A tris(benzyltriazolemethyl)amine-based cage as a CuAAC ligand tolerant to exogeneous bulky nucleophiles

Article abstract of DOI:10.1039/d0cc08005e

The archetypal tris(benzyltriazolemethyl)amine (TBTA) ligand was equipped with a bowl-shaped cap in the cage Hm-TBTA. Hm-TBTA accelerates CuAAC reactions without suffering from product inhibition. Furthermore, this shielded ligand efficiently protects the

Full text of DOI:10.1039/d0cc08005e

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A tris(benzyltriazolemethyl)amine-based cage
as a CuAAC ligand tolerant to exogeneous bulky
nucleophiles†
Cite this: Chem. Commun., 2021,
57, 2281
Received 9th December 2020,
Accepted 25th January 2021
Gege Qiu, Paola Nava,
Alexandre Martinez * and Cedric Colomban
*
´
DOI: 10.1039/d0cc08005e
rsc.li/chemcomm
The archetypal tris(benzyltriazolemethyl)amine (TBTA) ligand was metal-binding in a confined space,13 or as highly efficient chlorine
equipped with a bowl-shaped cap in the cage Hm-TBTA. Hm-TBTA receptors through C–H hydrogen bonding.14 However well-defined
accelerates CuAAC reactions without suffering from product inhibition. organic cages based on the TBTA unit have not been evaluated in
Furthermore, this shielded ligand efficiently protects the copper center CuAAC reactions. On this basis, we resonated that the covalent
from deactivation by the CuI–chelator glutathione, opening the way to capping of TBTA by another C3 symmetrical unit will represent a
novel approaches for efficient CuAAC reactions in complex media.
particularly useful variation of the canonical ligand allowing for the
evaluation of the benefits of controlling the second coordination
The Cu-catalyzed azide–alkyne cycloaddition reaction (CuAAC) sphere level.
is the most popular reaction of the ‘‘click chemistry’’ toolbox.1
In this communication, we present the preparation and
Due to its high versatility, tolerance, and efficiency under mild evaluation as a CuAAC ligand of the novel hemicryptophane
conditions, this transformation belongs to the most broadly Hm-TBTA that represents an unprecedented organic cage built
applicable methods to prepare covalent linkages.2 Thus, CuAAC from the TBTA unit (Fig. 1). We demonstrate the ability of the
applications range from chemical biology,3 and medicinal caged-ligand to accelerate the reaction. Furthermore, we com-
chemistry,4 to materials science and interlocked structures.5 pare the performances of TBTA and Hm-TBTA in the presence
Many efforts have been dedicated to the development of assisting- of two bulky Cu–chelators: the picrate anion (Picꢀ) and the
ligands to improve the reaction efficiency.6 In their pioneering glutathione biothiol (GSH).
studies, Sharpless and Fokin report the tris(benzyltriazolemethyl)-
Hemicryptophanes are organic cages constituted of a northern
amine (TBTA) architecture as a remarkable CuI-stabilizing and rate- C3 symmetrical cyclotryveratrylene (CTV) unit, connected to
accelerating ligand.7 Since then, TBTA has become the most widely another tripodal unit.15 These structures have been recently
used CuAAC ligand and has been applied in bioconjugation reported as a new class of tridimensional ligands allowing for
reactions.8 Several structural variations of tetradentate triazole- the improvement of bioinspired oxidations involving metal-based
based coordinating structures have been described,6 including a catalysts.15,16 In line with this, we have designed the new hemi-
very recent report of superior water-soluble bioconjugaison CuAAC cryptophane Hm-TBTA where the archetypal TBTA is connected to
ligand.9 But despite recent progress, the development of catalysts
that remain active in complex mixtures, such as living systems
containing biological CuI ligands, is still needed.10 Deactivation of
several CuAAC catalysts via coordination of the copper ion by
external nucleophiles is indeed proposed to explain their low
in vivo catalytic efficiency.11 In this context, providing a second
coordination sphere to metal-based catalysts appears as a promising
approach due to the crucial benefits offered by confined catalysis in
terms of stability and efficiency.12 The tris(triazolemethyl)amine
structure has been introduced into organic cages and applied in
Aix Marseille Univ., CNRS, Centrale Marseille, iSm2, Marseille, France.
Fig. 1 DFT-optimized structures (BP86-D3/def2-SV(P)) of the canonical cop-
per complex CuI(TBTA), and the hemicryptophane complex CuI(Hm-TBTA),
which is target of this study.
