Magnetically recyclable γ-Fe2O3-HAP nanoparticles for the cycloaddition reaction of alkynes, halides and azides in aqueous media

Article abstract of DOI:10.1039/c3ra00038a

A simple and novel γ-Fe₂O₃ supported on hydroxyapatite (HAP) heterogeneous catalytic system is described that is useful in the synthesis of disubstituted 1,2,3-triazoles from terminal alkynes and in situ generated organic azide in aqueous media; this green methodology emerges as this new catalyst for the one-pot synthesis of 1,2,3-triazoles can be used in water without the addition of any reagent/base. The catalyst can be successfully recycled five times without significant loss of activity. It is important to note that 100% regioselectivity was observed for the cycloaddition reaction.

Full text of DOI:10.1039/c3ra00038a

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Magnetically recyclable c-Fe2O3–HAP nanoparticles for
the cycloaddition reaction of alkynes, halides and azides
in aqueous media3
Cite this: DOI: 10.1039/c3ra00038a
Received 4th January 2013,
Accepted 3rd April 2013
Sandip R. Kale,a Sandeep S. Kahandal,a Manoj B. Gawandeb and Radha V. Jayaram*a
DOI: 10.1039/c3ra00038a
www.rsc.org/advances
A simple and novel c-Fe2O3 supported on hydroxyapatite (HAP)
heterogeneous catalytic system is described that is useful in the
synthesis of disubstituted 1,2,3-triazoles from terminal alkynes
and in situ generated organic azide in aqueous media; this green
methodology emerges as this new catalyst for the one-pot
synthesis of 1,2,3-triazoles can be used in water without the
addition of any reagent/base. The catalyst can be successfully
recycled five times without significant loss of activity. It is
important to note that 100% regioselectivity was observed for
the cycloaddition reaction.
Among the various methods for the regioselective synthesis of
triazoles, the most commonly employed metals are copper10 and
ruthenium.11 After the discovery of click chemistry, these reactions
have recorded tremendous growth in organic synthesis and drug
discovery.12 The development of novel transition metal catalysts
and environmentally benign catalytic protocols for the cycloaddi-
tion of alkynes to in situ generated azides in aqueous media is still
an active area of research.
Nano-catalysis has become an active area of research, as these
materials exhibit better catalytic activity due to the enhanced
surface area towards organic reactions;13 copper-supported
magnetic nanoparticles have been investigated for the synthesis
of disubstituted 1,2,3-triazoles.14
In a continuation of our efforts towards the design of magnetic
nanocatalysts and their sustainable synthetic applications,15–17 we
herein present for the first time, c-Fe2O3 supported on hydro-
xyapatite (HAP) as a hitherto unknown novel catalyst for the
cycloaddition reactions culminating in the synthesis of 1,2,3-
triazoles and their derivatives in water. Notably, no ligands, base
or extra additives/reagents are needed for the reaction (Scheme 1).
To the best of our knowledge this is the first report of a copper-free
iron-based nanoparticle catalyst used in the cycloaddition reaction
for the synthesis of triazoles.
Over the past few decades iron-based catalysts and reagents have
been found to be effective and promising alternatives to expensive
transition metal catalysts, the major attributes being their
inexpensive and sustainable nature, their relative nontoxicity and
environmentally benign characteristics.1 A simple magnetically
driven separation of the catalyst from the reaction mixture makes
the catalyst reusability easier than filtration or centrifugation and
this accounts for the development of a plethora of synthetic
methods employing such magnetically recovered catalysts.2 One-
pot synthetic operations using heterogeneous acid/base catalysts
have attracted the attention of organic chemists because of their
synthetic utility and ease of operation that includes easy isolation
of product and recovery and recyclability of the catalysts.3
Triazoles are an important class of heterocyclic compounds
and have a wide range of biological activities such as anti-
allergenic,4 anti-infective agents,5 anti-HIV,6 and anti-bacterial.7
Consequently, many newer synthetic methodologies have been
advanced for the generation of triazoles using various catalysts8
including some greener protocols.9
aDepartment of Chemistry, Institute of Chemical Technology (Autonomous), N.
Parekh Marg, Matunga, Mumbai 400 019, India.
E-mail: rv.jayaram@ictmumbai.edu.in; 1sandipkale@gmail.com;
Fax: +91(22)24145614
b
ˆ
REQUIMTE, Department of Chemistry, Faculdade de Ciencias e Tecnologia,
Universidade Nova de Lisboa, Quinta da Torre, 2829-516 Caparica, Lisbon, Portugal.
