Copper salt of 12-tungstophosphoric acid: An efficient and reusable heteropoly acid for the click chemistry

Article abstract of DOI:10.1002/cjoc.201200939

Three components coupling of alkyl bromide, sodium azide and alkyne has been achieved using a catalytic amount of copper-exchanged phosphotungstic acid (Cu-TPA) in the presence of triethyl amine in DMF to afford substituted triazoles in good yields with h

Full text of DOI:10.1002/cjoc.201200939

FULL PAPER
DOI: 10.1002/cjoc.201200939
Copper Salt of 12-Tungstophosphoric Acid: An Efficient
and Reusable Heteropoly Acid for the Click Chemistry
K. V. Purnima,a D. Sreenu,a N. Bhasker,a K. Nagaiah,a N. Lingaiah,b
B. V. Subba Reddy,a and J. S. Yadav*,a
a Division of Organic Chemistry, Indian Institute of Chemical Technology, Hyderabad 500 007, India
b Division of Inorganic and Physical Chemistry, Indian Institute of Chemical Technology,
Hyderabad 500 007, India
Three components coupling of alkyl bromide, sodium azide and alkyne has been achieved using a catalytic
amount of copper-exchanged phosphotungstic acid (Cu-TPA) in the presence of triethyl amine in DMF to afford
substituted triazoles in good yields with high selectivity. Interestingly, the coupling of alkyl azide with alkyne pro-
ceeds readily at room temperature to furnish 1,2,3-triazoles in excellent yields. The catalyst can be recovered and
reused for three to four subsequent runs with a minimal decrease of activity. The use of copper modified heter-
opolyacids makes this procedure simple, convenient and environmentally friendly.
Keywords Cu-exchanged heteropoly acids, click reaction, three-component reactions, triazoles
Introduction
bility, etc.[27-30] However, there have been no examples
of the use of copper salt of heteropoly acid for the cou-
pling of alkynes with azides via 1,3-dipolar cycloaddi-
tion.
1,2,3-Triazoles are potential targets for drug discov-
ery because of their wide range of biological properties
such as antibacterial, antiviral, antiepileptic and anti-
allergic behavior.[1-5] They are also being used as optical
brighteners, light stabilizers, fluorescent whiteners and
corrosion retarding agents.[6] The Cu(I)-catalyzed azide-
alkyne cycloaddition (CuAAC),[7-9] one of the most re-
liable click reactions,[10-12] has enabled practical and
efficient preparation of 1,4-disubstituted-1,2,3-triazoles
from a wide range of substrates with excellent selectiv-
ity, which cannot be attained with the traditional Huis-
gen uncatalyzed thermal approaches.[13,14] This powerful
and reliable Cu-catalyzed 1,3-dipolar cycloaddition has
found widespread applications in combinatorial chemis-
try for drug discovery,[15,16] material science,[17-19] and
bioconjugation.[20-23] Since triazoles have become in-
creasingly useful and important targets in drugs and
pharmaceuticals, the development of simple and effi-
cient methods for their synthesis in a single step opera-
tion is desirable. Recently, the use of solid acids as het-
erogeneous catalysts has received considerable interest
in different areas of organic synthesis.[24-26] Among
various heterogeneous catalysts, heteropoly acids are
the most attractive, because of their unique properties
such as well-defined structure, Bronsted acidity, possi-
bility to modify their acid-base and redox properties by
changing their chemical composition (substituted HPAs),
ability to accept and release electrons, high proton mo-
Results and Discussion
In this article, we report a novel procedure for the
one-pot synthesis of 1,2,3-triazoles from halides, so-
dium azide and alkynes by means of three component
reaction.[31-37] In a preliminary experiment, benzyl bro-
mide (1) was treated with sodium azide and phenyl
acetylene (2) in the presence of triethyl amine and 10
mol% of Cu-TPA in DMF. The reaction went to comple-
tion in 8.0 h and the desired product, 1-benzyl-4-
phenyl-1H-1,2,3-triazole (3a) was obtained in 85%
yield (Scheme 1).
