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Silver nanoparticles as an efficient, heterogeneous and recyclable catalyst for synthesis of β-enaminones
Catalysis Communications 11 (2010) 1233–1237
Contents lists available at ScienceDirect
Catalysis Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c a t c o m
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
Article history: 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
a b s t r a c t 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.
1. Introduction 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. Metal nanoparticles have high surface to volume ratio and unresidual energy which are mainly responsible for their catalytic
activity in various different organic transformations [18]. Nanosize catalysts can act as a bridge between homogeneous and heterogeneous 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. 2. Experimental 2.1. General
⁎ Corresponding author. Tel.: +91 22 24145616; fax: +91 22 24145614. E-mail address: [email protected] (B.M. Bhanage). 1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2010.06.011
AgNO3 (A.R. grade, 99% pure) was purchased from Sigma Aldrich. All other chemicals with their highest purity were purchased from S.D.
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K.D. Bhatte et al. / Catalysis Communications 11 (2010) 1233–1237
Fig. 1. TEM of silver nanoparticles with SAED pattern inside. TEM scale corresponds to 100 nm.
Fine Chemicals Pvt. Ltd. Mumbai, India and were used without further purification. The conversion was based on GC analysis (Chemito 1000). All products were characterized by GC-Ms analysis (Shimadzu QP 2010). 2.2. Synthesis of silver nanoparticles 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
Fig. 2. Histogram of silver nanoparticles obtained with DLS.
Fig. 3. XRD pattern of silver nanoparticles.
absence of other impurities (see Fig. 4). UV–Vis spectroscopy of silver nanoparticles showed characteristic plasmon band at 420 nm (Fig. 5). 2.3. Typical procedure for synthesis of enaminones and enamino esters 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). 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. 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 Catalyst
Catalyst loading (mmol)
Temp (°C)
Yieldb(%)
Effect of catalyst 1 Methanol 2 Methanol 3 Methanol 4 Methanol 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 Traces
Effect of solvent 6 Ethanol 7 Acetonitrile 8 Toluene 9 Solvent free
Ag Np Ag Np Ag Np Ag Np
0.2 0.2 0.2 0.2
60 60 60 60
86 90 68 Traces
Effect of catalyst loading 10 Methanol 11 Methanol 12 Methanol
Ag NP Ag Np Ag Np
0.10 0.15 0.25
60 60 60
63 70 92
Effect of temperature 13 Methanol 14 Methanol 15 Methanol
Ag Np Ag Np Ag Np
0.2 0.2 0.2
r.t. 45 Reflux
55 70 84
Entry
Fig. 5. UV–Vis spectrum of silver nanoparticles.
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
Solvent
a Reaction conditions: acetyl acetone(1 mmol), aniline (1 mmol), solvent (5 mL), and time (8 h). b Yield is based on GC analysis.
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 temperature, 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 concentration 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, morpholine, 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 benzalacetone was also studied (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 Time (h)
Yieldb (%)
Ref.
1
8
90c
[10]
2
4
86
[25]
3
2
92
[10]
4
2
96
[10]
5
2
90
[24]
6
1
93
[10]
7
2
83
[24]
8
6
70
[24]
9
2
80
[26]
10
3
88
[24]
11
3
90
[10]
No.
a b c
Dicarbonyl compound
Amine
Product
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.
K.D. Bhatte et al. / Catalysis Communications 11 (2010) 1233–1237
1237
Acknowledgements
Table 3 Recyclability of catalyst.a Entry
Run
Yieldb (%)
1 2 3 4
Fresh First Second Third
90 88 85 85
The financial support from UGC-Green Technology Center, University of Mumbai and collaborative support from DST-JSPS for IndoJapan project no. DST/INT/JAP/P-67/08 are gratefully acknowledged. References
a
Acetyl acetone (1 mmol), aniline (1 mmol), Ag Np (0.2 mmol), and temperature (60 °C). b Yield based on GC analysis.
with aliphatic, aromatic and alicyclic amines under optimized reaction conditions (Table 2, entries 7–10). The bulky diketone, benzalcetone also reacted smoothly with butyl amine to furnish a product with a 90% yield (Table 2, entry 11). An important criterion for heterogeneous catalysis is the reusability of catalyst to make the process more economical which suggests us to study the recyclability of silver nanoparticles. The series of reaction cycles were ran in order to investigate the efficiency of the catalytic system for reaction 1 in Table 2. During each cycle the catalyst was separated by centrifugation method, and then used for the next reaction after washing with distilled water and ethanol. The catalyst revealed a remarkable activity and was reused up to three consecutive cycles without any significant loss in catalytic activity (Table 3). 4. Conclusion In conclusion, we have developed an efficient protocol for synthesis of β-enaminones and β-enamino esters using silver nanoparticles as a heterogeneous, recyclable and moisture stable catalyst. The reaction was optimized with respect to various parameters and could be employed for the condensation of different dicarbonyl compounds and amines. The advantages offered by this protocol include high yields of desired products, under ambient conditions with diverse substrate compatibility making it an important supplement to the existing methods.
