Copper nanoparticles in an ionic liquid: an efficient catalyst for the synthesis of bis-(4-hydroxy-2-oxothiazolyl)methanes

Article abstract of DOI:10.1016/j.tetlet.2007.11.106

Copper nanoparticles were synthesized and characterized by TEM, XRD and UV-vis techniques. The copper nanoparticles in an ionic liquid were employed as a recyclable catalyst for the synthesis of bis-(4-hydroxy-2-oxothiazolyl)methanes in excellent yields a

Full text of DOI:10.1016/j.tetlet.2007.11.106

Available online at www.sciencedirect.com
Tetrahedron Letters 49 (2008) 727–730
Copper nanoparticles in an ionic liquid: an efficient catalyst
for the synthesis of bis-(4-hydroxy-2-oxothiazolyl)methanes
Prashant Singh b,c, Anju Katyal b, Rashmi Kalra d, Ramesh Chandra a,b,
*
a Department of Chemistry, University of Delhi, Delhi 110 007, India
b Dr. B.R. Ambedkar Center for Biomedical Research, University of Delhi, Delhi 110 007, India
c A.R.S.D. College, University of Delhi, Delhi, India
d A.N.D. College, University of Delhi, Delhi, India
Received 31 August 2007; revised 24 October 2007; accepted 20 November 2007
Available online 23 November 2007
Abstract
Copper nanoparticles were synthesized and characterized by TEM, XRD and UV–vis techniques. The copper nanoparticles in an
ionic liquid were employed as a recyclable catalyst for the synthesis of bis-(4-hydroxy-2-oxothiazolyl)methanes in excellent yields and
in short reaction times.
Ó 2007 Elsevier Ltd. All rights reserved.
Keywords: Copper nanoparticles; Ionic liquid; Thiazolidine-2,4-dione; Aromatic aldehydes
Recently, research has been directed towards the synthe-
sis and application of metal nanoparticles in view of their
unique properties compared to the bulk metals.1,2 Among
various metal nanoparticles, copper nanoparticles have
received considerable attention because of their unusual
properties and potential applications in diverse fields.3
The various synthetic procedures for their synthesis include
microemulsion techniques,4 the use of reverse micelles,5
reduction of aqueous copper salts,6 UV-light irradiation,7
physical vapour deposition8 and impregnation methods.9,10
The core–shell particles are of great interest due to their
potential applications in diverse fields including catalysis,
drug delivery, photonics, sensors, etc.11 Copper nanoparti-
cles, in particular, being cheap, require only mild reaction
conditions to produce high yields of products in short reac-
tion times compared to traditional catalysts and can also be
recycled. Rothenberg12 and co-workers recently reported
the use of copper nanoparticles, which are less harmful to
the environment than any other metals in Suzuki cross-
coupling reactions. The condensation of aromatic halides
(except fluorine) catalyzed by copper, the Ullmann reac-
tion, has gained tremendous laboratory and industrial
interest due to its wide applicability in the synthesis of sym-
metrical and unsymmetrical biaryls and polyaryls with dif-
ferent functional groups which are otherwise difficult to
obtain. The adsorption of aromatic halides on metal parti-
cles is strong because of the interaction of delocalized p-
electrons with the metal. The mechanism of the Ullmann
condensation reaction is believed to be a nucleophilic
attack of copper on an aromatic carbon carrying a halogen
substituent. Cu nanoparticles are also used as efficient and
selective catalysts in aza-Michael reactions of N-alkyl- and
N-arylpiperazines with acrylonitrile.13
There is considerable interest in the use of room temper-
ature ionic liquids as substitutes for volatile organic sol-
vents.14 Ionic liquids have been employed as reaction
media for several organic reactions, including alkylation,15
hydrogenation16 and oxidation.17 As a result of their tun-
able polarity and hydrophobicity, they can solvate various
organic, inorganic and polymeric compounds, and have
been used as green solvents for liquid–liquid separations,
extractions and recycling in homogeneous catalysis.
*
Corresponding author. Tel./fax: +91 011 276666248.
E-mail address: rameshchandra57@yahoo.co.in (R. Chandra).
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.11.106

