What are alkenes?
Alkenes are hydrocarbons that contain C=C bonds (carbon-carbon double bonds). It belongs to unsaturated hydrocarbons and can be divided into alkenes and cycloalkenes. Alkenes have the general formula CnH2n and have always played the most important role in the field of synthesis.
New application of alkenes in latest research
The Kolbe coupling reaction (reported by Kolbe in 1849) is one of the oldest electrochemical reactions and probably the first in the history of organic synthesis to build a C–C bond. Generally speaking, in the Kolbe reaction, two molecules of carboxylic acid undergo electrochemical decarboxylation coupling on the electrode, release carbon dioxide, and form Csp3–Csp3 bonds. However, due to harsh reaction conditions, low chemoselectivity, and reliance on noble metal electrodes, the scope of application of this reaction has been rather limited for many years, and it is very rare in mainstream organic synthesis. In recent years, synthetic chemists have recognized the use of different waveforms to deliver electrical current as a means of enhancing reactivity and selectivity.
Recently, some researchers have provided a simple and easy-to-operate solution for the Kolbe coupling reaction by using waveform-controlled electrosynthesis technology. Based on this reaction, researchers have prepared a variety of valuable molecules, including unnatural amino acids and polymer building blocks, using readily available raw materials such as alkenes and carboxylic acids. Mechanistic studies reveal that the waveform plays an important role in modulating the local pH around the electrode, and acetone also plays a key role as an unconventional reaction solvent.
First, the researchers used biomass-derived carboxylic acid-10-undecenoic acid as the template substrate to screen the reaction conditions. Preliminary studies have shown that direct current electrolysis of Pt electrodes under traditional conditions did not obtain dimerization products due to electrode passivation. The screening of the electrodes found that the reaction can be carried out smoothly without Pt electrodes, and the type of carbon electrodes is crucial to the reaction, among which reticulated vitreous carbon (RVC) works best. In addition, this reaction can also use cheap and readily available potassium hydroxide (KOH) instead of Me4N•OH. Secondly, the researchers also explored the rAP-Kolbe reaction using 4-phenylbutyric acid as a template substrate, the results showed that under the traditional Kolbe reaction conditions, the passivation of the Pt electrode led to the cessation of the reaction, while in the RVC electrode, the oxidation products of aromatic hydrocarbons were dominant.
Under optimal conditions, the researchers explored the substrate scope of the rAP-Kolbe reaction, and the results showed that a series of primary carboxylic acids substituted with different groups can carry out the reaction smoothly, including: ester group, carbamate, hydroxyl, ketocarbonyl, electron deficient and electron rich arenes, aryl halides, terminal and internal alkenes.
In addition, this method can also be applied to convert cheap natural amino acids (glutamic acid and aspartic acid) into valuable unnatural amino acids without racemization, which greatly simplifies the traditional synthetic methods. For example, using this method, alkene-containing amino acids (~$1,000/g) can be obtained in one step, while previous methods require laborious preparation of key intermediates or custom ligands for asymmetric coupling prior to asymmetric hydrogenation.
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Reference
- Overcoming the limitations of Kolbe coupling with waveform-controlled electrosynthesis
Science, 2023, 380, 81-87.
