VIETNAM JOURNAL OF CHEMISTRY VOL. 50(5) 601-608 OCTOBER 2012
MICROWAVE-ASSISTED SUZUKI REACTION USING PALLADIUM COMPLEX IMMOBILIZED ON SBA-15 AS AN EFFICIENT
CATALYST
Phan Thanh Son Nam, Nguyen Thi Quynh Ngoc Ho Chi Minh City University of Technology, Vietnam
Received 14 January 2012 Abstract
Highly ordered mesoporous silica SBA-15 was synthesized and functionalized with Schiff-base groups on the surface to form immobilized bidentate ligands. The fiinctionalized SBA-15 was complexed with palladium acetate, affording the immobilized palladium catalyst with a palladium loading of 0.49 mmol/g. The catalyst was characterized by X-ray powder diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), thermogravimctric analysis (TGA), Fourier transform infrared (FT-IR), nitrogen physisorplion measurements, and elemental analysis (EA). The catalyst was used as an efficient catalyst for the Suzuki cross-coupling reaction of iodobenzene and phenylboronic acid under microwave irradiation. The reaction rate was dramatically enhanced compared with that of the reaction under conventional heating. It was also observed that the modified SBA-15 catalyst could be ^cilely separated from the reaction mixture by centrifugation, and could be reused in subsequent reactions without significant degradation in activity.
Keywords: SBA-15, palladium, Suzuki reaction, microwave.
1. INTRODUCTION
Transition metal-catalyzed cross-coupling reactions have gained popularity over the past thirty years in organic synthetic chemistry, as they represent key steps in the building of more complex molecules from simple precursors [1]. A wide variety of cross- coupling methodologies have been developed to achieve the most powerful and useful tool for the elaboration of molecular architecture, in which the Suzuki reactions have attracted significant interests as convenient techniques for the formation of biaryl derivatives [2, 3]. These biaiyl units have exhibited practical applications in the production of phamiaceuticals, herbicides, as well as engineering materials such as conducting polymers and liquid crystals [3]. Catalysts used in the standard Suzuki processes are generally based on homogeneous palladium phosphine complexes, which are rarely recoverable widiout elaborate and wasteful procedures, and therefore commercially undesirable [4].
In this context, heterogeneous palladium catalysts have recently emerged as a greener alternative to homogeneous processes so that catalysts can be recovered and reused [5]. At the same time, the catalyst recovery also decreases contamination of the desired products with
hazardous or harmful heavy metals [5]. SBA-15 has emerged as one of the most common mesoporous silica catalyst supports. It is known to be a well- defined, hexagonal mesoporous silica material with straight mesopores that are connected through small micropores [6]. In this paper, we wish to report for the first time in Vietnam, to our best knowledge, the microwave-assisted Suzuki reaction of iodobenzene and phenylboronic acid to form biphenyl as the principal product, using palladium complex immobilized on SBA-15 as an efficient catalyst.
Almost quantitative conversion was achieved within 60 min by using microwave irradiation, compared to conversions obtained after 6h under conventional heating conditions.
2. EXPERIMENTAL
2.1. Materials and instrumentation
Chemicals were purchased from Sigma-Aldrich and Acros, and used as received without further purification. The synthesis of SBA-15 and diamino- functionaiized SBA-15 was carried out at the School of Chemical and Biomolecular Engineering, the Georgia Institute of Technology, USA. Fourier transform infrared (FT-IR) spectra were obtained on
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a Bruker TENSOR37 instrument. Scanning electron microscope (SEM) studies were performed on a JSM 740. Transmission electron microscope (TEM) studies were performed using a JEOL JEM 1400. A Netzsch Thermoanalyzer STA 409 was used for simultaneous thermal analysis combining thermogravimctric analysis (TGA) and differential thermal analysis (DTA). Nitrogen physisorplion measurements were conducted using a Micromeritics Chem BET 3000 system. X-ray powder diffraction (XRD) patterns were recorded using Cu Ka radiation source on a D8 Advance Bruker powder diffractometer. GC-MS analyses were performed using an Agilent GC-MS 6890. GC analyses were performed using a Shimadzu GC-17A equipped with a FID detector and a 30 m x 0.25 mm X 0.25 pm DB-5 column.
