Pyridinyl-triazole ligand systems for highly efficient CuI-catalyzed azide-alkyne cycloaddition

Article abstract of DOI:10.1016/j.catcom.2020.106165

Pyridinyl-triazole ligand systems (including N²-2-pyridinyl 1,2,3-triazoles and N¹/N²-substituted 2-(NH-1,2,3-triazol-4-yl)pyridines) were found to be superior ligands for CuI-catalyzed azide-alkyne cycloaddition (CuAAC) r

Full text of DOI:10.1016/j.catcom.2020.106165

CatalysisCommunications148(2021)106165
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Catalysis Communications
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Short communication
Pyridinyl-triazole ligand systems for highly efficient CuI-catalyzed azide-
alkyne cycloaddition
⁎
Lei Zhenga, Ye Wanga, Xianggao Mengb, Yunfeng Chena,
a School of Chemistry and Environmental Engineering, Wuhan Institute of Technology, Wuhan 430073, PR China
b Department of Chemistry, Central China Normal University, Wuhan 430079, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Pyridinyl-triazole ligand systems (including N2-2-pyridinyl 1,2,3-triazoles and N1/N2-substituted 2-(NH-1,2,3-
triazol-4-yl)pyridines) were found to be superior ligands for CuI-catalyzed azide-alkyne cycloaddition (CuAAC)
reactions. Low catalyst loadings, short reaction times, facile catalyst recyclability, ambient temperature, and
open-flask conditions made this catalytic system very practical. The iodide anions could form iodine bridges to
construct stable dinuclear Cu(I) complexes with these ligands, which was the key to achieve high catalytic
activities. While CuBr and CuCl were not suitable for this ligand system because of the improper size of Br and Cl
atoms for the formation of the corresponding dinuclear Cu(I) complexes.
1,2,3-Triazole
Ligands
Dicopper complex
Click chemistry
1. Introduction
Later, an alcohol variant of TBTA, tris(1-benzyl-1H-1,2,3-triazol-4-yl)me-
thanol ligand, together with CuCl also exhibited high catalytic activities
Copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction [1,2],
as a typical “click” reaction [3], has been widely used in drug discovery
[4,5], materials science [6], bioconjugation [7], and many other fields.
The development of highly active catalysts is the key to this reaction. The
initial catalytic systems were mainly based on the reduction of Cu(II)
salts in aqueous phase, where the active Cu(I) species were generated in
situ. Later, it was found that ligands could stabilize the Cu(I) species,
increase the solubility, and enhance the catalytic performance. There-
fore, various ligands [8–10] such as nitrogen ligands (including amine or
N(sp2)-type) [11,12], phosphorous ligands [13], carbene ligands [14],
and sulfur ligands [15] were developed to promote the CuAAC reactions.
Among them, Schiff-based ligands, including pyridinyl derivatives,
have been reported as ligands for the Cu-catalyst of this “click” reaction
[16]. Similarly, pyridinyl oxazoles have also been described as efficient
ligands [17]. On the other hand, some 1,2,3-triazole based ligands, re-
sulting from the CuAAC reactions, also demonstrated excellent catalytic
activities for the CuAAC reactions, which were regarded as “click ligands
for click reactions” (Scheme 1) [10,18]. For example, di-(benzyl-
triazolylmethyl) amine and tris-(benzyltriazolylmethyl)amine (TBTA),
discovered by Sharpless and Fokin [19], could bind Cu(I) to form a five-
member chelate ring between the N3-atom of the 1,2,3-triazoles and the N-
atom of the amine, in favor of stabilizing Cu(I) species towards highly
efficient CuAAC reactions. Finn et al. developed oligobenzimidazole and
related heterocycle variants for highly efficient CuAAC reactions [11].
[20]. For these triazoles ligands, two or more triazole rings were beneficial
to chelate the copper species; in particular, the N3 atoms could form a
strong coordinate bond with Cu(I) due to their high electron density.
Motivated by the good performance of pyridine/triazole-based li-
gands and our interest in the 1,2,3-triazole ligand chemistry [21], we
attempted to utilize high electron densities of triazolyl N1 or N3-atom
and pyridinyl N-atom to form bidentate chelating coordination mode,
so a series of pyridinyl-triazole ligands L1-L18 were designed which
could be easily obtained by triazole post-N- functionalization [22,23]
(Scheme 1). Herein, we reported a new catalytic system, N2-pyridinyl-
1,2,3-triazoles or N1/N2-substituted 2-(NH-1,2,3-triazol-4-yl)pyridines,
for highly effective CuI-catalyzed AAC reactions.
2. Results and discussion
2.1. Click reaction: reaction conditions
To initiate our studies, benzyl azide (1a) and phenylacetylene (2a)
were selected as model substrates to evaluate the catalytic activity of
designed ligands for [3 + 2] cycloaddition reactions. First, the re-
presentative N1 and N2-pyridinyl 1,2,3-triazole ligands L1-L4 were se-
lected, and the reactions were carried out in CH3CN under open-flask
conditions. The time-dependent yield profile (Fig. S1) showed that the
reaction exhibited excellent performance when L1 was employed as
⁎ Corresponding author.
E-mail address: yfchen@wit.edu.cn (Y. Chen).
https://doi.org/10.1016/j.catcom.2020.106165
Received 7 June 2020; Received in revised form 7 September 2020; Accepted 8 September 2020
Availableonline19September2020
1566-7367/©2020ElsevierB.V.Allrightsreserved.

