The use of redox mediators is a well-established approach in electrosynthesis to lower kinetic barriers, to improve product selectivity and to suppress electrode fouling.[1] Recently we have developed a new series of metal‐free and easy-to-synthesize redox mediators based on the phenanthro[9,10-d]imidazole framework (see figure below, left).[2] Upon anodic oxidation these compounds form stable radical cations which can be used to catalyze the oxidation of benzylic alcohols and ethers. By selection of a suitable substitution pattern the oxidation potential of the mediator can be adapted to the redox potential of a specific redox reaction.

Our efforts are currently focused on the immobilization of alkyne-modified phenanthroimidazoles on carbon electrodes via Cu(I)-catalyzed azide-alkyne cycloaddition (see figure below, right). Our goal is to simplify the product separation after an electrolysis and to improve the long-term stability of the mediator. By attachment to the electrode surface we were able to increase the turnover numbers by several orders of magnitude while maintaining reasonable reaction rates.

Further studies are directed towards a better understanding of the mechanism of the charge transfer between mediator and substrate as well as the chemical follow-up reactions. A combination of spectroelectrochemical experiments and computational studies will be used to discuss possible intermediates and transition states.

References:

[1] R. Francke, R. D. Little, Chem. Soc. Rev. 2014, 43, 2492.

[2] R. Francke, R. D. Little, J. Am. Chem. Soc. 2014, 136, 427.