Electrosyntheses of α ‐Hydroxycarboxylic Acids from Carbon Dioxide and Aromatic Ketones Using Nickel as the Cathode

Abstract

With Ni as the cathode and Al as the anode, aromatic ketones (acetophenone, benzophenone, 6-methoxy-2-acetonaphthone, 4-methylacetophenone and 4-methoxyacetophenone) could be successfully electrocarboxylated with CO 2 (4 MPa) in an undivided cell containing n-Bu 4 NBr-DMF electrolyte at a constant current under room temperature, affording the corresponding α-hydroxycarboxylic acids in good to excellent yields (56%-90%). The experimental results showed that the yields of the target products were strongly dependent on the nature of cathode materials and aromatic ketones as well as electrolytic conditions (such as electricity, substrate concentrations, conducting salts and solvents, CO 2 pressure). In addition, it was found that the presence of protic agent such as water would lead to the increase of pinacol byproducts. The reaction mechanism was briefly discussed by CV experiments.

Synthetic Organic Electrochemical Methods Since 2000: On the Verge of a Renaissance

“…A recent report by Yuan and co-workers detailed a protocol for the reductive carboxylation of ketones (Figure 46F, top). 808 Although the scope of substrates is limited to aromatic ketones, α -hydroxycarboxylic acids were generally obtained in good yields together with small amounts of pinacol byproducts. Since the reduction potentials of ketones are close to that of CO 2 , it is proposed that the reduction of both CO 2 and ketones could occur under galvanostatic conditions.…”

Electrocarboxylation: towards sustainable and efficient synthesis of valuable carboxylic acids

“…This offers an electrochemical route for several commercially relevant α-aryl propionic acids, used as non-steroidal anti-inflammatory drugs (NSAIDs) [80]. Therefore, the electrocarboxylation of aromatic ketones with sacrificial anodes has been extensively investigated [75–77 81–98]. Some researchers focused on replacing toxic and volatile organic solvents with ionic liquids [81–83].…”

“…The cathode material again plays an important role in the electrocarboxylation of carbonyl compounds. Toxic lead cathodes [85] and expensive platinum cathodes [87] can easily be replaced by better performing stainless steel [90] and nickel [75,89]. Concerning the reactivity of carbonyl compounds, it has been demonstrated that the carboxylation rate of benzophenones is decreased by electron donating substituents [90].…”

“…One starts with CO 2 reduction; another starts with reduction of the substrate ( Scheme 11 ). The second route is considered as the predominant one [ 75 ]. In case the carbonyl or imine compound is reduced first, the negative charge may reside either on the carbon or on the heteroatom.…”

“…In both cases, a second electron reduction in presence of CO 2 yields a carboxylate intermediate with an additional carbonate or carbamate group. The latter is converted to the corresponding α-hydroxy acid or to an α-amino acid after acid hydrolysis in the product work-up [ 75 – 77 ].…”