728
P. Singh et al. / Tetrahedron Letters 49 (2008) 727–730
As a part of our ongoing research on the synthesis and
catalytic applications of metal nanoparticles, we herein
report a novel and simple method for the synthesis of Cu
nanoparticles. The nanoparticles were synthesized and
characterized by XRD, TEM and UV–vis techniques and
were employed as a recyclable catalyst for the condensation
between thiazolidine-2,4-dione 1 and aromatic aldehydes 2
(Scheme 1)20–23 to produce the bis-(4-hydroxy-2-oxothia-
zolyl)methanes 3 in very high yields and in short reaction
times.
The Cu nanoparticles were prepared as follows: In a
round-bottom flask, 1 ml of ionic liquid 4 and 5 mg of cop-
per nitrate was treated with excess NaBH4 in methanol. A
black coloured solution was obtained rapidly indicating the
presence of Cu(0) particles. Stirring was continued for 6 h.
The solution was taken in ethanol (20 ml) and then centri-
fuged at 3000 rpm for 10 min and the supernatant was dec-
anted to afford the Cu nanoparticles as a solid.
The Cu nanoparticles were characterized by TEM, XRD
and UV–vis techniques. The X-ray powder diffraction pat-
tern of the catalyst showed a signature for Cu(0) as shown
in Figure 1. The peaks at 44.580° and 60.956° confirmed
the presence of Cu(0) in the sample. Recycling of the cata-
lyst five times did not result in any changes in the structure
and the morphology of the catalyst during the course of the
reaction and is a sign of its stability.
Fig. 1. Powder-XRD curves of Cu nanoparticles: (i) fresh Cu nanopar-
ticles and (ii) used Cu nanoparticles.
Transmission electron microscope (TEM) studies of the
Cu nanoparticles were carried out to investigate the shape
and the size of the particles. Figure 2 shows the transmis-
sion electron micrographs for fresh and used Cu nanopar-
ticles of 50–60 nm in diameter with spherical shape.
The UV–vis absorption spectrum of Cu nanoparticles
dispersed in ethanol showed a band around 583 nm due
to surface plasma resonance, which is within the range
500–600 nm reported as the band characteristic of copper
nanoparticles, see Figure 3.12 The observed band for the
Cu nanoparticles suggests that the effective particle size
of the Cu nanoparticles is within the range 50–60 nm as
observed from TEM studies. The shifts in the plasmon
band depend on the size and the shape of the nanoparti-
cles.24–26 Copper nanoparticles of 30–50 nm diameter show
a plasmon band at 579 nm, whereas nanorods show a plas-
mon band at 586 nm.27 The observed plasmon band at
583 nm for the Cu nanoparticles suggests that the effective
particle size of the Cu nanoparticles is within the range
50–60 nm.
OH
O
-
Br
A
=
N
N
NC
N
N
Ionic Liquid 4
We have found that copper nanoparticles in ionic liquid
18 act as an efficient catalyst for the condensation reaction
between thiazolidine-2,4-dione 1 and benzaldehyde, see
Table 1.19
4
O
O
O
O
S
S
S
Cu nanoparticles in
Ionic Liquid.
2
+
The activity of the synthesized Cu nanoparticles for the
conjugate addition of a variety of aromatic aldehydes with
thiazolidine-2,4-dione in the presence of 5 mg of Cu nano-
particles in ionic liquid 4 at room temperature was studied
and the results are summarized in Table 2. All the reactions
gave the corresponding products within 5–30 min and the
yields of the desired products were good to excellent.
In summary, we have shown that Cu nanoparticles in
ionic liquid 4 catalyzed the reaction between thiazolidine-
HN
r.t.
NH
HN
OH
OH
O
R
R
1
2
3
Scheme 1.

P. Singh et al. / Tetrahedron Letters 49 (2008) 727–730
729
Fig. 2. TEM of the synthesized copper nanoparticle synthesized: (i) fresh Cu nanoparticles and (ii) used Cu nanoparticles.
Table 2
Plot of wavelength vs Absorbance
Synthesis of bis-(4-hydroxythiazol-2-one)methanes by reaction between
thiazolidine-2,4-dione and aromatic aldehydes in the presence of copper
nanoparticles in an ionic liquid
0.8
0.6
0.4
0.2
0
Entry
R
Product
Yielda (%)
1
2
3
4
5
6
7
8
9
10
11
12
a
C6H5
3a
3b
3c
3d
3e
3f
92
88
85
89
82
88
85
92
90
95
91
89
4-OCH3C6H4
4-OHC6H4
2-OHC6H4
2,4-Cl2C6H3
2-ClC6H4
0
200
400
600
800
Wavelength (nm)
4-FC6H4
3g
3h
3i
4-N(CH3)2C6H4
3,4-(OCH3)2C6H3
3-OHC6H4
2-OHC10H7
C4H3CHOb
Fig. 3. UV–vis absorption spectrum of copper nanoparticles in ethanol.
3j
3k
3l
Table 1
Optimization of the catalyst for the synthesis of bis-(4-hydroxy-2-
oxothiazolyl)methanes 3a
Yield from GC.
2-Formylthiophene.
b
Entry
Catalyst
Time (min)
Yield (%)
1
2
3
4
5
a
—
45
8
15
5
65
75
70
92
75
Cu nanoparticles
Ionic liquid 4
Cu nanoparticles in ionic liquid 4
Boric acid
References and notes
15
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2,4-dione and aromatic aldehydes in high yields and in
short reaction times. Our protocol avoids the use of expen-
sive reagents, high temperatures and the reaction being per-
formed in an ionic liquid and serves as an efficient method
and also allows easy recyclability of the catalyst. Further
catalytic applications of Cu nanoparticles for condensation
reactions with complex structures of biological significance
are currently under investigation.
Acknowledgement
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This work is dedicated to the late Dr. N.N. Ghosh, my
mentor and my supervisor (P.S.).

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P. Singh et al. / Tetrahedron Letters 49 (2008) 727–730
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the catalyst was preserved for the next run. Structural assignments of
the products were based on 1H NMR, 13C NMR, IR and mass
analysis. Analysis of spectral and compositional data revealed the
formation of bis-(4-hydroxy-2-oxothiazolyl)methane 3a in 92% yield.
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round-bottom flask, 10 mmol of 1-H-tetrazole-5-acetic acid and
15 mmol of acrylonitrile in 20 ml of acetonitrile were stirred for 2 h.
The extent of the reaction was monitored by TLC. On completion, the
solvent and excess acrylonitrile were removed under reduced pressure.
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of bromobutane was added. The mixture was refluxed for 6 h and
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nanoparticles (0.005 g), thiazolidine-2,4-dione (20 mmol) and benzal-
dehyde (10 mmol) in ionic liquid 4 (3 mL) was stirred at room
temperature for the appropriate time. After completion of the
reaction, as indicated by TLC, the product was extracted with ethyl
acetate (3 Â 10 mL). The combined organic extracts were concen-
trated in vacuum and the resulting product was purified by column
chromatography on silica gel with ethyl acetate and n-hexane (3:7) as
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