2.2. Synthesis of SBA-15 and amino- functionalized SBA-15
Mesoporous SBA-15 was synthesized using poly(elhylene oxide)-poly(propylene o.xide)- poly(ethylene oxide) (EO-PO-EO), 1,3,5- trimethylbenzene (TMB), and tetraethyl orthosilica (TEOS), according to a previously reported literature procedure [7]. Prior to functionallzation, the SBA-15 was dried under vacuum at 200°C for 3h and stored in a dry box. Diamino-functionalized SBA-15 was synthesized by stirring a toluene (30 ml) suspension of SBA-15 (1 g) and Af-[3-(trimethoxysilyl)propyl]- ethylenediamine (1 g) at room temperature for 24 h under an argon atmosphere. The solid was then filtered and washed with copious amounts of toluene, hexanes, methanol, and ether in a dry nitrogen glove box and dried under vacuum at room temperature overnight, yielding approximately I g of diamino-functionalized SBA-15.
2.3. Synthesis of palladium catalyst immobilized on SBA-15
The diamino-functionalized SBA-15 (0.80 g) was added to a round-bottom flask containing ethanol (99.5 %, 80 ml) and 2-acetyl pyridine (14 ml 0.125 mmol). The resulting mixture was then heated at reflux with rapid stirring for 32 h. After that, the reaction mixture was cooled to room temperature, centrifuged, washed with copious amounts of ethanol and n-hexane, and dried under vacuum at room temperature to yield the immobilized Schiff base (0.81 g). The immobilized Schiff base (0.7558 g) was then added to the round-bottom flask containing the solution of palladium acetate (0.1051
Phan Thanh Son Nam, et al.
g, 4.68 mmol) in acetone (80 ml). The mixture was then stirred vigorously at room temperature for 30 h.
The solid was then separated by centrifugation, washed with copious amounts of acetone and dried at room temperature to yield the immobilized palladium catalyst (0.7570 g).
2.4. Catalysis studies
Unless otherwise stated, a mixture of iodobenzene (0.12 ml, 1.08 mmol), phenylboronic acid (0.1976 g, 1.62 mmol), KjPO* (0.8628 g, 3.24 mmol), and hexadecane (0.12 ml) as the internal standard in dimethylformalmide (5 ml) were added lo a round-bottom flask containing the required amount of the immobilized palladium catalyst. The flask was heated in a modified household microwave oven (Sanyo-800W) at 800 W. Reaction conversions were monitored by withdrawing aliquots (0.1 ml) from the reaction mixture at different time intervals, and quenching with dilute Na2C03 solution. The organic components were extracted into diethylether, dried over Na^SO^ and analyzed by gas chromatography (GC) with reference to hexadecane.
Product identity was also further confirmed by gas chromatography - mass spectroscopy (GC-MS).
3. RESULTS AND DISCUSSION
Mesoporous SBA-15 was synthesized according to a literature procedure as previously reported, utilizing the triblock poly(ethylene oxide)- poly(propylene oxide)-poly(ethyIene oxide) (EO- PO-EO) nonionic surfactant as the structure- directing agent and 1,3,5-trimethylbenzene (TMB) as a swelling cosolvent [7]. The as-synthesized silica was then functionalized via the reaction of these silanol groups with Af-[3-(trimethoxysilyl)propyl]- ethylenediamine to create surface amino groups. The amino-functionalized SBA-15 was allowed to react with 2-acetyl pyridine to form an immobilized bidentate iminopyridine ligand, which was complexed with palladium acetate using a literature procedure previously reported by Clark and co- workers, affording the immobilized palladium complex catalyst (Scheme 1) [8]. The modified SBA-15 was characterized by X-ray powder diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), thermogravimctric analysis (TGA), Fourier transform infrared (FT-IR), nitrogen physisorplion measurements, and elemental analysis (EA), which were in good agreement with the literature [9].
