A simple, efficient thermally promoted protocol for Huisgen-click reaction catalyzed by CuSO4·5H2O in water

Article abstract of DOI:10.1016/j.tetlet.2014.02.108

A thermally promoted and CuSO₄-catalyzed new version of the Huisgen-click reaction is presented in this Letter. Notably, this protocol was suitable not only for the reactions between organic azides and alkynes, but also for one-pot three-component reactions among alkyl halides, NaN₃, and alkynes.

Full text of DOI:10.1016/j.tetlet.2014.02.108

Tetrahedron Letters 55 (2014) 2410–2414
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Tetrahedron Letters
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A simple, efficient thermally promoted protocol for Huisgen-click
reaction catalyzed by CuSO4Á5H2O in water
a
a
a
b
a
a,
Yuqin Jiang a, , Duanyang Kong , Jinglin Zhao , Weiwei Zhang , Wenjing Xu , Wei Li , Guiqing Xu
⇑
⇑
a School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang 453007, Henan, PR China
b Department of Chemistry, Jiaozuo Teachers’ College, Jiaozuo 454001, Henan, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A thermally promoted and CuSO4-catalyzed new version of the Huisgen-click reaction is presented in this
Letter. Notably, this protocol was suitable not only for the reactions between organic azides and alkynes,
but also for one-pot three-component reactions among alkyl halides, NaN3, and alkynes.
Ó 2014 Elsevier Ltd. All rights reserved.
Received 28 November 2013
Revised 17 January 2014
Accepted 27 February 2014
Available online 6 March 2014
Keywords:
Huisgen-click reaction
Water
Cycloaddition
Triazoles
Catalysts
It is well known that 1,2,3-triazoles have found wide
applications in pharmaceuticals, agrochemicals, dyes, corrosion
inhibitors, biochemicals, polymers, and functional materials.1 As
for their preparation, the most straightforward method is perhaps
the Huisgen cyclization of azide with alkyne. However, there are
some drawbacks such as high reaction temperatures and lack of
regioselectivity associated with this classical method. To circum-
vent these problems, Sharpless2 and Meldal3 independently dis-
covered that Cu(I) catalysts could significantly facilitate the
Huisgen cycloaddition in a regiospecific manner to give only 1,4-
disubstituted triazoles. Following these pioneering works, numer-
ous new protocols for the preparation of 1,2,3-triazole derivatives
have also been developed.
So far, the copper catalysts used in Huisgen-click reaction
include Cu(I) salts (usually in presence of bases and/or ligands),4
Cu(II) salts together with a reducing agent (sodium ascorbate,
metallic copper, hydrazine monohydrate, etc.),2,5 and metallic cop-
per.6 Recently, protocols directly using Cu(II) salts for the Huisgen-
click reaction have been reported and their mechanism has been
also discussed.7–12,6j In 2005, Reddy and co-workers have revealed
a simple protocol for the synthesis of 1,4-disubstituted 1,2,3-tria-
zoles in water at room temperature using Cu(OAc)2ÁH2O
(20 mol %) as the catalyst resulting in a good yield in 20 h.7
Recently, it was reported that Cu(OAc)2ÁH2O could also accelerate
the Huisgen-click reaction in alcoholic solvents in the absence of
reductants. In that case, the required Cu(I) might be generated
in situ via the oxidation of alcohol or homocoupling of terminal
alkyne by Cu(OAc)2.8,9 Moreover, solid supported Cu(II) as recycla-
ble and reusable catalysts in the Huisgen-click reaction have also
been prepared and used. In 2007, an alginate-supported Cu(II) cat-
alyst for the Huisgen-click reaction was reported by Reddy and
coworkers.10 In 2009, Cu(II)-Hydrotalcite was reported as an effi-
cient heterogeneous catalyst for the Huisgen-click reaction in ace-
tonitrile at room temperature in 6–12 h.11 In 2011, Masuyama
et al. reported hydroxyapatite-supported Cu(II) as a reusable heter-
ogeneous catalyst with neither reducing agents nor bases for the
Huisgen-click reaction in water under air at 50–70 °C.12 Very re-
cently, Varma and co-workers reported CuSO4 on chitosan as a
recyclable heterogeneous catalyst for azide-alkyne cycloaddition
reactions in water at room temperature and the reaction could
be finished in 4–12 h.6j In addition, Cu(II)-catalysts were also used
for the three-component Huisgen-click reaction.13 In all the
reported Cu(II)-catalyzed Huisgen-click reactions, the reaction
time is relatively longer than that of Cu(I)-catalyzed ones. Under
this circumstance, we were interested in the study on how to
accelerate Cu(II)-catalyzed Huisgen-click reaction while the good
regioselectivity is still retained. Herein, we wish to present a ther-
mal-promoted, simple, and efficient protocol for the Huisgen-click
reaction by using CuSO4Á5H2O as the catalyst in water at 100 °C
without any additives or supports.
⇑
Corresponding authors. Tel.: +86 3733326335 (Y.J.).
E-mail addresses: jiangyuqin@htu.cn (Y. Jiang), guiqingxu@163.com (G. Xu).
http://dx.doi.org/10.1016/j.tetlet.2014.02.108
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

