Silica functionalized Cu(I) as a green and recyclable heterogeneous catalyst for the huisgen 1,3-dipolar cycloaddition in water at room temperature

Article abstract of DOI:10.1007/s10562-010-0330-3

Silica functionalized Cu(I) is reported as a green and recyclable heterogeneous catalyst for the regioselective synthesis of 1,4-disubstituted-1, 2,3-triazoles via Huisgen 1,3-dipolar cycloaddition reaction between terminal alkynes, benzyl/allyl/alkyl halides and NaN₃ in water at room temperature. The azide intermediate do not need to be isolated, which make this procedure highly economical. Further, silica functionalized Cu(I) could be recovered by simple filtration and recycled up to 7 consecutive runs without loss of significant activity. The catalyst is characterized by FTIR, thermal analysis, SEM, TEM and AAS.

Full text of DOI:10.1007/s10562-010-0330-3

Catal Lett (2010) 136:260–265
DOI 10.1007/s10562-010-0330-3
Silica Functionalized Cu(I) as a Green and Recyclable
Heterogeneous Catalyst for the Huisgen 1,3-Dipolar Cycloaddition
in Water at Room Temperature
•
Tahira Shamim Satya Paul
Received: 17 February 2010 / Accepted: 12 March 2010 / Published online: 30 March 2010
Ó Springer Science+Business Media, LLC 2010
Abstract Silica functionalized Cu(I) is reported as a
green and recyclable heterogeneous catalyst for the regio-
selective synthesis of 1,4-disubstituted-1,2,3-triazoles via
Huisgen 1,3-dipolar cycloaddition reaction between ter-
minal alkynes, benzyl/allyl/alkyl halides and NaN3 in water
at room temperature. The azide intermediate do not need to
be isolated, which make this procedure highly economical.
Further, silica functionalized Cu(I) could be recovered by
simple filtration and recycled up to 7 consecutive runs
without loss of significant activity. The catalyst is charac-
terized by FTIR, thermal analysis, SEM, TEM and AAS.
stability (both chemical and thermal), high surface area,
good accessibility, and organic groups can be robustly
anchored to the surface, to provide catalytic centres [3–10].
Recently, water has been found to be a promising medium
for heterogenised homogeneous catalysis. Catalysts heter-
ogenised through covalent or non-covalent attachment to
either inorganic or organic materials, particularly, silica
have been successfully employed in aqueous media [11–14].
Huisgen 1,3-dipolar cycloaddition of azides and alkynes
[15–18] to afford 1,2,3-triazole ring system via ‘click
chemistry’ has been attracting great attention in recent
times. The enormous attention recently gained by this
reaction began with the pivotal discovery by the groups of
Meldal [19] and Sharpless [20], in which copper(I) catal-
ysis was found to dramatically accelerate the reaction
under mild conditions at the same time that a high regi-
oselectivity was achieved towards the 1,4-regioisomer
of the triazole product. Recently, copper-free Huisgen
1,3-dipolar cycloadditions have been reported, but reactions
are usually slow and the regioselectivity is unpredictable
[21, 22]. Therefore, copper catalyzed processes are still being
preferred. The sources of copper(I) include: (a) copper(I)
salts, normally in the presence of a base and/or a ligand,
(b) insitu reduction of copper(II) salts and (c) compropor-
tionation of copper (0) and copper(II), generally limited to
special applications (e.g., biological systems) [23, 24].
