Silver nanoparticles as an efficient, heterogeneous and recyclable catalyst for synthesis of β-enaminones

Article abstract of DOI:10.1016/j.catcom.2010.06.011

A simple and efficient protocol has been developed for synthesis of β-enaminones and β-enamino esters catalyzed by silver nanoparticles as a novel, heterogeneous, moisture stable and recyclable catalyst under mild reaction conditions. The reaction was optimized with respect to various parameters such as catalyst screening, catalyst loading, different solvents and temperature. The silver nanoparticle exhibited excellent activity and the methodology is applicable to diverse substrates providing good to excellent yields of desired products. The catalyst was recycled for three consecutive cycles without any significant loss in activity.

Full text of DOI:10.1016/j.catcom.2010.06.011

Catalysis Communications 11 (2010) 1233–1237
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Silver nanoparticles as an efficient, heterogeneous and recyclable catalyst for
synthesis of β-enaminones
⁎
Kushal D. Bhatte, Pawan J. Tambade, Kishor P. Dhake, Bhalchandra M. Bhanage
Department of Chemistry, Institute of Chemical Technology (Autonomous), N.M. Parekh Marg, Matunga, Mumbai – 400 019, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A simple and efficient protocol has been developed for synthesis of β-enaminones and β-enamino esters
catalyzed by silver nanoparticles as a novel, heterogeneous, moisture stable and recyclable catalyst under
mild reaction conditions. The reaction was optimized with respect to various parameters such as catalyst
screening, catalyst loading, different solvents and temperature. The silver nanoparticle exhibited excellent
activity and the methodology is applicable to diverse substrates providing good to excellent yields of desired
products. The catalyst was recycled for three consecutive cycles without any significant loss in activity.
© 2010 Elsevier B.V. All rights reserved.
Received 4 January 2010
Received in revised form 11 June 2010
Accepted 18 June 2010
Available online 1 July 2010
Keywords:
Silver nanoparticles
β-Enaminones
β-Enamino esters
Heterogeneous
Recyclable catalyst
1. Introduction
activity in various different organic transformations [18]. Nanosize
catalysts can act as a bridge between homogeneous and heterogeneous
The enaminones and enamino esters are very stable structural motifs
which are prepared by inexpensive raw materials. Hence they are
considered as excellent starting materials in organic synthesis.
Enaminones and enamino esters are used as versatile building blocks
for synthesis of important heterocyclic compounds, nitrogen containing
compounds, naturally occurring alkaloids, pharmaceutical drugs with
antiepileptic, anticonvulsant and antitumor properties [1–8]. They even
served as synthons for γ-aminoalcohol, β-aminoacids which are a class
of very stable compounds useful in asymmetric catalysis as a chelating
agent [9]. Condensation reactions of carbonyl compounds with amines
in the presence of various acid catalysts like metal triflates viz, Yb(OTf)3
[10], perchlorates [11], Zn(OCl)4 [12], InBr3 [13], CoCl2 [14] are well
reported for the synthesis of β-enaminones and enamino esters.
Additionally, non-conventional techniques such as microwave and
ultrasound are also reported [15–17]. However, these methods are
associated with certain drawbacks like use of moisture sensitive metal
triflates and tedious workup [10], requirement of drastic conditions and
use of harmful reagents [11], use of homogenous catalyst [12–14], and
non recyclable catalyst [14]. Hence there is sufficient scope for the
development of heterogeneous, reusable catalytic system, which could
catalyze the synthesis of enaminones and enamino esters at milder
operating conditions.
catalysis. It offers the advantage of homogeneous and heterogeneous
catalysis disciplines and provides unique activity with high selectivity in
catalysis [19]. Application of free nanoparticles as a catalyst is an
important vital route to understand the basics in catalysis. However less
attention has been paid towards free nanoparticle catalysis. Hence, the
development of free nanoparticles with versatile catalytic activity is still
an attractive and challenging task from the point of view of academia
and industry. Silver nanoparticles have been used as catalyst for
oxidation and reduction of certain molecules and demonstrating their
antimicrobial activity in colloidal state [20–22]. Recently, the colloidal
silver nanoparticles have been reported for C–C coupling reaction [24].
To the best of our knowledge free silver nanoparticles have not been
explored yet as a catalyst for any condensation transformation.
We herein report an efficient protocol for the synthesis of
enaminones and enamino esters using silver nanoparticles as a
heterogeneous, moisture stable and recyclable catalyst with a simple
workup procedure. The catalyst exhibited a remarkable activity and
the system tolerates a wide variety of amines and dicarbonyl
compounds with good to excellent yields of desired products. The
catalyst was also found to be reusable up to three consecutive cycles
without any significant loss in activity.
Metal nanoparticles have high surface to volume ratio and
unresidual energy which are mainly responsible for their catalytic
2. Experimental
2.1. General
AgNO3 (A.R. grade, 99% pure) was purchased from Sigma Aldrich. All
other chemicals with their highest purity were purchased from S.D.
⁎
Corresponding author. Tel.: +91 22 24145616; fax: +91 22 24145614.
E-mail address: bhalchandra_bhanage@yahoo.com (B.M. Bhanage).
1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2010.06.011