L. Zheng, et al.
CatalysisCommunications148(2021)106165
Scheme 1. The design and synthesis of 1,2,3-triazole ligands for the CuAAC reactions.
ligand. However, the use of ligands L2-L4 resulted in low yields, and
the solution gradually turned green, which meant that the Cu(I) species
should be oxidized. As expected, the 1,2,3-triazoles ligands provide N3
or N1 atoms as one of the coordination sites, and the five-membered
chelate ring might be more suitable to stabilize the Cu(I) species [19].
By using L1 as the ligand, other reaction parameters, such as sol-
vents, reaction temperature, and other copper sources were subse-
quently explored (Table 1). Firstly, it showed that the solvent could
affect the reaction rates, and the reaction was accelerated in polar
solvents (DMF, CH3CN, EtOH), which might be attributed to the de-
polymerization of CuI and the subsequent formation of Cu(I) complex
(entries 1–6) [24]; moreover, 3aa could be obtained with 95% isolated
yield under solvent-free conditions in 2 h (entry 7). It was found that
recovered catalysts (see Supporting Information) could also promote the
reaction and give 3aa with 95% yield in 2 h (entry 8), which revealed
that the catalyst was very stable. Then, we tried to lower the catalyst
loading of L1/CuI. The reaction was also completed in 2 h by reducing
the catalyst loading to 0.5 mol% (entry 9). Importantly, it was found
that the choice of the copper salts was critical to this reaction. Other Cu
(I) salts, such as CuCl or CuBr, exhibited inferior reactivities with lower
conversion rates, and the reactions were gradually terminated with the
mixture turning to green (entries 10–11). If bipyridine was used instead
of L1, 3aa was isolated with only 25% yield (entry 12). Furthermore,
the reaction did not work without the addition of CuI or L1 (entry 13).
These results indicated that both CuI and chelate ligand were crucial for
achieving high reactivities in these catalytic systems.
2.2. Click reaction: substrate reactivity
With the optimized reaction conditions in hand, the scope of the
cycloaddition was explored (Table 2). A broad variety of alkynes and
azides, both equipped with various functional moieties such as arenes,
heterocycles, alcohols, alkanes, and esters, were successfully employed.
The reaction of azides (including benzyl, α‑carbonyl, aryl, and aliphatic
substituted) with 2a afforded the corresponding 1,4-disubstituted tria-
zoles 3aa-3ag with excellent yields (up to 99%). Similar results were
obtained with alcohol and alkane terminal alkyne substrates, affording
1,4-disubstituted triazoles in 90–99% yields. Further, the reaction of
2

