Accepted Manuscript Title: Cellulose-supported N-Heterocyclic Carbene-palladium Catalyst: Synthesis and its applications in the Suzuki Cross-coupling Reaction Author: Xiaoxia Wang Peibo Hu Fengjun Xue Yuping Wei PII: DOI: Reference:
S0144-8617(14)00799-1 http://dx.doi.org/doi:10.1016/j.carbpol.2014.08.030 CARP 9167
To appear in: Received date: Revised date: Accepted date:
16-3-2014 23-7-2014 6-8-2014
Please cite this article as: Wang, X., Hu, P., Xue, F., and Wei, Y.,Cellulosesupported N-Heterocyclic Carbene-palladium Catalyst: Synthesis and its applications in the Suzuki Cross-coupling Reaction, Carbohydrate Polymers (2014), http://dx.doi.org/10.1016/j.carbpol.2014.08.030 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Cellulose-supported N-Heterocyclic Carbene-palladium Catalyst:
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Synthesis and its applications in the Suzuki Cross-coupling
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Reaction
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Xiaoxia Wang1, Peibo Hu1, Fengjun Xue, Yuping Weia,b,*
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a
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China
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b
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300072, P.R. China
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Department of Chemistry, School of Science, Tianjin University, Tianjin 300072, P.R.
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Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin
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1
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*Corresponding Author. Tel and Fax: +86 22 27403475; E-mail: [email protected]
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Abstract
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Both authors contributed equally to this work.
A cellulose-supported
N-methylimidazole-palladium catalyst
(Cell-NHC-Pd)
was
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synthesized and used for Suzuki cross-coupling reactions between aryl halides and
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phenylboronic acids to create the corresponding coupling products in good to excellent
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yields. Moreover, the catalyst is easily recovered using only a few cycles of simple filtration.
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Keywords: Cellulose, N-methylimidazole, Palladium, Suzuki cross-coupling reaction
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1. Introduction
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Palladium-catalyzed Suzuki cross-coupling reactions of aryl halides with arylboronic
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acids are among the most efficient methods to selectively construct biaryl compounds in
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organic synthesis (Littke & Fu, 2002; Martin & Buchwald, 2008; Yin & Liebscher, 2007;
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Old et al., 1998; Zhang et al., 1999; Lemo et al., 2005; Li et al., 2000). To date, Suzuki
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reactions have been extensively used for the synthesis of natural products, herbicides,
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pharmaceuticals, and other products (Corbet & Mignani, 2006; Hassan et al., 2002; Suzuki,
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1999; Miyaura & Suzuki, 1995; Meijere & Diederich, 2004). Commonly, phosphine ligands
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are employed to facilitate the corresponding transformations. However, a critical setback for
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phosphine ligands is the fact that they are toxic and air-sensitive (Birkholz et al., 2009; Fu,
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2008). Recently, N-heterocyclic carbenes (NHCs) have attracted many chemists’ attention as
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transition metal ligands (Wang et al., 2012; Gao et al., 2012; Fleckenstein et al., 2007) after
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they were first reported by Herrmann and co-workers (Herrmann et al., 1995; Herrmann et
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al., 1998). Compared with phosphine ligands, NHCs have the same σ-donor and low
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π-acceptor ability while they can form stronger bonds with the majority of metals. More
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importantly, they are air and moisture stable. Nowadays, a wide range of NHC ligands are
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commercially available which exhibit high activities in various important organic
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transformations when combined with metallic pre-catalysts (Drge & Glorius, 2010; Dorta et
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al., 2005; Marion & Nolan, 2008). Despite the significant progress that has been made in the
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design of these NHC ligands, the desired products are always inevitably contaminated by
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catalyst residues during these homogeneous reactions (Blaser, 2002; Cole-Hamilton, 2003),
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which greatly limits the use of NHCs-related organic transformations in industrial
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applications.
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The immobilization of metal catalysts on solid supports has been considered an efficient
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approach to address the aforementioned issues. Numerous solid supports have been utilized
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as anchors for NHC and their metal complex such as polystyrene (Kang et al., 2005; Lo &
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Luo, 2012), polysilylethers (Lázaro et al., 2013) and mesoporous silica (Pozo et al., 2011).
