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Actual Structure, Thermodynamic Driving Force, and Mechanism of Benzofuranone-Typical Compounds as Antioxidants in Solution

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journal contribution
posted on 07.04.2011, 00:00 by Xiao-Qing Zhu, Jian Zhou, Chun-Hua Wang, Xiu-Tao Li, Sha Jing
5,7-Ditert-butyl-3-(3,4-dimethylphenyl)benzofuran-2(3H)-one (HP-136) (1H) and its 30 analogues (2H−5H) as benzofuranone-typical antioxidants were synthesized. The structures of the benzofuranones in solid and solution were examined by using experimental and theoretical methods. The results show that the dominant structure is the lactone form rather than the enol form both in solid and solution. The thermodynamic driving forces of the 31 benzofuranone-typical compounds to release protons [ΔGPD(XH)], hydrogen atoms [ΔGHD(XH)], and electrons [Eox(XH)] and the thermodynamic driving forces of the anions (X) of the benzofuranones to release electrons [Eox(X)] were determined for the first time in DMSO. The ΔGHD(XH) scale of these compounds in DMSO ranges from 65.2 to 74.1 (kcal/mol) for 1H−4H and from 73.8 to 75.0 (kcal/mol) for 5H, respectively, which are all smaller than that of the most widely used commercial antioxidant BHT (2,6-ditert-butyl-4-methylphenol, 81.6 kcal/mol), suggesting that the 31 XH could be used as good hydrogen-atom-donating antioxidants. The ΔGPD(XH) were observed to range from 11.5 to 16.0 (kcal/mol) for 1H−4H and from 18.6 to 22.4 (kcal/mol) for 5H, indicating that benzofuranones (1H−4H) are good proton donors, and their analogues (5H) should belong to middle-strong proton donors. Eox(XH) of the 31 XH to release an electron vary from 1.346 to 1.962 (V versus Fc+/0), implying that the 31 XH are weak electron donors, whereas the quite negative Eox(X) show that X are good electron donors. The Gibbs free-energy changes of the radical cations (XH+•) to release protons [ΔGPD(XH+•)] were evaluated according to the corresponding thermodynamic cycle, and the results reveal that XH+• are good proton donors. Further inspection of our experimental results showed the ΔGHD(XH), ΔGPD(XH), ΔGPD(XH+•), Eox(XH), and Eox(X) of the five chemical and electrochemical processes are all linearly dependent on the sum of Hammett substituent parameters σ with very good correlation coefficients, indicating that for any one- or multisubstituted species at the para- and/or meta-position of benzofuranones and their various reaction intermediates, the five thermodynamic driving force parameters all can be easily and safely estimated from the corresponding Hammett substituent parameters. The rates of hydrogen atom transfer from XH to DPPH were determined by using the UV−vis absorption spectroscopy technique. Combining these important thermodynamic parameters and dynamic determination results, the mechanism of hydrogen transfer from HP-136 and its analogues to DPPH was studied. The results suggest that the hydrogen transfer from HP-136 and its analogues 2H to DPPH actually includes two steps, proton transfer and the following electron transfer, but the proton transfer is rate-determined.