Carbon free-radicals are used as soluble redox mediators to catalyze ORR and enhance the discharge of Li–O₂ batteries.

We have employed the rotating ring disk electrode (RRDE) technique to study the oxygen reduction reaction (ORR) on gold and glassy carbon cathodes in dimethyl sulfoxide (DMSO) electrolytes containing lithium salts. At the gold ring electrode at 3.0 V vs. Li/Li⁺ (0.1 M LiPF₆) soluble superoxide radical anion undergoes oxidation to O₂ under convective-diffusion conditions. For both glassy carbon and gold cathodes, typical oxygen reduction current-potential curves are sensitive to rotation speed and undergo a maximum and further electrode passivation by formation of Li₂O₂ while the Au ring electrode currents follow the same peak shape with detection of soluble superoxide at the ring downstream in the electrolyte solution. Unlike the behavior in acetonitrile-lithium solutions, LiO₂ is more stable in DMSO and can diffuse out in solution and be detected at the ring electrode. While in cyclic voltammetry both time and potential effects are convoluted, we have carried out RRDE chrono-amperometry experiments at the disk electrode with detection of superoxide at the Au ring so that thus potential and time effects were clearly separated. The superoxide oxidation ring currents exhibit a maximum at 2.2 V due to the interplay of O₂⁻ formation by one-electron O₂ reduction, Li₂O₂ disproportionation and two-electron O₂ reduction.

Objectives: Many natural antioxidants have poor pharmacokinetic properties that impair their therapeutic use. For hydroxycinnamic acids (HCAs) and other phenolic antioxidants, their major drawback is their low lipophilicity and a rapid metabolism. The difluoromethyl group may be considered as a ‘lipophilic hydroxyl' due to its hydrogen bond donor and acceptor properties; this prompted us to assess it as a bioisosteric replacement of a phenolic hydroxyl for increasing the lipophilicity of HCAs. Methods: Six difluoromethyl-substituted methyl cinnamates (4a-c, 5a-c) related to caffeic acid were synthesized and their antioxidant activity evaluated by chemical (FRAP, DPPH scavenging, inhibition of β-carotene bleaching, at 1–200 μm), electrochemical (differential pulse voltammetry, cyclic voltammetry) and cell-based (inhibition of lipid peroxidation in erythrocytes, at 1 and 50 μm) assays. Key fndings: Analogues 4a-c and 5a-c were inactive in FRAP and DPPH assays and only those containing a free phenolic hydroxyl (4a and 5a) exhibited electrochemical activity although with high redox potentials. Compounds 4a,b and 5a,b were active in the inhibition of β-carotene bleaching assay and all analogues inhibited lipid peroxidation in the human erythrocytes assay. Conclusions: Lipophilic difluoromethyl-substituted cinnamic esters retain radical scavenging capabilities that prove useful to confer antioxidant properties in a non-polar environment.