E-mail: cedric.colomban@univ-amu.fr, alexandre.martinez@centrale-marseille.fr
† Electronic supplementary information (ESI) available: Experimental procedure
and spectral data. See DOI: 10.1039/d0cc08005e
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Table 1 Comparison of CuAAC accelerating ligand Hm-TBTA and TBTAa
a CTV unit by three methylene –CH2– bridges (Fig. 1). Hm-TBTA
was obtained in a five-step synthetic procedure (Fig. 2 and Scheme
S1, ESI†). The CTV(OH)3 unit was derivatized by compound 1 in
DMF, using Cs2CO3 as a base, to yield the CTV derivative 2, which
presents three azidomethyl–benzene moieties, in 79% yield.
Hm-TBTA was then obtained following a ‘‘1+1’’ coupling strategy
under diluted conditions, which allows for the closure of the cage
and the formation of the TBTA unit in a single step. The CuAAC
reaction between an equimolar amount of 2 and tripropargyla-
mine, using a catalytic quantity of the CuSO4 salt in combination
with the TBTA ligand (10 mol%), and sodium ascorbate NaAsc
(reducing agent, 0.5 equiv.) leads to the target cage in 22% yield.
The 1H NMR characterization of Hm-TBTA reveals sharp, identical
and well defined signals for the protons belonging to the southern
[Cu] source
Entry (1 mol%)
Reducing agent Ligand
(5 mol%)
Time Yieldc
(1 mol%) (h) (%)
1
2
3
4
5
6
7
[CuI(CH3CN)4]PF6
—
—
—
—
—
—
—
NaAsc
NaAsc
NaAsc
NaAsc
none
Hm-TBTA
TBTA
Hm-TBTA 24
TBTA 24
Hm-TBTA 0.5
TBTA
Hm-TBTA
TBTA
Hm-TBTA 16
TBTA 16
2
2
2
o1
17
35
99
99
6
12
43
64
85
95
[CuI(CH3CN)4]PF6
[CuI(CH3CN)4]PF6
[CuI(CH3CN)4]PF6
[CuI(CH3CN)4]PF6
[CuI(CH3CN)4]PF6
[CuI(CH3CN)4]PF6
CuIISO4
0.5
2
2
8b
9b
10b
11b
CuIISO4
0
N–CH2– link (Ha), the TBTA unit (Hb, Hc,c , Hd and He), the –CH2–
CuIISO4
0
0
link (Hf,f ), and the CTV unit (Hg, Hh, Hi and Hj,j ), attesting to an
averaged C3 symmetrical structure in CDCl3, at room temperature
(Fig. 2 and Fig. S3, S4, ESI†).
CuIISO4
a
Reaction conditions: benzyl azide (0.2 mmol), phenylacetylene (1.0 equiv.),
[CuI(CH3CN)4]PF6 (1 mol%), in a 1 : 10 MeOH/CH2Cl2 solvent mixture, at
Next, the capability of Hm-TBTA to coordinate a Cu(I) metal–ion
in its interior was explored. The copper complex CuI(Hm-TBTA)(PF6)
was prepared by reacting one equivalent of the CuI(CH3CN)4(PF6)
salt in CH3CN, at room temperature, under an inert atmosphere (see
the ESI†). The formation of the complex was confirmed by High-
Resolution Mass Spectrometry (ESI-HRMS) analysis (Fig. S5, ESI†).
b
25 1C, under an argon atmosphere. Benzyl azide (0.2 mmol), phenyl-
acetylene (1 equiv.), CuIISO4 (1 mol%), sodium ascorbate (NaAsc, 5 mol%) in
a 2:1:1 THF/H2O/tBuOH solvent mixture, at 25 1C. c Yields determined by
1H NMR using 2,4-dibromomesitylene as an internal standard.
Interestingly, the 1H NMR analysis of CuI(Hm-TBTA)(PF6) reveals a with an endohedral functionalization of the copper complex.17 The
C3 symmetrical spectrum (on average) in CD3CN, indicating a performance of our capped ligand Hm-TBTA was then evaluated
retention of the cage symmetry upon copper coordination. Its and compared with the one of its related open analogue, in a model
1H NMR spectra display identical and well defined signals. Com- CuAAC transformation. We have studied the cycloaddition reaction
pared to the 1H-NMR spectrum of the free ligand in CD3CN, that of between benzyl azide and phenylacetylene 3, catalyzed by 1.0 mol%
CuI(Hm-TBTA)(PF6) displays signals corresponding to the triazole of the CuI(CH3CN)4(PF6) metal salt (see the ESI†). The yields of the
(Hb) and the –CH2–Ar (Hc,c’) moieties that appear shifted and desired product 4 have been determined for reactions performed
broader (Fig. S9, ESI†), in good agreement with a binding of the in the presence of 1.0 mol% of the tris(triazole)-based structures
Cu(I) ion at the southern coordinating unit. These NMR observa- Hm-TBTA or TBTA, as well as in the absence of a stabilizing ligand
tions, which reveal that the C3 symmetry of Hm-TBTA is retained (Table 1, entries 1–3). For a more rigorous comparison, reaction
upon metal coordination at its tris(triazolemethyl)amine unit, yields have been compared at a short reaction time of 2 hours, when
strongly suggest that the coordination of the CuI metal ion occurs a third of the maximum yield was obtained in the case of the TBTA
ligand (35% yield, Table 1, entry 3). At longer reaction time (24 h)
quantitative yields were obtained for both TBTA and Hm-TBTA
(Table 1, entries 4 and 5).
