J. Phys. Chem. C 2008, 112, 10827–10832
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Suzuki Reaction within the Core-Corona Nanoreactor of Poly(N-isopropylacrylamide)Grafted Pd Nanoparticle in Water Guanwei Wei, Wangqing Zhang,* Fei Wen, Yao Wang, and Minchao Zhang Key Laboratory of Functional Polymer Materials of Ministry of Education, Institute of Polymer Chemistry, Nankai UniVersity, Tianjin 300071, China ReceiVed: January 21, 2008; ReVised Manuscript ReceiVed: April 24, 2008
A nanoreactor of poly(N-isopropylacrylamide)-grafted Pd nanoparticle (Pd@PNIPAM) is proposed for the Suzuki reaction performed in the sole solvent of water. The Pd@PNIPAM nanoparticle has a core of Pd nanoparticle and a corona of poly(N-isopropylacrylamide) brushes. The Pd@PNIPAM nanoparticle can act as a nanoreactor for the Suzuki reaction since the grafted poly(N-isopropylacrylamide) brushes provide a nanoenvironment for guest molecules. Both hydrophilic and hydrophobic reactants can be enriched in the nanoreactor of Pd@PNIPAM, and therefore the Suzuki reaction within the nanoreactor is performed in water at room temperature or above the phase-transition temperature of the corona-forming brushes of poly(Nisopropylacrylamide). Besides, the nanoreactor of Pd@PNIPAM can be recycled due to the reversible phasetransition of the poly(N-isopropylacrylamide) brushes. 1. Introduction The Suzuki reaction of aryl halide with arylboronic acid is a fundamental transformation in modern organic synthesis and offers a powerful pathway for the construction of the aryl-aryl bond.1,2 The traditional Suzuki reaction generally employs homogeneous palladium catalyst in the presence of various ligands such as phosphanes,3 N-heterocyclic carbenes,4 oxime carbapalladacycle,5 imidazolium,6,7 and Schiffs bases.8 Since the initial investigation by Miyaura et al. in 1981,1 various efficient homogeneous palladium catalysts have been proposed, and now it is easy to perform the Suzuki reaction of various compounds such as unactivated aryl chlorides and hindered boronic acids,9,10 heteroaryl halides and heteroaryl boronic acids,11 alkenyl derivatives,12 and so forth at mild conditions. However, the drawbacks of these homogeneous palladium catalysts are apparent. For example, homogeneous palladium catalysts are usually not reusable and the products are frequently contaminated by the residual palladium or ligands. Besides, some ligands are usually expensive and toxic, and in large-scale applications they may be more costly than the noble metal itself.13 Transition metal nanoparticles are known to be effective catalysts for chemical transformations due to the high surfaceto-volume ratio. The common method to prepare transition metal nanoparticles involves the reduction of metal ions in the presence of stabilizers such as surfactants,14 functionalized polymers,15 dendrimers,16 block copolymer micelles,17 hydrogel,18 and nanoand microspheres.19 It has been reported that Pd nanoparticles are efficient catalysts for some Suzuki reactions in both aqueous and nonaqueous solvents.20–29 Our recent study also suggests that the catalyst of Pd nanoparticles loaded on polymeric colloid has the potential to combine the easy reuse of heterogeneous catalyst and the high efficiency of the homogeneous one.30,31 The use of water as solvent has important advantages in largescale processes due to nontoxic, nonflammable, and inexpensive features. However, when the Suzuki reaction catalyzed with palladium is performed in water, surfactants such as tetrabuty* Corresponding author. E-mail:
[email protected]. Phone: 8622-23509794. Fax:86-22-23503510.
