Pyrazole-4-boronic acid pinacol ester
[CAS 269410-08-4]

Identification

• Classification: Chemical reagent >> Organic reagent >> Borate
• Name: Pyrazole-4-boronic acid pinacol ester
• Synonyms: 4-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)-1H-pyrazole
• Molecular Formula: C₉H₁₅BN₂O₂
• Molecular Weight: 194.04
• CAS Registry Number: 269410-08-4
• EC Number: 629-131-7
• SMILES: B1(OC(C(O1)(C)C)(C)C)C2=CNN=C2

Properties

• Solubility: 2.76e+004 mg/L (25 °C water)
• Density: 1.1±0.1 g/cm³ Calc.^*
• Melting point: 142 - 146 °C (Expl.)
• Boiling point: 335.4±15.0 °C 760 mmHg (Calc.)^*
• Flash point: 156.6±20.4 °C (Calc.)^*
• Index of refraction: 1.487 (Calc.)^*

^* Calculated using Advanced Chemistry Development (ACD/Labs) Software.

Safety Data

• Hazard Symbols: GHS07 Warning
• Risk Statements: H315-H319-H335
• Safety Statements: P261-P264-P264+P265-P271-P280-P302+P352-P304+P340-P305+P351+P338-P319-P321-P332+P317-P337+P317-P362+P364-P403+P233-P405-P501

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Skin irritation	Skin Irrit.	2	H315
Specific target organ toxicity - single exposure	STOT SE	3	H335
Eye irritation	Eye Irrit.	2	H319
Acute toxicity	Acute Tox.	4	H302
Acute toxicity	Acute Tox.	4	H332
Eye irritation	Eye Irrit.	2A	H319
Acute toxicity	Acute Tox.	4	H312

chemBlink Chemical Story

Pyrazole-4-boronic acid pinacol ester, CAS 269410-08-4, is a heteroaryl boronic ester widely used as a synthetic building block. Fisher Scientific records the molecular formula C9H15BN2O2, molecular weight 194.04 and the systematic structure 4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-1H-pyrazole. The molecule combines an intact pyrazole ring with a pinacol-protected boron group at the 4-position.

Its principal value is in carbon-carbon bond construction. Boronic acids and boronic esters are standard partners in Suzuki-Miyaura cross-coupling, where an organoboron group reacts with an aryl or heteroaryl halide under palladium catalysis. In this reagent, the transferable carbon fragment is the 4-pyrazolyl unit. After coupling, the boron-containing group disappears and a new C-C bond connects pyrazole to the partner molecule.

The pinacol ester form is important for practical reasons. Free heteroaryl boronic acids can sometimes suffer from limited stability, dehydration or protodeboronation. Converting boronic acid to a cyclic pinacol ester often improves handling, crystallinity and storage stability while retaining access to Suzuki coupling. It is therefore common for medicinal chemists to keep boron in the Bpin form until the carbon-carbon bond is needed.

Pyrazole is itself a privileged heterocycle in medicinal and agrochemical chemistry. Its two adjacent nitrogen atoms create distinctive hydrogen-bonding and electronic properties, and substitution around the five-membered ring allows broad exploration of molecular shape. A 4-boronate reagent is valuable because it provides a standardized route to install pyrazole at a defined position in a larger scaffold.

CAS 269410-08-4 is therefore not important because the Bpin group is expected to remain in most final products. Its value is precisely the opposite: boron is a temporary synthetic handle. The reagent carries a useful heteroaryl fragment in a stable, transferable form, ready to exchange the C-B bond for a new carbon-carbon connection.

References:
1. Fisher Scientific, CAS 269410-08-4, PubChem CID 2774010.
2. Miyaura N, Suzuki A. Chem Rev. 1995;95:2457-2483.
3. Hall DG, ed. Boronic Acids. 2nd ed. Wiley-VCH, 2011.

Read more stories