A variety of different transition metals have been used as highly active catalysts for organic synthesis, and these transition metal-catalyzed reactions play an important role in the synthesis of fine chemicals, pharmaceuticals, and advanced materials [1, 2]. Pd is one of the most popular of the transition metals, and Pd-based catalysts have been used for a variety of organic transformations, especially cross-coupling reactions for the formation of C-C, C-N, and C-O bonds [3, 4]. In particular, the Suzuki reaction for the formation of biaryl compounds via the coupling of an aryl halide with an organoboronic acid has a high functional group tolerance towards the presence of other substituents on the reacting substrates [1, 5]. Many heterogeneous and homogeneous Pd catalysts have been developed for these reactions. Although toxic and expensive homogeneous Pd phosphine complexes generally show higher reaction rates and greater selectivity towards the desired reaction than heterogeneous catalysts, they can be difficult to recover and reuse, and catalysts of this type can also be sensitive to oxygen and/or moisture [6, 7]. Furthermore, the contamination of the reaction products with trace amounts of metal represents another major issue associated with the use of transition metal catalysts, and this issue can be particularly problematic following the use of expensive and/or toxic heavy metal complexes [8, 9]. To alleviate these problems, significant research needs to be conducted towards the development of new heterogeneous catalyst systems that can be readily isolation and recycled without any significant loss in their efficiency.
Numerous studies have been conducted towards the development of Pd-based catalysts that work in ligand-free environments, such as carbon nanotubes, magnetic nanoparticles, silicates, graphene, lignin, metal oxides, and polymers [10, 11, 12, 13, 14, 15, 16, 17]. However, the weakly bonded Pd particles in these heterogeneous systems can undergo significant leaching during their use in chemical reactions, which can result in a decrease in their catalytic activity and the contamination of the product with Pd [18]. Furthermore, the use of toxic organic solvents during synthesis represents another environmental and economic challenge. Hence, the use of water as a reaction medium for coupling reactions catalyzed by heterogeneous catalysts is widely acknowledged as a desirable alternative to conventional synthetic practices because it is much more environmentally friendly [19, 20]. Given that the leaching of Pd from heterogeneous catalysts represents a major challenge for chemical reactions conducted in organic solvents, the leaching of Pd could become even more severe when the same reactions are conducted in water, because Pd catalysts are generally more soluble in water than they are in organic solvents [21]. With this in mind, there is therefore an urgent need for the development of novel water resistant heterogeneous catalysts to allow for the Suzuki reaction to be conducted in water. Several researchers have developed water resistant heterogeneous catalysts, which have been reported to exhibit excellent performance during their first reaction cycle. Unfortunately, the activities of these catalysts tend to drop after they have been reused several times [9, 22, 23]. The falling activity of these catalysts is probably caused by the leaching of active Pd from their surfaces during the reaction, because the activity in the recycling experiments is strongly dependent upon the recycling procedure. Physical losses are also encountered during the separation of these catalysts from the aqueous reaction mixture [24, 25].
HypoGel has recently attracted considerable attention as an alternative solid support for heterogeneous catalysis, as well as being useful for the formation of cross-linked captured Pd catalysts. It has also been reported that HypoGel can be used as solid support for the removal of arsenic from polluted water [26, 27]. Furthermore, Bagheri et al. [28] and Costa et al. [29] reported the immobilization of antimicrobial peptides with HypoGel for clinical applications. Polystyrene-based HypoGel resins combine the advantages of higher reaction capacities with the unique chemical properties of grafted gel-type supports containing hydrophilic glycol spacers. In this study, we have incorporated Pd(OAc)2 into HypoGel to prepare a novel water resistant cross-linked captured Pd catalyst (XL-HGPd) that can be used for Suzuki reactions in water. HypoGel was selected as a support for the immobilization of the Pd catalyst, because it can be readily separated from the reaction media by solid phase extraction (SPE) with minimum catalyst weight loss. It was also envisaged that the immobilization of the Pd catalyst on HypoGel would lead to lower levels of metal leaching.
XL-HGPd was synthesized according to a previously published procedure from the literature [17, 30] with minor modifications. A mixture of HypoGel and 5 mol% Pd(OAc)2 in toluene was heated at 80 °C for 10 min, and the mixture was then cooled to room temperature and stirred for 2 h. The resulting brown resin was cross-linked with succinyl chloride and triethylamine in dry DMF before being filtered. The solid material was then treated with hydrazine hydrate in methanol (10 w/v%) at room temperature to yield a black resin. The change in the color of the resin from brown to black was attributed to the immobilization of Pd(0) nanoparticles on the solid support. Similar changes in color have been reported by several other researchers [30, 31, 32, 33].