L. Zheng, et al.
CatalysisCommunications148(2021)106165
Table 1
Optimization of the click reaction conditionsa.
Entry
CuX
Catalyst
(mol %)
Solvent
Yield(%)b
1 h
Yield(%)b
2 h
1
CuI
CuI
CuI
CuI
CuI
CuI
CuI
–
1
CH3CN
DMF
99
95
85
95
80
60
93
90
95
40
25
10
0
99
99
90
99
85
75
95
95
98
45
30
15
0
2
1
3
1
CH3OH
EtOH
4
1
5
1
CH2Cl2
toluene
neat
6
1
7
1
8c
9
–
EtOH
CuI
CuBr
CuCl
CuI
–
0.5
0.5
0.5
0.5
0.5
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
10
11
12d
13e
a
b
c
d
e
Reaction conditons: benzyl azide 1a (1.0 mmol), phenylacetylene 2a (1.1 mmol), L1 (1,1 of the CuX) and solvent (1 mL) in room temperature (25 °C).
Isolated yields.
Catalyst recovered from entry 4.
Bipyridine was used instead of L1.
Without CuI or L1.
propargylamine substrates (R = CH2NH2, CH2NHTs) with azides also
was very fast and quantitative yellow solids were formed when CuI was
used. However, the solid product was decomposed, colors changed, and
the ligand was released in the solution while the CuBr or CuCl were
used (see SI). Finally, three related complexes 1–3 were characterized
by single crystal X-ray diffraction analyses. The molecular structure of
complex 1 (CCDC: 1886196) revealed a neutral dinuclear unit con-
taining a [Cu2(μ-I)2] core. Each Cu(I) cation was bonded to one iodide
and two nitrogen atoms (N3 and N4) from a chelating L1, and the di-
meric CueCu distance was 2.578(1) Å. It was found that the model
reaction could give 3a with quantitative yield using 1% complex 1 in
1 h with CH3CN as solvent at room temperature. Conversely, for com-
plexes 2 (CCDC: 1886197) and 3 (CCDC: 1886198), the original Cu(I)
were oxidized to Cu(II), and the coordination modes of L1 were also
changed (Scheme 2). The two Cu(II) centers are six-coordinate with two
gave satisfactory yields within 100–120 min. In some cases, the free
amino groups in the substrate could not drive the CuAAC reactions to
completion because of the strong coordinating tendency of amines with
Cu(I), subsequently deactivating the catalyst [20]. These results re-
vealed that the tight chelation of the L1 ligand with CuI and the super
stability of the copper complex ensured the high reactivity of the cat-
alytic system in the CuAAC reactions.
2.3. Catalyst synthesis and structure
To further investigate the related catalytic species, a solution of L1
with an equivalent amount of copper salt (CuI, CuBr, or CuCl) were
stirred in DMF (Scheme 2). It was found that the coordination reaction
Table 2
Synthesis of different 1,2,3-triazoles by click [3 + 2] cycloadditiona.
R = Ph
3aa
3ab
3ac
3ad
3ae
3af
3ag
55 min, 99%b
3ba
50 min, 96%
3ibb
80 min, 92%
3jbc
60 min, 94%
3bd
60 min, 93%
3be
45 min, 94%
3bf
90 min, 92%
R = CH2OH
R = C(CH3)2OH
R = (CH2)4CH3
R = CH2NHX
60 min, 97%
3ca
60 min, 99%
3cb
90 min, 94%
70 min, 91%
3cd
65 min, 92%
3ce
60 min, 94%
3cf
80 min, 98%
3da
85 min, 98%
3db
65 min, 96%
3dd
60 min, 91%
3de
60 min, 92%
3df
70 min, 97%
3fa (X = Ts)
100 min, 92%
65 min, 93%
90 min, 91%
3fd (X = Ts)
120 min, 91%
110 min, 90%
3ee (X = H)
120 min, 86%
3fe (X = Ts)
100 min, 91%
85 min, 90%
a
b
Reaction conditions: azide reagents (0.50 mmol), alkynes (0.50 mmol) and CuI/L1 (0.0025 mmol) in 1 mL of CH3CN, rt.
Isolated yield.
3

L. Zheng, et al.
CatalysisCommunications148(2021)106165
Scheme 2. The ORTEP molecular structures of complex 1–3.
Table 3
Catalyst performance of the complex 1a.
Entry
Cat. (mol %)
Temperature
(°C)
Time
Yieldb
(%)
TON
TOF
(min)
(h−1
)
1
0.5
25
25
50
25
50
25
55
99
75
95
45
78
92
198
216
375
950
450
780
384
2
0.1
120
60
750
3
0.1
950
4
0.05
0.05
0.01
120
120
24 (h)
900
5
1560
9200
6c
a
b
c
Reaction conditons: benzyl azide 1a (1.0 mmol), phenylacetylene 2a (1.0 mmol), complex 1 (0.5–0.01 mol%), and CH3CN (1 mL).
Isolated yields.
Large scale: phenylacetylene 1a (10.0 mmol), benzyl azide 2a (10.0 mmol).
terminal halides (Br for 2; Cl for 3), and four nitrogen atoms (N1 and
electron density, indicating that there might be two possible coordinated
modes. Notably, N3-atom had a higher electron density cloud than N1,
which could be attributed to the electron donation of the C4-Aryl. Based
on these, we speculated that Cu(I) tended to coordinate with the N3 and
N4-atom of the L1 to form the Cu(I)-L1 species (Scheme S1). The iodide
anions could form a tight Cu-I-Cu iodine bridge to construct a stable
dinuclear copper complex. Conversely, the CuBr and CuCl failed to sta-
bilize the Cu(I)-L1 species (color changed, and ligand released) and re-
sulted in a change in the coordination mode. The DFT calculation of the
energy differences between the geometries of complexes [Cu2(L1)2X2]
and [Cu2(L1)2X2] with different halogens (X = Cl, Br, and I) also
N4) coming from two L1 ligands to form octahedral coordination geo-
metries (Scheme 2). Many attempts to obtain L1 and CuBr/CuCl com-
plex under inert atmosphere also failed, and we proposed that the
corresponding CuBr or CuCl complexes were very unstable and could
undergo self-decomposition to form Cu(II) complexes quickly.
In order to determine the formation of the complexes, the charge of
the nitrogen atoms of L1 and L2 were studied using Gaussian 09 pro-
grams (see SI, Table S1). First, compared with L1, the low electron
density of N2 (L2) confirmed that L2 had weak ability to coordinate and
stabilize the Cu(I) species. Secondly, N1 or N3-atom of L1 had high
4