lammonium bromide (TBAB) or polar organic cosolvent such as N,N-dimethylformamide (DMF) are usually needed to improve solvation of organic substrates in water.32–34 Furthermore, the activity of the Pd nanoparticles usually seems not as efficient as the homogeneous one. Thus, more effort should be made to design an efficent and reusable catalyst of Pd nanoparticles, and to conduce the Suzuki reaction in the sole solvent of water, although a few successful examples have been reported.30,31,34 Chemical conversion in a confined nanoreactor is of growing interest to chemists as it is expected to increase conversion efficiency. Up to now, various nanoreactors have been designed and used in catalysis.35 Recently, Yeung and Crooks found that the Heck reaction of unactivated aryl halide with butylacrylate within a dendritic nanoreactor could be achieved in a highly selective manner.36 Furthermore, Lee et al. found a way to catalyze the Suzuki reaction in water at room temperature.37 They achieved this by hosting the hydrophobic reactants in a nanoreactor of block copolymer micelles, which contain a hydrophobic core in which the Suzuki reaction takes place. These results suggest that chemical conversion within a nanoreactor has advantages such as high efficiency and high selectivity. Herein, we propose a core-corona nanoreactor of poly(N-isopropylacrylamide)-grafted Pd nanoparticle (Pd@PNIPAM) for the Suzuki reaction performed in the sole solvent of water. The Pd@PNIPAM nanoreactor is synthesized by grafting the thiol-terminated poly(N-isopropylacrylamide) brushes to the Pd nanoparticle as described elsewhere.38 The smart polymer of poly(N-isopropylacrylamide) (PNIPAM)39 is chosen as the grafting brushes for two reasons. First, the PNIPAM brushes grafted to the Pd nanoparticle can provide a nanoenvironment for reactants of both hydrophilic and hydrophobic aryl halides, which conduces the Suzuki reaction with high yield of biaryls in water at mild conditions. Second, the nanoreactor of Pd@PNIPAM can be recycled due to the reversible phase transition of the PNIPAM brushes.39,40 The catalysis demonstrates that the Suzuki reaction within the nanoreactor of Pd@PNIPAM can be efficiently performed in water at room temperature, especially for the Suzuki reactions of hydrophilic aryl halides with benzeneboronic acid.
10.1021/jp800741t CCC: $40.75 2008 American Chemical Society Published on Web 06/25/2008
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2. Experimental Section 2.1. Materials. The nanoreactor of Pd@PNIPAM was synthesized by grafting the thiol-terminated PNIPAM (Mn ) 5.7 × 103 g/mol, Mw/Mn ) 1.14) to Pd nanoparticles according to our previous publication.38 Benzeneboronic acid (>99%, Beijing Wisdom Chemicals Co., Ltd.), 4-bromophenol (>99%, Tianjin Guangfu Fine Chemical Research Institute), 4-bromoacetophenone (>99%, Alfa Aesar), iodobenzene (>98%, Alfa Aesar), 4-iodophenol (>99%, Shanghai Haiqu Chemical Co., Ltd.), 4-iodobenzoic acid (>99%, Shanghai Bangcheng Chemical Co., Ltd.), 4-iodobenzaldehyde (>99%, Shanghai Bangcheng Chemical Co., Ltd.), 4-iodoanisole (>99%, Shanghai Bangcheng Chemical Co., Ltd.), 4-bromobenzoic acid (>99%, Beijing Henye Fine chemical Co., LTD), 4-bromoanisole (>99%, Tianjin Chemical Company), and 2-bromopyridine (>98%, Alfa Aesar) were used as received. 2-Bromo-1-methylpyridinium iodide (BMPI) was synthesized as described elsewhere.41 Other analytical reagents were used as received. 