VJCVol, 50(5), 2012 EO-PaEO
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Scheme I: Synthesis of the immobilized palladium catalyst on SBA-15
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Figure I: XRD patterns (a), TEM (b) and SEM (c) micrographs of the functionalized SBA-15 Low angle XRD profiles of the modified SBA-
15 exhibited reflections in the 29 range of 0.7-2°
attributable to 2D hexagonal symmetry (figure la).
The patterns were consistent with the literature with no impurity peak being observed in the XRD diffractogram [10]. Nitrogen physisorption measurements of the modified SBA-15 showed BET surface areas of 449 mVg. The TEM micrograph of the modified SBA-15 showed the honey-comb like structure, typical of an hexagonal array with highly
regular parallel layers [U] (Figure lb). The SEM micrograph revealed that the modified SBA-15 consisted of several rod-like domains with relatively uniform sizes of 2-3 |.im, which were aggregated into wheat-like macrostructures (figure lc). FT-IR spectra of the immobilized palladium catalyst showed an O-H stretching vibration due to physisorbed water and potentially surface hydroxyls near 3436 cm"', an 0-H deformation vibration near 1636 cm"', and an Si-0 sfretching vibration near
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1076 cm', respectively. The significant feature was the appearance of the peaks near 2950-3050 cm"' due to the -CH; and aromatic C-H stretching vibrations, and the presence of the imine C=N
Phan Thanh Son Nam, et al stretching vibration near 1558 cm"' (figure 2).
However, the FT-IR spectra exhibited little meaningful data due to the low loading of the palladium complex on the SBA-15.
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Figure 2: FT-IR spectrum of the immobilized palladium complex catalyst
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0.50 0.00 -0.50 0 Figure 3: TGA graph of the immobilized bidentate iminopyridine ligand TGA analyses of the amino-functionalized SBA-
15 and the immobilized bidentate iminopyridine ligand showed that 1.30 mmol/g of the diamine and 0.83 mmol/g of the Schifl" base (figure 3), respectively, were supported on the SBA-15. This indicated that approximately 64% of the surface amino groups were converted to the Schiff base. As expected, AAS analysis of the immobilized palladium complex catalyst exhibited a palladium loading of 0.49 mmol/g. It was previously reported that increasing the catalyst loading on the solid support to over 0.5 mmol/g could make a number of active sites inaccessible to the reactants [4]. As the
catalyst was designed for the Suzuki reaction where a base was required, it was unnecessary to block the free amino groups on the surface of the catalyst.
Indeed, it was previously reported that the presence of an amine could increase the stability of the palladium catalyst in the Heck and the Suzuki reactions [5]. However, the effect of free amino groups on the activity of the catalyst still needs further investigation.
The immobilized palladium complex catalyst was assessed for its activity initially in the Suzuki reaction between iodobenzene and phenylboronic acid to form biphenyl as the principal product
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(scheme 2). As DMF is normally the solvent of choice for several palladium-catalyzed cross- coupling reactions [4], it was decided to carry out the Suzuki reaction in DMF. The efficiency of microwave irradiation in accelerating organic transformations has recently been proven in several different fields of organic chemistry, in which reaction times can be dramatically reduced from
Microwave-assisted Suzuki reaction using...
days and hours to minutes [12-14]. Microwave- assisted chemistry is usually performed in high boiling polar solvents such as DMSO, NMP and DMF due to their high dipole moments [15]. The Suzuki reaction using the immobilized palladium catalyst was therefore carried out in a modified household microwave oven (Sanyo-800W) at 800 W.
B(0H)2 X I, Br. a
6
R H. O b , CXX>b [Rd]
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Scheme 2- The Suzuki reaction between aryl halides and phenylboronic acid
10 20 30 40 50 60 Time (min) Figure 4: Effect of different bases on reaction
conversions
10 20 30 40 50 60, Time (min) Figure 5: Effect of catalyst concentration on
reaction conversions It is generally accepted that a base is obviously
necessary to accelerate the transmetallation step in the catalytic cycle of the Suzuki reaction [5].
