Conjugated polymer stabilized palladium nanoparticles as a versatile

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synthesis.17–19 The use of palladium nanoparticles in catalysis is not only industrially ... acquired using the green (514.5 nm) line of an argon ion laser.

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Conjugated polymer stabilized palladium nanoparticles as a versatile catalyst for Suzuki cross-coupling reactions for both aryl and heteroaryl bromide systems Rafique Ul Islam,a Michael J. Witcomb,b Michael S. Scurrell,c Elma van der Lingen,d Willem Van Otterloc and Kaushik Mallick*d Received 1st December 2010, Accepted 19th January 2011 DOI: 10.1039/c0cy00071j A simple and efficient procedure for Suzuki coupling of aryl bromides with phenylboronic acid, catalyzed by an in situ-generated palladium(0)–polymer composite in the absence of any phosphine ligand, has been reported. The catalyst is remarkably active having a high TOF value for both aryl and heteroaryl systems, and is recyclable up to three runs with minimum loss of efficiency.

Introduction Nanoparticles are of considerable interest in catalysis and as a result this topic has undergone tremendous growth over the past few years.1–3 It has been reported that supported nanosized metal catalysts with specific sizes and shapes exhibit improved catalytic performance over conventional catalysts.4–6 As a consequence, the incorporation of such metal nanoparticles in polymers has attracted much attention and research interest. Composite architectures of polymers and metal nanoparticles synergistically provide both useful functionality and mechanical integrity. Metal nanoparticles combined with a suitable conjugated polymer can form a unique composite both with interesting physical properties and potential applications. Such composites have been shown to exhibit various properties directly relevant to and of benefit to dielectrics, energy storage and catalytic activity.7 Many investigations have been published regarding the development of the incorporation of the metal nanoparticles into a polymer matrix. Three different approaches have been utilized to make such a composite material: (a) in situ preparation of the nanoparticles in the matrix,which is achieved either by the reduction of metal salts dissolved in the polymer matrix8–10 or by the evaporation of metals on a heated polymer surface;11 (b) polymerizing the matrix around the nanoparticles;12 (c) in situ formation of the polymer from its monomer and the formation of the metal nanoparticles from their ionic precursor.13 a

Department of Applied Chemistry, Birla Institute of Technology, Mesra, Ranchi-835215, India Microscopy and Microanalysis Unit, University of the Witwatersrand, Private Bag 3, WITS 2050, South Africa c Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, Private Bag 3, WITS 2050, South Africa d Advanced Materials Division, Mintek, Private Bag X3015, Randburg 2125, South Africa. E-mail: [email protected]; Fax: +27 (0)11 709 4480; Tel: +27 (0)11 709 4768 b

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The latter method is likely to provide a high degree of synthetic control over both the size of the nanoparticles and the morphology of the polymer matrix, which in turn may be expected to exert a strong influence on the metal–polymer interaction. Polymer stabilized noble metal nanoparticles have attracted much attention recently as a new research direction in catalysis.14–16 The polymer matrices serve both as the support as well as the stabilizer of the nanoparticles thus providing a mechanism to prevent aggregation. Palladium based catalysts, in particular nanoscale palladium particles, have recently drawn enormous attention due to their versatile role in organic synthesis.17–19 The use of palladium nanoparticles in catalysis is not only industrially important,20–22 but also scientifically interesting since they provide details of the sensitive relationship between the catalytic activity and the nanoparticle size and shape as well as the nature of the surrounding media.23 The well-known application of palladium nanoparticles as a catalyst in synthetic organic chemistry is in reactions involving carbon–carbon bond formation which are commonly used for the synthesis of natural products, pharmaceutical products, fine chemicals and the manufacturing of long chain organic molecules for organo-electronics applications. Among the various carbon–carbon bond formation reactions the Suzuki coupling reaction, also known as the Suzuki–Miyaura reaction, is one of the most important methods for the synthesis of biaryl products in a single step process.24 In this paper, we describe the synthesis method for a palladium–polymer hybrid material in which the metal nanoparticles are stabilized in a polymer matrix and the subsequent application of the nanocomposite to Suzuki coupling reactions as a catalyst. We have found that the hybrid material is very active as a catalyst in the absence of a phosphine ligand for Suzuki reactions both with regard to aryl and heteroaryl systems. This journal is