2.2. Typical Procedures for the Suzuki Reaction. To a screw-capped vial with a side tube were added 2.0 mmol of aryl halide, 6.0 mmol of K2CO3, 3.0 mmol of benzeneboronic acid, and 6.0 mL of water. The mixture was degassed under nitrogen purge for 10 min at room temperature. Subsequently, 2.0 mL of predegassed colloidal dispersion of the Pd@PNIPAM nanoaprticles was added. The vial content was placed in a preheated oil bath at a given temperature and magnetically stirred under nitrogen. After the reaction was completed, the reaction mixture was cooled to room temperature by immediately immersing the vial in water (∼20 °C). Subsequently, the product was extracted from the reaction mixture with diethyl ether (3 × 20 mL), and then the organic phase was collected, washed with water, and concentrated. The resulting product was dried under vacuum at 40 °C, weighed, and analyzed by 1H NMR. For the substrates of 4-halogenated phenols and 4-halogenated benzoic acids, the reaction mixture was first acidified with 1 mol/L of HCl aqueous solution and then the product was extracted with diethyl ether. For the substrate of BMPI, the catalyst of Pd@PNIPAM was first separated by filtration at temperatures above 50 °C, then the aqueous phase was concentrated. Subsequently, 20 mL of methanol was added, the insoluble inorganic salts were discarded, and the methanol phase was concentrated. The resulting product was dried under vacuum at 40 °C, weighed, and analyzed by 1H NMR. 2.3. Recycling of the Pd@PNIPAM Catalyst. After extraction of the synthesized product with diethyl ether, the aqueous phase containing the Pd@PNIPAM catalyst was heated to a temperature above 50 °C. At this temperature, the Pd@PNIPAM catalyst existing as a fine floccule-like suspension was collected with great care. The collected catalyst was reused in another run of the Suzuki reaction of 4-bromophenol with benzeneboronic acid in water at 90 °C with the same amount of reactants and water being added to keep the same concentration of reactants as in the fresh run. 2.4. Characterization. Transmission electron microscopy (TEM) measurement was conducted by using a Philips T20ST electron microscope at an acceleration of 200 kV, whereby a small drop of the colloidal dispersion was deposited onto a piece of copper grid and then dried at room temperature under vacuum. The powder X-Ray Diffraction (XRD) measurement was performed on a Rigaku D/max 2500 X-ray diffractometer. The 1H NMR measurement was carried out on a UNITY PLUS400 NMR spectrometer with CDCl3 or dimethyl-d6 sulfoxide (DMSO) as solvent.
Figure 1. TEM image of the core-corona Pd@PNIPAM nanoparticles. Inset: Schematic structure of the core-corona Pd@PNIPAM nanoparticles.
3. Results and Discussion 3.1. Synthesis and Characterization of the Core-Corona Poly(N-isopropylacrylamide)-Grafted Pd Nanoparticles. The core-corona Pd@PNIPAM nanoparticles, the average size of which is about 3 nm (Figure 1), were synthesized by covalently grafting the thiol-terminated PNIPAM to Pd nanoparticles according to our previous publication.38 Both TEM and XRD measurements confirm formation of the Pd@PNIPAM nanoparticles. The Pd@PNIPAM nanoparticle has a core-corona structure as the schematic inset in Figure 1, where the Pd nanoparticle forms the core and the PNIPAM brushes form the corona. On the basis of the molecular weight Mn of the coronaforming PNIPAM brushes, 5.7 × 103 g/mol, the fully extended length of the PNIPAM brushes is calculated appropriately to be 16 nm. Thus, the PNIPAM corona provides the potential to form a nanoenvironment for guest molecules around the Pd nanoparticle core. However, if the PNIPAM brushes are soluble in aqueous medium, the guest molecules can easily access to or escape from the Pd nanoparticle core and no nanoenvironment forms at this condition. When the PNIPAM brushes become hydrophobic and collapse onto the Pd nanoparticle core, the nanoenvironment forms and therefore the core-corona Pd@ PNIPAM nanoparticles can act as a nanoreactor. It is wellknown that PNIPAM is a typical thermoresponsive polymer, which undergoes a coil-to-globule transition at the lower critical solution temperature (LCST) at about 32 °C.39 Our previous study shows that the PNIPAM brushes grafted on the Pd nanopartilces have a phase-transition temperature at about 33 °C in water.38 This suggests that the grafted PNIPAM brushes become hydrophobic and collapse on the