Y. Jiang et al. / Tetrahedron Letters 55 (2014) 2410–2414
2411
Table 1
To investigate the effect of CuSO4Á5H2O on the regio-selectivity
of Huisgen-click cyclization, the reaction between benzyl azide and
p-nitrophenyl propargyl ether was selected as another model reac-
tion as this reaction usually produces a mixture of 1,4- and the 1,5-
substituted regioisomers. The reaction was carried out at 100 °C in
the presence or absence of Cu(II) catalysts respectively. It turned
out that without CuSO4Á5H2O, the reaction gave 1,4-isomer and
1,5-isomer in a ratio of 75 to 25 (Table 2, entry 1). When it was car-
ried out in the presence of CuBr2, Cu(OAc)2ÁH2O, and CuSO4Á5H2O
with a 10À7 mol % loading at 100 °C, it gave the 1,4-isomers and
1,5-isomers in a ratio of 78 to 22, 89 to 11, and 95 to 5, respectively
(Table 2, entries 2, 4 and 7). These results indicated CuSO4Á5H2O
showed the best regioselectivity among the tested Cu(II) catalysts.
Further studies showed that increasing the loading of Cu(II) could
improve the regioselectivity accordingly. In fact, the ratio of 1,4- to
1,5-isomers could reach to near 100:0 when the loading of CuBr2,
Cu(OAc)2ÁH2O, and CuSO4Á5H2O increased to 1 mol %, 1 mol %, and
10À2 mol %, respectively (Table 2, entries 3, 5, and 8). It was also
found that a further increase of the loading of CuSO4Á5H2O to
1 mol % or 10 mol % did not improve the reaction obviously. Then,
VcNa/CuSO4Á5H2O and CuCl were also tested as possible catalysts
for this reaction. We observed that under similar conditions the
regioselectivity is almost the same as that of CuSO4Á5H2O (seeing
Supporting information). Therefore, the optimized conditions are
as follows: CuSO4Á5H2O (1 mol %) as the catalyst, water as the sol-
vent, and 100 °C as the reaction temperature. By the way, the plau-
sible mechanism of CuSO4-catalyzed Huisgen-click reactions
between organic azides and alkynes might be similar with litera-
ture report.13f
The model reaction between propargyl phenyl ether and ethyl azidoacetate catalyzed
by CuSO4Á5H2O in water
O
O
N
+
N3
O
N
N
O
O
O
Entry
Copper sourcea
T (°C)
Times (min)
Yieldc (%)
1
2
3
4
5
6
7
None
None
25
100
25
40
60
20 (h)
20 (h)
20 (h)
10 (h)
4 (h)
70
4
38
83
84
82
87
85
CuSO4Á5H2Ob
CuSO4Á5H2O
CuSO4Á5H2O
CuSO4Á5H2O
CuSO4Á5H2O
80
100
30
a
Reaction conditions: terminal alkyne (1 mmol), Azide (1.2 mmol), CuSO4Á5H2O
(1 mol %), H2O (2 mL).
b
CuSO4Á5H2O (20 mol %).
c
Isolated yields.
Firstly, the reaction between propargyl phenyl ether and ethyl azi-
doacetate was selected as a model reaction to investigate the effect of
CuSO4Á5H2O and thermal condition on the Huisgen-click reaction. As
shown in Table 1, when the reaction was carried out in water in the
absence of CuSO4Á5H2O at 25 °C or 100 °C for 20 h, a yield of 4% or
38% was obtained respectively. When it was run in the presence of
20 mol %CuSO4Á5H2Oinwaterat 25 °Cfor20 h,ayieldof83%resulted
(Table 1, entries 1–3). Next, it was tried with 1 mol % CuSO4Á5H2O at
40 °C, 60 °C, 80 °C, and 100 °C respectively (Table 1, entries 4–7).
The results in Table 1 showed that the higher the reaction tempera-
ture the shorter the reaction time needed for a complete transforma-
tion. For example, the reaction could complete within 30 min at
100 °C to give the corresponding product in a yield of 85%. In contrast,
the reactionneeded10 htocompleteat40 °C. Basedonthe resultsob-
tained so far, it is concluded that either heating or addition of CuSO4-
Á5H2O could accelerate this reaction remarkably and a combination of
CuSO4Á5H2O and thermal condition could afford the desired product
in high efficiency within short reaction period (Table 1, entry 7).
To study the scope the above reaction, a wide range of diversely
substituted terminal alkynes and azides were tried. As shown in
Table 3, the reactions work well not only with alkyl azides, but also
with aryl azides. It is worth to mention that the reaction could pro-
ceed smoothly even when both the alkynes and the azides are in
solid states. (Table 3, entry 11).
In a further aspect, as organic azides are generally unstable and
potentially explosive, we then tried
a one-pot synthesis of
1,4-disubstituted triazoles directly from alkyl halides, NaN3, and
Table 2
Synthesis of 1,2,3-triazole from p-nitrophenyl propargyl ether and benzyl azide
N
N
N3
NO2
N
NO2
+
+
O
O
O
N
N
N
O2N
1,4-isomer
1,5-isomer
Entry
Copper sourcea
Catalyst loading (mol %)
T (°C)
1,4-Isomer:1,5-isomerb
Yieldc (%)
1
2
3
4
5
6
7
8
9
None
CuBr2
CuBr2
100
100
100
100
100
25
100
100
100
100
100
100
75:25
78:22
96:4
54
82
82
85
83
84
86
87
88
87
90
89
10À7
1
Cu(OAc)2ÁH2O
Cu(OAc)2ÁH2O
CuSO4Á5H2O
CuSO4Á5H2O
CuSO4Á5H2O
CuSO4Á5H2O
CuSO4Á5H2O
CuSO4Á5H2O/VcNa
CuCl
10À7
1
89:11
100:0d
100:0d
95:5
20
10À7
10À2
1
10
1
100:0d,e
100:0d,f
100:0d
100:0d
100:0d
10
11
12
1
a
b
c
d
e
f
Reaction conditions: terminal alkyne (1 mmol), azide (1.2 mmol), H2O (2 mL).
NMR ratio of the crude product (seeing Supporting information).
Isolated yields.
Approximate 100:0.
The reaction time is 5 h.
The reaction time is 50 min.