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Experimental Materials and instrumentation

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(A) Chemicals. p-Amino acetanilide (FW: 150.18) was purchased from BDH. Pd-acetate from Next Chimica was dissolved in toluene (Merck). All the other chemicals were purchased from Aldrich and used as received. (B) In situ synthesis of a Pd–poly(amino acetanilide) [Pd–poly(AA)] composite. p-Amino-acetanilide (0.307 g) was dissolved in methanol. Pd-acetate (1.510 3 mol dm 3, 12 ml) was added drop by drop to the p-amino-acetanilide in methanol solution. The solution turned to a dark brown colour, the precipitation occurring after the complete addition of the Pd-acetate. TEM specimens were prepared by pipetting 2 mL of the colloid solution onto lacey carbon coated copper grids. Subsequently, the TEM grids were sputter coated with a conducting layer of 15 nm thick Au–Pd and viewed in the SEM. A small portion of the colloidal sample was used for optical characterization. The solid sample was used as catalyst for the Suzuki–Miyaura coupling reaction. The amount of Pd present in the catalyst was quantified by the ICPMS technique which indicated that the metal loading in the sample was B1.5 wt%. (C) Procedure for Suzuki coupling reactions catalyzed by the Pd–poly(AA) composite. In a typical experiment, aryl halide (1.0 mmol), phenylboronic acid (1.5 mol), K2CO3 (1.5 mmol) and the catalyst, Pd–poly(AA), were added to the solvent (5 ml) in a small round bottom flask with a magnetic stirring bar. The reaction mixture was placed in an oil bath at 70–90 1C and stirred for 4.5–14 h depending on the aryl halides used. The reaction was monitored by thin layer chromatography (TLC) and GC. After completion of the reaction, the mixture was extracted three times utilizing ethyl acetate. At the end of the reaction, the reaction mixture was cooled, diluted with Et2O, filtered through a pad of silica gel with copious washings and purified by flash chromatography on silica gel. (D) Characterization techniques. For UV–vis spectra analysis, a small portion of the solid sample was dissolved in methanol and scanned within the range 300–800 nm using a Varian, CARY, 1E, digital spectrophotometer. Raman spectra were acquired using the green (514.5 nm) line of an argon ion laser as the excitation source. Light dispersion was carried out via the single spectrograph stage of a Jobin–Yvon T64000 Raman spectrometer. Power at the sample was kept very low (0.73 mW), while the laser beam diameter at the sample was B1 mm. SEM studies were carried out in a FEI FEG Quanta ESEM at 5 kV. The X-ray photoelectron spectra (XPS) were collected in a UHV chamber attached to a Physical Electronics 560 ESCA/SAM instrument. Transmission electron microscopy studies of the composite were carried out at an accelerating voltage of 197 kV using a Gatan GIF Tridiem on a Philips CM200 TEM equipped with a LaB6 source.

compounds by the oxidative polymerization of their monomer.28 According to the most widely accepted mechanism, the first step during the polymerization of aniline or substituted aniline involves the formation of a radical cation accompanied by the release of an electron. This step is the initiation process of the polymerization reaction. In the present work, palladium acetate was used as an oxidizing agent and during polymerization the released electrons reduced the Pd2+ ions to form palladium atoms. The coalescence of these atoms ultimately forms nanoparticles, which are encapsulated and stabilized by the polymer. Spectroscopic analysis confirmed that aniline or a substituted aniline oxidation product only has a head-to-tail (–N–Ph–N–Ph–)-like arrangement rather than a head-to-head (–Ph–N|N–Ph–) type orientation. Conjugated polymers are highly ordered, well-defined synthetic macromolecules that can be synthesized in the nanometre range. The most common method for the synthesis of a metal nanoparticle-conjugated polymer composite material is the ‘in situ polymerization and composite formation (IPCF)’ technique in which both the polymer and the nanoparticles are produced simultaneously so facilitating an intimate contact between both the particles and the polymer through functionalization (Fig. 1).13,25 An ideal composite material having such a nanolevel interaction might be expected to exhibit an improved performance with wider applications in various systems. The incorporation of metal nanoparticles into the conjugated polymers provides enhanced performance for both the ‘host’ and the ‘guest’, and this can lead to interesting physical properties and important potential applications.26 To exploit the full potential of such a technological application for such a composite material, it is important to characterize the different components of the composite material. Fig. 2A shows the Raman spectra recorded for poly-AA obtained by the chemical synthesis route in the presence of amino acetanilide and Pd-acetate. In the range between 1700 and 1000 cm–1, the bands are sensible to the poly-AA oxidation state. The spectrum reveals the C–C deformation bands of the benzenoid ring at 1610 cm–1, which are characteristic of the quinoid rings.27,28 The peak appearing at 1507 cm–1 corresponds to the N–H bending deformation band due to the interaction of the palladium particles with nitrogen, which also indicates the functionalization of the metal nanoparticles with the polymer through the nitrogen, a fact corroborating