Pd core, and therefore provide a nanoenvironment for hydrophobic guest molecules in water just by increasing the temperature above the phasetransition temperature of the PNIPAM brushes. When the core-corona Pd@PNIPAM nanoparticles are used as a nanoreactor for the Suzuki reaction, it is necessary that the reagents can penetrate the PNIPAM corona and access the Pd catalyst to arouse reaction. The driving force for the hydrophobic guest molecules to diffuse into the nanoreactor of Pd@PNIPAM is mainly ascribed to the hydrophobic-hydrophobic interaction. For the hydrophilic guest molecules such as phenol, benzoic acid, and their substitutes, the PNIPAM brushes have the ability to entrap them due to strong hydrogen bonding between the PNIPAM brushes and the hydrophilic guest molecules.42,43 The entrapment or diffusion of the guest reactants into the nanoen-
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SCHEME 1: Schematic Entrapping of Guest Molecules within the Nanoreactor of Pd@PNIPAM Due to the Hydrogen Bonding or Hydrophobic-Hydrophobic Interaction
vironment offers an extremely high concentration around the Pd nanoparticle as shown in Scheme 1; therefore the Suzuki reaction within the nanoreactor of Pd@PNIPAM is expected to be efficient in water at mild conditions. 3.2. The Suzuki Reaction within the Core-Corona Nanoreactor of Pd@PNIPAM in Water. To confirm the hypothesis proposed above, Suzuki reactions of several typical hydrophilic and hydrophobic aryl halides with benzeneboronic acid are carried out in water at temperature ranging from 25 to 90 °C. As discussed above, the hydrophilic substrates such as 4-bromophenol, 4-bromobenzoic acid, 4-iodophenol, and 4-iodobenzoic acid can be entrapped in the nanoenvironment of the PNIPAM brushes due to the strong hydrogen bonding at temperature whether below or above the phase-transition temperature (∼33 °C).42,43 Therefore, as shown in Table 1, all the coupling reactions of these hydrophilic aryl halides with benzeneboronic acid within the nanoreactor of Pd@PNIPAM are quite efficient and the yields of biaryls are higher than 70% at room temperature in 1 h (entries 1-4). Furthermore, the results also show that the coupling reactions at room temperature are almost as efficient as at high temperature up to 90 °C (entries 1-4). To the best of our knowledge, the turnover frequency (TOF) values for the four hydrophilic substrates, especially for the two aryl bromides, may be the highest for all Suzuki reactions performed in the sole solvent of water at room temperature.30,31,34 Just as expected, the Suzuki reactions of hydrophobic aryl halides with benzeneboronic acid at room temperature are unsatisfactory and the yields are relatively low (entries 5-9), since no nanoenvironment forms at this condition. For example, the Suzuki reaction of the electron-deficient and therefore active hydrophobic aryl halide of 4-iodobenzaldehyde with benzeneboronic acid just affords 35% yield of 4-biphenylcarboxaldehyde at room temperature in 2 h (entry 6). However, when temperature increases to 90 °C, the PNIPAM brushes collapse on the Pd core and provide a nanoenvironment for the hydrophobic substrates, and high yields of biaryls are achieved (entries 5-9). As described above, the highly efficient Suzuki reactions of 4-halogenated phenols and 4-halogenated benzoic acids with benzeneboronic acid in water at room temperature are ascribed to the entrapment of the hydrophilic reactants in the nanoreactor of Pd@PNIPAM. To confirm this hypothesis, the Suzuki reactions of 2-bromopyridine and BMPI, both of which are hydrophilic but cannot form a hydrogen bond with the PNIPAM brushes, are tested. Just as expected, the Suzuki reaction of 2-bromopyridine or BMPI in water is inefficient either at room temperature or at high temperatures up to 90 °C (entries 10 and 11), possibly since these hydrophilic reactants cannot be entrapped in the nanoreactor of Pd@PNIPAM.
