Copper nanoparticulates in Guar-gum: A recyclable catalytic system for the Huisgen [3 + 2]-cycloaddition of azides and alkynes without additives under ambient conditions

Article abstract of DOI:10.1039/c2gc35070j

Cu-nanoparticulates in Guar-gum at room temperature were investigated for the first time as a recyclable catalytic system in organic synthesis. The catalytic potential of these materials were evaluated in the Huisgen [3 + 2]-cycloaddition of azides and al

Full text of DOI:10.1039/c2gc35070j

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COMMUNICATION
Copper nanoparticulates in Guar-gum: a recyclable catalytic system for the
Huisgen [3 + 2]-cycloaddition of azides and alkynes without additives under
ambient conditions†
Ajeet Kumar,a Swati Aerry,a Amit Saxena,a Arnab deb and Subho Mozumdar*a
Received 15th January 2012, Accepted 6th March 2012
DOI: 10.1039/c2gc35070j
catalysts to overcome the above drawbacks have been
proposed.8–11 Herein, we describe the virtues of copper nanopar-
ticulates in Guar-gum polymer (Cu/GG) as a simple, inexpen-
sive, and especially general and efficient heterogeneous catalyst
for use in this emerging area. It is now well established that
polymers are excellent host materials for the preparation of metal
nanoparticles. Polymers act as a surface-capping agent when the
nanoparticles are embedded or encapsulated in a polymer.
Various synthetic polymers, such as poly(vinyl alcohol),12,13
poly(vinyl pyrrolidone),14 polystyrene,15 and poly(methyl
methacrylate),16 have been used in the synthesis of metal nano-
particles. However, very little work has been carried out in
which a natural polysaccharide has been used as a capping agent
for the stabilization of metal nanoparticles. Recently, the syn-
thesis of natural-polymer-stabilized selenium nanoparticles by
the reduction of selenious acid by ascorbic acid was reported.17
Natural polymers, because of their large abundance, biodegrad-
ability, and reactivity (due to the presence reactive functional
groups), may show a better ability to stabilize nanoparticles.
Guar-gum (GG) is a naturally occurring edible carbohydrate
polymer found in the seeds of Cyamopsis tetragonolobus. It is a
non-ionic, branched-chain polymer, consisting of straight-chain
mannose units joined by β-D-(1–4) linkages having a-D-galacto-
pyranose units attached to this linear chain by (1–6) linkages.
The structure of Guar-gum is very complex and it has several
hydroxyl groups, which can effectively bind metal clusters. So
this property of Guar-gum can be exploited for the stabilization
of copper nanoparticles generated in situ. Guar-gum has a very
strong tendency to form gel in aqueous solution. This gel is
highly viscous in nature and hence, can entrap, protect and
stabilize the synthesized nanoparticles in its strong gelly mesh
for a longer period of time and in the process act as an excellent
surface capping agent. Guar-gum is not soluble in organic sol-
vents and so this is an added advantage as the catalyst can be
recovered from the organic reaction media by mild centrifugation
and can be reused again and again, retaining the catalytic
efficiency of the entrapped Cu-nanoparticles. As it is not soluble
in organic solvents, so it does not interfere with the reaction
pathway thus helps in maintaining the purity of the reaction. As
both copper and copper oxide nanoparticles are expected as the
species present within the Guar mesh, a reducing agent may not
be needed to carry out the proposed cycloaddition reaction.
In this communication, we wish to draw attention towards an
efficient and environmentally benign protocol for an alternative
Cu-nanoparticulates in Guar-gum at room temperature were
investigated for the first time as a recyclable catalytic system
in organic synthesis. The catalytic potential of these
materials were evaluated in the Huisgen [3 + 2]-cycloaddition
of azides and alkynes without additives under ambient con-
ditions which offers several advantages, viz. high yields,
clean reactions, short reaction times, recyclability of the cata-
lyst and a simple workup procedure.
Introduction
The chemistry of azides has come alive after the discovery of
“Click” chemistry, discovered a few years back. “Click” chem-
istry is an alluring concept proposed by Sharpless et al. in
2001.1 Since its discovery “Click” chemistry has generated inter-
est to chemists in areas of catalysts, polymers, material science,
synthesizing libraries of compounds and more importantly in
drug discovery.2,3 This approach, based on the joining of smaller
units mimics the approach used by nature to generate substances.
