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

• Published in: Journal of organic chemistry Vol. 67; no. 15; pp. 5190 - 5196
• Main Authors: Foti, Mario C, Johnson, Erin R, Vinqvist, Melinda R, Wright, James S, Barclay, L. Ross C, Ingold, K. U
• Format: Journal Article
• Language: English
• Published: WASHINGTON American Chemical Society 26.07.2002; Amer Chemical Soc
• Subjects: Antioxidants - chemical synthesis; Antioxidants - chemistry; Antioxidants - pharmacology; Catechols - chemical synthesis; Catechols - chemistry; Catechols - pharmacology; Chemistry; Chemistry, Organic; Exact sciences and technology; Hydrogen - chemistry; Hydrogen Bonding; Kinetics; Kinetics and mechanisms; Molecular Structure; Naphthalenes - chemical synthesis; Naphthalenes - chemistry; Naphthalenes - pharmacology; Organic chemistry; Physical Sciences; Reactivity and mechanisms; Science & Technology; Thermodynamics; Vitamin E - pharmacology; OXIDATION; ESTROGENS; CHAIN-BREAKING ANTIOXIDANTS; PHENOLIC ANTIOXIDANTS; INHIBITED AUTOXIDATION; REACTIVITY; FLAVONOIDS; PEROXIDATION; DISSOCIATION ENTHALPIES; CYTOTOXICITY; Hydrogen transfer; Diol; Stabilization; Pyrocatechol; Antioxidant; Dissociation energy; Naphthalene derivatives; Chemical reaction kinetics; Free energy; Autoxidation; Reactivity; Organic free radical; Density functional method; Rate constant; Intramolecular hydrogen bond
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo020184v

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-t-octylphenyl)-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-di-tert-butylcatechol, 3. Compound 6 has almost twice the antioxidant activity, k ArOH/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, (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, αDOPPH • = 0.48 > αROO • = 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.