Usually, the activity of aryl halides decreases in the order of I > Br > Cl and the electron-deficient aryl halide is generally more active than the electron-rich one.44,45 However, as shown in Table 1, the Suzuki reaction within the nanoreactor of Pd@PNIPAM in water seems to break the normal rule. In the present study, the Suzuki reaction of hydrophilic aryl halides with benzeneboronic acid within the nanoreactor of Pd@PNIPAM at 25-90 °C is much more efficient than that of the hydrophobic ones at 90 °C. We think the great difference in efficiency is partly ascribed to the mass-transfer of the aryl halides and the resultant biaryls between the nanoreactor of Pd@PNIPAM and the solvent of water. When a hydrophilic aryl halide is used, the hydrophilic aryl halide is entrapped in the nanoreactor until the mass-distribution balance between the nanoreactor and the solvent of water is reached. As for the resulting hydrophilic biaryl in the nanoreactor, it is also easy to diffuse through the nanoreactor into the solvent to reach mass-distribution balance, whereas it is much more difficult for the hydrophobic substrates and the resultant hydrophobic biaryls to diffuse from the nanoreactor into water even at high temperature. The high concentration of the hydrophobic biaryls enriched in the nanoreactor therefore decelerates the Suzuki reaction. Thus, we conclude that the efficiency of the Suzuki reaction within the Pd@PNIPAM nanoreactor is not only determined by the activity of the substrates, but also incumbent upon their hydrophilicity. To confirm the conclusion, a competition Suzuki reaction of a binary mixture of the hydrophobic 4-bromoanisole and the hydrophilic 4-bromophenol (1:1 by mole number) within the Pd@PNIPAM nanoreactor in water is studied. These two electron-rich aryl bromides are chosen since they are similarly active in a general Pd-catalyzed Suzuki reaction (see entries 8 and 9, Table 2). It is found that the competition Suzuki reaction affords a much higher yield of 4-hydroxybiphenyl than that of 4-methoxybiphenyl whether at room temperature or at 90 °C (see the Supporting Information). These confirm the abovementioned conclusion that the hydrophilicity of the substrates strongly affects the Suzuki reaction within the Pd@PNIPAM nanoreactor in water. 3.3. Comparison of Suzuki Reactions within the CoreCorona Nanoreactor of Pd@PNIPAM in Water and via the General Catalyst of Pd@PNIPAM in the Solvent Mixture of DMF/Water. To further explore the catalysis within the nanoreactor of Pd@PNIPAM, the Suzuki reaction performed in the solvent mixture of DMF/water (3:1 by volume) at 90 °C is studied. At these conditions, the PNIPAM brushes are soluble and no nanoenvironment forms,46 therefore the aryl halides and the resultant biaryls are easy to access to or go off the catalyst of Pd nanoparticles. As shown in Table 2 (entries 1-9), the activity of the aryl halides is not determined by the hydrophilic
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TABLE 1: Suzuki Reactions within the Pd@PNIPAM Nanoreactor in Watera
a Reaction conditions: aryl halide (2.0 mmol) and benzeneboronic acid (3.0 mmol, 1.5 equiv.), K2CO3 (6.0 mmol, 3 equiv), 8.0 mL of water containing 0.20 mol % Pd@PNIPAM nanoreactor. b Isolated yield and purity of the isolated product being confirmed by 1H NMR. c 0.050 mol % Pd catalyst being used. d No biaryl being detected by 1H NMR.
or hydrophobic nature, but generally obeys the normal rule that the activity of the aryl halides decreases in the I > Br order and the electron-deficient aryl halide is generally more active than the electron-rich one.44,45 Moreover, as shown in Table 2, the Suzuki reactions of hydrophobic aryl halides with benzeneboronic acid in the solvent mixture of DMF/water (entries 1-7) are almost as efficient as that in water, whereas for the hydrophilic aryl bromides (entries 8 and 9), the coupling reaction via the general catalyst of Pd@PNIPAM in the solvent mixture of DMF/water is much less efficient than those within the nanoreactor in water. These results possibly suggest that the high concentration of the hydrophilic aryl halides entrapped in the nanoreactor of Pd@PNIPAM may be the key to accelerate the Suzuki reaction, whereas for the hydrophobic aryl halides, in addition to the nanoreactor effect, the mass transfer of the hydrophobic reactants between the nanoreactor of Pd@PNIPAM and the solvent also exerts great influence on the Suzuki reaction.47