Although organic azides are generally safe compounds,
but low molecular weight can be unstable and therefore,
difficult to handle [25]. Thus, a method that avoids isola-
tion of organic azides is desirable. In situ generation
of organic azides from suitable precursors followed by
addition of alkyne in one-pot, to form the corresponding
1,2,3-triazole would avoid the difficulties associated with
the explosive nature of azides. Recently, copper catalyzed
one-pot synthesis of 1,2,3-triazoles from aryl/alkyl halides,
Keywords Silica functionalized Cu(I) Á
1,4-Disubstituted-1,2,3-triazoles Á
Huisgen 1,3-dipolar cycloaddition Á
Heterogeneous catalysis Á Recyclability Á
Aqueous conditions
1 Introduction
Owing to increasing concern about environmental impact,
tremendous efforts have been made towards the develop-
ment of new processes that minimize pollution in chemical
synthesis. For this reason and others (catalyst removal,
recovery and recycling), heterogeneous catalysis is clearly
on the rise, including in industry [1, 2]. Among various
heterogeneous catalysts developed over the past decades,
metallic species supported on silica take the lead, since
silica displays many advantageous properties––excellent
T. Shamim Á S. Paul (&)
Department of Chemistry, University of Jammu,
Jammu 180 006, India
e-mail: paul7@rediffmail.com
123

SiO2–CuI Catalyzed Huisgen 1,3-Dipolar Cycloaddition
261
alkynes and sodium azides have been reported [26].
However, these homogeneous processes suffer from one or
more disadvantages like, difficulty in separation of the
product from the reaction medium, catalysts are generally
non-recyclable, low regioselectivity and long reaction
times. In order to address these problems, recently, heter-
ogeneous copper(I) catalysts based on silica [27, 28],
polymer [29, 30], montmorillonite [31], copper nanoparti-
cles [32, 33], zeolites [34], Cu in charcoal [35] have been
developed. These catalysts possess increased advantages
over their homogeneous counter parts, however, a hetero-
geneous catalytic system which could be used under
aqueous conditions and yield products in minutes at room
temperature is still required to be developed. In this paper,
we wish to report a silica functionalized Cu(I) catalyzed
one-pot synthesis of 1,2,3-triazoles from terminal alkynes,
benzyl/allyl/alkyl halides and sodium azide in water at
room temperature (Scheme 1).
Australia. The catalyst was stirred in dil. HCl for 10 h and
then subjected to AAS analysis.
2.2 Preparation of Silica Functionalized Cu(I) Catalyst
[SiO2–CuI]
Silica (K100, 0.063–0.200 mm) was activated by refluxing
in a mixture of conc. HCl and distilled water (1:1) for 24 h
and then washed thoroughly with distilled water and dried
at 110 °C for 12 h. Activated silica (10 g) was added to a
solution of 3-aminopropyl(trimethoxy) silane (1.79 g,
10 mmol) in dry toluene and refluxed for 24 h. The
3-aminopropyl silica (AMPS) was filtered off, washed with
hot toluene and dried at 110 °C for 5 h to give the surface
bound amino groups (AMPS). The oven dried aminopropyl
silica (AMPS, 5 g) was added to sodium dried toluene
(50 mL) in a round-bottom flask (100 mL) followed by
2-acetylpyridine (0.605 g, 5 mmol). The reaction mixture
was refluxed at 120 °C and water formed during the
reaction was removed using dean-stark apparatus. The
ligand-grafted silica (imine) was filtered at the reaction
temperature and washed with hot toluene thoroughly to
remove unreacted 2-acetylpyridine. It was dried in an oven
at 100 °C for 10 h.
The mixture of ligand-grafted silica (imine, 4 g) and CuI
(1.528 g, 2 mmol/g) was stirred in acetonitrile (60 mL) at
room temperature for 18 h. The solid was filtered off at
pump and washed with acetonitrile till washings were
colourless. It was dried in an oven at 95 °C for 5 h and then
conditioned for a total of 8 h (2 9 2 h each refluxing in
acetonitrile and toluene). The conditioned catalyst was
dried under vacuum for 5 h before use.
2 Experimental
2.1 General Remarks
2.1.1 General
Silica gel (K100, 0.063–0.200 mm) was purchased from
Merck (Germany) and 3-aminopropyl(trimethoxy) silane
and 2-acetyl pyridine from Aldrich Chemical Company.
