The details of the radical - scavenging mechanism and activity of antioxidants became the subject of the wide interest. One of the most important ways of radical desactivation is Hydrogen Atom Transfer (HAT) from a phenol hydroxyl group to a radical: PhOH + X^· --> PhO^· + XH. For the long time HAT was considered as one step process and, in fact, such opinion is valid for non-polar solvents. However, the surprisingly high values of the rate constant for the reaction of phenols with 2,2-diphenyl-1-picrylhydrazyl radical carried out in alcohols and in other polar solvents lead to the discovery of Sequential Proton Loss Electron Transfer mechanism (SPLET) [1]. The first step of SPLET is the phenol deprotonation: PhOH = PhO^- + H⁺, then the fast electron transfer from the phenol anion to radical: PhO^- + X^· --> PhO^· + X^-, and the last step involves protonation of X^- to form neutral XH molecule. The SPLET mechanism can be clearly observed for the reactions carried out in ionization supporting solvents (notably methanol, ethanol and the mixtures of organic solvents with water) for phenols with low pK_a values and for electron-deficient radicals.

In our studies we described SPLET mechanism for the reactions of 2,2-diphenyl-1-picrylhydrazyl radical with a number of synthetic phenols [1, 2] and natural antioxidants (like curcumin and Vitamin E) [3]. In our presentation the above results of the studies on SPLET / HAT competition will be compared with the recent findings. Currently we study the structure-reactivity relationship for flavonoids and the simple phenols like catechols and resorcinols. The pK_a values measured in water-methanol mixtures (1:1) are correlated with the rate constants for phenol + 2,2-diphenyl-1-picrylhydrazyl reaction in alcohols indicating the crucial role of phenol acidity in the reaction kinetics governed by HAT / SPLET mechanisms. It was demonstrated, that depending on the favoured mechanism, the different hydroxyl groups in flavonoids are the most active sites in flavonoids structure.

[1] G. Litwinienko, K.U. Ingold, J. Org. Chem. 2004, 69, 5888-5896.

[2] G. Litwinienko, K. U. Ingold, J. Org. Chem. 2005, 70, 8982-8990.

[3] a) G. Litwinienko, K. U. Ingold, J. Org. Chem. 2005, 70, 8982-8990. b) M. Musialik, G. Litwinienko, Org. Letters 2005, 7, 4951-4954.

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