L. Zheng, et al.
CatalysisCommunications148(2021)106165
Scheme 3. Some other pyridinyl-triazole ligands and the ORTEP molecular structure of complex 4 and 5.
revealed that complexes 2 and 3 were more stable than the dicopper
structure (see SI, Table S2). We proposed that these differences may be
due to the smaller sizes of Br or Cl atoms, which made it difficult to form
a compact Cu(I) species. Therefore, the species were further decomposed
and oxidized to Cu(II). The Cu(II) coordinated N1 and N4-atoms of the L1
to form Cu(L1)2Br2 or Cu(L1)2Cl2 because of the steric hindrance of the
C4-aryl. Moreover, a control experiment showed that complexes 2 or 3
could not catalyze the click reaction, and the combination of L1 and CuBr
or CuCl also led to a low yield even under an inert atmosphere.
combined with 0.5 mol% of equivalent CuSO4/ascorbate, provided
triazoles with 99% yield in 24 h [19]. Daniel et. al reported that Copper
(II) complexes supported by N,N, N,O, and N,S- 1,2,3-triazole-based li-
gands were catalytically active at room temperature in the CuAAC re-
action; further, using sodium ascorbate as a reducing agent and water-
ethanol as a solvent mixture, 5 mol% of the catalyst led to a 95% yield in
16 h [25]. Recently, Mahmoud et al. reported Cu(I)-DAPTA complexes
viz. [CuX(DAPTA)3] and dicopper complexes [Cu(μ-X)(DAPTA)2]2 (X = I
or Br), which served as efficient catalysts for the one-pot three-compo-
nent Huisgen cycloaddition reaction [13]. The turnover number (TON) of
the dicopper complexes with iodine and bromine bridges were 26 and 46
in MW irradiation conditions, respectively, while the [CuI(DAPTA)3]
could reach up to 190. We were pleased to find that the TON value was
900 and the turnover frequency (TOF) value was 450 h−1 in 25 °C using
0.05 mol% of complex 1 (Table 3, entry 4). Notably, the TOF could reach
up to 950 h−1 in 50 °C when the catalyst loading was 0.1 mol% (entry 3).
2.4. Catalyst performance and the mechanism discussions
A survey of the literature revealed that the combination of CuI/
DIPEA/HOAc (0.01/0.02/0.02) could catalyze the cycloaddition of
benzyl azide with phenylacetylene affording the triazoles a 96% yield in
12 min [24]. The polytriazole ligands such as TBTA or a variant of TBTA,
5