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However, the progress in the quality of catalysts on solid support has been relatively slow
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compared to soluble homogeneous complexes because of the deactivation that results from
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particle aggregation, metal leaching and matrix degradation (Molnár & Papp, 2014). Another
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major problem is the environmental concern arising from the use of poisonous and often
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costly materials applied in catalyst synthesis. In this regards, cellulose is an attractive and
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promising candidate that could be used as an efficient, renewable, and biodegradable support
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for catalysts (Jamwal et al., 2011). In addition, cellulose is stable in most organic solvents,
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inexpensive, and has a high surface area. Typically, functional groups present in cellulose
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backbone participate in bonding with metals. Compared with widely used PS support,
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cellulose can easily be functionalized due to its abundant hydroxyl groups and can create
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either soluble or insoluble supports through chemical modification. Cellulose and its
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derivatives have also been widely used for coatings, optical films, pharmaceuticals, and
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consumables (Klemm et al., 2005). The use of cellulose as an efficient support for Pd
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nanoparticles in cross-coupling reactions has also been widely reported (Jamwal et al., 2011;
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Reddy et al., 2006; Cirtiu et al., 2011; Xu et al., 2008). As demonstrated in a recently
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accepted review publication (Molnár & Papp, 2014), the most studied catalysts are still
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cellulose deposited Pd particles. In contrast, the use of immobilized Pd complexes has not
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been fully exploited. According to the literature, only three examples for such support
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materials have been synthesized: cellulose-tagged diphenylphosphinate (Du & Li, 2012; Du
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& Li, 2011; Keshipour et al., 2013), triphenylphosphine (Xu et al., 2014) and
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ethylenediamine (Zheng et al., 2009). More versatile functionalization techniques should be
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developed to achieve stable, immobilized cellulose metal complexes, allowing for the
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improvement of catalytic activities. With all these considerations in mind, in this paper, we present the anchoring of N-
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methylimidazole onto a cellulose backbone as a precursor, followed by coordination with
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Pd(OAc)2 to prepare a novel cellulose-NHC-palladium catalyst (Cell-NHC-Pd). In addition,
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we explore its catalytic activity and recycling performance using the Suzuki reaction as an
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example.
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2. Experimental
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2.1. Materials
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Microcrystalline cellulose AVICEL PH-101 (Microcrystalline cellulose, degree of
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polymerization = 180) was purchased from Shandong Liaocheng A Hua Pharmaceutical Co.,
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Ltd. (Shandong Province, P.R. China). Triethylamine (Et3N), anhydrous lithium chloride
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(LiCl), potassium carbonate (K2CO3), dimethylformamide (DMF), all of which are analytical
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grade (AR), were obtained from Tianjin Guangfu Fine Chemical Research Institute (Tianjin,
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P.R. China). N-methylimidazole was purchased from Sinopharm Chemical Reagent Beijing
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Co., Ltd. Acetone was obtained from Tianjin Jiangtian Chemical Technology Co., Ltd. The
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cellulose, anhydrous lithium chloride, and potassium carbonate were vacuum dried for 48 h
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at 50 °C, 40 °C and 30 °C respectively, before use. Dimethylacetamide (DMAc),
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dimethylsulfoxide (DMSO), dichloromethane (CH2Cl2) and DMF were desiccated over
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CaH2 overnight, then filtered through silica gel; they were then vacuumed distilled and
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stored in brown bottles with 4A molecular sieves under N2. Other reagents were only
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redistilled.
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2.2. Instruments NMR spectra were recorded on a Bruker Avance 400 MHz spectrometer in CDCl3 using
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TMS as an internal standard. The particle sizes of the samples were observed using a Tecnai
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G2 F20 transmission electron microscope (TEM). The elemental content of palladium was
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determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) using a
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Thermo Jarrell-Ash corporation, ICP-9000 (N+M) instrument. X-ray photoelectron
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spectrometry (XPS) was conducted using a PHI1600 ESCA System (Perkin-Elmer, United
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States). The content of sulfur and nitrogen were determined by a Vario Micro cube elemental
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analyzer. Thermogravimetric analysis (TGA) was measured with a STA 409 PC thermal
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analyzer (NETZSCH, Germany). Fourier transform infrared spectroscopy (FTIR) analysis
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was performed on a BIO-RAD FTS3000 IR Spectrum Scanner.