Importantly, after 2 hours, the model CuAAC reaction in the
presence of Hm-TBTA (1 mol%) gave 17% yield while the
ligand-free experiment only produces trace of 4 (Table 1, entries
2 and 1 respectively). Therefore Hm-TBTA clearly acts as an
accelerating ligand for the CuAAC reaction, allowing for the
observation of several turnover numbers without suffering from
product inhibition effect. It should be noted that inhibition by
a product that ends up trapped in the catalyst’s tridimentional
structure, is a common issue of supramolecular catalysis.18
These results therefore highlight the flexible nature of the
Hm-TBTA ligand that does not become eventually blocked by
the triazole product 4. This flexibility, which also guarantees
the access of both substrates to the copper active site, contrasts
with other confined CuI-catalysts. For example, the encapsula-
tion of a CuI–carbene catalyst within a cucurbit[7]uril host has
been very recently used to fully suppress the CuAAC reaction.19
Flexibility of Hm-TBTA has been further demonstrated by
Fig. 2 (a) Synthesis of the hemicryptophane Hm-TBTA along with (b) its
1H NMR spectra (CDCl3, 400 MHz), at 298 K (* = CH2Cl2).
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Fig. 3 Preparation of triazole products Pn in quantitative yield using the
CuAAC ligand Hm-TBTA. [a] reaction conditions: see Table 1.
extending the substrate scope to hindered phenylacetylenes.
Reactions between para (5, 6, 7), meta (8), and ortho-substituted
(9) alkynes and benzyl azide, indeed yield the corresponding
products P5–9 with quantitative NMR-yields, after 24 h reaction
time (Fig. 3). The reaction is not suppressed by theses sub-
strates and Hm-TBTA does not end up blocked by the bulky P5–9
products. In addition, competitive CuAAC reactions of systems
containing two alkynes (at 2 hours reaction time) reveal modest
substrate-selectivity improvements in the case of Hm-TBTA
(compared to the open TBTA ligand) (see Fig. S12, ESI†).
Fig. 4 CuAAC reaction between benzyl azide (0.2 mM) and phenylacetylene
(1.0 equiv.), catalyzed by 1.0 mol% of CuI(CH3CN)4(PF6), in the presence of 0,
10, 30, and 70 mol% of potassium picrate (K+Picꢀ), using 1.0 mol% of Hm-TBTA
(blue circle) or TBTA (purple square), yields determined by 1H NMR.
The direct comparison between Hm-TBTA and its related ligand TBTA was strongly affected by the Picꢀ nucleophile: the
‘‘uncaged’’ model reveals a 2-fold decrease of the catalytic yield of triazole product 4 dramatically drops from 35% (in the
efficiency. Indeed, 17% and 35% yields were obtained after 2 absence of external nucleophile), to 17%, 9%, and 7% yields in
hours using respectively Hm-TBTA and TBTA ligands (Table 1, the presence of respectively 10, 20 and 70 mol% of K+Picꢀ (Fig. 4,
entries 2 and 3). Comparison of the DFT-optimized structures and Table S2, ESI†).
of the two copper complexes (Fig. 1) clearly shows a widely
Clearly, the CTV-capped Hm-TBTA structure protect the
exposed CuI-center in the case of CuI(TBTA), while a much copper complex from the competitive binding of the bulky
more hindered metal core is observed in CuI(Hm-TBTA). picrate anion. Inspired by this observation, we next sought to
Furthermore it has been proposed that the TBTA-assisted explore the possibility of using Hm-TBTA to shield the metal
CuAAC reaction involves a dinuclear copper intermediate ion from its binding by bulky biological Cu(I) ligands. The
allowed by the dissociation of triazole arms.20 Therefore, a glutathione reduced (GSH) thiol was selected as a model
more difficult formation of such a dinuclear complex for the biological nucleophile because this tripeptide is particularly
capped ligand might account for its slower catalytic perfor- abundant in the biological milieu, with concentrations that
mance. Studies of the influence of the Hm-TBTA:CuI ratio could range from micromolar to 10 mM,10b,23 and its coordina-
reveal a strong enhancement of the catalytic efficiency upon tion to the copper is considered as the major factor that greatly
addition of a second copper ion (Table S1, ESI†). Although the hampered bioconjugation reactions. Inhibition of tris(triazole-
existence of a monometallic mechanism could not be excluded, methyl)amine-based CuAAC catalysts in the presence of 23,23
these results strongly suggest that Hm-TBTA is flexible enough and even 2 equivalents of GSH,10a with respect to copper, have
to allow a bimetallic mechanism. Interestingly, Hm-TBTA been reported. As expected, when TBTA (1 mol%) was applied
(1 mol%) was also an effective accelerating ligand for aqueous in the model CuAAC reaction, in the presence of GSH
CuAAC reaction using a CuII salt (CuII(SO4), 1 mol%) in combi- (20 mol%), a marked decrease in efficiency was observed. After
nation with a reducing agent (NaAsc, 5 mol%) in a 2 : 1 : 1 THF/ 2 hours, the reaction yield dramatically drops from 35% (in the
H2O/tBuOH solvent mixture (Table 1 entry 8).
