Cooperative electrocatalytic alcohol oxidation: quest for an optimal mediator system
23 June 2017
Campus Groenenborger, V0.08 - Groenenborgerlaan 171 - 2020 Antwerpen (route: UAntwerpen, Campus Groenenborger
Kourosch Abasspour Tehrani & Karolien De Wael
PhD defence Jeroen Lybaert - Faculty of Science, Department of Chemistry
In the future it is important to obtain a more sustainable organic synthesis. A possible strategy to make oxidation reactions more environmentally friendly is the use of electrochemical recycling of the catalyst.
Electrosynthesis in general, agrees well with the principles of sustainable chemistry, as it actually uses the electron as reagent. The surface of the electrode is at a certain potential capable of adding or taking away an electron and in this manner inducing the desired chemical modification.
By directly using electrons, it is possible to avoid waste as no stoichiometric oxidant or reductant has to be added.
Three different catalytic systems are investigated. It was evaluated if these systems can be used in an electrochemical approach for the oxidation of alcohols.
First system uses the popular tetrapropylammonium perruthenate (TPAP) catalyst. An electrochemical study of this catalyst is performed. Next the use of the electrochemical recycling of the catalyst is evaluated by performing electrolysis reactions. Unfortunately, it was concluded that there was a lack of selectivity when this system was used.
Therefore a different system for the oxidation of alcohols in the presence of a quinone and the Shvo catalyst is investigated. In this reaction, 2,6-dimethoxybenzoquinone serves as a mediator, transferring electrons from the reduced Shvo complex to the electrode surface. By means of electrochemical techniques the type of solvent, temperature and the influence of additives has been investigated. The electrochemical oxidation of hydroquinones in combination with the Shvo catalyst is an interesting alternative for environmentally friendly, high yield conversion of secondary alcohols into ketones.
To expand the scope of the electrolysis mediated by the hydroquinone/benzoquinone couple, the use of neocuproine ligated palladium catalysts is also examined. This catalyst is interesting as it is able to oxidize alcohols at room temperature and more important shows good selectivity for the oxidation of vicinal diols.