Results and discussion A wide variety of methods have been applied to the preparation of conjugated polyaniline or substituted polyaniline-type This journal is

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Fig. 1 Polymer functionalized metal nanoparticle.

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Fig. 3 SEM image of the Pd–poly(amino acetanilide) composite revealing its three dimensional morphology.

Fig. 2 (A) Raman spectrum of the Pd–poly(amino acetanilide) composite material. (B) The UV–vis spectrum of poly(amino acetanilide) showing a sharp absorption band with a peak position at 345 nm due to the p–p* transition of the benzenoid rings. A shoulder-like appearance within the range between 400 to 500 nm is due to the polaron–bipolaron transition.

with previous results.25 The typical Raman spectrum of the resultant polymer includes a band at 1470 cm–1, which corresponds to the C|N stretching mode of the quinoid units. A broad band with a doublet peak with the peak positions at 1212 and 1227 cm–1 along with an individual peak at 1267 cm–1 can be assigned to the C–N stretching mode of the polaronic units. In the semiquinone radical units the C–N bond order is intermediate between those of amine ({C–Nz) and imine ({C|N–) groups. The peaks due to the C–N stretching deformations (1315 and 1367 cm–1) are therefore expected between the positions of the amine group peaks and imine group ones, i.e. in the spectral region 1300–1400 cm–l. Further, the position of the C–H benzene deformation mode is clearly visible at 1165 cm–1 indicating the presence of the quinoid rings. The electronic absorption spectrum of polyaniline and its derivatives (Fig. 2B) show three absorption peaks at 310–360, 400–440, and above 600 nm. The absorption peak at 310–360 nm is due to the p–p* transition of the benzenoid rings. The peak at 400–440 nm results from the polaron–bipolaron transition, whereas the absorption band above 600 nm is due to the benzenoid-to-quinoid excitonic transition. The polyaniline and its derivatives formed were either in the neutral or basic condition since the benzenoid–quinoid excitonic transition shows the absorption peak at lower wavelength region,29 whereas, a 310

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broad absorption band at higher wavelength region is observed when the polymer is synthesized under lower pH conditions. The shifting of the excitonic peak depends on various factors such as counter ions, solvent, chemical structure and the morphology of the polymer.30,31 In this study, the UV–vis spectrum of poly-AA (Fig. 2B) showed a sharp absorption band with a peak position at 345 nm due to the p–p* transition of the benzenoid rings. A shoulder-like appearance within the range between 400 to 500 nm results from the polaron–bipolaron transition. The benzenoid-to-quinoid excitonic transition is not obvious or suppressed in the UV–vis spectrum of the polymer possibly due to the high peak intensity caused by the p–p* transition. The SEM image in Fig. 3 shows that the resultant product consists of regular straight nanofibers up to 2.0 mm in length and of an average diameter of 100 nm. The fibers are almost identical in morphology and very straight suggesting a high level of rigidity. Fig. 4A displays a TEM image of the composite. This image was taken from a less dense area with some of the polymer fibers being less than 50 nm in diameter, indicated by the arrows. Higher magnification TEM images (Fig. 4B and C) show the high density distribution of dark spots in the polymer corresponding to the location of the palladium nanoparticles. Stereo images (not shown) clearly indicate that the B3 nm diameter nanoparticles are both encapsulated and well dispersed within the polymer matrix. A typical EDX spectrum (Fig. 5A) obtained from the electron beam being focused onto a dark spot in the polymer confirms that these spots are palladium particles. Focusing the beam between the dark spots near the thin edge of the polymer yielded no palladium X-ray peaks. The palladium particles were found to be homogeneously distributed within and throughout the polymer. Thus, we can envisage that the polymer acts as a cage and the Pd-nanoparticles are encapsulated and highly dispersed within that cage. Selected area diffraction (SAD) patterns indicated that the Pd nanoparticles were amorphous. To identify the chemical state of the polymer stabilized palladium particles, X-ray photoelectron spectroscopy (XPS) measurements were carried out. The Pd 3d region of the XPS spectrum of the Pd–poly(AA) composite is given in Fig. 5B revealing the presence of both the Pd 3d5/2 and 3d3/2 peaks at binding energies of 335.83 and 341.05 eV This journal is