Samples of the catalyst were analyzed based on their N2 adsorption-desorption isotherms (ASAP 2010, Micromeritics) using the BET method to determine their specific surface area and porosity properties. Field emission scanning electron microscope (FE-SEM) measurements were conducted using a SUPRA 55VP (Carl Zeiss, Oberkochen, Germany) electron microscope. The presence of Pd on the surface of the XL-HGPd resin was confirmed and quantified using energy-dispersive X-ray (EDX) analysis (equipped with an SEM device). Platinum was used as a coating agent to prevent charging effects during the SEM-EDX analysis. The X-ray powder diffraction (XRD) pattern of the catalyst was recorded using a D8-Advance X-ray diffractometer (Bruker, Germany). Diffraction data were recorded using continuous scanning at a rate of 8°/min with a step value of 0.025°. NMR data for the products were recorded on a JEOL ECP 400 MHz Superconductor spectrometer (Japan) and a Bruker Avance 111 600 MHz spectrometer using CDCl3 as the solvent. The amount of metal leaching from the support was determined by inductively coupled plasma mass spectrometry (ICP-MS) using a PerkinElmer Optima 2000 DV system (USA).
A mixture of phenylboronic acid (0.75 mmol, 1.5 equiv), aryl halide (0.5 mmol), K2CO3 (0.75 mmol, 1.5 equiv), tetrabutylammonium bromide (1.5 equiv), water (2 mL), and XL-HGPd (27.8 mg, 2.4 mol% of aryl halide) in a 10-mL vial was heated at 90 °C with stirring for 6 h. The XL-HGPd catalyst was subsequently recovered by filtration, and washed sequentially with ethyl acetate (3×10 mL) and acetone (3×10 mL) before being dried in an oven at 80 °C for 24 h. The reaction mixture was extracted with ethyl acetate (3 × 10 mL), and the combined organic layers were washed with distilled water, dried over MgSO4, and concentrated under reduced pressure to give the crude product as a residue. The crude product was further purified by column chromatography over silica gel to yield the final product, which was characterized by NMR spectroscopy.
Phenylboronic acid (91.5 mg, 0.75 mmol), tetrabutylammonium bromide (1.5 equiv), K2CO3 (103.5 mg, 0.7 mmol, 0.7 equiv) and the XL-HGPd catalyst (27.8 mg, 2.4 mol% of the aryl halide) were added to a solution of 1-bromo-4- methoxybenzene (0.5 mmol) in water (2 mL), and the resulting mixture was heated at 90 °C with stirring for 6 h. The Pd catalyst was subsequently removed from solution with tweezers, and washed sequentially with ethyl acetate (3×10 mL) and acetone (3×10 mL) before being dried in an oven at 80 °C for 24 h. The reaction mixture was extracted with ethyl acetate (3 × 10 mL), and the combined organic layers were washed with distilled water, dried over MgSO4, and concentrated under reduced pressure to give the crude product as a residue. The crude product was purified by column chromatography over silica gel to yield the final product, which was characterized by NMR spectroscopy. The recovered catalyst was reused in the same reaction to investigate its recyclability.
XL-HGPd was readily prepared via the 3-step procedure shown in Scheme 1 [27]. This synthetic procedure was based on the physical adsorption of Pd by the polymer, as well as the formation of electronic interactions between the electrons belonging to the aryl rings of the polystyrene-based polymer and the vacant orbitals surrounding the catalysts. Pd(OAc)2 was initially immobilized on the surface of the HypoGel support, and the catalyst was then cross-linked with succinyl chloride to enhance its mechanical stability during the organic reaction. The material was then treated witha 10% solution of hydrazine hydrate in methanol to reduce the Pd(II) particles to Pd(0), which resulted in the trapping of the Pd(0) within the resin, as indicated by the change in the color of the resin from brown to black.
The surface properties of fresh HypoGel and Pd-loaded HypoGel (XL-HGPd) are shown in Table 1. The BET surface area, average pore diameter, and pore volume of fresh HypoGel were determined to be 1.1717 m2/g, 1.9556 nm, and 3.33×10-4 cm3/g, respectively. The loading of the Pd catalyst onto the HypoGel led to a decrease in surface properties, and the surface area in particular dropped from 1.1717 to 0.4716 m2/g, which confirmed that Pd had been successfully immobilized onto the surfaces and channels of the support.
The XRD patterns of HypoGel and the XL-HGPd(II) and XL-HGPd(0) resins are shown in Fig. 1. Notably, no discernible differences were observed between the XRD patterns of the HypoGel and XL-HGPd(II) samples. However, specific peaks were observed at 2θ = 40.1°, 46.6°, and 68.3° following the reduction step, which were attributed to the (111), (200), and (220) planes of the face-centered cubic lattice, respectively. This result confirmed the presence of Pd(0) on the XL-HGPd(0) surface. These data were consistent with those provided for Pd(0) by ASTM, and were also in agreement with the results reported by Shen et al. [34]. The average size of the Pd nanoparticles in XL-HGPd(0) was determined to be 8.14 nm using the Scherrer formula.