Scheme 1 Preparation of triazole 3a via three-component reac-
tion
10 mol%
N
N
N
Cu-TPA/NaN3
Br
Ph
+
Et3N, DMF,
90 oC
Ph
2
1
3a
Similarly, other alkynes such as propargyl alcohol,
(prop-2-ynyloxy)naphthalene, 1-decyne, p-methoxyphe-
nylacetylene, (prop-2-ynyloxy)benzene and homopro-
pargyl alcohol also reacted effectively with halides and
*
E-mail: basireddy@iict.res.in; Fax: 0091-040-27160512
Received December 18, 2012; accepted January 24, 2013; published online XXXX, 2013.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201200939 or from the author.
Chin. J. Chem. 2013, XX, 1—5
© 2013 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1

Purnima et al.
FULL PAPER
sodium azide under identical conditions to produce
1,2,3-triazoles in good yields (Entries b－k, Table 1).
Aliphatic substrates such as (2-bromoethyl)benzene and
1-bromopentane also participated well in this reaction.
However, in the absence of Cu-TPA, the reaction did not
give the expected triazole even after long reaction times
(8－12 h). To know the effect of solvent, the reactions
were conducted in various solvents such as acetonitrile,
1,4-dioxane and DMF, but surprisingly, no cycloaddition
was observed in 1,4-dioxane. In contrast to acetonitrile,
enhanced reaction rates and improved yields are the
remarkable features obtained in DMF. This is probably
due to the high solubility of sodium azide in DMF. The
scope and generality of this process is illustrated with
respect to various alkynes, alkyl bromides and sodium
azide and the results are presented in Table 1. The ef-
fects of various copper(II) salts such as Cu-TPA,
Cu(acac)2, Cu(OTf)2, Cu(OAc)2 and CuSO4 were
screened in the preparation of 3a. Of these copper salts,
Cu-TPA was found to give the best results and the re-
sults are presented in Table 2. Furthermore, we per-
formed the two component coupling reactions between
alkyl azides and alkynes using Cu-TPA as a catalyst
(Scheme 2).
Scheme 2 Preparation of triazoles via two-component reaction
N
10 mol% Cu-TPA/Et3N
N
N
R
N3
R
R'
+
1,4-Dioxane, 25 oC
R'
the desired triazoles were obtained in high yields in
short reaction times and the results are summarized in
(Entries l－t, Table 1). Since the reaction mixture is
heterogeneous in 1,4-dioxane, the catalyst could be eas-
ily separated by simple filtration. The recovered catalyst
was further washed with dioxane, dried at 120 ℃ un-
der reduced pressure and reused in three to four succes-
sive runs with only a minimal decrease in activity. For
example, the coupling of benzyl azide with phenyl
acetylene in the presence of triethyl amine and 10 mol%
of Cu-TPA in 1,4-dioxane gave 92%, 87%, 85% and
80% yields over four cycles.
In summary, we have developed an efficient protocol
for the preparation of 1,2,3-triazoles by means of Click
Chemistry using copper exchanged heteropoly acid
(Cu-TPA) as a heterogeneous catalyst. This method of-
fers significant advantages such as high conversions,
short reaction times, and ease of recovery and reusabil-
ity of the catalyst, which make it a useful and attractive
strategy for the preparation of a wide range of triazoles
in a single-step operation.