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Fig. 4 TEM images of the Pd–poly(amino acetanilide) composite. (A) An image taken from a less dense area revealing that some of the polymer fibers are less than 50 nm in diameter, indicated by the arrows. Higher magnification TEM images (B and C) showing the high density distribution of B3 nm diameter dark spots in the polymer corresponding to the location of the palladium nanoparticles.

respectively. These binding energy values are in accordance with those reported for metallic palladium.32 There was no evidence of Pd2+ in the XPS spectrum, which suggests that the palladium acetate was completely converted into metallic nanoparticles. The catalytic property of the polymer-encapsulated palladium nanoparticles for a carbon–carbon bond formation reaction was the subject of the present investigation. Among the various carbon–carbon bond formation protocols Suzuki cross-coupling is one of the important reactions from a standpoint of its versatile applications. The original recommendation for the Suzuki reaction involved a phosphine-based palladium catalyst, it being commonly believed that one role of the phosphine ligand was the formation of catalytically active zero-valent palladium species. Recently, there has been considerable interest in the development of new catalysts that are environmentally benign and efficient. The coupling reaction under phosphine-free conditions is a topic of interest resulting from both economic and environmental considerations. Again, for the success of Suzuki reaction the presence of a base in the reaction media is very important. In this study, a series of various bases (K2CO3, Na2CO3, K3PO4, Cs2CO3, KF and NaOAc) have been utilized in order to determine the This journal is

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optimum condition of the reactions. Among them, K3PO4 was found to be highly efficient for the Suzuki coupling reaction. We have investigated the Suzuki reaction for the coupling of aryl bromides with phenylboronic acid in the presence of a Pd–polymer hybrid catalyst (Table 1). High catalytic activity for both the deactivated and activated aryl bromides was observed with the formation of the corresponding biphenyl compounds with excellent yields. Aryl bromides with electronwithdrawing groups (entries 1–3) showed slightly higher reactivity than those possessing electron-donating groups (entries 4–6). It was found that the turnover frequency (TOF) was as high as 3343 h 1 for the coupling of 4-nitro bromobenzene with phenylboronic acid (entry 2). When the electron-donating group (–OCH3) was attached at the para position of the bromobenzene the TOF value was found to be as high as 2744 h 1 for the coupling with phenylboronic acid (entry 5). Again, due the presence of the amino group the TOF value dropped to 1496 h 1 with a yield of 62% for the coupled product (entry 6). A higher yield was achieved at a higher Pd loading (0.9 mol%) for a longer reaction time, but the TOF value further decreased to 874 h 1 (entry 7). The Suzuki reaction was again tested for the derivatives of phenylboronic acids (entries 8, 9). The results revealed that the Catal. Sci. Technol., 2011, 1, 308–315

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View Online Table 1 Suzuki coupling reaction between aryl bromides and substituted/unsubstituted phenylboronic acidsa

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Entry Aryl bromide

Fig. 5 (A) EDX spectrum derived from placing the focused electron beam onto a single dark spot in Fig. 4C, clearly showing the presence of palladium. The copper peak comes from the TEM copper mesh support grid. (B) Palladium 3d XPS spectrum of the palladium–polymer composite. The peaks at binding energies at 335.55 eV for 3d5/2 and 340.85 eV for 3d3/2 are indicative of metallic palladium.