E-mail: m.gawande@fct.unl.pt
Electronic supplementary information (ESI) available: Experimental details,
3
characterization data and 1H NMR, 13C NMR and GC-MS spectra of all prepared
compounds. See DOI: 10.1039/c3ra00038a
Scheme 1 c-Fe2O3–HAP catalyzed cycloaddition reaction of alkynes and in situ
generated azides in water.
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Fig. 1 FT-IR spectrum of the c-Fe2O3–HAP catalyst.
Result and discussion
The iron nanoparticles supported on hydroxyapatite (c-Fe2O3–
HAP) were prepared by a simple co-precipitation method and
characterized using various techniques such as XRD, FT-IR, SEM,
TEM, TGA, ICP-AES, EDAX and BET surface area (ESI ).
3
The characteristic absorption bands due to the bending
vibration mode of O–P–O surface phosphate groups in the
hydroxyapatite shell were observed at 570 and 602 cm21 which
were overlapping with the Fe–O stretching bands. Also, the
stretching of the P–O bond appeared at 1041 cm21, overlapping
with the S–O stretching peak (Fig. 1).
The TEM images (10–35 nm) of the freshly prepared catalyst
and the catalyst recovered after the fifth cycle are depicted in Fig. 2
indicating no change in the morphology of the c-Fe2O3–HAP
nanocatalyst occurred.
Fig. 2 TEM images of the c-Fe2O3–HAP nanocatalyst: (a) ‘‘fresh’’ catalyst and (b)
‘‘recovered’’ catalyst after the fifth run.
The EDAX spectrum of the c-Fe2O3–HAP nanocatalyst shows
the presence of Fe from the c-Fe2O3, and Ca and P from the HAP
(Fig. 3).
The XRD spectrum of the c-Fe2O3–HAP is depicted in Fig. 4
which shows the crystalline nature of the nanocatalyst. The
observed diffraction peaks agree with those of the tetragonal
structure of c-Fe2O3.
Nitrogen adsorption–desorption isotherms are depicted in
Fig. 5 and reveal that the adsorption–desorption process is not
reversible. The surface area, calculated using the BET method,
revealed a value of 103 m2 g21 for c-Fe2O3–HAP.
cycloaddition reaction. It was observed that an increase in the
catalyst loading (Fe mol%) from 1 mol% to 5 mol%, resulted an
increase in the yield of 1-benzyl-4-phenyl-1H-1.2.3-triazole up to
94% (Table 1, entries 2–4). The yield of the triazole reaction
remained unchanged beyond 5 mol% (Table 1, entry 5). To check
the influence of temperature on the yield of the triazole product,
we carried out our model reaction at varying temperatures in the
To examine the catalytic activity of c-Fe2O3–HAP, we began our
investigation with the optimization of the reaction parameters.
The reaction of phenyl acetylene, sodium azide and benzyl
bromide in the presence of c-Fe2O3–HAP in an aqueous medium
was chosen as a model reaction.
The hydroxyapatite acts as a base and removes the acetylenic
proton to form a Fe–HAP acetylide complex.18 The Fe–HAP
acetylide complex undergoes cycloaddition with an in situ formed
azide to form a triazole. To probe the active species involved in the
reaction, we carried out the reaction of diphenyl acetylene with
benzyl bromide and sodium azide, but the reaction did not
proceed, indicating the involvement of the acetylenic hydrogen in
the mechanism of the reaction.
The various reaction parameters such as the influence of
catalyst loading, time and temperature were investigated for the
Fig. 3 EDAX spectrum of the c-Fe2O3–HAP catalyst.
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Table 1 Screening of different reaction parameters for the one-pot synthesis of
1,2,3-triazoles using c-Fe2O3–HAP in watera
Entry
Fe (mol%)
Time (h)
Temp. (uC)
Yieldb (%)
Effect of catalyst loading
1
2
3
4
5
0
1
2
5
10
24
5
5
5
5
100
100
100
100
100
0
40
82
94
94
Effect of temperature
Fig. 4 XRD pattern of the c-Fe2O3–HAP catalyst.
6
7
8
9
5
5
5
5
5
5
5
5
30
60
80
0
54
82
94
120
range of 30 uC to 120 uC (Table 1, entries 6–9), showing that at 100
uC, the yield of the product was maximised to 94% (Table 1, entry
5). A further increase in temperature had no profound effect on
the yield of the reaction.
