Introduction to Click Reactions and Click Chemistry Tools

Click Reactions and Click Chemistry Tools

In-Situ Chemistry to Support Click Reactions


What Is Click Chemistry?

Click chemistry is a term that describes a family of chemical reactions that are designed to be efficient, selective, and simple. These chemical reactions are designed to be modular, wide in scope, and generate minimal by-products. Click chemistry reactions are widely used in many areas of chemistry, including drug discovery, materials science, and bioconjugation.

The concept of “click chemistry” was introduced in 2001 by Nobel Prize laureate, K. Barry Sharpless. The terms "click chemistry" and "click reaction" are often used interchangeably, but there is a subtle difference between the two.

What Is a Click Reaction?

Click reactions refer to chemical reactions that meet the criteria of click chemistry. These reactions involve the joining of two molecules through a specific chemical bond, often involving azides, alkynes, or cyclooctynes. Click reactions are typically fast, high-yielding, and occur under mild conditions, making them ideal for a variety of applications. 

In click reactions, scientists utilize special molecules that can easily snap together, like a seat belt perfectly buckling together. They are particularly important considering the concept is applicable regardless of scale. In the past few decades, click reactions have become widely used in specialty chemical, pharmaceutical, biomolecular, biomedical, and polymer applications. 

Click chemistry has revolutionized the way chemists approach the synthesis of complex molecules and led to the development of new materials, drugs, and other products that have important applications in various fields.

Click Reaction Criteria

The goal is to develop an expanding set of powerful, selective, and modular “blocks” that work reliably in both small- and large-scale applications. We have termed the foundation of this approach “click chemistry,” and have defined a set of stringent criteria that a process must meet to be useful in this context. The reaction must be modular, wide in scope, give very high yields, generate only inoffensive by-products that can be removed by nonchromatographic methods, and be stereospecific (but not necessarily enantioselective). The required process characteristics include (Kolb et al. 2.1): 

  1. Simple reaction conditions and preferably, the process should be unaffected by the presence of oxygen and water
  2. Starting materials and reagents that are easily accessible
  3. Reactions are either solvent-free, or use benign or easily eliminated solvent
  4. Simple product isolation 

Click reactions have certain key properties that make them effective. They rely on a strong thermodynamic driving force, usually over 20 kcal mol-1, which means they can occur quickly and produce only one desired product. We can think of click reactions as being ready to go in a single direction, like a loaded spring. It's important to understand these properties to use click reactions effectively.

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CuAAC (Copper-Catalyzed Azide-Alkyne Cycloaddition) Reaction

The copper-catalyzed azide-alkyne cycloaddition was the first click reaction developed independently by Nobel Prize laureates, Sharpless and Meldal. This reaction, coined “the crown jewel of click chemistry,” uses a copper catalyst to form a new bond between azide and alkyne functional groups. The result is a triazole ring that acts like a Lego block or seat belt buckle to “click” one molecule to another. 

CuAAC reactions can be carried out in a one-pot procedure, meaning that all of the reactants can be combined at the beginning of the reaction, simplifying the process and making it more efficient. The products of CuAAC reactions are structurally pure, high molecular weight polymers. 

Due to the remarkable properties of copper catalysts, such as its stability in many reaction conditions, e.g. hydrolysis, oxidation, and reduction, there has been considerable research into the development of various copper catalysts (and other metals, e.g., ruthenium) for azide-alkyne synthesis of polytriazole polymers as well as post functionalization of polymers.

CuAAC Reaction diagram

Thiol-Ene Reaction

In a thiol-ene reaction, a thiol reacts with an alkene to form a carbon-sulfur bond, and a new carbon-carbon double bond is formed in the process. Thiol-ene reactions have several advantages over traditional reactions, including the ability to:

  • Proceed under mild conditions
  • Tolerate a wide range of functional groups
  • Be carried out in a one-pot procedure, simplifying the process and increasing efficiency  

The thiol-yne reactions are similar to the thiol-ene reactions, but involve the reaction of a thiol with an alkyne to form a carbon-sulfur bond, with a new carbon-carbon triple bond formed in the process. These reactions can be used for building dendrimers, hydrogels, and nanoparticles, as well as post-functionalizing polymers chains. Terminal alkene and thiol groups can be easily introduced, and reactions can be performed free of toxic catalysts via photochemistry.

