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Efficient clay supported Pd nanoparticles as heterogeneous catalyst for arylation of alkenes
Catalysis Communications 21 (2012) 82–85
Contents lists available at SciVerse ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Efficient clay supported Pd nanoparticles as heterogeneous catalyst for arylation of alkenes Mehran Ghiaci a,⁎, Fatemeh Ansari a, Zahra Sadeghi b, A. Gil c a b c
Department of Chemistry, Isfahan University of Technology, Isfahan 8415683111, Iran Payam-Nor University, Isfahan, Iran Department of Applied Chemistry, Los Acebos Building, Public University of Navarra, Campus of Arrosadia, 31006-Pamplona, Spain
a r t i c l e
i n f o
Article history: Received 6 January 2012 Received in revised form 31 January 2012 Accepted 1 February 2012 Available online 13 February 2012
a b s t r a c t A new pincer-type ligand containing Pd(II) was immobilized on a modified bentonite. The material was evaluated as catalyst in the arylation of iodobenzene and several alkenes under room conditions. The catalyst combines high activity and selectivity, and can be reused three times with no observed deactivation. © 2012 Elsevier B.V. All rights reserved.
Keywords: Amine modified bentonite Palladium catalyst Alkenes arylation
1. Introduction
2. Experimental section
The arylation of alkenes under a palladium catalyst is referred as the Heck reaction. This reaction has become an important tool in organic synthesis [1–3]. The homogeneous palladium-based catalysts suffer from low stability and high costs, which prevent their application in industrial processes. As a metal, palladium is also highly undesirable as a contaminant of pharmaceutical products. In efforts to develop a heterogeneous catalyst for the Heck reaction, palladium salts or its complexes have supported on materials such as active carbon [4,5], mesoporous silica [6], inorganic oxide [7,8], molecular sieves [9], polymers [10], zeolites [11], hydrotalcites [12,13], and clays [14,15]. Our interest is the use of a modified clay material as a catalyst for the arylation of olefins. For that, we report the immobilization of a nonionic surfactant with multiple N groups on a bentonite modified with a cationic surfactant and the use of this material as support for the immobilization of palladium acetate. The amino groups in the surfactant were designed to coordinate with Pd(ΙΙ) and additional amino groups could be varied in order to improve the performance of the complexes in catalytic reactions and prevention of palladium leaching. The new catalyst reported in this work does not require addition of phosphins, commonly introduced in to palladium catalyzed reactions, and exhibit high activity toward the Heck reaction of iodobenzene with olefins with small amount of palladium.
2.1. Preparation of immobilized palladium (II) on a modified bentonite
⁎ Corresponding author. E-mail address: [email protected] (M. Ghiaci). 1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2012.02.001
Synthesis, characterization, and immobilization of 3,3′(dodecylazanediyl)bis(N-(2-(2-aminoethylamino)ethyl)propanamide) (DAEP) on a modified bentonite have been described in a previous work [16]. The modified bentonite at this stage has a layer of cationic surfactant (cetyl pyridinium) and a layer of DAEP. With the multiple amine groups on the surface of the bentonite, a suitable pincer-type ligand for immobilization of palladium nanoparticles have been designed (see Scheme 1). In a typical experiment to prepare Pd(II)-bentonite composite, 0.5 g of the modified bentonite (DAEP-bentonite) was dispersed in 100 cm 3 of n-hexane to which 10 − 4 dm 3 of 10 − 2 mol/dm 3 of a aqueous solution of palladium acetate were added in ten steps to the suspension during 72 h, under vigorous stirring in a Morton flask. The resulting product was separated via centrifugation, and washed few times with distilled water. The solid was dried under vacuum at room temperature, before characterization and catalytic activity measurements. The amount of palladium in the catalyst was determined by ICP-AES method. The maximum palladium load achieved was 164 μmol/gcatalyst. 2.2. Characterization techniques X-ray diffraction (XRD) patterns were recorded by using a Siemens D-500 diffractometer, at 40 kV and 30 mA, and employing filtered Cu Kα radiation over a 2θ range between 5 and 80°. Scanning electron micrographs were obtained using a Cambridge Oxford 7060 Scanning Electron Microscope (SEM) connected to a
M. Ghiaci et al. / Catalysis Communications 21 (2012) 82–85
83
Scheme 1. Modification of the clay surface with a surfactant and impregnation of Pd particles.
four-quadrant backscattered electron detector with resolution of 1.38 eV. The samples were dusted on a double sided carbon tape placed on a metal stub and coated with a layer of gold to minimize charging effects. FTIR spectra of the catalyst were recorded on a JASCO FTIR 680 plus spectrometer with the KBr pellet method.
the support with palladium, the interlayer space increased to 13.21 Å, which probably means that some of the palladium ions with their water shells penetrated into the bentonite layers.