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Contents lists available at ScienceDirect
Catalysis Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c a t c o m
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
Article history: 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
a b s t r a c t 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.
1. Introduction 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. Metal nanoparticles have high surface to volume ratio and unresidual energy which are mainly responsible for their catalytic
activity in various different organic transformations [18]. Nanosize catalysts can act as a bridge between homogeneous and heterogeneous 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. 2. Experimental 2.1. General
⁎ Corresponding author. Tel.: +91 22 24145616; fax: +91 22 24145614. E-mail address: [email protected] (B.M. Bhanage). 1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2010.06.011
AgNO3 (A.R. grade, 99% pure) was purchased from Sigma Aldrich. All other chemicals with their highest purity were purchased from S.D.
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K.D. Bhatte et al. / Catalysis Communications 11 (2010) 1233–1237
Fig. 1. TEM of silver nanoparticles with SAED pattern inside. TEM scale corresponds to 100 nm.
Fine Chemicals Pvt. Ltd. Mumbai, India and were used without further purification. The conversion was based on GC analysis (Chemito 1000). All products were characterized by GC-Ms analysis (Shimadzu QP 2010). 2.2. Synthesis of silver nanoparticles 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
Fig. 2. Histogram of silver nanoparticles obtained with DLS.
Fig. 3. XRD pattern of silver nanoparticles.
absence of other impurities (see Fig. 4). UV–Vis spectroscopy of silver nanoparticles showed characteristic plasmon band at 420 nm (Fig. 5). 2.3. Typical procedure for synthesis of enaminones and enamino esters 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). 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. 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 Catalyst
Catalyst loading (mmol)
Temp (°C)
Yieldb(%)
Effect of catalyst 1 Methanol 2 Methanol 3 Methanol 4 Methanol 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 Traces
Effect of solvent 6 Ethanol 7 Acetonitrile 8 Toluene 9 Solvent free
Ag Np Ag Np Ag Np Ag Np
0.2 0.2 0.2 0.2
60 60 60 60
86 90 68 Traces
Effect of catalyst loading 10 Methanol 11 Methanol 12 Methanol
Ag NP Ag Np Ag Np
0.10 0.15 0.25
60 60 60
63 70 92
Effect of temperature 13 Methanol 14 Methanol 15 Methanol
Ag Np Ag Np Ag Np
0.2 0.2 0.2
r.t. 45 Reflux
55 70 84
Entry
Fig. 5. UV–Vis spectrum of silver nanoparticles.
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
Solvent
a Reaction conditions: acetyl acetone(1 mmol), aniline (1 mmol), solvent (5 mL), and time (8 h). b Yield is based on GC analysis.
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 temperature, 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 concentration 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, morpholine, 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 benzalacetone was also studied (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 Time (h)
Yieldb (%)
Ref.
1
8
90c
[10]
2
4
86
[25]
3
2
92
[10]
4
2
96
[10]
5
2
90
[24]
6
1
93
[10]
7
2
83
[24]
8
6
70
[24]
9
2
80
[26]
10
3
88
[24]
11
3
90
[10]
No.
a b c
Dicarbonyl compound
Amine
Product
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.
K.D. Bhatte et al. / Catalysis Communications 11 (2010) 1233–1237
1237
Acknowledgements
Table 3 Recyclability of catalyst.a Entry
Run
Yieldb (%)
1 2 3 4
Fresh First Second Third
90 88 85 85
The financial support from UGC-Green Technology Center, University of Mumbai and collaborative support from DST-JSPS for IndoJapan project no. DST/INT/JAP/P-67/08 are gratefully acknowledged. References
a
Acetyl acetone (1 mmol), aniline (1 mmol), Ag Np (0.2 mmol), and temperature (60 °C). b Yield based on GC analysis.
with aliphatic, aromatic and alicyclic amines under optimized reaction conditions (Table 2, entries 7–10). The bulky diketone, benzalcetone also reacted smoothly with butyl amine to furnish a product with a 90% yield (Table 2, entry 11). An important criterion for heterogeneous catalysis is the reusability of catalyst to make the process more economical which suggests us to study the recyclability of silver nanoparticles. The series of reaction cycles were ran in order to investigate the efficiency of the catalytic system for reaction 1 in Table 2. During each cycle the catalyst was separated by centrifugation method, and then used for the next reaction after washing with distilled water and ethanol. The catalyst revealed a remarkable activity and was reused up to three consecutive cycles without any significant loss in catalytic activity (Table 3). 4. Conclusion In conclusion, we have developed an efficient protocol for synthesis of β-enaminones and β-enamino esters using silver nanoparticles as a heterogeneous, recyclable and moisture stable catalyst. The reaction was optimized with respect to various parameters and could be employed for the condensation of different dicarbonyl compounds and amines. The advantages offered by this protocol include high yields of desired products, under ambient conditions with diverse substrate compatibility making it an important supplement to the existing methods.
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