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K.D. Bhatte et al. / Catalysis Communications 11 (2010) 1233–1237
Fig. 3. XRD pattern of silver nanoparticles.
Fig. 1. TEM of silver nanoparticles with SAED pattern inside. TEM scale corresponds to
100 nm.
absence of other impurities (see Fig. 4). UV–Vis spectroscopy of silver
nanoparticles showed characteristic plasmon band at 420 nm (Fig. 5).
Fine Chemicals Pvt. Ltd. Mumbai, India and were used without further
purification. The conversion was based on GC analysis (Chemito 1000).
Allproducts werecharacterized by GC-Ms analysis(Shimadzu QP2010).
2.3. Typical procedure for synthesis of enaminones and enamino esters
2.2. Synthesis of silver nanoparticles
In a 10 mL round bottom flask, dicarbonyl compound (1 mmol),
amine (1 mmol) and silver nanoparticles (0.2 mmol) were added in
3–5 mL of methanol and stirred at 60 °C for a desired time. Progress
of reaction was monitored by TLC and Gas chromatography. On
completion of reaction, 5 mL of water was added to the reaction
mixture and centrifuged at 5000 rpm for 20 min to separate the silver
nanoparticles. The supernatant was slowly decanted and the
sediment silver nanoparticles were allowed to dry and were further
collected. The crude product was extracted from mixture using ethyl
acetate (5 mL×3) and dried using Na2SO4. All the products are well
known and were characterized using GC-MS and were also compared
with the authentic samples (Scheme 1).
Silver nanoparticles were synthesized by reduction of AgNO3 to Ag
by reverse micellar system using hydrazine and Triton X-100 as
surfactant under nitrogen atmosphere as a reported procedure for the
preparation of metal nanoparticles [23]. Metallic silver nanoparticles
were confirmed with a UV spectrophotometer (Shimadzu 1650) and
XRD (Mini Flex Rigaku model with Cu Kα source). The particle size of
silver in nano region was confirmed by a Transmission Electron
Microscope (Philips model CM 200). EDAX analysis was carried out
using JEOL-JFC 1600 with platinum coating. Particle size histogram
was obtained with photon correlation spectroscopy (DLS) using
Beckman coulter Delsa Nano C instrument.
TEM image of silver nanoparticles showed that, nanoparticles are
well dispersed and non-agglomerated (see Fig. 1). The average particle
size of silver nanoparticles was found to be 40 nm with histogram
obtained by DLS (see Fig. 2). The X-ray diffractogram of silver
nanoparticles shows an FCC cubic structure with an average crystalline
size of 40 nm using Scherrer's equation showing prominent peaks at 2
values about 38°, 44.2°, 64.4°, and 77° representing (111), (200),
(220), and (311) planes (see Fig. 3). The EDAX analysis of silver
nanoparticles showed that the sample is essentially pure with the
3. Results and discussion
Initially condensation of aniline and acetyl acetone was selected as
a model reaction, and the influence of various reaction parameters like
solvent, catalyst, catalyst loading, temperature and time was
Fig. 2. Histogram of silver nanoparticles obtained with DLS.
Fig. 4. EDAX analysis of silver nanoparticles.