2.3. Arylation of alkenes with iodobenzene
The SEM images of the modified bentonites with or without palladium nanoparticles are shown in Fig. 1. Palladium nanoparticles had a strong influence on the morphology of the support. The Na-bentonite after modification with cetyl pyridinium bromide has found a pack type aggregation of the particles [18]. This packed structure completely changed when it was modified with DAEP in the next step (Fig. 1B). The hydrophilic nature of the head of DAEP seems to
The reactions were carried at 130 °C in a sealed reactor immersed in a thermo-stated bath. A typical reaction run was as follows: 0.05 g of catalyst, 1 mmol of iodobenzene, 2 mmol of alkene, 2 mmol of base (NaOAc or triethylamine) and 3 cm 3 of DMF. The reaction mixture was heated to the desired temperature and stirred for 20–24 h. When the reaction was completed, the reaction mixture was centrifuged and the reaction products were analyzed by gas chromatography (Agilent, model 6890, equipped with a wide bone OV-17 capillary column and a FID detector). The products were identified by GC–MS (Fisons Instruments, model 8060).
3.3. SEM
3. Results and discussion 3.1. Synthesis of the catalyst DAEP-dendrite was immobilized on a modified bentonite starting from bentonite with monolayer cetyl pyridinium cation coverage. This process was monitored by the adsorption isotherm of DAEP on modified bentonite. The FTIR spectrum of the catalyst contains peaks corresponding to the primary amino groups of the dendrite which appear at 3390 cm − 1 and 3344 cm − 1, and shifted to 3358 cm − 1 and 3300 cm − 1 after complexation with metal [16]. The electronic spectrum of the complex showed a broad band at 450 nm, which may be due to charge transfer transition, and a band at 650 nm due to first excited transition [16]. 3.2. X-ray diffraction analysis The interlayer spacing of the original bentonite (Na-bentonite) is 12.3 Å [17], which has changed to 12.95 Å after adsorption of cationic surfactant, cetyl pyridinium bromide. However, by adsorption of the pincer-type surfactant (DAEP) onto the monolayer modified bentonite, the interlamellar distances did not change to a meaningful value (13.0 Å). Therefore, it was presumed that DAEP molecules adsorbed on the external surface of the monolayer modified bentonite and edges of the layers, and could not penetrate into the interlamellar spaces of the bentonite. Also it should be mentioned that by loading
Fig. 1. SEM images of (A) Na-bentonite; (B) DAEP-bentonite with Pd(II).
84
M. Ghiaci et al. / Catalysis Communications 21 (2012) 82–85
change the porosity of the support by allowing it to adsorb more water. These changes were continued by loading the modified bentonite with palladium nanoparticles.