“Click” chemistry uses only the most practical and reliable
chemical reactions to connect a diversity of structures bearing a
wide variety of functional groups. However, the early Huisgen
cycloaddition process required a strong electron-withdrawing
substituent either on azide or on alkyne, and were often con-
ducted at high temperature for a prolonged period of time, and
usually led to the isolation of a mixture of 1,4-disubstituted and
l,5-disubstituted-l,2,3-triazoles regioisomers and the electronic
effect of the substituent groups on phenyl ring were not known
clearly.4–6 The increasing environmental consciousness of the
chemical community has led to the search for more efficient and
environmentally friendly methods for chemical syntheses.7
Therefore, it is desirable to develop a new, convenient and
regio-controlled synthetic approach for the formation of tria-
zoles. Recently, some important concepts and transition-metal
aDepartment of Chemistry, University of Delhi, Delhi 110007, India.
E-mail: subhoscom@yahoo.co.in; Tel: +919810728438
bDepartment of Microbiology and Immunology, Columbia University
Medical Centre, USA
†Electronic supplementary information (ESI) available: Procedural
details for the synthesis and characterization of copper nanoparticulates
in Guar-gum associated with this article can be found, in the online
version. See DOI: 10.1039/c2gc35070j
1298 | Green Chem., 2012, 14, 1298–1301
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Table 1 Huisgen [3 + 2]-cycloaddition of azides and alkynes using copper nanoparticulates in Guar-gum
Entry
1
Azide
Alkyne
Product
Time (h)
Yield (%)
98
5
2
5
94
3
4
5
5
95
92
5
6
5
5
98
97
7
8
5
5
98
95
9
5
5
92
94
10
Reaction conditions: The reaction has been performed using the azide (1 mmol), alkyne (1.2 mmol), catalyst (10 mg) in ethanol (5 mL) at room
temperature.
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Table 2 Comparison at various conventional protocols used for
Huisgen [3 + 2]-cycloaddition of azides and alkynes
Sr.
Reaction
time
No. Protocol
References
Ref. 18
1
2
Copper-in-charcoal at room
temperature
PVP-stabilized copper-np at room
temperature
10 h
15–45 min Ref. 19
3
4
Copper-in-zeolite at room temperature 15 h
Ref. 20
Ref. 21
Copper catalyst (Cu/AlO(OH) in
pluronics P123
12 h
5
CuI with N,N′-
dimethylethylenediamine (DMEDA)
as stabilizing ligand
20 h
Ref. 22
Fig. 1 Recycling of Guar-gum stabilized copper nanoparticles for the
cycloaddition of benzyl azide and phenyl acetylene in ethanol (Table 1,
entry 1).
6
7
8
9
Cu-np with Et3N at 65 °C at room
temperature
CuSO4·5H2O/sodiumascorbate at room 8 h
temperature
125 °C heating using 100 W
microwave irradiation
Cu-particulates in Guar-gum at room
temperature
30 min
Ref. 23
Ref. 11
10–15 min Ref. 24
the reaction and the catalytic activity in terms of yield is pre-
sented in Fig. 1. The catalytic system worked extremely well
even up to five subsequent cycles with almost 95% yield. It has
been observed that with the increasing number of cycles of the
reaction, the catalytic activity of the nanoparticles decreases
insignificantly while the reaction time was delayed. The differ-
ence between delayed time in the second and fourth cycle is
much higher than that corresponding to fifth cycle. These obser-
vations suggest that after few cycles the catalytic efficiency
could be preserved although, at higher reaction times when com-
pared with the first cycle. It has been reported that some
polymer-encapsulated nanoparticulates do aggregate after the
catalytic reaction.25 Thus the above result may be supposed to be
induced by the agglomeration of the Guar-gum containing
copper nanoparticles as catalyst. However, this point requires
stricter experimental confirmation. The difference in QELS data
before and after reaction (see ESI†) clearly shows the agglomera-
tion of the catalyst. However, agglomeration of the catalyst does
not affect the catalytic efficiency to a large extent. In order to
check the leaching and stability of the catalyst the reaction
mixture was further characterised after recovery of the catalyst
(Fig. 6 in ESI†) and no trace of leaching was observed which
was further confirmed by allowing the recovered reaction solvent
to react further without adding catalyst and even after 24 hours
the reactants remain unreacted. Thus, the proposed methodology
is very clean and ‘green’.