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2.3. Preparation of Cell-NHC-Cl
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Cellulose p-toluenesulfonate (Cell-OTs) was prepared according to methods reported in
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the literature (Rahn et al., 1996). Cell-OTs (1.91 g, 10.0 mmol) was then added to DMSO
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(10 mL) in a 25 mL flask, with degree of substitution (DS) of 0.14, and the mixture was
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stirred for 24 h at 60 °C followed by overnight stirring at room temperature to dissolve the
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Cell-OTs completely. N-methylimidazole (0.86 g, 10.0 mmol) was then added to the reaction
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mixture under nitrogen and the viscous black mixture was stirred for 12 h at 100 °C. After
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cooling to room temperature, the mixture was added dropwise into acetone in a beaker and
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stirred vigorously for approximately 6 h. The reaction mixture was filtered and the resulting
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[Cell-NHC][OTs]- was obtained. [Cell-NHC][OTs]- was then dissolved in a solution of NaCl
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(1.15 g, 20.0 mmol) in acetone (10 mL) to replace OTs- with Cl-. The precipitate
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(Cell-NHC-Cl) was collected via filtration. The obtained brown powder was vacuum dried
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for 24 h at 40 °C. The DS of Cell-NHC was 0.08 determined on the base of nitrogen content.
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2.4. Preparation of Cell-NHC-Pd complex
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Under nitrogen, Cell-NHC-Cl (0.50 g) was added to a solution of Pd(OAc)2 (0.10 g, 0.45
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mmol) in DMF (10 mL) and the mixture was stirred for 24 h at 60 °C. After cooling to room
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temperature, the reaction mixture was filtered. The obtained solid black product was washed
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carefully with distilled water (3×25 mL), absolute ether (2×25 mL) and absolute ethyl
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alcohol (2×25 mL) successively then dried under reduced pressure at room temperature to
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give the dark palladium complex (Cell-NHC-Pd).
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2.5 General procedure for the Suzuki reaction of aryl halides with arylboronic acids
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The aryl halide (0.5 mmol), arylboronic acid (0.75 mmol), Cell-NHC-Pd (0.75 mol%),
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K2CO3 (1 mmol) and DMF : H2O (1:1) (2.5 mL) were successively added into a 25 mL
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round bottom flask. The mixture was stirred vigorously at 80 °C for the varying specific
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length of time based on each different substrate. After reaction completion, the solid catalyst
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was filtered off. The filtrate was extracted with ethyl acetate (3×5 mL), washed with water
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and brine three times, and then dried over anhydrous MgSO4. The MgSO4 was then filtered
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off. After removal of the solvent, the crude product was purified by preparative TLC (eluent:
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petroleum ether/ethyl acetate, 20/1) to give the desired product and calculate the yield.
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2.6 Separation of the catalyst and recycling tests
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After completion of the Suzuki–Miyaura reaction, the liquid phase was removed by
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centrifugation. The solid catalyst was simply recovered by filtration and washing. Without
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drying, the recovered catalyst was directly used in next run with new portions of reactants.
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3. Results and Discussion
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3.1. Synthesis and characterization of Cell-NHC-Pd
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Cellulose-supported NHC (Cell-NHC) was synthesized via the nucleophilic substitution of
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Cell-OTs with N-methylimidazole as shown in Scheme 1. The tosylate acted as an effective
140
leaving group, allowing the SN2 reaction with N-methylimidazole to create Cell-NHC-OTs.
141
In order to improve the catalytic activity, OTs- was exchanged by Cl- (Kim et al., 2005). The
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resulting Cell-NHC-Cl was then coordinated with palladium acetate in DMF to give the final
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catalyst Cell-NHC-Pd complex. The Pd loading was determined to be 0.24 mmol/g by
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inductively coupled plasma-atomic emission spectroscopy (ICP-AES), which was higher
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than that of other polymer-supported NHC palladium complex reported in previous literature
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(Lo & Luo, 2012; Kim et al., 2006).
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Scheme 1. Reaction scheme for the formation of catalyst (Cell-NHC-Pd).
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The FTIR spectra of cellulose, cellulose derivatives Cell-OTs, Cell-NHC-Cl, and
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Cell-NHC-Pd are shown in Fig. 1. The bands at 1359 cm-1 and 1179 cm-1 corresponds to the
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O=S asymmetric and symmetric stretching vibration of Cell-OTs, respectively, which are
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greatly weakened after substitution with N-methylimidazole as shown in Fig. 1b-c.
