Angewandte
Chemie
DOI: 10.1002/anie.201310671
Bioorthogonal Click Chemistry Very Important Paper
Copper-Chelating Azides for Efficient Click Conjugation Reactions in
Complex Media**
Valentina Bevilacqua, Mathias King, Manon Chaumontet, Marc Nothisen, Sandra Gabillet,
David Buisson, Cꢀline Puente, Alain Wagner, and Frꢀdꢀric Taran*
Abstract: The concept of chelation-assisted copper catalysis
was employed for the development of new azides that display
unprecedented reactivity in the copper(I)-catalyzed azide–
alkyne [3+2] cycloaddition (CuAAC) reaction. Azides that
bear strong copper-chelating moieties were synthesized; these
functional groups allow the formation of azide copper com-
plexes that react almost instantaneously with alkynes under
diluted conditions. Efficient ligation occurred at low concen-
tration and in complex media with only one equivalent of
Scheme 1. Examples of efficient N-donor ligands for CuAAC reactions.
copper, which improves the biocompatibility of the CuAAC
reaction. Furthermore, such a click reaction allowed the
localization of a bioactive compound inside living cells by
fluorescence measurements.
TBTA=tris[(1-benzyl-1H-1,2,3-triazol-4-yl)methyl]amine, Bn=benzyl,
BTTE=2-[4-({bis[1-tert-butyl-1H-1,2,3-triazol-4-yl)methyl]amino}-
methyl)-1H-1,2,3-triazol-1-yl]ethanol.
T
he copper(I)-catalyzed azide–alkyne [3+2] cycloaddition
oxygen species (ROS) from O2,[6] several CuAAC procedures
have been reported in the literature. They are usually based
on the use of water-soluble copper ligands and additives that
both accelerate the reaction and act as ROS scavengers to
decrease the cell toxicity of copper.[7] A second approach uses
highly reactive azides designed according to the concept of
chelation-assisted metal catalysis. Picolyl azides have been
reported to react much faster than standard azides thanks to
internal chelation of the copper catalyst, which is supposed to
enhance the electrophilicity of the azido group and facilitate
the formation of the metallacycle intermediate.[8] This con-
cept was recently successfully exploited by Ting et al. to
perform site-specific protein labelling on the surface of living
cells at low concentrations (50 mm) of the copper tris(triazole)
complex using pyridine-based copper-chelating azides as
substrates.[9] Nevertheless, the use of the CuAAC reaction
in the cellular context is still limited to cell-surface labelling
and, aside from the above-mentioned examples, is often
discouraged in favor of copper-free click reactions.[10]
Herein, we describe our efforts to improve the perfor-
mance of the CuAAC reaction in complex media using new
azides, which include a complete copper-chelating system in
their structure (Scheme 2). These azides were designed to
form strong, active copper complexes and should therefore be
considered both as reactant and catalyst in the CuAAC
reaction. Using this type of azides, the CuAAC should thus
become a bimolecular reaction and display much faster
kinetics than classic CuAAC reactions.
(CuAAC)[1] reaction can be considered as the archetype of
click chemistry.[2] The many reviews that relate the wide
variety of applications of this relatively young reaction bear
witness to its huge impact.[3] In this context, the use of copper
ligands that are able to stabilize and modulate the catalytic
activity of the CuI center contributed to the success of this
reaction by allowing for smoother reaction conditions and
broader applicability. Some of the most representative ligands
are the commercially available sulfonated bathophenantro-
line BPDS, the tris(benzimidazole) (BimC4A)3 and the tris-
(triazole) TBTA ligands (Scheme 1), which greatly accelerate
the reaction and stabilize the CuI oxidation state.[4] Analogues
of TBTA that bear bulky tert-butyl groups, such as BTTE and
BTTES, were recently found to be the most efficient ligands
for the CuAAC reaction described thus far.[5]
Despite the high reactivity of such copper complexes, the
copper source, ligands, and sodium ascorbate are usually
required in excess amounts to reach efficient ligation under
the diluted conditions that are typically used in the fields of
bioconjugation or cell labelling. To limit the damages that are
caused by the copper(I)-mediated generation of reactive
[*] V. Bevilacqua, Dr. M. Chaumontet, M. Nothisen, S. Gabillet,
D. Buisson, C. Puente, Dr. F. Taran
CEA, iBiTecS, Service de Chimie Bioorganique et de Marquage
91191 Gif sur Yvette (France)
E-mail: frederic.taran@cea.fr
To evaluate this strategy, we first synthesized various
chelating azides with increasing copper-chelation capabilities
(Scheme 3; see the Supporting Information for detailed
synthetic procedures). Azides A1, A3, and A4 were designed
for negative-control experiments; all other azides were
chosen for their potential ability to coordinate copper as bi-
(A2, A5, A6–A10, and A12), tri- (A11 and A13), or
Dr. M. King, Dr. A. Wagner
Laboratory of Functional Chemo Systems, Facultꢀ de Pharmacie
67000 Strasbourg (France)
[**] This work was supported by the European community through the
BioChemLig project.
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1
These are not the final page numbers!