Carbodiimide

A carbodiimide or a methanediimine is a functional group consisting of the formula RN=C=NR. Carbodiimides hydrolyze to form ureas, which makes them uncommon in nature. From the perspective of small molecule activation, carbodiimides are isoelectronic with carbon dioxide.

Carbodiimides are formed by dehydration of ureas or from thioureas. They are also formed by treating organic isocyanates with suitable catalysts (generally based on phosphine oxides); in this process, carbon dioxide evolves from the isocyanate.

In synthetic organic chemistry, compounds containing the carbodiimide functionality are dehydration agents and are often used to activate carboxylic acids towards amide or ester formation. Additives, such as N-hydroxybenzotriazole or N-hydroxysuccinimide, are often added to increase yields and decrease side reactions.

Carbodiimides can also react with amines to form guanidines.

The formation of an amide using a carbodiimide is straightforward, but with several side reactions complicating the subject. The acid 1 will react with the carbodiimide to produce the key intermediate: the O-acylisourea 2, which can be viewed as a carboxylic ester with an activated leaving group. The O-acylisourea will react with amines to give the desired amide 3 and urea 4.

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