153
Correspondingly, the N-methylimidazole graft is visible from appearance of a new band at
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1580 cm−1, which corresponds to the stretching vibration of N–sp2 C bond (Zhao et al., 1995;
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Kurt et al., 2000). In addition, the absorption in the 1220 cm−1 region is interpreted as the
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vibration of N–sp3 C bond (Kurt et al., 2000; Rodil et al., 2000). As shown in Fig. 1c-d, the
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band at 632 cm−1 region corresponds to the vibration of the imidazole ring, and the intensity
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of its absorption is reduced after reaction with palladium acetate, suggesting the formation of
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the NHC-Pd complex (He & Cai, 2011).
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Fig. 1. FT-IR spectra of Cell (a), Cell-OTs (b), Cell-NHC-Cl (c), and Cell-NHC-Pd (d).
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As the Suzuki reaction usually requires heating, thermogravimetric analysis (TGA) was
163
performed to evaluate the thermal stability of the cellulose-supported catalyst in air and high
164
temperature. As shown in Fig. 2, TGA of the catalyst system showed high thermal stability
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with decomposition at around 225 °C in air. The result demonstrated that cellulose was
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considerably stable in an oxygen-containing atmosphere as a catalyst support in
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experimental conditions.
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Fig. 2. TGA traces of Cell-NHC and Cell-NHC-Pd catalyst under air flow. XPS spectroscopy was employed to investigate the elemental coordination state of Pd
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species. As shown in Fig. 3a, the binding energies of Pd3d5/2 in the Cell-NHC-Pd catalyst
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were 335.3 eV and 337.4 eV, respectively (Xu et al., 2008), which revealed that both Pd (0)
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and Pd (II) were present. Based on previous literature reports (Wu et al., 2013; Xiong et al.,
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2013), parts of Pd (II) species were likely reduced in the existence of cellulose backbone.
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Fig. 3. XPS spectrum of fresh Cell-NHC-Pd catalyst: Pd 3d (a), C 1s (b), N 1s (c).
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As shown in Fig. 3b, the status of C in Cell-NHC-Pd can be also explored by XPS
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analysis of C 1s. The C 1s peak was divided into four components at binding energies of
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282.8, 284.8, 286.5 and 288.0 eV, which were attributed to C=C, C–C, C–O and carbene
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carbon to Pd bonds, respectively (Khabashesku et al., 2000; Mao et al., 2011).
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Correspondingly, XPS of N 1s (Fig. 3c) showed 401.3 eV as the major peak and 397.4 eV as
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a minor peak, which were assigned to N–sp2C and N–sp3C bonds (Gammon et al., 2003; Li
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et al., 2009). Combining these results with those obtained from FTIR, it can be speculated
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that N-methylimidazole on cellulose backbone captured the Pd species successfully, as
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shown by the model in scheme 1.
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Surface morphology of the cellulose fibers was investigated with transmission electron
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microscopy (TEM). TEM images (Fig. 4) revealed the presence of palladium nanoparticles
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of about 9 nm in size, which proved the excellent dispersion of palladium sites on the surface
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of cellulose.
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Fig. 4. TEM images of the fresh Cell-NHC-Pd (a and c) and recovered (b and d).
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3.2. Applying the catalyst to Suzuki cross-coupling reactions
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3.2.1 Evaluation of its catalytic activity
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Initial examination was carried out using p-bromoanisole (0.250 mmol, 1.0 equiv) and
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phenylboronic acid (0.375 mmol, 1.5 equiv) as a model reaction in the presence of
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Cell-NHC-Pd. Several variables such as the amount of catalyst, temperature, solvent and
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base were screened to optimize the reaction conditions. Results were summarized in Table 1.
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As shown in Table 1, the product yield increased with the increase of the amount of
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catalyst (Table 1, entries 1-3). However, the yield decreased when the amount of catalyst
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increased from 0.75 to 1.25 mol% (Table 1, entries 3-5). Therefore, 0.75 mol% of the
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catalyst (Table 1, entry 3) was sufficient to efficiently catalyze the reaction. The coupling
205
yield increased slightly from 60 °C to 80 °C (Table 1, entries 6 and 7). However, the yield
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decreased when the temperature rose to 100 °C (Table 1, entry 8). We tried a variety of
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solvent systems, going from pure water (Table 1, entry 12) to a mixture of an organic solvent
208
with water. The organic solvents tried were dimethylformamide (DMF), dimethylacetamide
209
(DMAc) and Ethanol (EtOH) (Table 1, entries 6-11, 13-17). Results showed that DMF : H2O
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(1:1) was the most favorable system. Using potassium carbonate and potassium tert-butoxide
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(Table 1, entries 7 and 16) both gave excellent yields. However, due to the hygroscopic
212
nature of potassium tert-butoxide and considering its higher cost, we opted for potassium
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carbonate. When we increased the amount of K2CO3 to 3 equivalents, the yield rose to 98%
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(Table 1, entry 13).