Diels-Alder Reaction

Diels-Alder cycloaddition reactions are a class of chemical reactions that involve the formation of cyclic compounds from a conjugated diene (a molecule containing two alternating double bonds) and a dienophile (a molecule containing a double bond). The diene and dienophile undergo a concerted reaction in which a new bond is formed between the diene and dienophile to produce a new cyclic compound.  

While the Diels-Alder reaction is a powerful synthetic tool and can be highly selective, it is not always fast or high-yielding. In some cases, the reaction conditions need to be carefully controlled to obtain the desired product. Additionally, some Diels-Alder reactions may result in side reactions or formation of unwanted by-products.

Diels-Adler Reaction diagram

SPAAC (Strain-Promoted Azide-Alkyne Cycloaddition) Reaction

Strain-promoted azide-alkyne cycloaddition (SPAAC) is a type of click reaction that does not require a metal catalyst. Instead, the reaction is driven by the inherent strain energy of cyclooctynes and their derivatives, which react with azides to form a stable triazole product. 

The reaction was published in 2004 by Nobel Prize laureate Carolyn Bertozzi, who won the Nobel Prize in Chemistry with Sharpless and Meldal. Bertozzi knew that copper is toxic to living things, so she searched the literature for an alternative to copper-catalyzed azide-alkyne cycloaddition (CuAAC) click reactions. She found that azides and alkynes can react together if the alkyne is forced into a ring-shaped chemical structure. 

The SPAAC reaction worked well in cells, so Bertozzi demonstrated that it can be used to track glycans, special carbohydrates located on the surface of cells. This is because SPAAC is highly selective and bioorthogonal, meaning it can occur in biological systems without interfering with other biological processes. 

SPAAC is widely used in chemical biology and bioconjugation, enabling the labeling and imaging of biomolecules like glycans, as well as the development of targeted therapies. SPAAC is a useful alternative to CuAAC reactions in some applications due to its mild reaction conditions and lack of catalyst. For example, the reaction also allowed Bertozzi and scientists to study disease processes. 

Tetrazine Click Chemistry

Tetrazine click chemistry is a type of click reaction that involves the reaction of tetrazines with strained alkenes, such as trans-cyclooctenes, to form a stable product. The reaction is bioorthogonal, meaning it can occur in biological systems without interfering with other biological processes. 

Tetrazine click chemistry is particularly useful for in vivo imaging and drug delivery applications, as it allows for the selective labeling and targeting of biological molecules. Additionally, tetrazine click chemistry is fast and efficient, occurring within seconds at room temperature. Its high selectivity and fast reaction kinetics make tetrazine click chemistry a powerful tool for chemical biology research and drug development.

Click Polymerization

Click polymerization is a type of click reaction that is used to synthesize polymers. This approach involves the rapid and efficient coupling of monomers using click chemistry reactions, such as CuAAC or thiol-ene chemistry. 

Click polymerization has several advantages over traditional polymerization method, including high efficiency, high selectivity, and mild reaction conditions. Additionally, click polymerization allows for the precise control of polymer structure and composition, enabling the creation of complex, multi-functional materials. 

Click polymerization has found numerous applications in materials science, including the development of advanced coatings, adhesives, and composites. Ease of use and versatility make click polymerization a valuable tool for the synthesis of functional polymers with tailored properties.


Click-to-release is a type of click reaction that triggers the release of bioactive molecules from a carrier molecule or scaffold. This approach involves the use of a linker molecule that can be cleaved by a click reaction, such as CuAAC or SPAAC, to release the cargo molecule. 

