Activation

Carboxylic acids react with amines at room temperature to give an ammonium salt instead of a carboxamide. For formation of the peptide bond to occur, activation of the carboxyl group of the N-a-protected amino acid must occur. Activation decreases the electron density of the C=O group by introduction of an electron-accepting species. This permits nucleophilic attack of the amino component that results in peptide bond formation.

Several main activation strategies commonly used for coupling reactions exist. 

Phosphonium + Uronium