absence of biothiol), to only 7% after addition of 20 equivalents
We then wondered if the steric shield offered by Hm-TBTA of GSH compared to the copper (Table 2). Remarkably, we
could protect the copper catalyst from its inhibition by external observed that our CTV-capped ligand was able to efficiently
bulky nucleophiles. Aiming at establishing a proof of our protect the active CuI ion as the catalytic performance was
concept, we first examine the putative shielding effect of retained in the presence of 20 mol% of the sulfur-containing
Hm-TBTA by studying our model CuAAC reaction in the amino acid. After 6 and 24 h of reaction in the presence of GSH
presence of the potassium picrate salt (K+Picꢀ). We chose the bulky (20 mol%), the transformation accelerated by Hm-TBTA was
picrate nucleophile because (i) it can bind to Cu-complexes,21 significantly more efficient than in the case of TBTA with
and (ii) it has been previously reported that this anion cannot respectively 30 and 99% yields, while no further formation of
access the cavity of hemycryptophane cages, due to its large size.22 product 4 was observed for the open ligand (6–7% yields,
Interestingly, the data in Fig. 4 revealed that the Hm-TBTA-assisted Table 2). Importantly, isolated yields obtained after 24 h con-
CuAAC reaction did not suffer from deactivation when performed in firm this remarkable behavior (Table 2, entries 4 and 8). The
the presence of up to 70 equivalents of K+Picꢀ compared to the CuI caged ligand Hm-TBTA therefore provides a powerful steric
source. Contrastingly, the catalytic performance of its parent open shield that prevents the competitive binding of a bulky exogeneous
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Table 2 Comparison of TBTA- and Hm-TBTA-assisted CuAAc reactions
in the presence of the biological thiol glutathione (GSH)a
Conflicts of interest
There are no conflicts to declare.
Notes and references
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Entry
Ligand
Glutathione (GSH)
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
TBTA
TBTA
TBTA
TBTA
Hm-TBTA
Hm-TBTA
Hm-TBTA
Hm-TBTA
None
2
2
6
24
2
2
6
24
35
7
20 mol%
20 mol%
20 mol%
None
20 mol%
20 mol%
20 mol%
6
7 (tracesc)
17
15
30
99 (83c)
a
Reaction conditions: benzyl azide (0.2 mmol), phenylacetylene
(1.0 equiv.), [CuI(CH3CN)4]PF6 1 mol%), in a 1 : 10 MeOH/CH2Cl2
solvent mixture, at 25 1C, under an argon atmosphere, in the presence
of GSH (20 mol%). b 1H NMR yields. Isolated yields.
c
CuI–ligand, preserving a high catalytic performance even in the
presence of 20 equivalents of the notorious CuAAC inhibitor GSH.
In summary, we described herein the first covalent capping
of the canonical CuAAC–ligand TBTA that has been equipped
with a CTV unit, providing a steric shield at the second
coordination sphere. We demonstrate that the resulting
cage Hm-TBTA coordinates a copper metal ion in its interior.
Hm-TBTA is a tridimensional CuAAC accelerating-ligand that
does not suffer from product inhibition. The Hm-TBTA-assisted
transformation is slower than in the case of the parent TBTA.
However, we evidence that our CTV-shielded ligand protects the
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Indeed, while the catalytic performance of the TBTA ligand
was drastically reduced in the presence of bulky Cu–chelators
´
19 T. G. Breve, M. Filius, C. Araman, M. P. van der Helm, P. Hagedoorn,
(Picꢀ, GSH), the Hm-TBTA-assisted CuAAC was not affected.
Remarkably, no significant change in the catalytic efficiency
was observed even in the presence of 20 equivalents (with
respect to the copper) of the notorious biological CuAAC
inhibitor glutathione. We envisioned that our approach could
find applications in the field of CuAAC chemistry in complex
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This work was supported by PRC ANR-19-CE07-0024-01.
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2284 | Chem. Commun., 2021, 57, 2281ꢀ2284
This journal is The Royal Society of Chemistry 2021