Pd–polymer hybrid material acted as a very competent catalyst for the coupling reaction of aryl bromides with different phenylboronic acids. Aryl bromides with an electron donating group (–CH3) also reacted efficiently with phenylboronic acid substituted with an electron-withdrawing group (–NO2) and produced the biaryl product with a TOF value of 2684 h 1 (entry 8). The coupling between the electron withdrawing nitro-substituted aryl bromides with a derivative of phenylboronic acid (para substituted with electron donating methoxy group, –OCH3) produced biaryl products (entry 9) with a good yield and a TOF value of 3250 h 1. The coupling between 2,6-dimethylbromobenzene and phenylboronic acids was not as effective as in the above mentioned examples due to the presence of two methyl groups next to the bromide group (entry 9). The sterically hindered bromide group of 2,6-dimethylbromobenzene efficiently coupled with phenylboronic acids with higher yields at elevated temperatures over a relatively longer time span. The presence of two methyl groups at the ortho positions restricts the access of phenylboronic acid, but at higher temperatures due to thermal agitation the reactant species has a greater opportunity to come closer so causing a higher yield at elevated temperatures. At 70 1C the yield of formation of the coupled product was low (yield = 48% and TOF = 965 h 1), but at higher temperatures (120 1C) in the presence of the same amount of catalyst (0.045 mol% of Pd) the formation of the coupled product was relatively fast (10 h) with an increased yield (69%) and an enhanced TOF value (2316 h 1). From the results it can be concluded that the thermal energy was responsible for the enhancement of the TOF value when other parameters remained the same for the two similar kind of reactions. When only 312

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PhBr(OH)2

Time/h Yield (%)

1

7.0

96

2

7.0

97

3

8.0

94

4

8.0

89

5

8.0

91

6

10.0

62

7

12.0

87

8

8.0

89

9

7.0

93

10

12.0

48

a

Reaction conditions: Bromobenzene (1.0 mmol), phenylboronic acid and derivatives (1.5 mmol), K2CO3 (1.5 mmol), toluene (5 ml). Entry 7, catalyst concentration was 0.9 mol% Pd.

palladium acetate, the precursor we used for the synthesis of polymer encapsulated palladium nanoparticles, was used as the catalyst the formation of the bi-aryl product was only achieved at higher temperatures, but with a lower yield. The Suzuki coupling reaction is generally performed in an organic solvent or in combination with a water–organic system as a solvent. The use of water as a solvent has attracted much attention due to increasing environmental awareness in recent years. We have tested the coupling of 4-bromonitrobenzene with phenylboronic acid in pure water and biphasic toluene–water system as a solvent for a model reaction (Table 2). The first set of reactions was performed using 0.045 mol% of Pd as a catalyst and K3PO4 as the base for only water as a solvent. A good conversion with high TOF value (B2000 h 1) was achieved at 90 1C (entry 1). In the case of the water– toluene system, a higher yield (91%, TOF = 3136 h 1) was achieved at relatively lower temperature and time (entry 2). When a phase transfer agent (PTA), tetrabutylammonium bromide (TBAB) (entries 3 and 4), was used in either water This journal is

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View Online Table 2 The effect of organic and aqueous solvents in the presence of a phase transfer agent on the Suzuki coupling reaction between 4-bromo nitrobenzene and phenylboronic acida

Entry Solvent 1 2 3 4

Temp./1C PTA

Water 90 Water–toluene (1 : 1) 70 Water 70 Water–toluene (1 : 1) 70

Time/h Yield (%)

— 10 — 7 TBAB 6 TBAB 6

81 91 93 95

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a

Nitrobromobenzene (1.0 mmol), phenylboronic acid (1.5 mmol), K2CO3 (1.5 mmol), solvent (5.0 ml), phase transfer agent (PTA) (0.5 mmol)