3.4. Kinetics of the Suzuki Reaction within the Nanoreactor of Pd@PNIPAM in Water. The kinetics of the Suzuki reaction in the nanoreactor of Pd@PNIPAM is further studied. Herein, the Suzuki reactions of the hydrophobic 4-bromoacetophenone and the hydrophilic 4-bromophenol with benzeneboronic acid in water are chosen as examples to study the kinetics of the coupling reaction within the nanoreactor of Pd@PNIPAM. To run the steady Suzuki reaction and achieve suitable yields of biaryls in a relatively long time such as 6 h, the Suzuki reactions of the hydrophobic 4-bromoacetophenone and the hydrophilic 4-bromophenol are performed at 80 °C employing 0.20 mol % Pd@PNIPAM and at 25 °C employing 0.050 mol % Pd@PNIPAM, respectively. As demonstrated above, the nanoreactor of Pd@PNIPAM forms at these conditions. As shown in Figure 2, the yield of 4-acetylbiphenyl in the initial 2 h (indicated by arrow) increases much faster than in the later period. At the beginning of the coupling reaction, the concentration of the substrate of 4-bromoacetophenone
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TABLE 2: The Suzuki Reactions via the Pd@PNIPAM Nanoparticles in the Solvent Mixture of H2O and DMF (1:3 by volume)a
a Reaction conditions: 4-bromophenol (2.0 mmol) and benzeneboronic acid (3.0 mmol, 1.5 equiv), K2CO3 (6.0 mmol, 3 equiv), 8.0 mL of solvent mixture of H2O and DMF (1:3 by volume) containing 0.20 mol % Pd catalyst. b Isolated yield and purity of the isolated product being confirmed by 1H NMR.
TABLE 3: Recycling of the Pd@PNIPAM Nanoreactor for the Suzuki Reaction Performed in Water at 90 °Ca run b
yield (%)
fresh use
1st recycle
2nd recycle
3rd recycle
96
89
77
67
a
Reaction conditions: 4-bromophenol (2.0 mmol) and benzeneboronic acid (3.0 mmol, 1.5 equiv), K2CO3 (6.0 mmol, 3 equiv), 8.0 mL of water containing 0.20 mol % Pd@PNIPAM nanoreactor, 90 °C. b Isolated yield and purity of the isolated product being confirmed by 1H NMR.
Figure 2. The kinetics of the Suzuki reaction of 4-bromoacetophenone (b) and 4-bromophenol (O) with benzeneboronic acid in the nanoreactor of Pd@PNIPAM. Reaction conditions for the Suzuki reaction of 4-bromoacetophenone: 2.0 mmol of 4-bromoacetophenone, 3.0 mmol of benzeneboronic acid, 6.0 mmol of K2CO3, 8.0 mL of water containing 0.20 mol % Pd catalyst, 80 °C. Reaction conditions for the Suzuki reaction of 4-bromophenol: 2.0 mmol of 4-bromophenol, 3.0 mmol of benzeneboronic acid, 6.0 mmol of K2CO3, 8.0 mL of water containing 0.050 mol % Pd catalyst, 25 °C.
within the nonoenvironment of Pd@PNIPAM is relatively high and more reactants access to the Pd nanoparticles, and therefore the coupling reaction at the initial reaction interval is fast as
shown in Figure 2. With the increase in time, the resulting hydrophobic 4-acetylbiphenyl is gradually enriched within the nanoreactor. Since the synthesized 4-acetylbiphenyl enriched in the nanoreactor is hydrophobic, its diffusion into the solvent of water is slow, and therefore it decelerates the Suzuki reaction in the later period. Furthermore, the yield of 4-acetylbiphenyl in the later period increases almost linearly with time, which seems abnormal to that of coupling reactions employing common catalyst.8,48,49 We think the possible reason is ascribed to the relatively constant concentration of the reactants within the nanoreactor of Pd@PNIPAM during the coupling reaction. The kinetics of the Suzuki reaction of the hydrophilic 4-bromophenol at 25 °C is similar to that of the hydrophobic 4-bromoacetophenone at 80 °C. That is, the yield fast reaches 38% in the initial 0.5 h, and then increases almost linearly with time in the later period. 