Click Chemistry Knowledge Base
A comprehensive reference for click reactions, reagents, mechanisms, and biomedical applications
Reaction Types
Explore the major families of click reactions, from copper-catalyzed cycloadditions to strain-promoted and bioorthogonal transformations.
CuAAC Reaction
Copper(I)-catalyzed azide-alkyne cycloaddition — the flagship click reaction forming 1,2,3-triazoles with exquisite regioselectivity under mild conditions.
CycloadditionSPAAC Reaction
Strain-promoted azide-alkyne cycloaddition using cyclooctynes (DBCO, BCN) — copper-free and bioorthogonal, ideal for live-cell labeling.
Strain-PromotedTetrazine / IEDDA
Inverse electron-demand Diels-Alder reaction between tetrazines and strained alkenes (TCO, BCN) — the fastest bioorthogonal reaction known (k > 104 M-1s-1).
Diels-AlderThiol-Ene
Radical-mediated addition of thiols across alkenes — high-yielding, oxygen-tolerant, and widely used in polymer and hydrogel synthesis.
Radical AdditionStaudinger Ligation
Phosphine-mediated reduction of azides to amines with concomitant amide bond formation — the first bioorthogonal ligation, pioneered by Bertozzi[4].
LigationClick-to-Release
IEDDA-triggered elimination reactions that uncage prodrugs or release payloads — extending click chemistry from labeling to therapeutic activation.
ReleaseNucleophilic Ring-Opening
Strain-driven opening of epoxides and aziridines by nucleophiles — a classical click family forming stable covalent bonds in aqueous media.
Ring-OpeningReagent Quick Reference
Key reagent classes used across click chemistry workflows, with structures, properties, and selection guidance.
Azides
Versatile 1,3-dipole partners for CuAAC and SPAAC. Small, stable, and bioorthogonal — the universal click handle.
Alkynes
Terminal and internal alkynes for CuAAC cycloadditions. Includes propargyl, hexynyl, and PEG-functionalized variants.
Cyclooctynes (DBCO/BCN)
Strained alkynes for copper-free SPAAC. DBCO and BCN enable bioorthogonal labeling without toxic catalysts.
Tetrazines
Electron-deficient dienes for IEDDA reactions. H-Tz, Me-Tz, and pyridyl-Tz variants with tunable reactivity.
TCO
trans-Cyclooctene — the fastest IEDDA dienophile. Enables ultra-fast tetrazine ligations for in vivo imaging.
Thiols
Nucleophilic sulfur reagents for thiol-ene and thiol-yne click reactions. Widely available as cysteine-containing biomolecules.
Phosphines
Reducing agents for Staudinger ligation. Triarylphosphines with electrophilic traps enable traceless amide bond formation.
Click Chemistry Classification
Sharpless and colleagues defined click chemistry[1] through four principal reaction categories, each encompassing multiple specific transformations.
1 Cycloadditions
Concerted pericyclic reactions forming heterocyclic rings with high atom economy and regioselectivity.
- 1,3-Dipolar cycloadditions — azide-alkyne (CuAAC, SPAAC), nitrile oxide-alkene
- Hetero-Diels-Alder — inverse electron-demand (IEDDA) with tetrazines and strained alkenes
- [4+2] and [2+2] variants — photochemical and thermal cycloadditions
2 Nucleophilic Ring-Openings
Strain-driven reactions where nucleophiles attack electrophilic strained heterocycles.
- Epoxides — ring-opening by amines, thiols, and alcohols
- Aziridines — nitrogen analogs with tunable N-substituent reactivity
- Lactones and cyclic sulfates — controlled polymerization and functionalization
3 Non-Aldol Carbonyl Chemistry
Thermodynamically driven condensations forming stable bonds without enolization side reactions.
- Amides — coupling reactions under near-neutral, aqueous conditions
- Hydrazones — hydrazine-aldehyde condensation with high equilibrium constants
- Oximes — aminooxy-aldehyde ligation, widely used in bioconjugation
4 Additions to C-C Multiple Bonds
Radical or base-catalyzed additions to alkenes and alkynes that proceed with anti-Markovnikov selectivity.
- Thiol-ene — radical-mediated hydrothiolation of unactivated alkenes
- Thiol-yne — double-addition to alkynes for network polymers
- Michael additions — conjugate additions of thiols and amines to acrylates
Biomedical Applications
Click chemistry has transformed the way researchers build and modify complex biomolecules, drug conjugates, and diagnostic probes.
Bioconjugation
Site-selective labeling of proteins, nucleic acids, and glycans for imaging, proteomics, and diagnostics. CuAAC and SPAAC are the dominant methods.
Drug Delivery
Click-to-release prodrug activation, antibody-drug conjugate (ADC) assembly, and modular synthesis of drug-loaded nanoparticles.
Materials Science
Thiol-ene hydrogels, click-crosslinked polymers, surface functionalization, and 3D-printable bioinks with spatially controlled gelation.