All melting points were determined on a Perfit melting
point apparatus and are uncorrected. 1H NMR spectra were
registered on a Bruker DPX-300 NMR spectrometer
(300 MHz) in CDCl3 ? DMSO-d6 using tetramethylsilane
as an internal standard. The IR spectra were recorded using
KBr disc on Perkin-Elmer FTIR spectrophotometer. Mass
spectral data were recorded on Esquire 3000 (ESI). Ther-
mal analysis was carried out on DTG-60 Shimadzu make
thermal analyzer with heating rate of 10 °C/min. SEM was
recorded using Jeol make T-300 Scanning Electron
Microscope and Transmission Electron Microscope (TEM)
on H7500 Hitachi. The amount of copper in SiO2–CuI was
determined on double beam Atomic Absorption Spectro-
photometer (AAS), GBC 932 AB manufactured in
2.3 General Procedure for The Synthesis
of 1,4-Disubstituted-1,2,3-triazoles
To a mixture of alkyne 1 (1 mmol), halide 2 (1 mmol),
NaN3 (1 mmol) and SiO2–CuI (0.1 g, 5 mol% Cu) in a
round-bottom flask (25 mL), deionized water (5 mL) was
added. The reaction mixture was then stirred at room
temperature for an appropriate time (Table 2). After com-
pletion of the reaction (monitored by TLC), the reaction
mixture was triturated with EtOAc (20 mL) and the SiO2–
CuI was filtered off. The product was obtained after
removal of the solvent under reduced pressure followed by
column chromatography. The SiO2–CuI was washed with
distilled water (200 mL) followed by methylene chloride
(3 9 15 mL) and dried at 110 °C for 2 h. It was used
further for carrying out the reaction (Fig. 3).
R'
R
SiO2-CuI
N
R'
NaN3
RX
Room Temperature
water, 15-20 min
N
N
3
1
2
The structures of the products were confirmed by IR,
1H & 13C NMR, mass spectral data and comparison with
authentic samples prepared according to the literature
methods.
Scheme 1 SiO2–CuI catalyzed one-pot regiospecific synthesis of
1,4-disubstituted-1,2,3-triazoles (3) by stirring at room temperature
under aqueous conditions
123

262
T. Shamim, S. Paul
1137, 1079, 762, 696; 1H NMR (CDCl3, 300 MHz): d 0.99
(t, J = 7.4 Hz, 3H), 1.34–1.41 (m, 2H), 1.87–1.96 (m, 2H),
4.40 (t, J = 6.5 Hz, 2H), 7.26–7.34 (m, 1H), 7.39–7.44
(m, 2H), 7.75 (s, 1H), 7.82 (d, J = 3.6 Hz, 2H); 13C NMR
(75 MHz, CDCl3): d 13.39, 19.24, 31.80, 49.78, 119.08,
125.56, 127.97, 128.59, 128.72, 130.63, 147.59; m/z
(%) = 198 (M?).
Table 1 Effect of solvent on the SiO2–CuI catalyzed synthesis of
1-benzyl-4-phenyl-1H-1,2,3-triazole at room temperature
Entry
Solvent
Time (h)
Yield (%)a
1
2
3
4
Ethanol
4
85
60
55
92
Methylene chloride
Acetonitrile
Water
4
4
0.25
2.4.5 1-Allyl-4-phenyl-1H-1,2,3-triazole
The reactions were carried out by stirring a mixture of phenyl
acetylene (1 mmol), benzyl bromide (1 mmol), NaN3 (1 mmol),
SiO2–CuI (0.1 g, 5 mol% Cu) and deionized water (5 mL) at room
temeparature
M. pt. 40–42 °C. IR (mmax in cm-1, KBr): 3122, 3080,
2926, 1640, 1483, 1465, 1453, 1360, 1150, 1077, 1050,
a
Yield refer to column chromatography yield
1
980, 817, 765, 730; H NMR (CDCl3, 300 MHz): d 5.45
(d, 2H), 5.52 (d, 2H), 6.21–6.43 (m, 1H), 7.24–7.40
(m, 3H), 7.68 (s, 1H), 7.81 (d, 2H, J = 6.75 Hz); 13C NMR
(75 MHz, CDCl3): d 56.1, 114.2, 128.1, 129.2, 129.7,
132.2, 133.2, 135.3; m/z (%) = 185 (M?).