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Table 1
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Cell-NHC-Pd catalyzed Suzuki cross-coupling reaction.
218 Entry a
Amount of catalyst (mol%)
Temperature (°C)
Solvent
Base
Yield (%) b
0.25
80
DMF : H2O (1:1)
K2CO3
79
0.50
80
DMF : H2O (1:1)
K2CO3
85
3
0.75
80
DMF : H2O (1:1)
K2CO3
94
4
1.00
80
DMF : H2O (1:1)
K2CO3
92
5
1.25
80
DMF : H2O (1:1)
K2CO3
85
1 2
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222
60
DMF : H2O (1:1)
K2CO3
89
7
0.75
80
DMF : H2O (1:1)
K2CO3
93
8
0.75
100
DMF : H2O (1:1)
K2CO3
84
9
0.75
80
DMAc : H2O (1:1)
K2CO3
91
10
0.75
80
DMF : H2O (1:3)
K2CO3
76
11
0.75
80
EtOH : H2O (1:1)
K2CO3
86
12
0.75
80
H2O
K2CO3
22
13 c
0.75
80
DMF : H2O (1:1)
K2CO3
98
14
0.75
80
DMF : H2O (1:1)
Cs2CO3
88
15
0.75
80
DMF : H2O (1:1)
K3PO4
90
16
0.75
80
DMF : H2O (1:1)
t-BuOK
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17
0.75
80
DMF : H2O (1:1)
Et3N
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Reaction conditions: 4-bromoanisole (0.250 mmol), phenylboronic acid (0.375 mmol), base (0.5 mmol),
solvent (2.5 mL), heating in air for 2 h, detected by TLC. b Isolated yield. c K2CO3 was used for 3 equiv (0.75 mmol).
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Overall, the optimized conditions were as follows: phenylboronic acid (0.375 mmol),
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4-bromoanisole (0.250 mmol), catalyst (0.75 mol%), K2CO3 (0.75 mmol) in DMF : H2O (1:1)
224
(2.5 mL) at 80 °C under air.
225
To extend the scope of the heterogeneous reaction, the Suzuki cross-coupling reaction of
226
aryl halides with various phenylboronic acids were examined as shown in Table 2. Most of
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the aryl bromides were converted to the corresponding biaryls with good to excellent yields
228
in reasonable reaction times. Coupling of 4-bromoanisole with sterically hindered
229
phenylboronic acids proceeded with different yields (Table 2, entries 3-5 and 6-8). The
230
reaction of electron-deficient aryl bromides (4-bromobenzonitrile as a model substrate) with 14 Page 14 of 26
phenylboronic acid occurred smoothly to afford the biaryls with excellent yields within 4 h
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at 80 °C (Table 2, entries 10-18). However, the reaction between benzene halides and
233
4-ethylphenylboronic acid was sluggish and gave only small amounts of products even with
234
prolonged reaction times (Table 2, entries 19-21). The reaction of electron-deficient
235
bromobenzene with 4-fluorine phenylboronic acid went smoothly. On the other hand, the
236
reaction of electron-rich bromobenzene with 4-fluorine phenylboronic acid was not
237
successful, with a yield of only 10%.
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238 Table 2
240
Suzuki Cross-coupling Reaction of Aryl halide with Phenylboronic acidsa.
