2-Chloro-5-Fluoro-4-Iodopyridine: Properties & Synthetic Uses
2-Chloro-5-Fluoro-4-Iodopyridine: Properties & Synthetic Uses
2-Chloro-5-fluoro-4-iodopyridine is a trihalogenated pyridine building block valued in medicinal chemistry and materials science for its orthogonal reactivity at three distinct ring positions, enabling selective palladium-catalyzed cross-coupling, nucleophilic aromatic substitution, and further functionalization.
- CAS Registry Number
- 884494-81-9
- Molecular Formula
- C₅H₂ClFIN
- Molecular Weight
- 257.43 g/mol
- Key Functional Handles
- C–I (position 4), C–Cl (position 2), C–F (position 5)
- Primary Reactivity
- C–I bond is preferentially engaged in Pd/Cu-catalyzed cross-coupling; C–Cl undergoes SNAr with hard nucleophiles; C–F is strongly electron-withdrawing
- Recommended Storage
- Cool, dry, well-ventilated area; protect from moisture and light
Chemical Identity and Physical Properties
2-Chloro-5-fluoro-4-iodopyridine (CAS 884494-81-9) is a pale solid pyridine derivative bearing three different halogen substituents: chlorine at the 2-position, fluorine at the 5-position, and iodine at the 4-position. Its molecular formula is C₅H₂ClFIN and its molecular weight is approximately 257.43 g/mol.
The presence of three electronegative substituents on the pyridine ring significantly lowers the electron density of the aromatic system, rendering the ring electrophilically deactivated and susceptible to nucleophilic attack at activated positions. The iodine substituent at the 4-position is the largest and most polarizable halogen in the molecule, making the C–I bond the weakest and most reactive toward oxidative addition by transition metals. The fluorine at the 5-position contributes a strong −I inductive effect without participating in cross-coupling under standard conditions, while the chlorine at the 2-position offers an intermediate reactivity handle for sequential functionalization.
Reactivity and Orthogonal Functionalization Strategy
The defining synthetic value of 2-Chloro-5-fluoro-4-iodopyridine lies in its capacity for orthogonal, sequential functionalization at three distinct positions. Researchers can exploit the differential reactivity of the C–I, C–Cl, and C–F bonds to introduce different functional groups in a controlled, stepwise manner.
C–I Bond (position 4): The aryl iodide is the most reactive site toward palladium- and copper-catalyzed reactions. It readily undergoes Suzuki–Miyaura (arylboronic acid), Sonogashira (terminal alkyne), Negishi (organozinc), Heck (alkene), and Buchwald–Hartwig (amine) couplings under mild conditions. Selectivity for C–I over C–Cl in cross-coupling is well established in the literature.
C–Cl Bond (position 2): After selective transformation of the C–I bond, the remaining aryl chloride can be engaged in Pd-catalyzed cross-coupling under more forcing conditions, or subjected to nucleophilic aromatic substitution (SNAr) with amines, alkoxides, or thiolates, facilitated by the electron-withdrawing pyridine nitrogen and adjacent fluorine.
C–F Bond (position 5): The C–F bond is the most kinetically inert toward oxidative addition under standard Pd catalysis but can participate in SNAr reactions with strongly activated nucleophiles under harsh conditions. More commonly, the fluorine serves as a metabolic-stability and lipophilicity modulator in drug candidates rather than as a leaving group.
Applications in Medicinal Chemistry and Drug Discovery
Halogenated pyridines such as 2-Chloro-5-fluoro-4-iodopyridine are indispensable building blocks in pharmaceutical research. The pyridine scaffold is present in numerous approved drugs and clinical candidates due to its favorable pharmacokinetic properties, water solubility, and ability to engage in hydrogen bonding with biological targets.
The fluorine atom at the 5-position is particularly prized in drug design. Fluorine substitution can improve metabolic stability by blocking cytochrome P450-mediated oxidation, modulate pKa of adjacent heteroatoms, enhance membrane permeability, and fine-tune binding affinity through electrostatic and steric effects. The combination of chlorine and iodine provides handles for rapid analog synthesis, enabling structure–activity relationship (SAR) exploration across large compound libraries.