It is important to note that reactions on only HAP did not take
place; this clearly indicates that the role of Fe2O3 is important in
catalysing the reaction. Notably, for the model reaction 100%
regioselectivity was observed.
Effect of time
10
11
5
5
1
3
100
100
35
72
a
Reactions were performed with phenyl acetylene (1 mmol), benzyl
b
halide (1 mmol) and sodium azide (1.2 mmol) in 5 mL water. GC
Yield
The reusability of the catalyst for the cycloaddition reaction of
phenyl acetylene, sodium azide and benzyl bromide, was
subsequently investigated (Fig. 6). It is noteworthy that the catalyst
was successfully reused for five cycles, without any significant loss
in activity (the c-Fe2O3–HAP was separated by an external magnet).
Based on ICP-AES analysis, the Fe content in the catalyst was
found to be 1.093 mmol g21 before the reaction and 1.091 mmol
With these optimized reaction conditions, the scope of the
reaction was probed with an array of alkynes, aliphatic as well as
aromatic halides and sodium azide (Table 2). Various aromatic
and aliphatic halides and alkynes afforded moderate to good
yields under these reaction conditions in an aqueous medium.
Trace amounts of the triazole product were observed by GC-MS,
when the reaction was carried out with hexyl and heptyl halides
with phenyl acetylene, which can be attributed to a steric
hindrance of the reaction centre by the alkane chain (entries not
mentioned in Table 2).
g
21, after the reaction, revealing that present reaction takes place
heterogeneously with negligible leaching of Fe. Standard leaching
experiments were conducted as reported earlier in the literature.19
The model reaction was conducted for 20 min at 100 uC, then the
hot reaction mixture was separated from the catalyst. The reaction
mixture was then heated for 24 h, but no enhancement in the
yield of the product was observed even after 24 h, and no Fe could
be detected in the filtered solution by ICP-AES analysis. This result
clearly verified that the reaction proceeded heterogeneously and
no homogeneous catalyst was involved during the course of the
reaction.
Both aliphatic as well as aromatic halides gave moderate to
good yields of the corresponding triazole derivatives (Table 2);
usually aliphatic halides are known to be less reactive then
aromatic ones.
To measure the ‘‘greenness’’ of the c-Fe2O3–HAP catalyzed
cycloaddition reaction, we calculated the E-factor for the various
iron catalyzed cycloaddition reactions. The comparison of
c-Fe2O3–HAP with different iron-based catalysts is depicted in
Table 3. It was observed that the heterogeneous catalyst (c-Fe2O3–
HAP) showed excellent relative performance compared to homo-
geneous iron catalysts presumably because of the large surface
area which could promote the reaction.
Experimental
NMR spectra were recorded on a Varian Mercury Plus NMR
spectrometer (1H NMR at 300 MHz and 13C NMR at 75 MHz) in
pure deuterated solvents. IR spectra were recorded using a Perkin
Fig. 5 Nitrogen adsorption–desorption isotherm plot of the c-Fe2O3–HAP catalyst.
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Table 2 Application of the optimized reaction protocol to various substratesab
Entry
1
Halide
Alkyne
Triazole
Time (h)
5
Isolated yield (%)
87
2
3
4
5
5
5
5
5
86
85
90
85
6
5
6
6
6
6
6
87
79
78
80
80
76
7
8
9
10
11
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Table 2 (Continued)
Entry
12
Halide
Alkyne
Triazole
Time (h)
5
Isolated yield (%)
81
13
14
15
16
5
5
5
7
83
84
72
81
17
18
19
20
7
7
7
7
84
81
85
84
21
22
7
7
89
80
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Table 2 (Continued)
Entry
23
Halide
Alkyne
Triazole
Time (h)
12
Isolated yield (%)
69
24
25
26
27
28
8
8
8
8
8
81
85
80
79
80
29
30
31
32
8
9
9
9
82
80
84
89
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Table 2 (Continued)
Entry
33
Halide
Alkyne
Triazole
Time (h)
9
Isolated yield (%)
78
34
35
9
9
75
72
36
10
74
a
Reactions were performed with phenyl acetylene (1 mmol), halide (1 mmol) and sodium azide (1.2 mmol) in 5 mL water, catalyst (0.05 g, 5
mol% Fe) 100 uC. The identity of the products was confirmed by IR, 1H NMR, 13C NMR and GC-MS.
