What is Click Chemistry?

On October 4, 2022, the Nobel Prize Committee announced that the 2022 Nobel Prize in Chemistry will be awarded to American chemist Carolyn R. Bertozzi, Danish chemist Morten Meldal and American chemist Karl Barry Sharpless in recognition of their outstanding contributions to click chemistry and bioorthogonal chemistry. As a set of efficient, robust, and stereoselective reactions, click chemistry can exploit simple reaction conditions and readily available starting materials to develop promising building blocks. Click chemistry has opened up a new method of combinatorial chemistry based on the synthesis of carbon-heteroatom bonds (C-X-C), and with the help of these reactions (click reactions) to obtain molecular diversity simply and efficiently.

How Does Click Chemistry Work?

Cu(I)-Catalyzed Azide-Alkyne Click Chemistry (CuAAC)

Immediately following the concept of click chemistry, the monovalent copper-catalyzed azide-alkyne cycloaddition reaction was independently reported by the Sharpless and Medal groups in 2002. This reaction can be regarded as the first classic work in click chemistry. Azides and terminal alkynes are stable under most chemical conditions, but can be efficiently and specifically converted to 1,3-substituted triazoles (Eq. 1) under the catalysis of monovalent copper. The advantages of this reaction are mild reaction conditions, high yield, high chemoselectivity and no interference from water and oxygen.

Strain-promoted Azide-Alkyne Click Chemistry (SPAAC)

The SPAAC reaction does not require metal catalysts, reducing agents, or stable ligands. This reaction utilizes the enthalpy released by ring strain to cyclooctynes (such as OCT, BCN, DBCO, DIBO, and DIFO) to form stable triazoles (Eq. 2). Although the reaction kinetics of SPAAC is slower than that of CuAAC, its biocompatibility in living cells is beyond doubt. So far, this reaction has been widely used in the fields of hybrid and block polymer formation, metabolic engineering, nanoparticle functionalization, oligonucleotide labeling, etc.

Inverse Electron Demand Diels-Alder (IEDDA)

The reaction of trans-cyclooctene (TCO) through IEDDA under physiological conditions has the characteristics of no catalyst, fast reaction rate and good biocompatibility. Tetrazine is a class of click chemistry labeling reagents containing reactive tetrazine groups, six-membered heterocyclic compounds containing four nitrogen atoms. Tetrazine has three isomers: 1,2,3,4-tetrazine, 1,2,4,5-tetrazine, 1,2,3,5-tetrazine. Tetrazine reagents are highly reactive with TCO in IEDDA reactions to eliminate nitrogen. Currently, the reaction of trans-cyclooctene with Tetrazine is widely used in the research of biology and material science, especially targeted medical imaging or other bioconjugation applications.

Characteristics of Click Chemistry Reaction

(1) Reaction modularization, such as azide and alkynyl can generate triazole compounds.

(2) Raw materials are easy to obtain and have a wide range of applications.

(3) High reaction yield, good regio- and stereoselectivity.

(4) Simple operation, mild reaction conditions, not afraid of water and oxygen.

(5) The product is easy to separate and purify, and can be separated by recrystallization or distillation without chromatographic column separation.

(6) Most reactions involve the formation of carbon-heteroatom (mainly nitrogen, oxygen, sulfur) bonds.

(7) The reaction requires a high thermodynamic driving force (>84 kJ/mol).

(8) Click reactions are generally chemical compounds (no by-products) or condensation reactions (products are small molecules such as water).

Applications of Click Chemistry

• Drug preparation
• Synthesis of lead compound libraries
• Target-directed active small molecule synthesis
• Glycoprotein
• Bioprobes and microarrays
• Immunofluorescence detection

Click Chemistry and Bioorthogonal Chemistry

Bioorthogonal chemistry refers to the set of chemical reactions that occur in living systems without interfering with natural biochemical processes. These reactions are often highly selective and can be performed under physiological conditions, allowing researchers to introduce specific chemical modifications in biological systems without causing major damage or disruption. Many click chemistry reactions are bioorthogonal and have been widely used in various fields such as bioimaging, drug delivery, proteomics, and chemical biology. For example, bioorthogonal reactions can be used to attach fluorescent dyes or other imaging probes to specific biomolecules, allowing researchers to visualize and track their localization and dynamics in living cells or organisms. Likewise, bioorthogonal chemistry can be used to selectively deliver drugs or therapeutics to specific targets in vivo, thereby minimizing off-target effects. Currently, the types of click reactions that can be used in bioorthogonal chemistry include strain-promoted azido-alkyne cyclodition (SPAAC), Cu-catalyzed azido-alkyne cyclodition (CuAAC), Staudinger ligation, cyclodition reactions between a tetrazine and a trans-alkene or a trained alkyne, reactions between an isocyanopropyl group and tetrazine, and Diels-Alder reactions between a cyclopentadienone and a trained alkyne. Each type of reaction has its own characteristics and is suitable for certain types of applications, such as drug delivery or bioimaging.

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