Table 2 Comparison of the data obtained in the present work with other studies in the literature. Entry
Haloarene
Alkene
Conversion (%)
1
Iodobenzene (0.5 mmol) Bromobenzene (1 mmol) Iodobenzene (1 mmol) Iodobenzene (1 mmol), Iodobenzene (1 mmol) Iodobenzene (0.5 mmol) Iodobenzene (2.23 mmol) Iodobenzene (1 mmol) Iodobenzene (1 mmol) Iodobenzene (1 mmol) Iodobenzene (1 mmol)
Methyl acrylate (0.75 mmol) Styrene (1.4 mmol) Butyl acrylate (1.5 mmol) Methyl acrylate (3 mmol) Butyl acrylate (1.5 mmol) Acrylic acid (6 mmol) Butyl acrylate (3.14 mmol) Methyl acrylate (2 mmol) methyl acrylate (1.5 mmol) Methyl acrylate (1.5 mmol) Methyl acrylate (2 mmol)
3.4. The arylation of alkenes In order to test the activity of the catalyst, the Heck reaction of iodobenzene with several alkenes, active and deactivated alkenes were considered. The results are shown in Table 1. An important point about these reactions is the small amount of Pd loaded on the catalyst. This scale of Pd was enough to perform the reaction between iodobenzene and alkenes with high yield and excellent selectivity. The reaction time was long but the preference of the amount of Pd, made it suitable catalyst for using in sensitive systems toward metals, especially for pharmaceutical systems. As shown in Table 1, in most cases (entries 1–4, and 7), the selectivity is 100% and only trans alkenes were found. The reaction of methyl acrylate and iodobenzene is also carried out with 0.05 g catalyst (8.2 μmol Pd) at several reaction temperature and several times. Maximum conversion for this reaction was achieved at 130 °C after 20 h . In all cases, the stability of the catalyst allowed us to work with it under non inert atmosphere and high temperature (130 °C). The catalytic activity was also studied in water as solvent. For the reaction of iodobenzene with methylacrylate in H2O only 60% of conversion was achieved at 130 °C after 20 h. For comparison of the effect of several bases, the reaction of iodobenzene with alkenes using NaOAc or Et3N were also examined (Table 1). Et3N exhibited fairly more selectivity toward the main product, in the Heck reaction between methyl vinyl ketone and iodobenzene (Table 1, entries 5 and 6). In this reaction, the second product was 4-phenyl-2-butanone which is produced by reduction of the double bond. For other alkenes, activity and selectivity were the same for both bases. The catalyst was also tested to catalyze the Heck reaction of bromobenzene with alkenes. Clearly, aryl bromides are much less reactive than aryl iodides. The reaction between methyl acrylate and bromobenzene with even 0.1 g catalyst (16.4 μmol Pd) just had 11% of conversion, so this catalyst is not suitable for less reactive halobenzenes. The catalyst used in the present study, and the other catalysts used for the Heck reaction under various environmental conditions reported in the literature are compared in Table 2. When the amount
2 3 4 5 6 7 8 9 10 11
Pd (mol)
Reference
99
0.5
[1]
89
0.1
[2]
99
1
[3]
95
0.5
[4]
99
0.5
[5]
94.5
0.1
[6]
87
1
[7]
95
1
[8]
4.36 × 10
−7
[9]
100
9.33 × 10
−5
[10]
100
8.2 × 10− 6
95
Current work
of palladium used in this study is compared with those published in the literature, it can be observed that the catalyst employed here behaves in an outstanding way. After each experiment, the Pd in solution after removal of the solid catalyst by centrifugation has been examined. The Pd content in solution was not detectable even after five runs. It's possible to consider that the decrease in activity could be related to leaching of Pd from the active sites of the catalyst and reprecipitation of palladium on the surface of bentonite. For recycling study, reaction of methyl acrylate and iodobenzene in the presence of NaOAc in DMF with 0.05 g of catalyst (8.2 μmol Pd) was performed. After each run of reaction, the catalyst was recovered by simple centrifugation and subsequently washed with water followed with chloroform, then dried in the oven at 100 °C and reused. The catalyst retained the same activity after three reuse runs and after five run only lost 12% of its activity (Table 3).
Table 1 The Heck reaction between iodobenzene and several alkenes.a. Entry 1 2 3 4
Substrate O O
O O O O O O
Base
Pd (μmol)
Conversion (%)
Selectivity (%)
NaOAc
8.2
100
100
Products O O O
Et3N
8.2
100
100
O O
NaOAc
8.2
100
100
O
O
Et3N
8.2
100
100
O O
5
O
NaOAc
8.2
100
67b
Et3N
8.2
100
80b
NaOAc
0.164
98
100
NaOAc
0.164
93
85c
O
6 7 8 a b c
O
OH O
O
Reactions condition: 1 mmol of iodobenzene, 2 mmol of alkene, 2 mmol of base, 3 cm3 of DMF, 130 °C, 20 h. Second product was 4-phenyl-2-butanone. Second product was cis stilbene.
OH
M. Ghiaci et al. / Catalysis Communications 21 (2012) 82–85 Table 3 Recycling and reuse of the catalyst a. Run
1
2
3
4
5
Conversion/selectivity (%)
100
100
100
97
88
a
Reactions condition: 0.05 g of catalyst (8.2 μmol Pd); 1 mmol of iodobenzene; 2 mmol of methyl acrylate; 2 mmol of NaOAc; 3 cm3 of DMF; 130 °C, 20 h.