This compound is frequently used as a key intermediate in the synthesis of kinase inhibitors, antiviral agents, and other biologically active heterocycles. Its well-defined reactivity profile allows medicinal chemists to design efficient, convergent synthetic routes with minimal protecting-group manipulations.
Handling, Storage, and Safety Considerations
As with all halogenated organic reagents, appropriate laboratory safety practices should be observed when working with 2-Chloro-5-fluoro-4-iodopyridine. Users should consult the current Safety Data Sheet (SDS) provided with the product before use.
Storage: Store in a tightly sealed container in a cool, dry, and well-ventilated area, away from moisture, direct sunlight, and incompatible materials such as strong oxidizers and strong bases. Maintaining low temperature storage (e.g., 2–8 °C or as specified on the product label) is recommended to ensure long-term stability and reagent quality.
Handling: Use appropriate personal protective equipment (PPE) including chemical-resistant gloves, safety goggles, and a laboratory coat. Work in a fume hood when weighing or transferring the compound to minimize inhalation exposure. Avoid contact with skin, eyes, and mucous membranes.
Disposal: Dispose of waste material in accordance with local, regional, and national regulations governing halogenated organic waste.
Frequently asked questions
What makes the C–I bond more reactive than C–Cl in cross-coupling reactions?
The C–I bond is longer and weaker than C–Cl due to the larger atomic radius and lower electronegativity of iodine, making oxidative addition to palladium(0) significantly faster. This kinetic preference allows selective functionalization of the aryl iodide position while leaving the aryl chloride intact for subsequent transformations.
Can 2-Chloro-5-fluoro-4-iodopyridine undergo nucleophilic aromatic substitution (SNAr)?
Yes. The electron-deficient pyridine ring, activated by the ring nitrogen and the electron-withdrawing halogen substituents, is susceptible to SNAr. The chlorine at the 2-position is particularly activated toward displacement by nitrogen and oxygen nucleophiles, aided by the ortho/para-directing influence of the pyridine nitrogen. Reaction conditions (solvent, temperature, nucleophile strength) must be optimized to control regioselectivity.
How should I select coupling conditions to achieve selectivity at the C–I position?
For selective cross-coupling at the C–I bond in the presence of C–Cl, use mild Pd(0) catalysts such as Pd(PPh₃)₄ or Pd(dba)₂ with monodentate phosphine ligands at room temperature to 60 °C. These conditions exploit the much faster oxidative addition of aryl iodides versus aryl chlorides. Avoid highly active bulky phosphine ligands (e.g., SPhos, XPhos) at elevated temperatures, which can also activate C–Cl bonds and erode selectivity.
What is the role of the fluorine substituent in drug candidates derived from this building block?
Fluorine at the 5-position acts primarily as a bioisostere and pharmacokinetic modulator rather than a synthetic handle. It enhances metabolic stability by blocking potential oxidation sites, can adjust the pKa of the pyridine nitrogen to optimize target binding, improves membrane permeability in some cases, and influences the overall lipophilicity (logP) of the molecule. These properties make fluorinated pyridines highly attractive scaffolds in medicinal chemistry.
References
- Johansson Seechurn, C. C. C.; Kitching, M. O.; Colacot, T. J.; Snieckus, V. Palladium-Catalyzed Cross-Coupling: A Historical Contextual Perspective to the 2010 Nobel Prize. Angew. Chem. Int. Ed. 2012, 51, 5062–5085. doi:10.1002/anie.201107017
- Meanwell, N. A. Synopsis of Some Recent Tactical Application of Bioisosteres in Drug Design. J. Med. Chem. 2011, 54, 2529–2591. doi:10.1021/jm101284e
- Gouverneur, V.; Müller, K. (Eds.) Fluorine in Pharmaceutical and Medicinal Chemistry. Imperial College Press, 2012.
- Clayden, J.; Greeves, N.; Warren, S. Organic Chemistry, 2nd ed. Oxford University Press, 2012. Chapter on nucleophilic aromatic substitution.
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