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

Abstract

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 [ΔG(PD)(XH)], hydrogen atoms [ΔG(HD)(XH)], and electrons [E(ox)(XH)] and the thermodynamic driving forces of the anions (X(-)) of the benzofuranones to release electrons [E(ox)(X(-))] were determined for the first time in DMSO. The ΔG(HD)(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 ΔG(PD)(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. E(ox)(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 E(ox)(X(-)) show that X(-) are good electron donors. The Gibbs free-energy changes of the radical cations (XH(+•)) to release protons [ΔG(PD)(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 ΔG(HD)(XH), ΔG(PD)(XH), ΔG(PD)(XH(+•)), E(ox)(XH), and E(ox)(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.

Dynamic covalent bond from first principles: Diarylbibenzofuranone structural, electronic, and oxidation studies

“…In Table , we present the bond lengths from our calculations; the geometry we obtained is in good agreement with previous studies on ABF analogs . The calculated bond length is 1.62 Å, which is also in agreement with the experimental value of 1.596 Å .…”

EPR spectrum of a radical from a nontypical antioxidant

“…Some controversy is found in the literature regarding the actual species that gives origin to radical 1 and whether hydrogen abstraction occurs from a keto – or from an enolic tautomer of HP‐136 . Whichever is its precursor, in both cases, radical 1 is postulated as the final product from the hydrogen‐abstraction process.…”

“…In spite of the applications of HP‐136 and of the interest generated by the radical formed from it, it is a bit surprising that direct and unequivocal evidence of radical 1 , provided by its electron paramagnetic resonance (EPR) spectrum, is not yet available. In this work, we therefore describe for the first time the EPR spectrum of radical 1 , analyzing its electron spin density distribution, in relation to its characteristically low reactivity.…”

Unusual Reaction Constant for Hydride Transfer from a Carbanion to 9‐Arylxanthyliums

“…For many important organic hydride-transfer reactions in CH 3 CN, thermodynamic problems have received good solutions, [2] because for many of the most important organic hydride donors and many typical organic hydride acceptors, such as olefins, imines, quinones, carbonyl compounds, and various organic cations, the free-energy changes associated with the release of hydride anions and the capture of hydride anions in CH 3 CN are available. [2][3][4][5][6][7][8][9][10][11] For kinetic problems, a classical but new kinetic equation to estimate activation energies of many hydride-transfer reactions has also been developed. [12] Thermodynamic analysis to elucidate the mechanism has been successfully applied in reaction systems such as caffeine, [6] vitamin C 4 , and carbanions.…”