4. Conclusions The commercial success of Heck reaction depends on two main objectives: (i) the use of cheap starting materials such as aryl bromides and aryl chlorides as the aryl halide source; and (ii) the use of active and recyclable palladium catalyst that can compensate for the cost of transition metal [19,20]. In this work, small amount of palladium are required to perform the reaction of iodobenzene with several alkenes with high yield and selectivity. Acknowlegdments Thanks are due to the Iranian Nanotechnology Initiative and the Research Council of Isfahan University of Technology and Center of Excellence in the Chemistry Department of Isfahan University of Technology for supporting of this work. References [1] I.P. Beletskaya, A. Cheprakov, Chemical Reviews 100 (2000) 3009–3066. [2] N.J. Whitcombe, K.K. Hii, S.E. Gibson, Tetrahedron 57 (2001) 7449–7476. [3] V. Farina, Advanced Synthesis and Catalysis 346 (2004) 1553–1582.
85
[4] R.G. Heidenreich, J.G.E. Krauter, J. Pietsch, K. Köhler, J. Mol. Catal.A 182-183 (2002) 499-509. [5] S. Mukhopadhyay, G. Rothenberg, A. Joshi, M. Baidossi, Y. Sasson, Advanced Synthesis and Catalysis 344 (2002) 348–354. [6] B. Pawelec, R. Mariscal, J.L.G. Fierro, Applied Catalysis A: General 225 (2002) 223–237. [7] S.H. Choi, J.S. Lee, Journal of Catalysis 193 (2000) 176–185. [8] S.S. Pröckl, W. Kleist, M.A. Gruber, K. Köhler, Angewandte Chemie International Edition 43 (2004) 1881–1882. [9] C.P. Mehnert, D.W. Weaver, J.Y. Ying, Journal of the American Chemical Society 120 (1998) 12289–12296. [10] M.R. Buchmeiser, T. Schareina, R. Kempe, K. Wurst, Journal of Organometallic Chemistry 634 (2001) 39–46. [11] A. Corma, H. Garcia, A. Leyva, A. Primo, Applied Catalysis A: General 247 (2003) 41–49. [12] T.H. Bennur, A. Ramani, R. Bal, B.M. Chanda, S. Sivasanker, Catalysis Communications 3 (2002) 493–496. [13] B.M. Choudary, S. Madhi, N.S. Chowdari, M.L. Kantam, B. Sreedhar, Journal of the American Chemical Society 124 (2002) 14127–14136. [14] M. Poyatos, F. Marquez, E. Peris, C. Claver, E. Fernandez, New Journal of Chemistry 27 (2003) 425–431. [15] Y. Cui, L. Zhang, Journal of Molecular Catalysis A: Chemical 237 (2005) 120–125. [16] M. Ghiaci, Z. Sadeghi, M.E. Sedaghat, H. Karimi-Maleh, J. Safaei-Ghomi, A. Gil, Applied Catalysis A: General 381 (2010) 121–131. [17] M. Ghiaci, R.J. Kalbasi, H. Khani, A. Abbaspur, H. Shariatmadari, Journal of Chemical Thermodynamics 36 (2004) 707–713. [18] P. Praus, M. Turicova, S. Studentova, M. Ritz, Journal of Colloid and Interface Science 304 (2006) 29–36. [19] M. Schlosser, Organometallics in Synthesis: A Manual, John Wiley and Sons, New York, 2002, pp. 1205–1208. [20] A. Zapf, M. Beller, Topics in Catalysis 19 (2002) 101–109.
Contents lists available at SciVerse ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Efficient clay supported Pd nanoparticles as heterogeneous catalyst for arylation of alkenes Mehran Ghiaci a,⁎, Fatemeh Ansari a, Zahra Sadeghi b, A. Gil c a b c
Department of Chemistry, Isfahan University of Technology, Isfahan 8415683111, Iran Payam-Nor University, Isfahan, Iran Department of Applied Chemistry, Los Acebos Building, Public University of Navarra, Campus of Arrosadia, 31006-Pamplona, Spain
a r t i c l e
i n f o
Article history: Received 6 January 2012 Received in revised form 31 January 2012 Accepted 1 February 2012 Available online 13 February 2012
a b s t r a c t A new pincer-type ligand containing Pd(II) was immobilized on a modified bentonite. The material was evaluated as catalyst in the arylation of iodobenzene and several alkenes under room conditions. The catalyst combines high activity and selectivity, and can be reused three times with no observed deactivation. © 2012 Elsevier B.V. All rights reserved.
