Carbon Dioxide, Part 3: Decarboxylation and Reduction

This presentation is about decarboxylations, that is reactions where an organic starting material releases CO2, and about the direct reduction of CO2 to formate, methanol, methane, and other small molecules. Some few examples have also been reported about the formation of oligomers upon reduction of carbon dioxide, like in the Fischer-Tropsch process, and I'll give you some references about this work, too. One type of decarboxylations are those, where release of carbon dioxide from a carboxylate would yield a well-stabilized carbanion of relatively low basicity. This is the reversal of the carboxylation of carbanions, and it proceeds more readily with decreasing basicity of the resulting carbanion. Typical examples would be the decarboxylation of malonic acids, beta-ketoacids, or nitroacetic acid. Intramolecular hydrogen bonding of the carboxyl group is not essential for these reactions, only a good electronic stabilization of the carbanion by electron-withdrawing substituents. A second group of carboxylic acids that readily undergo decarboxylation are those with a good leaving group in beta position. Here, bases can bring about decarboxylation and simultaneous elimination and formation of an alkene, as shown here. Carboxyl radicals readily decarboxylate because C-O pi bonds are stronger than C-C sigma bonds. Thus, decarboxylations can also be brought about by oxidizing carboxylic acids or their salts by single electron abstraction. If such an oxidation is performed electrochemically, for instance in methanol, then the resulting carbon-centered radicals formed on the anode surface can dimerize, in the so-called Kolbe reaction, not to be confused with the Kolbe carboxylation of phenols. Carboxyl radicals can also be generated chemically with some oxidant, and the radicals formed by decarboxylation can then be trapped with various reagents. In the Hunsdiecker reaction, for instance, these radicals are halogenated, but a number of other transformations have also been reported, including nitrations, aminations, cyanations, hydroxylations, thiolations, and arylations. You can find the references later in this presentation.

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