2412
Y. Jiang et al. / Tetrahedron Letters 55 (2014) 2410–2414
Table 3
Synthesis of triazoles using CuSO4Á5H2O as the catalyst
N
R1
CuSO4·5H2O
N
N
+
R1 N3
R2
H2O , 100ć
R2
Entry
1
Azide
Alkyne
Producta
Times (min)
30
Yieldb (%)
85
N
O
N
N
O
O
N3CH2COOEt
O
N
N
N
N3
2
30
83
Cl
N
O
N
N
N3
Cl
O
3
4
5
40
30
50
79
85
86
O
N
N
N
O
N3CH2COOEt
N3CH2COOEt
O
NO2
N
N
N
O
N
O
O
O2N
N
N
N3
6
7
30
60
85
86
O2N
N3
CH3
CH3
N
O2N
O2N
O2N
N
N
N
N
O
8
9
40
40
88
78
O
N
N3
Cl
N
N
O2N
Cl
O
N
N3
O
N
N
N
N3
10
11
30
80
89
85
NO2
NO2
O2N
N3
NO2
O
N
N
CH3
O
O
N
H3C
N
N
N
O
N3
12
13
50
30
88
82
O2N
N
O
N
N
N3
O
N
O
N
N
N3
O
O
14
40
81
O
N
N
O2N
O2N
N
N3
15
16
17
50
60
30
86
90
88
O2N
N3
CH3
N
C3H7
C3H7
CH3
O2N
N
N
N
O
N
N
O
N3