Keywords: Amine modified bentonite Palladium catalyst Alkenes arylation
1. Introduction
2. Experimental section
The arylation of alkenes under a palladium catalyst is referred as the Heck reaction. This reaction has become an important tool in organic synthesis [1–3]. The homogeneous palladium-based catalysts suffer from low stability and high costs, which prevent their application in industrial processes. As a metal, palladium is also highly undesirable as a contaminant of pharmaceutical products. In efforts to develop a heterogeneous catalyst for the Heck reaction, palladium salts or its complexes have supported on materials such as active carbon [4,5], mesoporous silica [6], inorganic oxide [7,8], molecular sieves [9], polymers [10], zeolites [11], hydrotalcites [12,13], and clays [14,15]. Our interest is the use of a modified clay material as a catalyst for the arylation of olefins. For that, we report the immobilization of a nonionic surfactant with multiple N groups on a bentonite modified with a cationic surfactant and the use of this material as support for the immobilization of palladium acetate. The amino groups in the surfactant were designed to coordinate with Pd(ΙΙ) and additional amino groups could be varied in order to improve the performance of the complexes in catalytic reactions and prevention of palladium leaching. The new catalyst reported in this work does not require addition of phosphins, commonly introduced in to palladium catalyzed reactions, and exhibit high activity toward the Heck reaction of iodobenzene with olefins with small amount of palladium.
2.1. Preparation of immobilized palladium (II) on a modified bentonite
⁎ Corresponding author. E-mail address: [email protected] (M. Ghiaci). 1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2012.02.001
Synthesis, characterization, and immobilization of 3,3′(dodecylazanediyl)bis(N-(2-(2-aminoethylamino)ethyl)propanamide) (DAEP) on a modified bentonite have been described in a previous work [16]. The modified bentonite at this stage has a layer of cationic surfactant (cetyl pyridinium) and a layer of DAEP. With the multiple amine groups on the surface of the bentonite, a suitable pincer-type ligand for immobilization of palladium nanoparticles have been designed (see Scheme 1). In a typical experiment to prepare Pd(II)-bentonite composite, 0.5 g of the modified bentonite (DAEP-bentonite) was dispersed in 100 cm 3 of n-hexane to which 10 − 4 dm 3 of 10 − 2 mol/dm 3 of a aqueous solution of palladium acetate were added in ten steps to the suspension during 72 h, under vigorous stirring in a Morton flask. The resulting product was separated via centrifugation, and washed few times with distilled water. The solid was dried under vacuum at room temperature, before characterization and catalytic activity measurements. The amount of palladium in the catalyst was determined by ICP-AES method. The maximum palladium load achieved was 164 μmol/gcatalyst. 2.2. Characterization techniques X-ray diffraction (XRD) patterns were recorded by using a Siemens D-500 diffractometer, at 40 kV and 30 mA, and employing filtered Cu Kα radiation over a 2θ range between 5 and 80°. Scanning electron micrographs were obtained using a Cambridge Oxford 7060 Scanning Electron Microscope (SEM) connected to a
M. Ghiaci et al. / Catalysis Communications 21 (2012) 82–85
83
Scheme 1. Modification of the clay surface with a surfactant and impregnation of Pd particles.
four-quadrant backscattered electron detector with resolution of 1.38 eV. The samples were dusted on a double sided carbon tape placed on a metal stub and coated with a layer of gold to minimize charging effects. FTIR spectra of the catalyst were recorded on a JASCO FTIR 680 plus spectrometer with the KBr pellet method.
the support with palladium, the interlayer space increased to 13.21 Å, which probably means that some of the palladium ions with their water shells penetrated into the bentonite layers.
