PyBOP (Benzotriazol-1-yloxytris(pyrrolidino)phosphonium hexafluorophosphate) is a phosphonium-based peptide coupling reagent commonly used in both solution-phase and solid-phase peptide synthesis for forming amide bonds with high efficiency. It activates carboxylic acids to form reactive OBt (benzotriazole) esters, similar to HBTU, but offers the distinct advantage of not generating guanidinium byproducts, which are difficult to remove and common with uronium-based reagents like HBTU and HATU. PyBOP also provides low racemization, good solubility in organic solvents, and high yields in coupling reactions, especially with sterically hindered substrates. Its balanced reactivity and cleaner reaction profile make it a preferred alternative when product purity is critical or when downstream purification needs to be simplified.