Click-to-release is highly selective and can be tailored to release the cargo molecule under specific conditions, such as in response to a particular enzyme or pH level. This approach has been used for targeted drug delivery, where the cargo molecule is released at the site of disease or injury, minimizing side effects and improving therapeutic efficacy. 

Click-to-release has also been used in materials science, where it allows for the controlled release of functional molecules from coatings, adhesives, and other materials. High selectivity and controllable release properties make click-to-release a powerful tool for targeted drug delivery and other applications.

Advantages of Click Reactions

  • High efficiency
  • High selectivity
  • Bioorthogonal reactivity
  • Mild reaction conditions

Limitations of Click Reactions

  • Requirement of toxic catalysts
  • Need for functional group tolerance
  • Incompatibility with some reaction conditions
  • Limited scope of applicability 

Future of Click Reactions

Click Chemistry Integrated with Emerging Technologies

Click chemistry has a promising future with continued development and refinement of existing click reactions, as well as the discovery of new click reactions with even greater efficiency, selectivity, and versatility. Process analytical technology (PAT) is poised to play a key role in this future, by enabling real-time monitoring and control of click reactions during synthesis and manufacturing processes. PAT allows for the rapid and continuous measurement of key process parameters, such as reaction kinetics, temperature, and concentration, providing valuable feedback for process optimization and control.

In the context of click chemistry, PAT can be used to monitor the progress of click reactions in real time, ensuring that the reaction proceeds efficiently and yields the desired product. Additionally, PAT can help identify potential sources of variability or impurities, allowing for early intervention and correction of any issues that may arise. As click chemistry continues to play an increasingly important role in chemical synthesis, materials science, and drug development, the use of PAT is likely to become more widespread, helping to ensure the consistent and efficient production of high-quality products.

in-situ ftir spectroscopy for click reaction profiling

Click Reaction Examples in Industry

PAT for Cycloaddition Click Reaction

Zhang, Y., Lai, W., Xie, S. Q., Zhou, H., & Lu, X. (2021b). Facile synthesis, structure and properties of CO2-sourced poly(thioether-co-carbonate)s containing acetyl pendants via thio-ene click polymerization. Polymer Chemistry, 13(2), 201–208.

Aliphatic polycarbonates are proving to be significant in biomedical applications and the synthesis of novel APCs are actively researched. In this work, poly(thioether-co-carbonate)s are synthesized bearing acetyl groups attached to vinyl-groups functionalized bis- and tris-β-oxo-carbonates. The aliphatic polycarbonates with thio-linkages in the main chain and acetyl pendants in each repeating unit were prepared via photochemical-induced thiol-ene click polymerization of the bis- and tris-vinyl-β-oxo-carbonates with primary bisthiols. These polycarbonates are readily depolymerized under mild conditions using t-butyl peroxide, producing peroxy-substituted cyclic carbonates and polyols. This degradation was demonstrated using in-situ FTIR.  

C=O stretching bands in the polymer were identified arising from the carbonate (1746 cm−1) and from the attached acetyl group (1723 cm−1). These C=O absorption bands diminished over time after 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) and tert-butyl hydroperoxide (TBHP) were added to the reaction system. The presence of a new top at 1809 cm−1 arising from the C=O stretching band of a cyclic carbonate was associated with the formation of peroxy-functionalized biscyclic carbonates, and reflective of polymer degradation. 

In-Situ FTIR for Thermoplastic Elastomers

Bretzler, V., Grübel, M., Meister, S., & Rieger, B. (2014b). PDMS-Containing Alternating Copolymers Obtained by Click Polymerization. Macromolecular Chemistry and Physics, 215(14), 1396–1406.