2.3. Arylation of alkenes with iodobenzene
The SEM images of the modified bentonites with or without palladium nanoparticles are shown in Fig. 1. Palladium nanoparticles had a strong influence on the morphology of the support. The Na-bentonite after modification with cetyl pyridinium bromide has found a pack type aggregation of the particles [18]. This packed structure completely changed when it was modified with DAEP in the next step (Fig. 1B). The hydrophilic nature of the head of DAEP seems to
The reactions were carried at 130 °C in a sealed reactor immersed in a thermo-stated bath. A typical reaction run was as follows: 0.05 g of catalyst, 1 mmol of iodobenzene, 2 mmol of alkene, 2 mmol of base (NaOAc or triethylamine) and 3 cm 3 of DMF. The reaction mixture was heated to the desired temperature and stirred for 20–24 h. When the reaction was completed, the reaction mixture was centrifuged and the reaction products were analyzed by gas chromatography (Agilent, model 6890, equipped with a wide bone OV-17 capillary column and a FID detector). The products were identified by GC–MS (Fisons Instruments, model 8060).
3.3. SEM
3. Results and discussion 3.1. Synthesis of the catalyst DAEP-dendrite was immobilized on a modified bentonite starting from bentonite with monolayer cetyl pyridinium cation coverage. This process was monitored by the adsorption isotherm of DAEP on modified bentonite. The FTIR spectrum of the catalyst contains peaks corresponding to the primary amino groups of the dendrite which appear at 3390 cm − 1 and 3344 cm − 1, and shifted to 3358 cm − 1 and 3300 cm − 1 after complexation with metal [16]. The electronic spectrum of the complex showed a broad band at 450 nm, which may be due to charge transfer transition, and a band at 650 nm due to first excited transition [16]. 3.2. X-ray diffraction analysis The interlayer spacing of the original bentonite (Na-bentonite) is 12.3 Å [17], which has changed to 12.95 Å after adsorption of cationic surfactant, cetyl pyridinium bromide. However, by adsorption of the pincer-type surfactant (DAEP) onto the monolayer modified bentonite, the interlamellar distances did not change to a meaningful value (13.0 Å). Therefore, it was presumed that DAEP molecules adsorbed on the external surface of the monolayer modified bentonite and edges of the layers, and could not penetrate into the interlamellar spaces of the bentonite. Also it should be mentioned that by loading
Fig. 1. SEM images of (A) Na-bentonite; (B) DAEP-bentonite with Pd(II).
84
M. Ghiaci et al. / Catalysis Communications 21 (2012) 82–85
change the porosity of the support by allowing it to adsorb more water. These changes were continued by loading the modified bentonite with palladium nanoparticles.
Table 2 Comparison of the data obtained in the present work with other studies in the literature. Entry
Haloarene
Alkene
Conversion (%)
1
Iodobenzene (0.5 mmol) Bromobenzene (1 mmol) Iodobenzene (1 mmol) Iodobenzene (1 mmol), Iodobenzene (1 mmol) Iodobenzene (0.5 mmol) Iodobenzene (2.23 mmol) Iodobenzene (1 mmol) Iodobenzene (1 mmol) Iodobenzene (1 mmol) Iodobenzene (1 mmol)
Methyl acrylate (0.75 mmol) Styrene (1.4 mmol) Butyl acrylate (1.5 mmol) Methyl acrylate (3 mmol) Butyl acrylate (1.5 mmol) Acrylic acid (6 mmol) Butyl acrylate (3.14 mmol) Methyl acrylate (2 mmol) methyl acrylate (1.5 mmol) Methyl acrylate (1.5 mmol) Methyl acrylate (2 mmol)
3.4. The arylation of alkenes In order to test the activity of the catalyst, the Heck reaction of iodobenzene with several alkenes, active and deactivated alkenes were considered. The results are shown in Table 1. An important point about these reactions is the small amount of Pd loaded on the catalyst. This scale of Pd was enough to perform the reaction between iodobenzene and alkenes with high yield and excellent selectivity. The reaction time was long but the preference of the amount of Pd, made it suitable catalyst for using in sensitive systems toward metals, especially for pharmaceutical systems. As shown in Table 1, in most cases (entries 1–4, and 7), the selectivity is 100% and only trans alkenes were found. The reaction of methyl acrylate and iodobenzene is also carried out with 0.05 g catalyst (8.2 μmol Pd) at several reaction temperature and several times. Maximum conversion for this reaction was achieved at 130 °C after 20 h . In all