K.D. Bhatte et al. / Catalysis Communications 11 (2010) 1233–1237
1235
Table 1
Study of reaction parameters on reaction of aniline with acetylacetone.a
Entry
Solvent
Catalyst
Catalyst loading
(mmol)
Temp (°C)
Yieldb(%)
Effect of catalyst
1
2
3
4
5
Methanol
AgNO3
Ag NP
Ni NP
Co Np
NIL
0.2
0.2
0.2
0.2
–
60
60
60
60
60
18
90
68
45
Methanol
Methanol
Methanol
Methanol
Traces
Effect of solvent
6
7
8
9
Ethanol
Acetonitrile
Toluene
Ag Np
Ag Np
Ag Np
Ag Np
0.2
0.2
0.2
0.2
60
60
60
60
86
90
68
Traces
Solvent free
Effect of catalyst loading
10
11
12
Methanol
Methanol
Methanol
Ag NP
Ag Np
Ag Np
0.10
0.15
0.25
60
60
60
63
70
92
Effect of temperature
13
14
15
Methanol
Methanol
Methanol
Ag Np
Ag Np
Ag Np
0.2
0.2
0.2
r.t.
45
Reflux
55
70
84
a
Reaction conditions: acetyl acetone(1 mmol), aniline (1 mmol), solvent (5 mL),
Fig. 5. UV–Vis spectrum of silver nanoparticles.
and time (8 h).
b
Yield is based on GC analysis.
examined (Table 1, entries 1–15). The effect of various free
nanoparticles of Ag, Ni and Co as a catalyst for condensation reaction
was investigated. Among which, Ag nanoparticles as a catalyst was
found to be the most effective catalyst providing an excellent yield of
90% (Table 1, entry 2). Reaction using bulk AgNO3 provided a low yield
(18%) of the desired product as compared to nanosize Ag particle
which states the influence of size to volume ratio. The control
experiment was carried out which resembles that the reaction did not
proceed without a catalyst (Table 1, entry 5). The influence of various
solvents like methanol, ethanol, acetonitrile, and toluene on the
condensation of aniline and acetyl acetone was then studied (Table 1,
entries 6–9). The reaction was also carried out under solvent free
conditions to evolve the influence of the solvent parameter but traces
of the product were formed (Table 1, entry 9). It was observed that the
reaction provided a good yield with the use of methanol and
acetonitrile as a solvent, employing best results with methanol (90%
yield), which was then employed for further study. In an effort to
demonstrate the catalytic activity of silver nanoparticles various
catalyst loadings were studied (Table 1, entries 2, 10–12). The yield
was observed to increase with the increase in catalyst loading in the
range of 0.05 mmol to 0.25 mmol of catalyst. Optimum result with
respect to yield was obtained when 0.2 mmol of silver nanoparticles
was used (Table 1, entry 2). There was no significant increase in yield
when catalyst loading increased up to 0.2 mmol to 0.25 mmol
(Table 1, entry 12). Effect of different temperatures on the activity
of silver nanoparticles for a desired synthesis was studied by varying
temperatures ranging from r.t. to reflux condition. It was observed
that with the increase in temperature the yield of desired product
initially increases (Table 1, entries 2, 13–15). However, at tempera-
ture, i.e., reflux condition the yield has decreased, reflecting that 60 °C
is the optimum temperature. Hence the optimum reaction conditions
are catalyst: Ag nanoparticles, solvent: methanol, catalyst concentra-
tion 0.2 mmol, temp: 60 °C and time: 8 h.
In order to test the generality and the efficiency of the protocol,
optimized reaction conditions were then applied for condensation of
various diketones with different amines, providing good to excellent
yields of desired products (Table 2, entries 1–11). Acetyl acetone was
found to give a good yield with aniline and p-toludine (Table 2, entries
1–2). Aliphatic and alicyclic amines such as n-butyl amine, morpho-
line, benzyl amine and cyclohexyl amine were found to give an
excellent yield of the condensed product using Ag nanoparticles as a
catalyst (Table 2, entries 3–6). It was observed that the reaction with
aliphatic amines proceeds easily in a short period as compared to
aromatic amines; this can be attributed to the higher nucleophilicity
of aliphatic amines as compared to aromatic amines.
The addition of amines to other dicarbonyl compounds like ethyl
acetoacetate and benzalacetonewasalsostudied (Table 2, entries 7–12).
Ethyl acetoacetate was found to deliver a good yield of desired products
Scheme 1. Synthesis of enaminones and enamino esters using silver nanoparticles.

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K.D. Bhatte et al. / Catalysis Communications 11 (2010) 1233–1237
Table 2
Condensation of dicarbonyl compounds with amines.a
No.
1
Dicarbonyl compound
Amine
Product
Time (h)
8
Yieldb (%)
90c
Ref.
[10]
2
3
4
2
86
92
[25]
[10]
4
2
96
[10]
5
6
2
1
90
93
[24]
[10]
7
8
2
6
83
70
[24]
[24]
9
2
80
[26]
10
3
3
88
90
[24]
[10]
11
a
Reaction conditions: dicarbonyl compound (1 mmol), amine (1 mmol), Ag Np (0. 2 mmol), methanol (5 mL), and temperature (60 °C).
Yield based on GC analysis.
In this reaction catalyst was recycled for three consecutive recycles which showed the same activity performance.
b
c

K.D. Bhatte et al. / Catalysis Communications 11 (2010) 1233–1237
1237
Table 3
Acknowledgements
Recyclability of catalyst.a
Yieldb (%)
The financial support from UGC-Green Technology Center, Univer-
sity of Mumbai and collaborative support from DST-JSPS for Indo-
Japan project no. DST/INT/JAP/P-67/08 are gratefully acknowledged.
Entry
Run
1
2
3
4
Fresh
First
Second
Third
90
88
85
85
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In conclusion, we have developed an efficient protocol for synthesis
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a heterogeneous, recyclable and moisture stable catalyst. The reaction
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