(entry 3) or toluene–water (1 : 1) (entry 4) solvent system an enhanced reactivity was observed with TOF values of 3740 and 3819 h 1 respectively. The coupling reaction using water as a solvent in the presence of a phase transfer agent thus was found to be very efficient within a short reaction time. The kinetics of the Suzuki reaction in the presence of a metal–polymer hybrid catalyst was further studied. Electrondeficient 4-bromo nitrobenzene (Table 1, entry 2), unsubstituted aryl bromide (Table 1, entry 3) and electron-rich 4-bromo aniline (Table 1, entry 6) with phenylboronic acid were chosen to study the kinetics of the coupling reactions (Fig. 6). For a steady reaction and in order to achieve maximum yields of biaryls in a relatively long time (10 h), the coupling reactions were performed at 70 1C employing 0.045 mol% Pd. However, aryl bromides with an electron donating group (–NH2) showed a drop of conversion (a) in comparison with the aryl bromide (b) and substituted aryl bromide with an electron withdrawing group (–NO2) (c). At the beginning of the coupling reaction, the conversion of 4-bromoaniline was relatively slow, but at higher temperatures the reaction between 4-bromo aniline and phenylboronic acid took place at a comparatively faster rate. In contrast, the coupling reaction between 4-bromo nitrobenzene and phenylboronic acid was fast and more than an 80% of conversion was achieved within 6 h. After that, a

Fig. 6 Comparative kinetic study of the Suzuki reaction involving 4-bromo aniline (Table 1, entry 6) (a), unsubstituted aryl bromide (Table 1, entry 3) (b) and 4-bromo nitrobenzene (Table 1, entry 2) (c) with phenylboronic acid using the Pd–poly(AA) hybrid catalyst (0.045 mol% Pd) at 70 1C.

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fatigue nature was observed in the curve which is probably due to the deposition of a solid biaryl product on the surface of the palladium particles that hindered the access of the reactants in contact with the catalyst. The recyclability of the Pd–poly(AA) composite catalyst was also investigated for the coupling of 4-bromo nitrobenzene with phenylboronic acid. The recovery of the catalyst is a tedious technique due to the minute quantity of the material used for the reaction and the nature of the support of the catalyst. After the first run the product was extracted with ethyl acetate and the residual catalyst was reused twice under the same conditions. The results indicated that the used material was also active as a catalyst without a significant loss of catalytic performance. At the end of the third cycle, a yield of 76% was achieved with a slight enlargement of the particles resulting from inter-particle agglomeration within the catalyst. No evidence was found for the formation of palladium black. The gradual decrease of the yield was mainly due to the loss of catalyst during the washing. In every cycle, the fresh reaction mixture had lower amount of palladium in the catalyst thus affecting the yield, but it may not have influenced the TOF value, which is difficult to calculate at that stage of the process. Considerable effort has been made over time to develop more and more active catalysts for the Suzuki coupling reaction. Most of them have involved the simple aryl halides– phenylboronic acid system. The study of the coupling reaction involving the heteroaryl systems has been less popular. References to the Suzuki reaction on the coupling between heteroaryl halides with heteroaryl boronic acids are thus less common.33–37 Such methods involved a palladium catalyst in the presence of phosphine ligands. A ligand-free Suzuki coupling of heterocycle-containing substrates by homogeneous palladium catalyst has been reported using a strong base,38 but the coupling between heteroaryl halides and heteroaryl boronic acids was not very efficient. A ligand-free heterogeneous Pd/C-catalyzed Suzuki coupling reaction for the efficient synthesis of heterobiaryl derivatives has also been reported with higher palladium loading in the catalyst.39,40 The Pd–poly(AA) hybrid material was further extended for the coupling of different heteroaryl compounds as a catalyst in the presence of iso-propyl alcohol as a solvent and K2CO3 as a base. The development of a universal catalyst for the crosscoupling of the simple aryl system as well as the heteroaryl substrates would be highly advantageous particularly in the area of new drug development based research. The scope of the reactions using a range of heteroaryl bromides and phenylboronic acids is summarized in Table 2 (entries 1–5). The reactions between bromopyridines with substituted and unsubstituted phenylboronic acids formed coupled products with more than 80% yields (entries 1–3). 5-Bromopyrimidine coupled with phenylboronic acid produced a moderately good yield (entry 4). 2-Bromo 5-methylthiophene reacted with methoxy substituted phenylboronic acid and produced a coupled product with the yield of 63% at 7 h. The turnover frequency values were within the range of 1700–2000 h 1. The Suzuki reactions between the heteroaryl bromides and heteroaryl boronic acids produced coupled product with a moderately good yield (Table 3, entries 1–4). The Pd–poly(AA) hybrid material with a Pd loading of 0.147 mol% was applied Catal. Sci. Technol., 2011, 1, 308–315