Interestingly, these two-component reactions pro-
ceeded rapidly at room temperature in 1,4-dioxane and
Table 1 CuTPA-catalyzed synthesis of triazoles via three component 1,3-dipolar cycloaddition
Alkyne Bromide/Azide Time
Producta
Entry
a
Yieldb/%
85
N
N
N
Ph
8 h
9 h
Ph
Ph
Br
Br
Ph
Ph
N
N
N
Ph
b
c
d
e
f
80
81
78
79
80
77
HO
HO
N
N
N
O
O
N
9 h
10 h
8 h
Ph
Ph
Ph
Br
Br
Br
Ph
N
N
Ph
C8H17
C8H17
N
N
N
Ph
MeO
MeO
N
N
N
Ph
Br
8 h
Ph
Ph
Ph
N
N
N
Ph
O
O
Br
g
9 h
Ph
N
N
N
Ph
Br
Br
h
i
10 h
9 h
81
81
BnO
Ph
Ph
BnO
N
N
Ph
N
HO
HO
2
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Chin. J. Chem. 2013, XX, 1—5

An Efficient and Reusable Heteropoly Acid for the Click Chemistry
Continued
Yieldb/%
Entry
j
Alkyne
Bromide/Azide
Producta
Time
10 h
N
N
Br
N
77
76
C5H11
Ph
Ph
N
N
k
l
10 h
Br
N
C5H11
HO
HO
92c
91c
N
Ph
N
20 min
25 min
N
Ph
Ph
N3
N3
Ph
Ph
N
N
m
Ph
HO
HO
N
N
N
N
O
n
o
20 min
20 min
89c
90c
O
Ph
N3
Ph
N
N
N
Ph
N
Ph
Ph
N3
MeO
MeO
85c
88c
N
p
q
30 min
25 min
Ph
N3
N
C8H17
C8H17
N
N
N
Ph
N3
Ph
Ph
Ph
N
N
Ph
90c
93c
O
N
r
30 min
O
N3
Ph
N
N
s
t
25 min
25 min
Ph
N3
N3
BnO
HO
N
Ph
Ph
BnO
88c
N
N
N
Ph
HO
a
b
c
The products were characterized by NMR, IR and mass spectroscopy. Yield refers to pure products after chromatograpy. Reactions
were carried out at 25 ℃ in 1,4-dioxane.
Table 2 Effect of various copper salts for the preparation of 3a
Experimental
Entry
Catalyst (10 mol%)
Yield/%
Time/h
Melting points were recorded on Buchi R-535 appa-
ratus and are uncorrected. IR spectra were recorded on a
Perkin-Elmer FT-IR 240-c spectrophotometer using KBr
optics. 1H NMR and 13C NMR spectra were recorded on
Gemini-200 and Varian Bruker-300 spectrometer in
CDCl3 using TMS as internal standard. Mass spectra
were recorded on a Finnigan MAT 1020 mass spec-
trometer operating at 70 eV.
1
2
3
4
5
6
CuTPA
82
60
52
35
25
20
8.0
12
12
12
12
12
Cu(acac)2
Cu(OTf)2
Cu(NO3)2
CuSO4
Cu(OAc)2
Chin. J. Chem. 2013, XX, 1—5
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3

Purnima et al.
FULL PAPER
Catalyst preparation
mmol), triethyl amine (0.27 mL, 1.96 mmol) and
CuTPA (10 mol%) in 1,4-dioxane (5 mL) was stirred at
room temperature for the appropriate time. After com-
pletion of the reaction as indicated by the TLC, the re-
action mixture was filtered to recover the catalyst and
washed with dioxane. The filtrate was concentrated in
vacuo and then diluted with water. The aqueous layer
was extracted with ethyl acetate (30 mL×3). The com-
bined organic layers were dried over anhydrous sodium
sulfate and concentrated in vacuo and then purified by
column chromatography on silica gel to afford pure tri-
azole.
The copper salt of the H3PW12O40 was prepared as
precipitate by adding 0.18 g of barium hydroxide (to
neutralize the three protons) to the aqueous solution
containing 2.0 g of H3PW12O40. Later 0.16 g of
CuSO4•5H2O was added to replace barium with copper
by eliminating the barium as BaSO4. Thus the
Cu1.5PW12O40 salt was recovered from the solution by
recrystallization. The catalyst mass was dried at 120 ℃
for 12 h in an oven and finally calcined at 300 ℃ for 2
h. The retention of Keggin structure after exchange of
copper ions is confirmed by XRD and FT-IT analysis
(Figures 1 and 2).
1-Benzyl-4-phenyl-1H-[1,2,3]triazole
(3a)[38]
Solid, m.p. 68－70 ℃; IR (neat) ν: 2923, 2853, 1457,
1
1181, 1074, 763, 726, 691 cm−1; H NMR (200 MHz,
CDCl3) δ: 7.76 (dd, J＝7.8, 1.5 Hz, 2H), 7.59 (s, 1H),
7.40－7.25 (m, 8H), 5.56 (s, 2H); LCMS m/z (%): 236.0
(M＋1)＋, 258.0 (M＋Na)＋.
