Click chemistry is an approach to chemistry that assembles molecules from modular building blocks through highly reliable, selective chemical reactions. Rather than naming one reaction, it describes a strategy for choosing transformations that give high yields, tolerate diverse chemical environments, and simplify product isolation. Its best-known example is copper-catalyzed azide–alkyne cycloaddition, which joins two complementary reactive groups into a stable ring. Applications extend from organic synthesis to materials construction and biological labeling. (onlinelibrary.wiley.com)
Origins and defining principles
K. Barry Sharpless, Hartmuth C. Kolb, and M. G. Finn systematically described the approach in a 2001 paper. Their central proposal was to construct useful compounds through a small repertoire of dependable linking reactions, rather than develop a separate synthetic solution for every target structure. This emphasizes molecular function and practical accessibility while making extensive use of existing carbon frameworks. (onlinelibrary.wiley.com)
The original criteria included modularity, broad scope, high yields, and byproducts removable without chromatographic purification. Ideal reactions use readily available starting materials, simple conditions, and either no solvent or a benign, easily removed one, such as water. Insensitivity to oxygen and moisture is desirable. Favorable thermodynamics helps drive these reactions toward stable products, although each reaction must also have a kinetically accessible pathway. These are selection criteria, not properties automatically shared by every transformation called “click.” (onlinelibrary.wiley.com)
In 2002, teams led by Morten Meldal and by Sharpless and Valery Fokin independently published copper-catalyzed azide–alkyne reactions that became central examples of the approach. The 2022 Nobel Prize in Chemistry was awarded jointly to Carolyn R. Bertozzi, Meldal, and Sharpless for developing click chemistry and bioorthogonal chemistry. (pubmed.ncbi.nlm.nih.gov)
Copper-catalyzed azide–alkyne cycloaddition
Copper-catalyzed azide–alkyne cycloaddition—usually abbreviated CuAAC—reacts an organic azide with a terminal alkyne. Copper(I) catalysis produces a 1,4-disubstituted 1,2,3-triazole, a five-membered ring containing three nitrogen atoms. The substituents carried by the starting materials consequently become connected through a defined molecular junction. Meldal’s initial publication demonstrated this transformation on solid-supported peptides with several classes of azide. (pubmed.ncbi.nlm.nih.gov)
The uncatalyzed azide–alkyne reaction belongs to the broader Huisgen cycloaddition family. Copper catalysis makes coupling practical under milder conditions and controls which ring positions bear the substituents. This regioselectivity reduces the formation of alternative positional isomers and simplifies the construction of structurally defined products. The copper-catalyzed process is stepwise rather than simply a faster version of a concerted thermal reaction. (pubmed.ncbi.nlm.nih.gov)
Azide and alkyne groups serve as complementary chemical handles that can be attached to larger structures before coupling. Their compatibility with many other groups allows the linking step to be separated from preparation of the molecular components. CuAAC therefore functions as a general assembly method rather than a reaction restricted to one class of final product. (sharpless.scripps.edu)
Bioorthogonal and copper-free reactions
Bioorthogonal chemistry concerns reactions that occur in biological environments without substantially interfering with native biochemical processes. It overlaps with click chemistry but is not synonymous with it: an efficient laboratory coupling need not be suitable for living cells. Copper-associated toxicity, for example, limits the direct use of conventional CuAAC in many living-system experiments. (nobelprize.org)
Strain-promoted azide–alkyne cycloaddition (SPAAC) avoids copper by using strained cyclic alkynes. Distortion of the alkyne within the ring increases its reactivity toward azides. Bertozzi and colleagues developed copper-free labeling methods based on this principle; a 2007 study used a fluorinated cyclooctyne to label cell-surface glycans and follow their movement in living cells. (pmc.ncbi.nlm.nih.gov)
Such experiments commonly separate labeling into two stages. A biological process first incorporates a small reactive handle into the target molecules; a complementary reagent then attaches a detectable probe. This enables studies of carbohydrate structures that cannot be labeled directly using genetically encoded fluorescent proteins. (pmc.ncbi.nlm.nih.gov)
Another important family is tetrazine ligation, including reactions between tetrazines and trans-cyclooctenes. These catalyst-free cycloadditions can proceed rapidly in water, cell media, and cell lysates. Their speed supports modification of proteins at low concentrations, where slower coupling reactions may be impractical. (pubmed.ncbi.nlm.nih.gov)
Additional reaction families and applications
The click concept extends beyond azide–alkyne chemistry. The 2001 formulation included selected cycloadditions, nucleophilic ring-opening reactions, carbonyl transformations, and additions to carbon–carbon multiple bonds. Membership depends on practical performance under specified conditions, not merely on the formal reaction category. (onlinelibrary.wiley.com)
Sulfur(VI) fluoride exchange (SuFEx), introduced as a click reaction family in 2014, uses sulfur(VI) fluoride groups as controlled linking sites. Suitable activation enables replacement of fluoride and formation of robust sulfur-containing connections. SuFEx has also been demonstrated in polymer synthesis, including the preparation of polysulfates and polysulfonates. (pubmed.ncbi.nlm.nih.gov)
In bioconjugation, click reactions connect probes or other components to biological molecules. In drug-discovery research, modular coupling allows related compounds to be assembled for experimental comparison. Materials applications include joining polymer components and altering material properties through attached molecular groups. Rapid radiolabeling provides another application, linking radioactive tags to molecular probes for imaging research. (sharpless.scripps.edu)
Practical boundaries
High coupling efficiency does not eliminate every experimental constraint. Copper toxicity can require a different reaction platform for living systems, while copper-free methods depend on the reactivity and availability of their specialized partners. Likewise, the original preference for simple isolation and benign solvents must be assessed for the complete synthesis, including preparation of the building blocks—not only the final joining step. (pmc.ncbi.nlm.nih.gov)