Figure 2 shows the ATR-FTIR spectra of HypoGel and the XL-HGPd(II) and XL-HGPd(0) resins. Consideration of the spectra revealed that four additional IR bands were observed in the ATR-FTIR spectrum of XL-HGPd(II), which were attributed to Pd(OAc)2 (i.e., 1617, 1467, 1385, and 1068 cm-1). The bands at 1617 and 1467 cm-1 corresponded to the asymmetric and symmetric stretching vibrations of the C=O and C-O moieties of the acetate group, respectively. The band at 1385 cm-1 was attributed to symmetric CH3 deformation vibrations, whereas the band at 1068 cm-1 corresponded to a CH3 rocking vibration [35]. The disappearance of this Pd(OAc)2 band in the XL-HGPd(0) resin demonstrated that the Pd(II) particles had been completely converted to Pd(0).
Figure 3 shows the SEM images of HypoGel, the fresh XL-HGPd(0) resin, and the used XL-HGPd(0) resin after its fifth cycle. The SEM images show that the HypoGel particles were spherical in shape both before and after the immobilization of Pd(0), with a diameter in the range of 120-150 µm. Particles of this size can be readily separated using a simple solid phase extraction (SPE) method. The SEM images also revealed that Pd nanoparticles were distributed over the entire surface of the HypoGel (Fig. 3(e)). The morphological characteristics of the used XL-HGPd(0) particles did not vary significantly from those of the original XL-HGPd(0) particles. Notably, however, the surfaces of the particles did become more homogeneous and compact after the fifth reaction cycle (Fig. 3(f)), although the average diameter did remain unchanged. The results of the SEM-EDX analysis of the fresh XL-HGPd(0) resin confirmed that Pd had been immobilized onto the surface of the resin without any observable impurities (Fig. (4)).
The TEM images and size distribution plots for the fresh XL-HGPd(0) and used XL-HGPd(0) resins are shown in Fig. 5(a) and 5(b), respectively. The TEM image of the fresh XL- HGPd(0) resin revealed that the spherical Pd(0) nanoparticles were well-dispersed within the resin. The average diameter of the Pd(0) nanoparticles in the fresh XL-HGPd(0) resin was found to be 8.5 ± 3 nm. The mean particle size of the used XL-HGPd(0) resin was found to occupy a narrow size distribution, which indicated the potential for the conservation of the XL-HGPd(0) resin during subsequent reaction cycles.
The catalytic activity of the XL-HGPd(0) resin (5 mol% based on its Pd content) was examined using the Suzuki reaction of phenylboronic acid with a variety of different aryl halides in water at 90 °C under atmospheric pressure (Table 2). Aryl bromides and aryl iodides bearing a variety of different electron-withdrawing (i.e., nitro and trifluoromethyl) or electron-donating (i.e., methyl and methoxy) groups were successfully coupled with phenylboronic acid using the XL-HGPd(0) resin as a catalyst to give the corresponding biaryl products in good yields and high purity (Table 2, entries 1-7). However, the use of aryl chlorides resulted in poor yields of the desired biaryl products (Table 1, entries 8 and 9). Aryl chlorides are well known to be least reactive of the aryl halides during the Suzuki reaction (relative reactivity: I > OTf > Br >>> Cl) because of the strength of the Ar-Cl bond (Ph-X: Cl (96) > Br (81) > I (65 kcal/mol)). The results of this study were therefore in accordance with those reported previously in the literature by Wu et al. [36]. Overall, the XL-HGPd(0) resin prepared in the current study behaved as a good heterogeneous catalyst for the Suzuki reactions of aryl bromides and aryl iodides with phenylboronic acid in water to give the corresponding biaryl products in high yields.
The recyclability of the XL-HGPd resin was investigated using the Suzuki reaction between phenylboronic acid and 1-bromo-4-methoxybenzene (Table 3). Upon completion of the initial Suzuki reaction, the resin was recovered by filtration and washed sequentially with ethyl acetate and acetone to remove any impurities. The catalyst was then dried and reused in the same Suzuki reaction. The results of these experiments revealed that the XL-HGPd catalyst retained its activity for five reaction cycles, which implied that the XL-HGPd resin could be reused at least five times without any significant loss in its catalytic activity. The leaching of metal into the reaction mixture from the recovered XL-HGPd resin support was monitored by ICP-MS. The results revealed that a small amount of Pd (0.55 ± 0.02 ppm) was leaching from the XL-HGPd resin during the reaction, with only a 0.25% weight loss of Pd from the supported catalyst following the first reaction cycle.
We have successfully used simple methods to synthesize a cross-linked resin-captured heterogeneous Pd catalyst (XL-HGPd). This new catalyst displayed high catalytic activity for the Suzuki reactions of phenylboronic acid with a variety of aryl bromides and aryl iodides in water under aerobic conditions. This catalyst also exhibited good recyclability properties as well as low levels of Pd leaching during its use as a catalyst in the Suzuki reaction in water. It is noteworthy that the catalyst could be recycled by simple filtration and washing steps. It is envisaged that these results will lead to the use of our newly developed XL-HGPd(0) resin as a catalyst in many other organic transformations.
2015, Vol. 36 Issue (5): 771-777
PDF (1870 KB)
2015, Vol. 36 Issue (5): 771-777
PDF (1870 KB)