupling via the 2- and 4-positions giving extended quinone dimers with intense transitions in the visible range, consistent with UV–vis and LFP data. Overall, these results allowed to elucidate the mechanism of oxidative polymerization of 1,8-DHN of possible relevance to melanogenesis in fungi and other processes of environmental and astrochemical relevance.

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“…k H values are in line with those determined in a previous study for HAT from activated phenolic systems. 29,30…”

Section: Results and Discussionmentioning

confidence: 99%

Characterization and Fate of Hydrogen-Bonded Free-Radical Intermediates and Their Coupling Products from the Hydrogen Atom Transfer Agent 1,8-Naphthalenediol

et al. 2018

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“…Furthermore, in the series of group transfers, the PyfS‐group showed the lowest activation barrier of 6.9 kcal mol −1 . At the same time, other factors such as solvation and hydrogen bonding of reacting species may play important role [14, 15] …”

Section: Methodsmentioning

confidence: 99%

Using the Thiyl Radical for Aliphatic Hydrogen‐Atom Transfer: Thiolation of Unactivated C−H Bonds

et al. 2020

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A metal‐ and catalyst‐free thiyl‐radical‐mediated activation of alkanes is described. Tetrafluoropyridinyl disulfide is used to perform thiolation of the C−H bonds under irradiation with 400 nm light‐emitting diodes. The key C−H activation step is believed to proceed via hydrogen‐atom abstraction effected by the fluorinated thiyl radical. Secondary, tertiary, and heteroatom‐substituted C−H bonds can be involved in the thiolation reaction. The resulting sulfides have wide potential as photoredox‐active radical precursors in reactions with alkenes and heteroarenes.

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“…Analyzing antioxidant protection in terms of the intrinsic reactivity of the antioxidant species is, by far, the theoretical approach most frequently found in the literature . There are several reactivity indexes that have been used for that purpose.…”

Section: The Computational Approachmentioning

confidence: 99%

Computational strategies for predicting free radical scavengers' protection against oxidative stress: Where are we and what might follow?

Galano

Álvarez-Idaboy

2018

Int J of Quantum Chemistry

213

247

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Oxidative stress, which is frequently induced by an overproduction of free radicals (FR), poses a high risk to human health. Thus, finding efficient strategies for scavenging FR is a research area of current interest. Among many other aspects, this involves identifying chemical compounds capable of offering antioxidant protection (AOP) and quantifying such protection. This review summarizes different computational approaches that can contribute to gain a deeper knowledge on this subject. Several reaction mechanisms that may contribute to AOP are discussed, as well as some key factors influencing their relative importance including the chemical nature of the reacting FR, the polarity of the environment and the pH in aqueous solution. Kinetics‐based analyses to characterize antioxidants, through their FR scavenging activity, are presented. Trends in such activity, from the data currently available in the literature are provided. Some key aspects, regarding AOP, that still deserves further investigation, are discussed.

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Importance of π-Stacking Interactions in the Hydrogen Atom Transfer Reactions from Activated Phenols to Short-Lived N-Oxyl Radicals

Cited by 31 publications

References 109 publications

Characterization and Fate of Hydrogen-Bonded Free-Radical Intermediates and Their Coupling Products from the Hydrogen Atom Transfer Agent 1,8-Naphthalenediol

Characterization and Fate of Hydrogen-Bonded Free-Radical Intermediates and Their Coupling Products from the Hydrogen Atom Transfer Agent 1,8-Naphthalenediol

Using the Thiyl Radical for Aliphatic Hydrogen‐Atom Transfer: Thiolation of Unactivated C−H Bonds

Computational strategies for predicting free radical scavengers' protection against oxidative stress: Where are we and what might follow?

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