Copper-Free Click Chemistry


Click chemistry involves the rapid generation of compounds by joining small units together via heteroatom links (C-X-C). The main objective of click chemistry is to develop a set of powerful, selective, and modular “blocks” that are useful for small- and large-scale applications. Reaction processes involved in click chemistry should conform to a defined set of stringent criteria such as being:1

  • Simple to perform
  • Modular
  • Wide in scope
  • High yielding
  • Stereospecific
  • Environmentally friendly by generating only harmless byproducts that can be removed by non‑chromatographic methods

Important characteristics of the reactions involved in click chemistry are:1

  • Simple reaction conditions
  • Readily and easily available starting materials and reagents
  • Use of no solvent, a benign solvent (such as water), or one that is easily removed
  • Simple product isolation
  • Product should be stable under physiological conditions

Click chemistry involves the use of a modular approach and has important applications in the field of drug discovery, combinatorial chemistry, target-templated in situ chemistry, and DNA research.2

A well-known click reaction is the Huisgen 1,3-dipolar cycloaddition of azides and alkynes.3 This reaction, yielding triazoles, has become the gold standard of click chemistry for its reliability, specificity, and biocompatibility.4 Such cycloadditions need high temperatures or pressures when the reaction involves simpler alkene or azides, since the activation energies are high (ΔG ≈ +26 kcal/mol). Sharpless & co-workers and Meldal & co-workers reported Cu(I) catalysts expedite the reaction of terminal alkynes and azides, thereby affording 1,4-disubstituted triazoles.4,5 This reaction is an ideal click reaction and is widely employed in material science, medicinal chemistry, and chemical biology.5

Scheme of the well-known Cu-catalyzed azide-alkyne cycloaddition reaction:

The cytotoxic nature of transition metals, employed as catalysts for the click reactions, precluded their use for in vivo applications. Alternative approaches with lower activation barriers and copper-free reactions were proposed. Such reactions were referred to as “copper-free click chemistry”.5 Copper-free click chemistry is based on a very old reaction, published in 1961 by Wittig et al.  It involved the reaction between cyclooctyne and phenyl azide, which proceeded like an explosion to give a single product, 1-phenyl-4,5,6,7,8,9-hexahydro-1H-cycloocta[d][1,2,3]triazole.6 The reaction is ultrafast due to the large amount of ring-strain (18 kcal/mol of ring strain) in the cyclooctyne molecule. Release of the ring-strain in the molecule drives the fast reaction. Cyclooctynes are reported to react selectively with azides to form regioisomeric mixtures of triazoles at ambient temperatures and pressures without the need for metal catalysis and no apparent cytotoxicity.7 Difluorinated cyclooctyne reagents have been reported to be useful for the copper-free click chemistry.8




Copper-Free Click chemistry has various diverse applications:

  • Labeling of biomolecules, such as glycans and lipids, selectively in living systems with no apparent toxicity.7
  • Labeling biomolecules in live mice, by an bioorthogonal reaction, the 1,3-dipolar cycloaddition of azides and cyclooctynes.9
  • Biarylazacyclooctynone (BARAC) can be readily synthesized and employed for live cell fluorescence imaging of azide-labeled glycans.10
  • In situ “click” cross-linking of azide-terminated photodegradable star polymers, by employing bifunctional, fluorinated cyclooctynes.11
  • Functionalization of new platinum(IV) [PtIV] prodrugs. These prodrugs may be utilized for the installation of targeting moieties, delivery systems and fluorescent reporters from a single precursor that are capable of releasing biologically active cisplatin.12
  • Functionalization of gold nanoparticles with monovalent maleimide, by the installation of a maleimide group on nanoparticle via copper-free click chemistry.13
  • Synthesis of biocompatible and biodegradable polysaccharide hydrogels derived from chitosan and hyaluronan have been achieved by copper-free click chemistry. These hydrogels have potential soft-tissue engineering applications.14
  • Triazole analogs of phthalate plasticizers (PVC-DEHT, PVC-DBT and PVC-DMT) have been prepared by copper-free azide-alkyne click reaction. Di(2-ethylhexyl)-1H-triazole-4,5-dicarboxylate (DEHT), di(n-butyl)-1H-1,2,3-triazole-4,5-dicarboxylate (DBT), and di(methyl)-1H-triazole-4,5-dicarboxylate (DMT) are covalently attached to azide-functionalized polyvinyl chloride (PVC) via copper free-click reaction.15

