Carbodiimides as important intermediates in the reaction of isocyanates with carboxylic acids

The reaction between isocyanates and carboxylic acids is known to lead to amides. We have studied the mechanism of catalysis of this reaction by phospholene oxides. The reaction between phenyl isocyanates and benzoic acid was monitored under various conditions using High-Pressure Liquid Chromatography. It was found that the catalyst reacts with the iso-cyanate under formation of a carbodiimide. The carbodiimide reacts with the carboxylic acid to form amide and isocyanate via an N-acylurea as intermediate. The catalyst is partly deactivated by the acid. The phospholene catalyst was only found effective in reactions with aromatic isocyanates.