Naphthalene diols: A new class of antioxidants intramolecular hydrogen bonding in catechols, naphthalene diols, and their aryloxyl radicals
1,8-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-toctylphenyl)-1-picrylhydrazyl radical (DOPPH ·), k ArOH/DOPPH ·, or as antioxidants during inhibited styrene autoxidation, k ArOH/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-ditert-butylcatechol, 3. Compound 6 has almost twice the antioxidant activity, k ArOH/ROO · = 6.0 × 10 6 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, (ΔBDEs), 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 k ArOH/DOPPH · and log k ArOH/ROO · versus ΔBDEs 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, α DOPPH · = 0.48 > α 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 k ArOH/ROO · versus ΔBDE plot.
|Journal||Journal of Organic Chemistry|
Foti, M.C. (Mario C.), Johnson, E.R. (Erin R.), Vinqvist, M.R. (Melinda R.), Wright, J.S, Barclay, L.R.C. (L. Ross C.), & Ingold, K.U. (2002). Naphthalene diols: A new class of antioxidants intramolecular hydrogen bonding in catechols, naphthalene diols, and their aryloxyl radicals. Journal of Organic Chemistry, 67(15), 5190–5196. doi:10.1021/jo020184v