2.4 Characterization Data
2.4.1 1-Benzyl-4-phenyl-1H-1,2,3-triazole
M. pt. 128–130 °C (Lit. 128–129 °C) [36]. IR (mmax in
2.4.6 (1-Allyl-1H-1,2,3-triazol-4-yl)methanol
cm-1, KBr): 3087, 2956, 1494, 1469, 1450, 1361, 1224,
1
1140, 1076, 1046, 972, 807, 769, 739; H NMR (CDCl3,
M. pt. 43–45 °C. IR (mmax in cm-1, KBr): 3266, 3139,
3088, 1820, 1650, 1498, 1467, 1450, 1365, 1222, 1138,
300 MHz): d 5.58 (s, 2H), 7.42–7.75 (m, 8H), 7.67 (s, 1H),
7.82 (d, J = 7.1 Hz, 2H); 13C NMR (75 MHz, CDCl3): d
53.66, 119.10, 124.74, 125.47, 126.08, 129.97, 134.17,
147.64; m/z (%): 235 (M?).
1
1010, 972, 813, 770, 700; H NMR (CDCl3, 300 MHz): d
3.12 (br s, 1H), 4.68 (s, 2H), 5.42 (d, 2H), 6.21–6.42
(m, 1H), 7.28–7.45 (m, 3H), 7.61 (s, 1H), 7.80 (d, 2H,
J = 6.74 Hz); 13C NMR (75 MHz, CDCl3): d 55.6, 62.3,
106.5, 130.3, 131.5, 147.2; m/z (%) = 139 (M?).
2.4.2 1-(4-Iodobenzyl)-4-phenyl-1H-1,2,3-triazole
M. pt. 153–155 °C (Lit. 154–156 °C) [36]. IR (mmax in
cm-1, KBr): 3142, 3029, 1497, 1468, 1449, 1360, 1221,
1139, 1077, 971, 806, 769, 732; 1H NMR (CDCl3,
300 MHz): d 5.62 (s, 2H), 7.17 (d, 2H, J = 8.25 Hz), 7.37
(d, 1H, J = 7.25 Hz), 7.43 (t, 2H, J = 7.25 Hz), 7.77
(d, 2H, J = 8.25 Hz), 7.86 (d, 2H, J = 7.5 Hz), 8.63
(s, 1H); 13C NMR (75 MHz, CDCl3): d 52.4, 94.4, 121.5,
125.1, 127.9, 128.8, 130.1, 130.5, 135.7, 137.5, 146.6; m/z
(%) = 361 (M?).
2.4.7 (1-Benzyl-1H-1,2,3-triazol-4-yl)methanol
M. pt. 77–78 °C (Lit. 76–77 °C) [30]. IR (mmax in cm-1
KBr): 3266, 3139, 3088, 1497, 1468, 1450, 1365, 1222,
1
1138, 1076, 1046, 972, 813, 770, 730; H NMR (CDCl3,
,
300 MHz): d 3.10 (br s, 1H), 4.71 (s, 2H), 5.46 (s, 2H),
7.20–7.39 (m, 6H); 13C NMR (75 MHz, CDCl3): d 57.2,
61.5, 125.6, 128.2, 129.1, 133.2, 137.7, 146.5; m/z
(%) = 189 (M?).
2.4.3 1-(4-Bromobenzyl)-4-phenyl-1H-1,2,3-triazole
3 Results and Discussion
M. pt. 150–151 °C (Lit. 150–152 °C) [36]. IR (mmax in
cm-1, KBr): 3138, 3016, 1495, 1465, 1447, 1358, 1223,
1137, 1075, 969, 804, 765, 726; 1H NMR (CDCl3,
300 MHz): d = 5.64 (s, 2H), 7.35–7.30 (m, 3H), 7.43
(t, 2H, J = 7.25 Hz), 7.61 (d, 2H, J = 8.25 Hz), 7.87
(d, 2H, J = 7.5 Hz), 8.65 (s, 1H); 13C NMR (75 MHz,
CDCl3): d 52.5, 121.3, 121.5, 125.0, 127.8, 128.8, 130.0,
140.5, 131.6, 135.3, 146.5; m/z (%) = 314 (M?).