Therefore, the effect of base on the reaction conversion was investigated, using four bases including K2CO3., K3PO4, piperidine, and triethylamine. The most commonly used base in the Suzuki reaction is Na2C03 or K2CO3, but stronger bases such as NaOH, K3PO4 and Ba(0H)2 were previously reported to give better results in some cases [16]. In this research, however, the reaction using K2CO3 afforded the coupling product in a lower conversion compared to the reaction using K3PO4 as a base (figure 4). A conversion of 80% was observed for the case of K2CO3 after 60 min, while the reaction using K3PO4 proceeded with up to more than 99% conversion being achieved after only 20 min at the palladium concentration of 0.1 mol%.
Organic bases such as friethyl amine and piperidine exhibited less reactivity compared to K3PO4. Indeed, Spring and co-workers also reported similar effects of bases in the Suzuki reaction, where the combinafion of DMF as the solvent and K3PO4 as the
base exhibited dramatically better conversion than other bases [17].
With these results in mind, we therefore studied the effect of catalyst concentration on reaction conversions, using DMF as the solvent and K3PO4 as the base under microwave irradiation. As with previous reports, the higher the catalyst concentration was used, the higher the reaction rate was observed. Almost quantitative conversion of iodobenzene to biphenyl was achieved after 20 min at the palladium concentration of 0.1 mol% relative to iodobenzene. Decreasing the catalyst concentration resulted in a drop in reaction rate, with 99% conversion being obtained after 30 min at palladium concentration of 0.03 mol%. The reaction using 0.01 mol% catalyst proceeded with slower rate, with a conversion of 99% being achieved after 60 min (figure 5). The catalyst concenfrations used in this study were comparable to those of several previous reports covering different aspects of the Suzuki reaction, where the palladium concenfrations varied from less than 0.1 mol% to more than 1
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inol%, depending on the nature of the catalysts as well as the substrates [4-5].
As mentioned cariier, reaction rale is dramatically increased under microwave irradiation as compared to that of reaction under conventional heating condition [12-14]. The microwave-assisted Suzuki reaction was therefore compared with the conventional reaction using the immobilized palladium catalyst. The reaction was carried out in DMF, using 0.05 mol% catalyst, in the presence of
Phan Thanh Son Nam, et al.
K3PO4 as the base. It was found that 99% conversion was achieved after 1 h for the reaction at 120 "C, while the reaction at 100 °C and 80 °C required a reaction time of 3 h and 6 h, respectively, to reach 99% conversion (figure 6). It should be noted that the microwave-assisted reaction using 0.05 mol%
catalyst and K3PO4 as the base could afford 99%
conversion after only 20 min. Indeed, it was previously reported that microwave technology could be applied effectively for Suzuki reaction [18].
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Figure 6: The Suzuki reaction under conventional heating condition
In order to investigate the effect of different substituents on reaction conversions, the study was then extended to the reaction of substituted iodobenzenes containing electron-donating (i.e. 4- iodotoluene) and electron-withdrawing (i.e 4- iodoacetophenone) groups. It was observed that the reaction of 4-iodotoluene with phenylboronic acid proceeded with slower rate than the Suzuki reaction of iodobenzene, with a conversion of 72% being
iodobenzene 4-iodotolucnc
•l-iodoacctophcnonc
10 20 30 40 50 60 Time (min) Figure 7: Effect of substituents on reaction
conversions
observed after 20 min. As expected, the reaction rate of the Suzuki between 4-iodoacetophenone and phenylboronic acid was higher than the case of iodobenzene (figure 7). This result indicated that the microwave-assisted Suzuki reaction using the immobilized palladium catalyst was favoured by electron-withdrawing groups on benzene ring, while electron-donating groups slowed down the cross- coupling processes.
