How a century-old solvent can enable efficient amide synthesis

Amide bonds are vital chemical connections found in biology, medicine and materials. These bonds link amino acids to form proteins and are crucial in pharmaceuticals and polymers. However, synthesizing amides efficiently while maintaining environmental safety remains a challenge in green chemistry. Most current methods require the use of hazardous coupling reagents that can be corrosive, toxic and generate substantial waste.

A research team led by Sunwoo Lee from Chonnam National University in South Korea has discovered that dichloromethane, a commonly used solvent, can be repurposed as a coupling reagent for the direct synthesis of amides from carboxylic acids and amines. Dichloromethane is frequently employed in various chemical processes, making it an ideal and cost-effective choice.

The researchers used benzoic acid and benzylamine as model substrates to optimize the reaction conditions. Under basic conditions, carboxylates can attack dichloromethane via an SN2 reaction, generating reactive chloromethyl ester intermediates. These intermediates can then undergo acyl substitution with amines to produce amides. The optimal conditions found were a temperature of 80 °C, a 12-hour reaction time and an excess of amine using sodium carbonate as the base and dimethyl sulfoxide as the solvent.

The method proved effective across a wide range of carboxylic acids and amines. The team successfully synthesized two pharmaceutical amides: procainamide with a 92% yield and moclobemide with a 76% yield. They also demonstrated the scalability of the approach by producing over 20 grams of moclobemide with greater than 99% purity in a single-step reaction using 100 millimoles of 4-chlorobenzoic acid and 2-morpholinoethanamine.

By avoiding traditional coupling reagents and reducing the generation of waste, this innovative approach using dichloromethane offers a practical and scalable alternative for amide synthesis in green chemistry.