The preparation procedure followed to obtain silica func-
tionalized Cu(I) [SiO2–CuI] is outlined in Scheme 2. It
consists of building up a suitable ligand structure on the
surface of commercial silica gel (K100) followed by
complexation with copper iodide. To make the process
completely heterogeneous, SiO2–CuI was conditioned for
8 h by refluxing each in acetonitrile (2 9 2 h) and toluene
(2 9 2 h) so as to remove the physisorbed copper iodide.
The characterization of SiO2–CuI was done on the basis of
thermal analysis (TGA), FTIR, Atomic Absorption Spec-
troscopy (AAS), Scanning Electron Microscopy (SEM) and
Transmission Electron Microscopy (TEM).
2.4.4 1-Butyl-4-phenyl-1H-1,2,3-triazole
M. pt. 46–48 °C (Lit. 46–47 °C) [36]. IR (mmax in cm-1
KBr): 3040, 2954, 2918, 2847, 1493, 1467, 1360, 1216,
,
123

SiO2–CuI Catalyzed Huisgen 1,3-Dipolar Cycloaddition
263
Scheme 2 Synthesis of silica
functionalized Cu(I) iodide
[SiO2–CuI]
O
O
O
OH
OH
OH
NH2
(MeO)3Si
Si
NH2
Toluene, Reflux, 24 h
AMPS
O
Toluene,
Reflux, 24 h
N
CuI
O
O
Acetonitrile, 18 h
O
O
Si
N
O
O
Si
N
N
Cu
N
I
Imine
SiO -CuI
2
Thermal analysis curve of SiO2–CuI showed an initial
weight loss up to 100 °C (attributed to the residual solvent
and water) and then from 175 to 400 °C was due to loss of
organic functionality. Thus, the catalyst is stable up to
175 °C. The FTIR of 3-aminopropylsilica displays CH2
stretching bands at 2933 and 2860 cm-1 and aliphatic
deformation bands at 1470 and 1445 cm-1. The FTIR of
chemically modified silica (imine) shows a sharp peak at
1640 cm-1 due to C=N bond, which on complexation with
copper disappears and appear as a band at 1635 cm-1. The
lowering in frequency of the C=N peak is indicative of
the formation of the metal–ligand bond [37]. The amount of
the copper loaded on the surface of silica gel was deter-
mined by AAS. The catalyst was stirred in dil. HCl and the
solution was then subjected to AAS analysis. SiO2–CuI was
found to contain 0.0335 g of Cu per gram of the catalyst.
However, the amount of copper before conditioning was
found to be 0.1082 g per gram of the catalyst. SiO2–CuI was
also examined by Scanning Electron Microscopy (SEM)
and Transmission Electron Microscopy (TEM). SEM image
shown in Fig. 1 indicate that catalyst is homogeneous
powder with porous structure. TEM image (Fig. 2) shows
the distribution of copper onto silica. Copper is uniformly
distributed with average diameter of about 2.25 nm.
Fig. 1 SEM image of SiO2–CuI
To optimize the reaction conditions, phenyl acetylene,
benzyl bromide and NaN3 for the synthesis of 1-benzyl-4-
phenyl-1,2,3-traizole were selected as the test substrates to
examine the effects of SiO2–CuI (5–20 mol%), at room
temperatures with different solvents (acetonitrile, methy-
lene chloride, ethanol and water). The results were evalu-
ated qualitatively through TLC (Table 1). The best
conditions employ 0.05:1:1:1 mol ratios of SiO2–CuI,
phenyl acetylene, benzyl bromide and NaN3 at room
temperature for 15 min using water as solvent.