cases, the stability of the catalyst allowed us to work with it under non inert atmosphere and high temperature (130 °C). The catalytic activity was also studied in water as solvent. For the reaction of iodobenzene with methylacrylate in H2O only 60% of conversion was achieved at 130 °C after 20 h. For comparison of the effect of several bases, the reaction of iodobenzene with alkenes using NaOAc or Et3N were also examined (Table 1). Et3N exhibited fairly more selectivity toward the main product, in the Heck reaction between methyl vinyl ketone and iodobenzene (Table 1, entries 5 and 6). In this reaction, the second product was 4-phenyl-2-butanone which is produced by reduction of the double bond. For other alkenes, activity and selectivity were the same for both bases. The catalyst was also tested to catalyze the Heck reaction of bromobenzene with alkenes. Clearly, aryl bromides are much less reactive than aryl iodides. The reaction between methyl acrylate and bromobenzene with even 0.1 g catalyst (16.4 μmol Pd) just had 11% of conversion, so this catalyst is not suitable for less reactive halobenzenes. The catalyst used in the present study, and the other catalysts used for the Heck reaction under various environmental conditions reported in the literature are compared in Table 2. When the amount
2 3 4 5 6 7 8 9 10 11
Pd (mol)
Reference
99
0.5
[1]
89
0.1
[2]
99
1
[3]
95
0.5
[4]
99
0.5
[5]
94.5
0.1
[6]
87
1
[7]
95
1
[8]
4.36 × 10
−7
[9]
100
9.33 × 10
−5
[10]
100
8.2 × 10− 6
95
Current work
of palladium used in this study is compared with those published in the literature, it can be observed that the catalyst employed here behaves in an outstanding way. After each experiment, the Pd in solution after removal of the solid catalyst by centrifugation has been examined. The Pd content in solution was not detectable even after five runs. It's possible to consider that the decrease in activity could be related to leaching of Pd from the active sites of the catalyst and reprecipitation of palladium on the surface of bentonite. For recycling study, reaction of methyl acrylate and iodobenzene in the presence of NaOAc in DMF with 0.05 g of catalyst (8.2 μmol Pd) was performed. After each run of reaction, the catalyst was recovered by simple centrifugation and subsequently washed with water followed with chloroform, then dried in the oven at 100 °C and reused. The catalyst retained the same activity after three reuse runs and after five run only lost 12% of its activity (Table 3).
Table 1 The Heck reaction between iodobenzene and several alkenes.a. Entry 1 2 3 4
Substrate O O
O O O O O O
Base
Pd (μmol)
Conversion (%)
Selectivity (%)
NaOAc
8.2
100
100
Products O O O
Et3N
8.2
100
100
O O
NaOAc
8.2
100
100
O
O
Et3N
8.2
100
100
O O
5
O
NaOAc
8.2
100
67b
Et3N
8.2
100
80b
NaOAc
0.164
98
100
NaOAc
0.164
93
85c
O
6 7 8 a b c
O
OH O
O
Reactions condition: 1 mmol of iodobenzene, 2 mmol of alkene, 2 mmol of base, 3 cm3 of DMF, 130 °C, 20 h. Second product was 4-phenyl-2-butanone. Second product was cis stilbene.
OH
M. Ghiaci et al. / Catalysis Communications 21 (2012) 82–85 Table 3 Recycling and reuse of the catalyst a. Run
1
2
3
4
5
Conversion/selectivity (%)
100
100
100
97
88
a
Reactions condition: 0.05 g of catalyst (8.2 μmol Pd); 1 mmol of iodobenzene; 2 mmol of methyl acrylate; 2 mmol of NaOAc; 3 cm3 of DMF; 130 °C, 20 h.
4. Conclusions The commercial success of Heck reaction depends on two main objectives: (i) the use of cheap starting materials such as aryl bromides and aryl chlorides as the aryl halide source; and (ii) the use of active and recyclable palladium catalyst that can compensate for the cost of transition metal [19,20]. In this work, small amount of palladium are required to perform the reaction of iodobenzene with several alkenes with high yield and selectivity. Acknowlegdments Thanks are due to the Iranian Nanotechnology Initiative and the Research Council of Isfahan University of Technology and Center of Excellence in the Chemistry Department of Isfahan University of Technology for supporting of this work. References [1] I.P. Beletskaya, A. Cheprakov, Chemical Reviews 100 (2000) 3009–3066. [2] N.J. Whitcombe, K.K. Hii, S.E. Gibson, Tetrahedron 57 (2001) 7449–7476. [3] V. Farina, Advanced Synthesis and Catalysis 346 (2004) 1553–1582.
85
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