Benzotriazole-1-yl-oxytripyrrolidinophosphonium hexafluorophosphate
[CAS 128625-52-5]

Identification

• Classification: Biochemical >> Amino acids and their derivatives >> Other protected amino acids
• Name: Benzotriazole-1-yl-oxytripyrrolidinophosphonium hexafluorophosphate
• Synonyms: PyBOP
• Molecular Formula: C₁₈H₂₈N₆OP.PF₆
• Molecular Weight: 520.40
• CAS Registry Number: 128625-52-5
• EC Number: 603-290-2
• SMILES: C1CCN(C1)[P+](N2CCCC2)(N3CCCC3)ON4C5=CC=CC=C5N=N4.F[P-](F)(F)(F)(F)F

Properties

• Melting point: 150 °C (Expl.)
• Solubility: 1 mm in 2 mL DMF (Expl.)

Safety Data

• Hazard Symbols: GHS07;GHS09 Warning
• Risk Statements: H302-H315-H317-H319-H335-H410
• Safety Statements: P261-P264-P264+P265-P270-P271-P272-P273-P280-P301+P317-P302+P352-P304+P340-P305+P351+P338-P319-P321-P330-P332+P317-P333+P317-P337+P317-P362+P364-P391-P403+P233-P405-P501

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Skin irritation	Skin Irrit.	2	H315
Eye irritation	Eye Irrit.	2	H319
Specific target organ toxicity - single exposure	STOT SE	3	H335
Acute toxicity	Acute Tox.	4	H302
Skin sensitization	Skin Sens.	1	H317
Chronic hazardous to the aquatic environment	Aquatic Chronic	1	H410
Acute toxicity	Acute Tox.	4	H332
Acute toxicity	Acute Tox.	4	H312
Acute hazardous to the aquatic environment	Aquatic Acute	1	H400

chemBlink Chemical Story

Peptide synthesis depends on a reaction that looks simple on paper: join a carboxylic acid to an amine and make an amide bond. In practice, the direct reaction is slow and often requires activation. PyBOP, CAS 128625-52-5, was introduced in 1990 as part of the long effort to make that activation fast, selective, and safer for routine peptide chemistry.

PyBOP stands for benzotriazol-1-yloxytripyrrolidinophosphonium hexafluorophosphate. It belongs to phosphonium coupling reagents that activate a carboxyl group and generate a reactive acylating species that can be attacked by an amine. Benzotriazole-derived leaving-group chemistry helps accelerate coupling and can reduce side reactions such as racemization compared with harsher activation methods. The reagent became popular because it works under relatively mild conditions and is compatible with many protected amino acids.

Its historical importance is easiest to understand by comparison with BOP, an earlier phosphonium coupling reagent. BOP was effective, but its use generated hexamethylphosphoramide, HMPA, a highly undesirable toxic by-product. Coste, Le-Nguyen, and Castro redesigned the phosphonium reagent by replacing dimethylamino groups with pyrrolidino groups. Their 1990 paper deliberately emphasized that the new reagent was 'devoid of toxic by-product.' PyBOP therefore represents an early example of improving a synthetic reagent not because the old one failed chemically, but because the overall process carried an unacceptable safety burden.

In a coupling reaction, PyBOP first transforms the carboxylate into an activated intermediate. The amine component then attacks to form the peptide bond. Base, solvent, protecting groups, and reaction time all influence yield and epimerization. Like all coupling reagents, PyBOP is not universally ideal: guanidination, incomplete coupling, side reactions, or difficult residues may require alternative reagents or additives. Modern peptide synthesis therefore treats coupling-agent choice as a problem in reaction design rather than as a single standard recipe.

The history continued after PyBOP. New uronium, phosphonium, and related coupling reagents were developed to improve speed, solubility, difficult-sequence performance, and safety. Reviews by El-Faham and Albericio and by Valeur and Bradley show how amide-bond formation evolved into a major technology supporting medicinal chemistry, peptide therapeutics, and automated solid-phase synthesis.

PyBOP is memorable because it captures a broader principle of green and responsible synthesis: the best reagent is not merely the one that gives product. Chemists must also consider toxic by-products, waste, racemization, handling, and purification. A small redesign around phosphorus turned a successful reaction into a safer practical tool and helped change how coupling reagents were judged.

References:
1. Coste J., Le-Nguyen D., Castro B. PyBOP: A new peptide coupling reagent devoid of toxic by-product. Tetrahedron Letters. 1990, 31, 205-208. DOI: 10.1016/S0040-4039(00)94371-5.
2. El-Faham A., Albericio F. Peptide coupling reagents, more than a letter soup. Chemical Reviews. 2011. DOI: 10.1021/cr100048w.
3. Valeur E., Bradley M. Amide bond formation: beyond the myth of coupling reagents. Chemical Society Reviews. 2009. DOI: 10.1039/B701677H.
4. Encyclopedia of Reagents for Organic Synthesis, PyBOP entry. DOI: 10.1002/047084289X.rn00198.

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