Beyond Kolbe and Hofer-Moest: Electrochemical Synthesis of Carboxylic Anhydrides from Carboxylic Acids

Abstract

Herein we report a conceptually new non-decarboxylative electrolysis of carboxylic acids to obtain their corresponding anhydrides as highly valuable reagents in organic synthesis. All carbon atoms of the starting material are preserved in the product in an overall redox-neutral reaction. In a broad substrate scope of carboxylic acids the anhydrides are generated with high selectivity, which demonstrates the versatility of the developed method. Beneficially, no dehydrating reagents are required in comparison to conventional methods and the synthesis is based on uncritical starting materials using graphite and stainless steel as very inexpensive and eco-friendly electrode materials.

Keywords: carboxylic acids; carboxylic anhydrides; electrochemistry; electrosynthesis; sustainable chemistry.

Scheme 1

Conventional synthesis approaches of carboxylic acid anhydrides and electrolysis of carboxylic acids in organic media towards Kolbe, non‐Kolbe, and Hofer–Moest products in comparison to this work.

Figure 1

Electrochemically generated carboxylic anhydrides; [a] reaction conditions: undivided cell, under air, 298 K, 0.7 m carboxylic acid 1 in 5.0 mL MeCN, 0.10 m KSCN, 1.1 F, 50 mA cm⁻², graphite anode, stainless steel cathode; quantification by ¹H NMR spectroscopy with 1,3,5‐trimethoxybenzene as internal standard; selectivity=yield/conversion; for individual deviations from standard conditions see Supporting Information; [b] quantification by GC‐FID with dibutyl fumarate or dibromomethane as internal standard; [c] quantification by inverse‐gated ¹³C NMR spectroscopy with 1,3,5‐trimethoxybenzene as internal standard; [d] isolated yield.

Scheme 2

Control experiments with deviations from standard conditions; Yields determined by ¹H NMR spectroscopy with 1,3,5‐trimethoxybenzene as internal standard; standard conditions: undivided cell, under air, 298 K, 0.70 m carboxylic acid in 5.0 mL MeCN, 0.10 m KSCN, 1.1 F, 50 mA cm⁻², graphite anode, stainless steel cathode.