Reactivity of the phosphonium salts in front of the aminuim salts in peptide synthesis

The peptide coupling reagent field has clearly evolved in the last decade from carbodiimides to onium (phosphonium and uronium) salts. The era of industrial coupling reagents began in 1955 with the introduction of dicyclohexylcarbodiimide (DCC), which at that time was already known and well studied, as a reagent for the formation of amide bond. Unfortunately, carbodiimides did not comply with the concept of ultimate coupling reagents because its high reactivity provokes racemization and side reactions during the coupling reaction.