This research highlights the advantages of thermoplastic elastomers (TPE) over chemically crosslinked elastomeric polymers that require costly catalysts and require additional considerations. TPE offers benefits in thermal processing, making them a valuable choice for applications such as 3D printing and injection molding. Notably, this study demonstrates that poly(dimethylsiloxane) can be used as segments in TPE, and CuAAC click reactions can construct linear polymers based on PDMS. 

The authors extend this research by showcasing the incorporation of various functionalities in a PDMS-containing alternating copolymer via the CuAAC reaction, which results in the formation of TPE with diverse properties. The authors explore structure-property relationships, which are dependent on the different azido-functionalized oligosiloxane segments, as well as the geometries of the various dialkyne comonomers used in the polymerizations

The ReactIR in-situ FTIR spectrometer provided insights into polymerization kinetics by tracking the decay of the azide functionality during the reaction. Furthermore, in-situ FTIR measurements demonstrated a significant enhancement in the polymerization reaction rate with the addition of one equivalent of the tridentate triazole ligand tris((1-benzyl-1H-1,2,3-triazol-4-yl)methyl)amine (TBTA) to the CuAAC reaction. Moreover, the optimal ratio of ligand-to-metal for achieving the accelerating effect was determined to be between 0.5 to 1.0 eq, resulting in the highest reaction rate. This study sheds light on the potential of TPE and provides a comprehensive understanding of their properties and capabilities.

Citations and References



What is the definition of a click reaction?

Click reactions are a family of chemical reactions that are rapid, efficient, and highly selective. They were first introduced by K. Barry Sharpless in 2001, and have since become a valuable tool for chemical synthesis, materials science, and bioconjugation. Click reactions typically involve the coupling of two molecular fragments through a specific reaction mechanism, such as cycloaddition, nucleophilic substitution, or Michael addition. These reactions are characterized by their high yield, simple reaction conditions, and ability to occur under biocompatible conditions. Click reactions have found numerous applications in chemical biology, where they are used for labeling, imaging, and drug delivery, as well as in materials science, where they are used for the synthesis of advanced coatings, adhesives, and composites.

Why is it called a click reaction?

In click reactions, scientists utilize special molecules that can easily snap together. This interlocking process allows scientists to build new things, much like constructing with Lego blocks.  

Click reactions have gained their name due to their simplicity, efficiency, and high selectivity. This term was introduced by K. Barry Sharpless in 2001, who described the ideal chemical reaction as "a process that is modular, wide in scope, gives a very high yield, generates only inoffensive by-products that are easily removed, and can be conducted under mild conditions, ideally in aqueous solution or in vivo."

Moreover, the ideal click reaction should also be highly selective and occur with a single step, eliminating the need for complex purification or isolation steps. The term "click" has now become widely used to describe a family of chemical reactions that fulfill these criteria.

Is click chemistry green chemistry?

Click chemistry has been recognized as a green chemistry technology due to its high efficiency, selectivity, and low waste production. The ideal click reaction should generate minimal or no waste, require minimal energy input, and proceed under mild reaction conditions (i.e. ambient temperature and pressure), making it an attractive option for sustainable chemistry. 

Click reactions can also be used in aqueous or other environmentally friendly solvents, further reducing their environmental impact. Furthermore, chemists combine click chemistry with other green chemistry techniques, such as flow chemistry, to further minimize waste and energy consumption. Click chemistry represents a promising avenue for the development of more sustainable and environmentally friendly chemical synthesis methods.

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What is the difference between click chemistry and click reactions?

Click chemistry is a type of chemical synthesis that aims to create new molecules quickly, easily, and selectively using only a small set of highly reliable and efficient reactions. Click reactions, on the other hand, are specific chemical reactions that use click chemistry to form a covalent bond between two functional groups.

In other words, click chemistry is a broad term that describes a general approach to chemical synthesis, while click reactions are specific reactions used within this approach. Click reactions are characterized by their high yield, high specificity, and mild reaction conditions, which make them ideal for applications such as drug discovery, materials science, and bioconjugation.