Elektrochemie als efficiënt middel voor organische synthese

Although the majority of scientists recognize the ecological impact of anthropogenic carbon dioxide, emissions are still increasing, mainly due to deforestation and burning of fossil fuels. In order to effectively reverse this process several parallel approaches are required. One interesting strategy consists in making carbon dioxide into a valuable resource for the production of fuels and chemicals, in an effort to close the carbon cycle. Carbon dioxide has a high thermodynamic stability, making it difficult to process in conventional thermocatalytic systems. However, an energy-efficient one-electron reduction allows to activate this highly stable molecule under mild and safe conditions. This work shows how carbon dioxide can be used as carbon source for the production of valuable carboxylic acids, through incorporation in organic substrates. The continuous and rapid improvement in electricity production from renewable resources (wind, water, sun, …) makes organic electrosynthesis into a very interesting and green alternative for traditional chemical processes. This synthetic strategy offers valorization routes for a wide range of renewable reactants. A first part focuses on the electrocarboxylation of conjugated dienes in the production of valuable dicarboxylic acids, which are potential polymer building blocks. The use of sacrificial anodes and careful optimization of reaction conditions allow to dicarboxylate internal conjugated dienes with good to excellent yields. Efficient CO2 fixation in conjugated linoleic acid is established for the first time, opening routes to other renewable dienes. The effect of molecular configuration and alkyl substitution on electrocarboxylation efficiency was studied. In a second part, an innovative, more sustainable, pathway was devised and elaborated for a related reaction, using an inert anode. A paired electrosynthesis of diacid and diol precursors from dienes and CO2 allows to increase the current and atom efficiency of the process. With tetraethylammonium trifluoroacetate both as electrolyte and reactant, 1,3-cyclohexadiene was converted to a dicarboxylate and diacetate product in a one-compartment setup. However, the reactivity of conjugated double bonds towards carboxylation and acetoxylation is highly dependent on alkyl substitution, complicating extension to other conjugated dienes. As a follow-up, a new paired electrosynthesis method was designed, allowing to convert carbon dioxide and alcohols into useful α-hydroxy acids, using inert electrodes. Anodic oxidation of alcohols to carbonyls and simultaneous cathodic carboxylation of these carbonyls with CO2 produces one sole product. Preliminary tests transformed 1-phenylethanol and benzhydrol to useful pharmaceutical intermediates in a one-compartment cell. A bromide-assisted decarboxylation of amino acids further demonstrates the potential of organic electrosynthesis in the valorization of renewable substrates. A wide range of naturally occurring amino acids can be converted efficiently to valuable nitriles in a single step, using bromide salts both as redox mediators and supporting electrolyte. Furthermore, the selectivity of the decarboxylation process can be tuned towards nitriles, amines or amides. In a last part, the electrochemical decarboxylation of amino acids with electrogenerated hypobromite was subjected to an electroanalytical study. By means of cyclic voltammetry, spectrophotometry and rotating ring disk electrode measurements, the effect of solvent, electrolyte and electrode material is confirmed and investigated. The water fraction and intrinsic acidity have a large effect on the hypobromite concentration. An efficient process is realized by finding a good balance between this concentration and the amino acid solubility.status: publishe