Upgrading of aromatics using organic electrosynthesis

Abstract

Electrochemical reactions provide an alternative route for the traditional organic synthesis. Electrosynthesis can be operated under relatively milder conditions without the need for highly reactive reagents and the selectivity can be controlled easily by manipulating the electric potential. Nevertheless, electrochemistry of organic compounds presents higher complexity and often causes undesired side reactions, leading to low selectivity toward the target product. Furthermore, the poor solubility of organic compounds in water usually requires the use of organic solvents, which aggravates the difficulty. As a result, the application of electrochemistry in organic synthesis is still constrained. Although previous studies have explored organic electrosynthesis, few has conducted comprehensive investigations in reaction condition optimization and the improvement of efficiency at a reactor scale. To gain better understanding of organic electrosynthesis, we investigated two examples of electrochemical upgrading of aromatic compounds. ☐ The first case employs the alkyl radicals generated through Kolbe electrolysis for radical addition reaction toward styrene, achieving side chain elongation. We optimize the electrolysis condition to maximize the production of the target addition product and minimize the side reaction of solvent oxidation and radical self-coupling. We also resolve the passivation problem by applying negative pulses. We study the reaction mechanism and propose the two electron pathway involving a radical and a carbocation as the intermediates. ☐ The second case deals with the electrochemical oxidation of p-xylene via two consecutive electrolysis. The first electrolysis achieves C-H activation of p-xylene using electrochemically in-situ produced bromine. The brominated aromatics are hydrolyzed to alcohol and aldehyde. The second electrolysis converts alcohol and aldehyde into terephthalate in alkaline media, followed by acidification of electrolyte to obtain terephthalic acid product. The acid and base needed in each step are prepared through the first electrolysis in a divided cell, avoiding the need of adding external acid and base, making p-xylene and water the only two substrates of the process. ☐ These two systems serve as representative of C-C coupling and C-H activation, respectively. In addition to the chemistry, often encountered problems in organic electrosynthesis, including electrode passivation, solvent involvement, side reactions, shuttling of active species toward counter electrode and cost of consumable co-reactants are thoroughly investigated and corresponding solutions are proposed.