Fig. 2 TEM micrograph of SiO2–CuI
123

264
T. Shamim, S. Paul
Table 2 SiO2–CuI catalyzed one-pot regiospecific synthesis of 1,4-disubstituted-1,2,3-trazoles (3) by stirring at room temperature under
aqueous phase conditions
Compounda
RX
R'
C
CH
Time (min)
15
Yield (%)b
92
m.p./Lit. m.p. (°C)
3a
CH2Br
CH2Cl
128–130/128–129 [36]
3b
3c
3d
3e
3f
17
15
16
18
20
20
20
20
20
20
18
18
15
15
91
92
90
85
92
90
91
91
91
90
90
91
92
91
128–130/128–129 [36]
153–155/154–156 [36]
150–151/150–152 [36]
140–141/140–142 [36]
59–61/61–62 [38]
46–48/46–47 [36]
71–73/70–71 [39]
77–79/79–80 [27]
77–78/78–79 [40]
68–69 [27]
I
CH2Br
Br
CH2Br
CH2Br
O2N
C2H5Br
3g
3h
3i
n-C4H9Br
n-C5H11Br
n-C6H13Br
n-c8H17Br
3j
3k
3l
C10H16Br
Cl
Cl
40–42
HOH2C
HOH2C
HOH2C
3m
3n
3o
43–45
CH2Br
CH2Cl
77–78/76–77 [30]
77–78/76–77 [30]
The reactions were carried out by stirring a mixture of alkyne 1 (1 mmol), halide 2 (benzyl, allyl or alkyl halide) (1 mmol), NaN3 (1 mmol) and
SiO2–CuI (0. 1 g, 5 mol% Cu) and deionized water (5 mL) at room temperature
Yield refer to column chromatography yield
Table 3 Effect of catalyst on the synthesis of 1,4-disubstituted-1,2,3-
triazoles in water at room temperaturea
Using these optimized conditions, the reaction of vari-
ous terminal acetylenes (phenyl acetylene, propargyl
alcohol), halides (benzyl, allyl and alkyl halides) and NaN3
was investigated (Scheme 1). It was found that all the
reactions proceeded smoothly to give the corresponding
1,4-disubstituted-1,2,3-triazoles in high yields (Table 2),
which clearly indicates the generality and scope of the
reaction with respect to various terminal alkynes and
halides. The reaction was also carried out using bromo-
benzene as halide but unfortunately very low conversion
was observed on TLC.
Entry Catalyst
Time (h) Yield (%)b
1
2
3
4
a
No catalyst
5
45
25
25
92
Activated silica
5
Organically modified silica (imine)
SiO2–CuI
5
0.25
Reaction conditions: Phenyl acetylene (1 mmol), benzyl bromide
(1 mmol), NaN3 (1 mmol), SiO2–CuI (0.1 g, 5 mol% Cu) or activated
silica (0.1 g) or organically modified silica (0.1 g), water (5 mL) at
room temperature
In order to find the role of SiO2–CuI for the Huisgen
1,3-dipolar cycloaddition reaction, we have carried out the
reaction in case of entry 1 (Table 2) with activated silica,
ligand grafted silica (imine) and in the absence of SiO2–
CuI under the similar conditions as applied for SiO2–CuI.
The results are summarized in Table 3. Thus, it was con-
cluded that SiO2–CuI catalyzes the reaction and the cor-
responding triazole was obtained in high yield and
b
Yield based on separation by column chromatography
regioselectivity. It is pertinent to mention that reaction did
take place without any catalyst but regioselectivity is very
poor (65% yield in 30 h at room temperature).
When using a heterogeneous catalyst, the important
point is the deactivation and recyclability of the catalyst.
To test this, a series of 7 consecutive runs in case of entry 1
123

SiO2–CuI Catalyzed Huisgen 1,3-Dipolar Cycloaddition
265
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In conclusion, we have developed a mild, simple, cost-
effective and green procedure for the one-pot regiospecific
synthesis of 1,4-disubstituted-1,2,3-triazoles via Huisgen
1,3-dipolar cycloaddition reaction between terminal
alkynes, halides and NaN3 in the presence of recyclable
SiO2–CuI using water as reaction medium at room tem-
perature. Moreover, the mild reaction conditions, high
yield of products, ease of work-up, and the ecologically
clean procedure, will make the present method a useful and
important addition to the present methodologies for the
synthesis of 1,4-disubstituted-1,2,3-triazoles.
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Acknowledgement We are grateful to SAIF, Punjab University,
Chandigarh for SEM and TEM; Head, Institute Instrumentation
Centre, Indian Institute of Technology, Roorkee for TGA; and SAIF,
Nagpur University, Nagpur for AAS analysis.
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