3.5. Recycling of the Nanoreactor of Pd@PNIPAM in Water. Lastly, the recycling of the Pd@PNIPAM nanoreactor is evaluated by using the Suzuki reaction of 4-bromophenol with benzeneboronic acid in water at 90 °C as the model reaction. At room temperature, the PNIPAM brushes are soluble and the Pd@PNIPAM nanoreactor exists as a colloid in water, whereas when reactants such as 4-bromophenol, benzeneboronic acid, and K2CO3 are added and the temperature is increased above the phase-transition temperature, the PNIPAM brushes become insoluble42,43,50 and collapse onto the Pd nanoparticle, and the Pd@PNIPAM nanoreactor becomes a fine floccule-like suspension. The floccule-like suspension is stable and no precipitate of black Pd is optically observed during the Suzuki reaction. After completion of the Suzuki reaction, the organic compounds are extracted into the diethyl ether phase and the catalyst of Pd@PNIPAM nanoparticles, which exists as a fine flocculelike suspension, remains in the water phase. The catalyst of the floccule-like suspension is carefully collected and reused in next run of the Suzuki reaction. As shown in Table 3 a 67% yield of 4-hydroxybiphenyl is achieved after 4 times of catalyst
10832 J. Phys. Chem. C, Vol. 112, No. 29, 2008 recycling. The gradual decrease in yield is partly ascribed to the catalyst deactivation, but mainly due to the loss of the catalyst during the collection of the floccule-like catalyst since it is found that part of the floccule-like suspension optically remains in the filter although it is carefully washed with 8.0 mL of water. It should be pointed out that the floccule-like suspension can be “homogeneously” dispersed in pure water at room temperature again due to the reversible phase transition of the PNIPAM brushes. Thus, all the floccule-like catalyst can be collected when more water is used to wash the catalyst-stuck filter. However, this washing will increase the volume of the reaction mixture and decrease the concentration of the reactants and the Pd catalyst. We believe the recycling of the Pd@PNIPAM nanoreactor can be improved when the Pd catalyst of the floccule-like suspension is centrifugally isolated at temperatures above the phase-transition temperature of Pd@PNIPAM. 4. Conclusions We have proposed an efficient and recyclable nanoreactor, the poly(N-isopropylacrylamide)-grafted Pd nanoparticle, for the Suzuki reaction in water. The poly(N-isopropylacrylamide)grafted Pd nanoparticle (Pd@PNIPAM) has a core-corona structure, where the Pd nanoparticle forms the core and the PNIPAM brushes form the corona. The corona-forming brushes of PNIPAM can form a nanoenvironment for guest molecules and the Pd@PNIPAM nanoparticle can act as a nanoreactor for the Suzuki reaction. For hydrophilic aryl halide, the substrates can be entrapped in the nanoreactor of Pd@PNIPAM and the Suzuki reaction within the nanoreactor can be efficiently performed in water even at room temperature. For hydrophobic substrates, the substrates can diffuse into the nanoreactor of Pd@PNIPAM and the Suzuki reaction within the nanoreactor is performed efficiently in water above the phase-transition temperature of the PNIPAM brushes. Besides, the nanoreactor of Pd@PNIPAM nanoparticles can be recycled due to the reversible phase transition of the PNIPAM brushes. We anticipate that a broader range of the Suzuki reaction of aryl halides within the nanoreactor of polymer-grafted Pd nanoparticle can be achieved when suitable functionalized graftingpolymeric brushes are used. Acknowledgment. The financial support by National Science Foundation of China (No. 20504016) and the Program for New Century Excellent Talents in University (No. NCET-06-0216) is gratefully acknowledged. Supporting Information Available: Text showing the synthesis of the Pd@PNIPAM nanoparticles and the competition Suzuki reaction of a binary mixture of the hydrophobic 4-bromoanisole and the hydrophilic 4-bromophenol within the Pd@PNIPAM nanoreactor in water. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Miyaura, N.; Yanagigand, T.; Suzuki, A. Synth. Commun. 1981, 11, 503. (2) Hassan, J.; Sevignon, M.; Gozzi, C.; Schulz, E.; Lemaire, M. Chem. ReV. 2002, 102, 1359. (3) Zapf, A.; Ehrentraut, A.; Beller, M. Angew. Chem., Int. Ed. 2000, 39, 4153. (4) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1291. (5) Baleizao, C.; Corma, A.; Garcia, H.; Leyva, A. J. Org. Chem. 2004, 69, 439.
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