Naphthalene Diols: A New Class of Antioxidants Intramolecular Hydrogen Bonding in Catechols, Naphthalene Diols, and Their Aryloxyl Radicals

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TypeArticle
Journal titleJournal Of Organic Chemistry
ISSN00223263
Volume67
Issue15
Pages51905196; # of pages: 7
Abstract1,8-Naphthalenediol, 5, and its 4-methoxy derivative, 6, were found to be potent H-atom transfer (HAT) compounds on the basis of their rate constants for H-atom transfer to the 2,2-di(4-t-octylphenyl)-1-picrylhydrazyl radical (DOPPH), kArOH/DOPPH, or as antioxidants during inhibited styrene autoxidation, kArOH/ROO, initiated with AIBN. The rate constants showed that 5 and 6 are more active HAT compounds than the ortho-diols, catechol, 1, 2,3-naphthalenediol, 2, and 3,5-di-tert-butylcatechol, 3. Compound 6 has almost twice the antioxidant activity, kArOH/ROO = 6.0 ? 106 M-1 s-1, of that of the vitamin E model compound, 2,2,5,7,8-pentamethyl-6-chromanol, 4. Calculations of the O-H bond dissociation enthalpies compared to those of phenols, (BDEs), of 1-6 predict a HAT order of reactivity of 2 < 1 < 3 4 < 5 < 6 in general agreement with kinetic results. Calculations on the diols show that intramolecular H-bonding stabilizes the radicals formed on H-atom transfer more than it does the parent diols, and this effect contributes to the increased HAT activity of 5 and 6 compared to the activities of the catechols. For example, the increased stabilization due to the intramolecular H-bond of 5 radical over 5 parent of 8.6 kcal/mol was about double that of 2 radical over 2 parent of 4.6 kcal/mol. Linear free energy plots of log kArOH/DOPPH and log kArOH/ROO versus BDEs for compounds 1-6 along with available literature values for nonsterically hindered monophenols placed the compounds on common scales. The derived Evans-Polanyi constants from the plots for the two reactions, DOPPH = 0.48 > ROO = 0.32, gave the expected order, since the ROO reaction is more exothermic than the DOPPH reaction. Compound 6 is sufficiently reactive to react directly with oxygen, and it lies off the log kArOH/ROO versus BDE plot.
Publication date
Linkhttp://pubs3.acs.org/acs/journals/doilookup?in_doi=10.1021/jo020184v
AffiliationNational Research Council Canada; NRC Steacie Institute for Molecular Sciences
Peer reviewedNo
Identifier10008226
NPARC number12328000
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Record identifier2caff009-1177-4e3f-8e87-dd0b9aee4733
Record created2009-09-10
Record modified2016-05-09
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