Electrochemical Synthesis of Quinones and Hydroquinones

Phenols and Related Starting Materials.The oxidative conversion of phenols can lead to a range of side-reactions resulting in oligomeric products. 67688]69 Most common quinones and hydroquinones are accessed through at least monooxygenated starting materials.This also holds true for their electrochemical synthesis.On the one hand side, this can be attributed to the abundance of phenolic starting materials, which often can be derived from naturally occurring sources, 707172 making them sustainable and inexpensive options.On the other hand side, it can be reasoned that the electrochemical synthesis is simplified by the presence of electron-donating moieties such as hydroxy groups since they lower the oxidation potential. 73,74In 2017 Raju and coworkers reported a method to oxidize hydroquinones into benzoquinones in a batch-type cell using biphasic media. 75The electrolysis occurred in a conducting aqueous phase with sulfuric acid and sodium bromide as the supporting electrolyte.Upon formation the non-polar quinones dissolve in the organic dichloromethane layer.For a future scale-up, the use of chlorinated solvents could pose issues due to health concerns.The authors argue that the product can be obtained using the theoretical amount of charge due to the decreased solubility of the quinone product in comparison to the hydroquinone starting material, which prevents its overoxidation.While the authors were successfully able to convert 1,4-hydroquinones and 1,4-naphthoquinones they also stumbled upon limitations.The conversion of 1,9-dihydroxyanthracene remained unsuccessful, while the application of catechol and resorcinol resulted in brominated side products (Scheme 4).The group of Prasad reported a combined approach of electrochemical and conventional chemical oxidation to obtain hydroquinones from phenols in 2022. 76Initially the authors attempted to Scheme 2. Some Naturally Occurring Quinones With Biological Relevance Scheme 3. Selected Examples of Pharmaceutical Agents with Quinone or Hydroquinone Motif Scheme 4. Electrochemical Synthesis of Quinones from Hydroquinones in a Biphasic System