As in all other metal nanoparticles catalyzed organic reactions,
the solvent used have a strong effect in deciding the reaction
path, time of completion of the reaction and the yield of the
product. Usually, the increase in polarity leads to increase in
reaction rate. This behaviour can be attributed to the influence of
the solvent on the transition state and to a change in the capacity
of the catalyst for proton transfer. When polar reagents are used,
the transition-state complex is better solvated by polar solvents
and the partition of the reactants at the solid–liquid interface is
higher, decreasing the activation free enthalpy and enhancing the
reaction rate.
5 h
Our
method
recyclable catalytic system for the Huisgen [3 + 2]-cycloaddition
of azides and alkynes without additives under ambient
conditions.
Results and discussion
The findings on the cycloaddition of different azides compounds
with different terminal alkynes at room temperature catalyzed by
Cu-nanoparticulates stabilized in Guar-gum have been presented.
The reaction takes place in a neutral medium like ethanol
without the use of any bases. It may be postulated that the pro-
posed catalyst nanoparticles play a redox role in accelerating the
cycloaddition reaction and thus may promote the formation of
triazole products. The classical cycloaddition of phenyl acety-
lene with benzyl azide has been used to explore the catalytic
efficiency of these Guar-gum stabilized Cu-nanoparticles
without any catalyst, this reaction did not take place in ethanol at
room temperature even after 72 hours of stirring and the starting
materials could be recovered. In order to further evaluate the
efficiency of this methodology, a number of azides and terminal
alkynes were further subjected to cycloaddition reaction using
Guar-gum stabilized Cu-nanoparticles at room temperature and
the products obtained have been tabulated in Table 1. It is impor-
tant to note that there is a lack of functional group interference.
Various substituted azides readily participated in this transform-
ation and tolerance for variations in the acetylene component
was also excellent. A comparison of some selected protocols in
literature and our methods is listed in Table 2. In case of nano-
particles, the surface of particles is considered to be more reac-
tive because of unfilled valencies of the surface atom. Also, the
surface area of the catalyst increases tremendously when size
approaches to nano level. To prove the efficacy of the catalytic
system and its potential experiment in industries recyclability of
the catalyst was checked. The catalyst could be recycled
efficiently by separating them from the reaction mixture by mild
centrifugation. They could be employed as a catalyst for the suc-
cessive reactions. The relation between the number of cycles of
The cycloaddition between benzyl azide and phenyl acetylene
has been investigated in various solvents with all the other par-
ameters kept constant and the results have been tabulated in
Table 3. Best results were obtained with ethanol and t-butanol :
water (3 : 1). In the presence of acetonitrile several products were
1300 | Green Chem., 2012, 14, 1298–1301
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Table 3 Effect of polarity of the solvent on Huisgen [3 + 2]-
cycloaddition of azides and alkynes using copper nanoparticulates in
Guar-gum
with ethyl acetate to ensure complete transfer. The volatile com-
pounds are removed in vacuum to give pure triazole. The struc-
tures of all the products were unambiguously established on the
1
basis of their spectral analysis (IR, H NMR and MS mass spec-
Yield (%)
Time (h)
tral data) (see ESI†).
Solvent
Ratio
Table 1 entry 1
t-Butanol
Water
Ethanol
Acetonitrile
THF
t-Butanol : water
t-Butanol : water
Ethanol : water
Neat
Neat
Neat
Neat
Neat
3 : 1
1 : 1
3 : 1
91
64
98
6
Acknowledgements
12 (catalyst degraded)
5
—
>72
10
5
This work is dedicated to Professor Sneh Kumar Dogra on his
70th birthday.
S. M., S. A and A. K. acknowledge the financial support from
the Department of Science & Technology (DST) and University
Grant Commission (UGC), Government of India, respectively.
Several spots
Incomplete
99
98
98
8
Reaction conditions: The reaction has been performed using the azide
(1 mmol), alkyne (1.2 mmol) and catalyst (10 mg).
Notes and references
1 H. C. Kolb, M. G. Finn and K. B. Sharpless, Angew. Chem., Int. Ed.,
2001, 40, 2004.
2 P. Wu and V. V. Fokin, Aldrichimica Acta, 2007, 40, 7.
3 H. C. Kolb and K. B. Sharpless, Drug Discovery Today, 2003, 8(24),
1128.
formed while the reaction in THF was not complete even after
72 hours of vigorous stirring.
4 S. Kamijo, T. Jin, Z. Huo and Y. Yamamoto, J. Org. Chem., 2004, 69,
2386.