(1-Benzyl-1H-[1,2,3]triazol-4-yl)methanol (3b)[39]
Solid, m.p. 75－77 ℃; IR (neat) ν: 3266 (br), 3139,
2925, 2853, 1455, 1222, 1130, 1061, 1014, 841, 719,
1
689 cm−1; H NMR (200 MHz, CDCl3) δ: 7.49－7.21
(m, 6H), 5.50 (s, 2H), 4.71 (s, 2H), 2.91－2.75 (br s,
1H); LCMS m/z (%): 212.0 (M＋Na)＋.
1-Benzyl-4-(naphthalene-2-yloxymethyl)-1H-[1,2,
3]triazole (3c) Solid, m.p. 156－158 ℃; IR (neat) ν:
3427, 3143, 2920, 2854, 1626, 1599, 1456, 1390, 1257,
10
20
30
2θ /( o)
40
50
60
1
1217, 1047, 822, 725, 695 cm−1; H NMR (300 MHz,
DMSO-d6) δ: 7.81－7.67 (m, 3H), 7.43－7.08 (m, 10H),
5.55 (s, 2H), 5.25 (s, 2H); LCMS m/z (%): 316.2 (M＋
1)＋, 338.1 (M＋Na)＋.
Figure 1 X-ray diffraction of CuTPA catalyst.
1-Benzyl-4-octyl-1H-[1,2,3]triazole (3d)[40] Solid,
m.p. 64－66 ℃; IR (neat) ν: 3111, 3063, 2956, 2922,
2851, 1461, 1211, 1052, 699 cm−1; 1H NMR (300 MHz,
CDCl3) δ: 7.41－7.31 (m, 3H), 7.28－7.21 (m, 2H),
7.10 (s, 1H), 5.47 (s, 2H), 2.65 (t, J＝7.5 Hz, 2H),
1.68－1.57 (m, 2H), 1.43－1.18 (m, 10H), 0.87 (t, J＝
7.5 Hz, 3H); LCMS m/z (%): 272.3 (M＋1), 294.1 (M＋
Na)＋.
1-Benzyl-4-(4-methoxyphenyl)-1H-[1,2,3]triazole
(3e)[40] Solid, m.p. 138－140 ℃; IR (neat) ν: 3424,
3134, 2924, 2853, 1611, 1499, 1456, 1248, 1024, 831,
Figure 2 FT-IR spectra of CuTPA catalyst.
1
793, 718 cm−1; H NMR (200 MHz, CDCl3) δ: 7.67 (d,
General experimental procedure for three-compo-
nent reaction
J＝6.8 Hz, 2H), 7.49 (s, 1H), 7.40－7.27 (m, 5H), 6.87
(d, J＝6.8 Hz, 2H), 5.54 (s, 2H), 3.81 (s, 3H); LCMS
m/z (%): 266.1 (M＋1)＋, 288.1 (M＋Na)＋.
A mixture of sodium azide (95 mg, 1.47 mmol),
benzyl bromide (0.17 mL, 1.47 mmol), phenyl acetylene
(100 mg, 0.98 mmol), triethyl amine (0.27 mL, 1.96
mmol) and Cu-TPA (10 mol%) in dimethylformamide
(5 mL) was heated to 90 ℃ for appropriate time (Table
1). After completion, as indicated by TLC, the reaction
mixture was diluted with water and extracted with ethyl
acetate (30 mL×3). The combined organic layers were
dried over anhydrous Na2SO4, concentrated in vacuo
and purified by column chromatography to afford pure
triazole.