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Table 3

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Entry

Suzuki coupling reaction between heteroaryl bromides with substituted/unsubstituted phenylboronic acidsa

Heteroaryl bromide

R

Product

Time/h

Yield (%)

1

4-CH3

5.0

81

2

H

4.5

81

3

4-NO2

4.5

83

4

H

5.0

72

5

4-OCH3

7.0

63

a

Heteroaryl bromide (1.0 mmol), phenylboronic acid and derivatives (1.5 mmol), K2CO3 (3.0 mmol), iso-propyl alcohol (5.0 ml).

Table 4

Entry

Suzuki coupling reaction between heteroaryl bromides with heteroaryl boronic acid (methyl-substituted thiopheneboronic acid)a

Time/h

Yield (%)

1

12

69

2

12

78

3

14

72

4

14

67

a

Heteroaryl bromide

Heteroaryl bronic acid

Product

Heteroaryl bromide (1.0 mmol), methyl-substituted thiopheneboronic acid (1.5 mmol), K2CO3 (5.0 mmol), iso-propyl alcohol (5.0 ml).

as a catalyst for the coupling of heteroaryl bromides with methyl-substituted thiopheneboronic acid. Bromo-pyrimidine, substituted bromo-thiophene, bromo-pyridine and bromoquinoline efficiently coupled with substituted thiopheneboronic acid in the presence of iso-propyl alcohol as a solvent and K2CO3 as a base at 90 1C. The kinetics for the heteroarylsystem followed the same trend as simple aryl-systems. 314

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The turnover frequency values were within the range of 500–850 h 1 for the coupling reactions between the heteroaryl bromides and the heteroaryl boronic acids (Table 4). The Pd–poly(AA) composite material performed well as a versatile catalyst for both aryl and heteroaryl systems. For the Suzuki cross-coupling reactions Pd(0) is the catalyst species and the possible mechanism is the interaction of aryl This journal is

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Scheme 1 Possible mechanism for the Suzuki coupling reaction.

halide (R1X) and Pd(0) to form the aryl–palladium halide complex [R1(Pd2+)X], which then couples with phenylboronic acid in the presence of a base to produce the [R1–(Pd2+)–R2] intermediate and finally produces the biaryl product (R1–R2) via the reductive elimination of Pd2+ to Pd(0) as outlined in Scheme 1. The TOF value [mol product/(mol catalyst)  hour] indicates that the polymer nanocomposite is a potential candidate for the Suzuki coupling reactions both for aryl and hetero-aryl systems.

Conclusions The present paper reports on the synthesis of an efficient polymer based metal–polymer composite catalyst for the Suzuki coupling reaction using the IPCF approach. The composite material served as an efficient and versatile catalyst for Suzuki coupling reactions both for aryl and hetero-aryl systems under a phosphine free condition, which has an enormous importance from environmental and economic points of view. The inventor of the coupling reaction, Professor Akira Suzuki, received the Noble Prize in Chemistry in 2010 in recognition of its potential magnitude and the wide range of applications. The composite catalyst was found to be very stable, stable in air and could be kept for several years without noticeable deactivation since the catalytically active species are encapsulated and protected from the air by a resistant polymer. We anticipate that this polymer based nanocomposite material should perform as a very promising candidate for other kinds of carbon–carbon coupling reactions.

References 1 A. Astruc, F. Lu and J. R. Aranzaes, Angew. Chem., Int. Ed., 2005, 44, 7852. 2 E. Moreno-Manas and R. Pleixats, Acc. Chem. Res., 2003, 36, 638.

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