Y. Jiang et al. / Tetrahedron Letters 55 (2014) 2410–2414
2413
Table 3 (continued)
Entry
Azide
Alkyne
Producta
Times (min)
30
Yieldb (%)
Cl
N
O
N
N
O
Cl
18
19
N3CH2COOEt
82
84
O
O
N
N
O
N3CH2COOEt
30
50
N
O
N3
O2N
20
21
88
O2N
N
N
N
NO2
N
O
N
N
N3
NO2
40
30
83
79
O
N
N
N
O
C3H7
22
N3CH2COOEt
C3H7
O
a
Reaction conditions: terminal alkyne (1 mmol), azide (1.2 mmol), CuSO4Á5H2O (1 mol %), H2O (2 mL), 100 °C.
b
Isolated yields.
Table 4
One-pot synthesis of triazoles from alkyl halides, NaN3, and alkynes
N
R1
CuSO4·5H2O
N
N
+
R1X
R2
+
NaN3
H2O , 100ć
R2
R2
Entry
1
R1X
Producta
Times (min)
60
Yieldb (%)
89
NO2
N
N
N
O
Cl
Cl
O
O2N
N
N
N
2
3
50
60
90
84
Cl
O
Cl
N
N
N
N
Cl
Cl
O
N
N
4
5
6
50
50
60
85
93
86
N
N
O
Cl
Cl
O
N
N
Cl
N
N
N
Cl
N
O2N
O2N
N
N
Cl
Cl
7
8
60
70
90
86
O2N
Cl
N
N
Cl
O
N
O2N
O2N
O
O
NO2
N
O
O
N
N
9
ClCH2COOEt
ClCH2COOEt
60
60
90
85
O
N
N
O
10
O
a
Reaction conditions: terminal alkyne (1 mmol), sodium azide (1.2 mmol), alkyl halide (1.2 mmol), CuSO4Á5H2O(1 mol %), H2O (2 mL), 100 °C.
b
Isolated yields.

2414
Y. Jiang et al. / Tetrahedron Letters 55 (2014) 2410–2414
alkynes. To our delight, under the conditions mentioned above, al-
kynes could react smoothly with the in situ generated organic
azides from alkyl halides and NaN3 to give the corresponding prod-
ucts in excellent yields (Table 4).
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To elucidate the mechanism of the Cu(II) catalyzed one-pot
three-component reaction of alkyl halides, NaN3, and alkynes,
the following control experiment was carried out. It was ob-
served that upon addition of an aqueous solution of NaN3 into
the solution of Cu(OAc)2ÁH2O in water, it turned into pale yel-
low colored suspension quickly. After a few minutes, red solids
precipitated. The structure of this solid compound was deter-
mined by XRD as Cu2O particles (seeing Supporting informa-
tion). According to our previous study,5c Cu2O should have
been generated from the hydrolysis of CuOAc, indicating that
during that process Cu(I) must have been in situ generated
through reaction of Cu(OAc)2 with sodium azide. In other
words, in the one-pot tandem process, Cu(II) was reduced to
Cu(I) by sodium azide. While our observation that sodium azide
could act as both an azidonation reagent and a reducing agent
in the three-component Huisgen-click reaction is actually con-
sistent with what was proposed by Bharate13a for a one-pot
three-component reaction among boronic acid, sodium azide,
and terminal alkyne, it is the first time that sodium azide has
been directly proved as a reducing agent for Cu(II) in water
by a visible chemical reaction.
In conclusion, we presented a simple protocol for the Huisgen-
click reaction using 1 mol % CuSO4Á5H2O as the catalyst at 100 °C in
water. The simple direct heat method could not only greatly accel-
erate the Huisgen-click reaction rate, but also retain its good regi-
oselectivity. Furthermore, the protocol was proved to be suitable
for one-pot three-component reactions of alkyl halides, NaN3,
and alkynes. In exploring the reaction mechanism, it was clearly
proved for the first time by a visible chemical reaction that sodium
azide acted as a reducing agent for the Cu(II)-catalyzed three-com-
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(2012QK11) of Henan Normal University, Foundation of Henan Sci-
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Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.tetlet.2014.
02.108.