1-Phenethyl-4-phenyl-1H-[1,2,3]triazole
(3f)[41]
Solid, m.p. 136－138 ℃; IR (neat) ν: 3081, 3028, 2924,
1
2854, 1455, 1221, 1081, 844, 763, 693 cm−1; H NMR
(300 MHz, CDCl3) δ: 7.71 (dd, J＝8.3, 1.5 Hz, 2H),
7.39－7.22 (m, 7H), 7.09 (dd, J＝8.1, 1.5 Hz, 2H), 4.60
(t, J＝7.3 Hz, 2H), 3.24 (t, J＝7.1 Hz, 2H); LCMS m/z
(%): 272.1 (M＋Na)＋.
4-(Naphthalene-2-yloxymethyl)-1-phenethyl-1H-
[1,2,3]triazole (3g) Solid, m.p. 118－120 ℃; IR
(neat) ν: 3424, 3064, 2921, 2858, 1627, 1598, 1458,
1
1254, 1215, 1182, 1017, 837, 809, 736, 698 cm−1; H
Two-component reaction: A mixture of benzyl azide
(199 mg, 1.47 mmol), phenyl acetylene (100 mg, 0.98
NMR (300 MHz, CDCl3) δ: 7.73－7.67 (m, 3H), 7.42－
4
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Chin. J. Chem. 2013, XX, 1—5

An Efficient and Reusable Heteropoly Acid for the Click Chemistry
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7.36 (m, 1H), 7.32－7.26 (m, 1H), 7.25 (d, J＝2.6 Hz,
1H), 7.19－7.13 (m, 4H), 7.09 (dd, J＝8.8, 2.6 Hz, 1H),
7.03－6.99 (m, 2H), 5.28 (s, 2H), 4.55 (t, J＝7.3 Hz,
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4-Benzyloxymethyl-1-phenethyl-1H-[1,2,3]triazole
(3h) Solid, m.p. 52－54 ℃; IR (neat) ν: 3447, 3121,
3069, 2946, 2856, 1455, 1214, 1094, 733, 697 cm−1; 1H
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7.03 (m, 2H), 4.68－4.50 (m, 6H), 3.20 (t, J＝7.1 Hz,
2H); LCMS m/z (%): 294.1 (M＋1)＋, 316.1 (M＋Na)＋.
2-(1-Phenethyl-1H-[1,2,3]triazol-4-yl)ethanol (3i)
Liquid; IR (neat) ν: 3380 (brs), 3141, 3029, 2929, 1454,
1218, 1056, 748, 700 cm−1; 1H NMR (300 MHz, CDCl3)
δ: 7.29－7.18 (m, 3H), 7.09－7.03 (m, 3H), 4.53 (t, J＝
7.1 Hz, 2H), 3.85 (t, J＝5.6 Hz, 2H), 3.18 (t, J＝7.1 Hz,
2H), 2.84 (t, J＝5.6 Hz, 2H); LCMS m/z (%): 240.4 (M
＋Na)＋.
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1-Pentyl-4-phenyl-1H-[1,2,3]triazole (3j)[42] Solid,
m.p. 55－57 ℃; IR (neat) ν: 3445, 3118, 2924, 2857,
1
1461, 1214, 1076, 838, 760, 694 cm−1; H NMR (300
MHz, CDCl3) δ: 7.92－7.64 (m, 3H), 7.49－7.25 (m,
3H), 4.38 (t, J＝7.1 Hz, 2H), 2.03－1.89 (m, 2H),
1.46－1.21 (m, 4H), 0.92 (t, J＝7.1 Hz, 3H); ESI m/z
(%): 216.1 (M＋1)＋.
2-(1-Pentyl-1H-[1,2,3]triazol-4-yl)ethanol (3k)[43]
Liquid; IR (neat) ν: 3332 (brs), 2957, 2926, 2856, 1464,
1239, 1058, 829, 760 cm−1; 1H NMR (300 MHz, CDCl3)
δ: 7.34 (d, J＝8.3 Hz, 1H), 4.33－4.22 (m, 2H),
3.92－3.83 (m, 2H), 2.91－2.82 (m, 2H), 1.98－1.80
(m, 2H), 1.44－1.15 (m, 4H), 0.89 (t, J＝6.8 Hz, 3H);
ESI m/z (%): 184.1 (M＋1)＋.
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(Pan, B.; Qin, X.)
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