Scheme of the above mentioned syntheses:15

  • Novel class of difluorinated cyclooctyne (DIFO) reagents were employed in copper-free click chemistry for the site-selective labeling of biomolecules in vitro and in vivo.16
  • Catalyst-free click reactions are useful tools for the preparation of radiometal-based pharmaceuticals. Radiotracer [64Cu]DOTA-ADIBON3-Ala-PEG28-A20FMDV2, [64Cu], used for positron emission tomography imaging of integrin αvβ6 expressing tumors, have been synthesized via copper-free click chemistry.17
  • Iodine radioisotope labeling of cyclooctyne-containing molecules by copper-free click reaction has been reported. Radioiodination using the tin precursor was carried out at room temperature to obtain 125I-labeled azide. Dibenzocyclooctyne (DBCO) containing cRGD peptide and gold nanoparticle were labeled by employing 125I-labeled azide to afford triazoles in good radiochemical yields (67–95%). This method is useful for both in vitro and in vivo labeling of DBCO group containing molecules with iodine radioisotopes.18
  • Protein site-specific labeling techniques involve copper-free strain-promoted azide–alkyne cycloaddition (SPAAC) reaction between dibenzocyclooctyne-fluor 545 (DBCO-fluor 545) and an azide-bearing unnatural amino acid (UAA).19



  1. Kolb, H. C; Finn, M. G. & Sharpless, K. B.  Angew. Chem., Int. Ed. 2001, 40, 2004.
  2. Kolb, H. C., & Sharpless, K. B. Drug discovery today  20038, 1128.
  3. Tornoe, C. W.; Christensen, C. and Meldal, M. J. Org. Chem. 2002, 67, 3057.
  4. Moses, J. E. & Moorhouse, A. D. Chem. Soc. Rev. 2007, 36, 1249.
  5. Rostovtsev, V. V.; Green, L. G.; Fokin, V. V. & Sharpless, K. B. Angew. Chem. 2002, 114, 2596.
  6. Akeroyd, N.; Postma, T. & Klumperman, B. Copper free click chemistry. Click chemistry for the preparation of advanced macromolecular architectures 2010.
  7. Lahann J. Click Chemistry for Biotechnology and Materials Science 2009.
  8. Himo, F.; Lovell, T.; Hilgraf, R.; Rostovtsev, V. V.; Noodleman, L.; Sharpless, K. B. & Fokin, V. V.  J. Am. Chem. Soc. 2005, 127, 210.
  9. Chang, P. V.; Prescher, J. A.; Sletten, E. M.; Baskin, J. M.; Miller, I. A.; Agard, N. J. & Bertozzi, C. R. Proc. Natl. Acad. Sci. 2010, 107, 1821.
  10. Jewett, J. C.; Sletten, E. M. & Bertozzi, C. R. J. Am. Chem. Soc. 2010, 132, 3688.
  11. Johnson, J. A.; Baskin, J. M.; Bertozzi, C. R.; Koberstein, J. T. & Turro, N. J. J. Chem. Commun. 2008, 26, 3064.
  12. Pathak, R. K.; McNitt, C. D.; Popik, V. V. & Dhar, S. Chemistry-A European Journal 2014, 20, 6861.
  13. Nieves, D. J.; Azmi, N. S.; Xu, R., Lévy, R.; Yates, E. A. & Fernig, D. G. Chem. Commun. 201450, 13157.
  14. Fan, M.; Ma, Y.; Mao, J.; Zhang, Z. & Tan, H. Acta Biomater. 2015, 20, 60.
  15. Earla, A. & Braslau, R. Macromol. Rapid Commun. 201435, 666.
  16. Codelli, J. A.; Baskin, J. M.; Agard, N. J. & Bertozzi, C. R.  J. Am. Chem. Soc. 2008130, 11486.
  17. Satpati, D.; Bauer, N.; Hausner, S. H. & Sutcliffe, J. L. J. Radioanal. Nucl. Chem. 2014, 302, 765.
  18. Jeon, J.; Kang, J. A.; Shim, H. E.; Nam, Y. R.; Yoon, S.; Kim, H. L.; Lee D. E. & Park, S. H.   Bioorg. Med. Chem. 2015.
  19. Zhang, G.; Zheng, S.; Liu, H. & Chen, P. R. Chem. Soc. Rev. 2015.


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