5 G. L’abbe, Chem. Rev., 1969, 69, 345.
Conclusions
6 K. V. Gothelf and K. A. Jorgensen, Chem. Rev., 1998, 98, 863.
7 P. T. Anastas and J. C. Warner, Green Chemistry: Theory and Practice,
Oxford University Press, Oxford, 1998.
8 Z. X. Wang and H. L. Lin, Chem. Commun., 2003, 2450–2451.
9 A. Krasinski, V. V. Fokin and K. B. Sharpless, Org. Lett., 2004, 6, 1237.
10 W. G. Lewis, L. G. Green, F. Grynszpan, Z. Radic, P. R. Carlier,
P. Taylor, M. G. Finn and K. B. Sharpless, Angew. Chem., Int. Ed., 2002,
41, 1053.
11 V. V. Rostovtsev, L. G. Green, V. V. Fokin and K. B. Sharpless, Angew.
Chem., Int. Ed., 2002, 41, 2596.
12 P. K. Khanna, N. Singh, S. Charan, V. V. V. Subbarao, R. Gokhale and U.
P. Mulik, Mater. Chem. Phys., 2005, 93, 117.
In conclusion, it can be said that highly efficient Click chemistry
between organic azides and terminal alkynes can be heteroge-
neously catalyzed by Cu-nanoparticulates embedded or
entrapped within the highly viscous mesh of Guar-gum in
ethanol at room temperature. Solubility issues, copper contami-
nation and modest yields usually associated with the choice of
copper salt are completely averted. External ligands and addition
of base known to accelerate Click reactions were not required.
The encouraging results of recyclability which appears to be
unaffected by exposure to air, makes this method facile, clean,
cost effective and ‘green’.
13 K. S. Chou and C. Y. Ren, Mater. Chem. Phys., 2000, 64, 241.
14 P. K. Khanna, R. Gokhale and V. V. V. Subbarao, J. Mater. Sci., 2004, 39,
3773.
15 D. Wu, X. Ge, Y. Huang, Z. Zhang and Q. Ye, Mater. Lett., 2003, 57,
3549.
16 O. L. A. Monti, J. T. Fourkas and D. J. Nesbitt, J. Phys. Chem. B, 2004,
108, 1604.
Experimental
Lyophilized powder of Guar-gum stabilized Cu-nanoparticulates
(10 mg) is added to a clean oven dried 25-mL round-bottom
flask fitted with a stir bar and septum. Absolute ethanol (5 mL)
is added slowly to the sidewalls of the flask to rinse the pow-
dered catalyst down. While the heterogeneous solution is stirred,
azide (1.0 mmol) and alkyne (1.2 mmol) are added. The round-
bottom flask is stirred at room temperature and the progress of
the reaction is monitored by TLC until complete consumption of
azide takes place. After the completion of the reaction, the reac-
tion mixture is centrifuged (5000 rpm, 10 min) to pellet out the
catalyst. The catalyst can then be washed with absolute ethanol
to remove all the organic impurities. These particles can be dried
and reused for the next reaction. The filtrate is further treated
17 H. Y. Zhang, J. Zhang, H. Y. Wang and H. Y. Chen, Mater. Lett., 2004,
58, 2590.
18 B. H. Lipshutz and B. R. Taft, Angew. Chem., Int. Ed., 2006, 45, 8235.
19 A. Sarkar, T. Mukherjee and S. Kapoor, J. Phys. Chem. C, 2008, 112,
3334.
20 S. Chassaing, M. Kumarraja, A. S. S. Sido, P. Pale and J. Sommer, Org.
Lett., 2007, 9, 883.
21 I. S. Park, M. S. Kwon, Y. Kim, J. S. Lee and J. Park, Org. Lett., 2008,
10, 497.
22 L. Ackermann, H. K. Potukuchi, D. Landsberg and R. Vicente, Org.
Lett., 2008, 10, 3081.
23 F. Alonso, Y. Moglie, G. Radivoy and M. Yus, Tetrahedron Lett., 2009,
50, 2358.
24 P. Appukkuttan, W. Dehaen, V. V. Fokin and E. V. Eycken, Org. Lett.,
2004, 6, 4223.
25 R. Narayanan and M. A. El-Sayed, J. Phys. Chem. B, 2005, 109, 12663.
This journal is © The Royal Society of Chemistry 2012
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