L. Zheng, et al.
CatalysisCommunications148(2021)106165
Table 4
The comparison of L5-L18 in the performancea.
Yieldb (%)
1 h
Entry
Catalytic systems
2 h
1
CuI/L5
60
75
2
CuI/L6
72
80
3
CuI/L7
90
95
4
CuI/L8
95
quant.
99
5
CuI/L9
92
6
CuI/L10
CuI/L11
CuI/L12
CuI/L13
CuI/L14
CuI/L15
CuI/L16
CuI/L17
CuI/L18
35
40
7
82
95
8
97
99
9
quant.
82
quant.
90
10
11
12
13
14
86
92
80
90
98
quant.
quant.
95
a
Reaction conditons: benzyl azide 1a (1.0 mmol), phenylacetylene 2a (1.1 mmol), Catalytic systems (CuI, L5-L16 or CuI-Ligand complex) (0.5 mol%) in CH3CN
(1 mL) rt.
Isolated yield.
b
In addition, a gram-scale reaction was performed, using only 0.01 mol%
loading in room temperature, 3aa was isolated in 92% yield in 24 h, and
the TON value was 9200 (entry 6).
functionalization (N1 or N2) of L10 was necessary for these catalytic
systems. Secondly, the ligands containing another pyridine (L12, L13,
L17, and L18) could shorten the reaction time significantly, which might
be helpful to the deprotonation of the terminal alkyne. Notably, N2-
pyridinyl ligand (L13) was the most efficient ligand in our catalytic
system (Table 4, entry 9). Finally, the single crystal of complex 5 (CCDC:
1889225) also proved that the N3-atom with a higher electron density
bridged the Cu(I) centers, forming a dinuclear Cu(I) complex, while the
N2-atom did not participate in the coordination. Moreover, the CueCu
distance was 2.670(1) Å, which was also in the range of 2.54–2.88 Å.
The CuAAC reaction mechanism was proposed to proceed via a di-
nuclear copper intermediate, which has been identified using methods
such as the isotope, kinetic, and other studies. [26–30]. Some pre-
formed dinuclear Cu(I) complexes acting as catalysts had also shown
excellent catalytic activities. The π,σ-bis(copper)acetylide complexes
were reported as the possible intermediates and “catalytically active
complex” in the azide–alkyne cycloaddition reaction [31–33]. The
CueCu distance of 2.54–2.88 Å in dinuclear Cu(I) complexes was fa-
vorable for the formation of the Cu(I)-alkynyl intermediates. Earlier,
the simplest dinuclear Cu(I) complex, [(CuOAc)2]n with a favorable
CueCu distance of 2.56 Å, was reported as an efficient catalyst in
CuAAC reaction [34]. Berg et al. described the systematic synthesis of
some bis-triazolylidene dicopper(I) complexes, in which the ligand
bridged the Cu(I) centers with an adequate CueCu distance (3.076 Å)
[33]. Recently, Mahmoud et al. demonstrated the efficiency of several
hydrosoluble Cu(I)-DAPTA complexes in azide–alkyne cycloaddition
[13]. The [Cu(μ-I)(DAPTA)2]2 complex also exhibited a [Cu2(μ-I)2]
core, and the distance between two copper centers was 3.224 Å. For
complex 1, the distance between two neighboring coppers was 2.578(1)
Å, which may be the reason for the efficient catalytic AAC reaction.
3. Conclusions
In conclusion, we developed a new catalytic system for the CuAAC
reactions, based on dinuclear Cu(I) complexes with N2-pyridinyl 1,2,3-
triazole and N1/N2-substituted 2-(NH-1,2,3-triazol-4-yl)pyridines as li-
gands. This system has a broad substrate scope (including free amines)
and high efficiency (low catalyst loadings, short reaction time, facile
catalyst recyclability, ambient temperature, and open-flask conditions).
The key factor was that the iodide anions could form iodine bridges to
construct stable neutral dinuclear Cu(I) complexes with these ligands,
while CuBr and CuCl were not suitable for these ligand systems due to
the inappropriate sizes of Br and Cl atoms.
Declaration of Competing Interest
2.5. Other pyridine-containing 1,2,3-triazole ligands
Some other N2-pyridinyl-1,2,3-triazole ligands L5-L9 (Scheme 3)
were also confirmed to be able to serve as efficient ligands to catalyze
the cycloaddition reaction in the presence of CuI (Table 4). The cata-
lytic activities were greatly reduced while the C4-aryl bears a CH3 or
OCH3 group (L5 and L6). Moreover, the single crystal of the CuI-L9
complex 4 (CCDC: 1896518) also exhibited a dinuclear Cu(I) core, and
the CueCu distance was 2.538(3) Å (Scheme 3).
The authors declare that there is no conflict of interest.
Acknowledgments
We are grateful to the National Natural Science Foundation of
China, China (21002076) and the Graduate Innovative Fund of Wuhan
Institute of Technology (CX2019166).
On the other hand, the ligands L10-L18, which could provide a N3-
coordinated site to form five-membered chelate mode, were also eval-
uated for these systems, and the results were shown in Table 4. Firstly, it
was found that L10 had a weak activity and that the synthesis of cor-
responding CuI complex also failed, which meant that the free NH-tria-
zole was not conducive to the stabilized Cu(I) species. Thus, the N-
Credit author statement
Yunfeng Chen directed the project. Lei Zheng and Ye Wang per-
formed all the experiments. Xianggao Meng joined in the discussion.
Yunfeng Chen and Lei Zheng prepared and revised the manuscript.
6

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CatalysisCommunications148(2021)106165
Appendix A. Supplementary data
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