Pd(o). X I. Br. a R K 0 ^ . 0 0 0 ^ R R Scheme 3: The rate-determining oxidative step in the catalytic cycle of the Heck reaction It was also previously reported that the use of
electron-withdrawing substituents normally lead to enhanced reactivity in palladium-catalyzed cross- coupling reactions [4, 17]. The effect of substituents on reaction conversions of iodobenzene derivatives observed in this research was therefore in good agreement with the literature. This could be rationalized based on the fact that oxidative addition is normally a rate-limiting step in the catalytic cycle of transition metal-catalyzed cross-coupling
reactions [4, 5]. The very first step in the catalytic cycle of the Suzuki reaction is the reduction of palladium (II) to palladium (0) as the active catalytic species. The next step of the catalytic cycle is the oxidative addition of the palladium (0) to the aryl halide to form the palladium (II) complex (scheme 3), where elecfron-withdrawing groups on the benzene ring facilitate the process. The similar trend in elecfronic effect of substituents observed in this research could be rationalized based on the same
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reasons, However, a complete catalytic pathway for the Suzuki reaction using the immobilized palladium
Microwave-assisted Suzuki reaction using...
catalyst still remains to be elucidated in further investigation.
0 10 20 30 40 Time (min)
Figure 8: Effect of halide on reaction conversions Although the Suzuki reaction of iodoarene derivatives with phenylboronic acid is successful in most cases, several efforts have been devoted to the investigation on the cross-coupling of bromoarene and chloroarene [4, 5]. The reason for this trend is that iodoarene derivatives are normally significantly more expensive than bromoarenes, while chloroarenes require lowest cost and therefore they are the most desirable starting materials. However, chloroarenes are unreactive in most cases, though the Suzuki reactions of activated chloroarene (i.e.
containing strong electron-withdrawing groups) are usually successful by using special catalyst systems.
We therefore decided to investigate the Suzuki reaction of bromobenzene and chlorobenzene with phenylbomic acid, respectively, using the immobilized palladium catalyst. As expected, it was observed that the Suzuki reaction of bromobenzene proceeded slower compared with the case of iodobenzene, with a conversion of 93% being observed after 60 min. The Suzuki reaction of chlorobenzene proceeded with difficulty, though the reaction still afforded a conversion of over 33% after 60 min (figure 8).
An important point concerning the use of a heterogeneous catalyst is its lifetime, especially for the reactions using expensive precious metal catalysts. In the best case the catalyst can be recovered and reused before it eventually deactivates completely. At the same time, the catalyst recovery can also reduce the environmental pollution caused by heavy metals used in the catalyst system [19].
The immobilized palladium catalyst was therefore investigated for recoverability and reusability. After the reaction, the catalyst was separated from the reaction mixture by simple centrifugation, washed several times with acetone and hexane to remove any physisorbed reagents. The recovered catalyst was then dried and reused in further reaction under
Figure 9: Catalyst recycling studies identical condition to the first run. Experimental results showed that the catalyst could be reused in further reaction without significant degradation in activity (figure 9).
4.X:0NCLUSI0NS
In summary, ordered mesoporous silica SBA-15 was synthesized and functionalized with Schiff-base groups on the surface to form immobilized bidentate ligands. The functionalized SBA-15 was complexed with palladium acetate, affording the immobilized palladium catalyst with a palladium loading of 0.49 mmol/g. The catalyst was characterized by X-ray powder diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), thermogravimefric analysis (TGA), Fourier transform infrared (FT-IR), nitrogen physisorption measurements, and elemental analysis (EA). The immobilized palladium catalyst was used as an efficient catalyst for the Suzuki cross-coupling reaction of iodobenzene and its derivatives with phenylboronic acid to form biphenyls under microwave irradition. The reaction rate was dramatically enhanced compared with that of the reaction under conventional heating. The modified SBA-15 catalyst could be facilely separated from the reaction mixture by centrifugation, and could be reused in subsequent reactions without significant degradation in activity. Current research in our laboratory has been directed to the design and immobilization of several homogeneous catalysts on SBA-15 for a wide range of organic transformations, and tesults will be published in due course.
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Corresponding author. Phan Thanh Son Nam
Ho Chi Minh City University of Technology, Vietnam 268 Ly Thuong Kiet, 10 District, Ho Chi Minh City Email: [email protected] / [email protected].