b
Elmer FT-IR spectrum 100 spectrophotometer. Mass spectra were
determined using a Shimadzu GCMS QP 2010 instrument. GC of
all compounds were obtained on a Thermo Scientific GC [capillary
column, 30 m 6 60 mm, ID-BP1 0.25 UM]. Melting points were
determined in capillary tubes using digital melting point
apparatus. Elemental analysis was done on a Harieusrapid
analyser. Column chromatography of some compounds was
carried out using silica gel with 60–120 mesh size. The chemicals
required were purchased from Sigma Aldrich and were used as
received. Wide angle XRD patterns of the catalyst were obtained
on a Rigaku, Japan, miniflex X-ray diffractometer with
a
monochromatic Cu-Ka beam (l = 0.154 nm). The diffractometer
was operated at 30 kV and 15 mA using a scanning step of 2 in two
theta and a dwell time of 1 s was used.
General procedure for the cycloaddition reaction
In a 10 ml round bottom flask fitted with a magnetic stirrer, the
catalyst (0.05 g), phenyl acetylene (1 mmol), sodium azide (1.2
mmol) and benzyl bromide (1 mmol) were stirred in water (5 ml)
at 100 uC for 5 h. The reaction progress was monitored by TLC and
the catalyst was recovered using an external magnet. The reaction
mixture was quenched and the product was extracted with ethyl
acetate. The organic layer was then separated out using a
separating funnel. The separated layer was washed with water
and dried over sodium sulfate. The ethyl acetate solvent was
removed using a rotary evaporator, which left the pure desired
product. The recovered catalyst was reused for further runs
without removing the catalyst from the flask. All of the prepared
1
compounds were confirmed by GC-MS, IR, H and 13C NMR.
Catalyst recycling procedure
An important aspect for heterogeneous catalysis is the reusability
of the catalyst. To achieve this goal, the reaction of phenyl
acetylene, benzyl bromide and sodium azide was chosen to test
the catalyst reusability. After completion of reaction, the c-Fe2O3–
Fig. 6 Reusability of the c-Fe2O3–HAP catalyst (reactions were performed with
phenyl acetylene (1 mmol), benzyl bromide (1 mmol) and sodium azide (1.2 mmol)
in 5 ml water, catalyst (0.05 g, 5 mol% of Fe), 100 uC.
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Table 3 The comparison of different iron containing catalysts for the formation of triazole in aqueous mediaa
Catalyst (mol% of Fe)
Time (h)
Temp (K)
GC yield (%)
E-Factor
Catalyst reusability
FeSO4(10)
12
12
12
12
5
373
373
398
298
373
18
15
22
35
94
381.95
572.42
295.11
122.97
13.63
No
No
No
No
Yes
FeCl3?6H2O (5)
FeCl3?6H2O (10)
NH4(FeSO4)2?12H2O (10)
c-Fe2O3/HAP (5)
a
Reactions were performed with phenyl acetylene (1 mmol), halide (1 mmol) and sodium azide (1.2 mmol) in 5 mL water, catalyst (0.05 g, 5
mol% Fe), 100 uC.
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HAP was separated using an external magnet and the reaction
mixture was extracted in ethyl acetate. The organic layer was
separated and passed through anhydrous sodium sulphate. After
separating the product, fresh aliquots of phenyl acetylene (1
mmol), benzyl bromide (1 mmol) and sodium azide (1.2 mmol)
were added to the reaction flask for the next cycle of the reaction.
It was observed that the catalyst showed excellent activity for the
cycloaddition reaction even after the fifth cycle in aqueous media.
Conclusions
In summary, c-Fe2O3–HAP nanoparticles have been developed as a
heterogeneous catalyst for the cycloaddition reaction of alkynes
and in situ formed organic azides in aqueous media. The use of an
iron-based heterogeneous catalyst instead of a more commonly
used copper or ruthenium catalyst renders the reported metho-
dology comparatively more economical, experimentally simple,
and, therefore, appealing for industrial use. Moreover, the
presented protocol has several advantages: (1) the catalyst can be
reused in up to five consecutive cycles without loss of catalytic
activity, (2) recovery of the catalyst can be accomplished easily
using a simple external magnet, (3) the starting materials are
readily available and are inexpensive, (4) the catalyst preparation is
simple and facile, (5) the present protocol is applicable to a
combination of substrates such as aromatic and aliphatic alkynes,
and halides containing aromatic and aliphatic groups. Further
studies are on going to understand the mechanism of the reaction
and other applications of the catalyst in various organic
transformations.
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Acknowledgements
SRK is thankful to the Council of Scientific and Industrial
Research, New Delhi for financial support in the form of a
CSIR-SRF fellowship
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