241 Aryl halide
1
Br
3 4 5
B(OH)2
OCH 3
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Phenylboronic acid
Br
OCH 3
F3 C
Br
OCH 3
H3 CO
Br
OCH 3
B(OH)2
B(OH)2
7
OCH 3
Br
OCH 3
F
B(OH) 2
Br
OCH 3
10
Br
CN
78
OCH 3
1.5
85
1.5
80
1.5
66
5.0
91
OCH 3
5.0
81
OCH 3
5.0
45
5.0
93
2.0
99
OCH 3
OCH 3 OCH 3
F
OCH 3
B(OH) 2 F
F
Br
4.5
F
OCH 3
9
OCH 3
H 3 CO
B(OH)2
F
8
97
H 3CO
B(OH)2
OCH 3
Br
2.0
OCH 3
F3C
OCH3
6
Yield (%)c
B(OH)2
H3 CO
Br
Time (h)b
Product
d
Entry
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C 2H 5
B(OH)2
C2 H5
B(OH)2
OCH 3
CN
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11
Br
CN
F3 C
B(OH)2
F3C
12
Br
CN
H3 CO
B(OH)2
H 3CO
13
Br
CN
CN
B(OH)2 H3 CO
CN
B(OH)2 OCH3
16
Br
CN
Br
F
F
B(OH) 2
Br
B(OH)2
19
B(OH) 2
CN
I
C 2H 5
B(OH)2
C2 H5
20
Br
C 2H 5
B(OH)2
C2 H5
21
Cl
C 2H 5
B(OH)2
CN
M
C2H5
C2 H5
F
Br
d
B(OH) 2
OCH 3
F
B(OH) 2
CH 3
242 243 244 245 246
a
96
95
83
2.3
98
4.0
61
4.0
53
4.0
33
3.5
76
6.0
10
OCH 3
F
Br
Ac ce pt e
23
2.6
F
B(OH)2
F
2.0
CN
C 2H 5
22
95
F
CN
CN
1.5 2.0
CN
F
18
97
OCH 3
F
17
4.0
ip t
CN
99
cr
Br
4.0
H3CO
CN
15
80
us
Br
CN
1.5
an
14
CN
CH 3
Unless otherwise specified, all the reactions were carried out using aryl halides (0.250 mmol), phenylboronic
acids (0.375 mmol), K2CO3 (0.75 mmol), Cell-NHC-Pd (0.75 mol%), DMF : H2O (1:1) (2.5 mL), under 80 °C in air. b Detected by TLC. c Isolated yield.
3.2.2 Separation of the catalyst and recycling tests.
247
The recyclability of the Cell-NHC-Pd catalyst was also investigated in the Suzuki
248
cross-coupling reaction of 4-bromoanisole with phenylboronic acid. After the reaction, the
249
liquid phase was removed by centrifugation and the catalyst was washed with ethyl acetate.
250
The catalyst was then used in the four additional reactions and the catalytic activity for the
251
five consecutive cycles was shown in Table 3. For the second and third reactions, only a
252
minor loss of catalytic activity was observed compared to the initial run. However, the 16 Page 16 of 26
reaction at the 4th and 5th recycled runs gave the products in 60% and 58% yields,
254
respectively. In addition, the filtrate resulting from the reaction mixture had no catalytic
255
activity under identical conditions. The total amount of palladium leaching after the 5th cycle
256
was 1.2% as detected by ICP-AES analysis (The amount of palladium is 2.55% before
257
reaction and 2.52% after 5 cycles). Therefore, the decrease of activity may be due to the
258
aggregation of palladium nanoparticles during the reaction as shown in TEM images of Fig.
259
4 (Narayanan & El-Sayed, 2003; Hu & Liu, 2005; Sin et al., 2010).
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260
266 267
an
Reusability of the Cell-NHC-Pd in Suzuki cross-coupling reactiona. 1st
2nd
3rd
Time (h) b
2.6
3
3
Yield (%) c
91
85
79
a
4th
M
Recycle
5th
3.5
4
60
58
d
263 264 265
Table 3
The reactions conditions: 4-bromoanisole (0.250 mmol), phenylboronic acid (0.375 mmol), K2CO3 (0.75
Ac ce pt e
261 262
mmol), Cell-NHC-Pd (0.75 mol%), DMF : H2O (1:1) (2.5 mL), under 80 °C in air. b Detected by TLC.
c
Isolated yield.
4. Conclusions
268
In conclusion, cellulose-supported N-methylimidazole was prepared by the nucleophilic
269
substitution reaction of N-methylimidazole with cellulose tosylate. The success of the
270
grafting of N-methylimidazole onto the cellulose backbone was confirmed by FT-IR and
271
XPS analysis. Cellulose-supported N-methylimidazole was then coordinated with Pd(OAc)2
272
in DMF to give the final cellulose-tagged N-methylimidazole palladium complex, which
273
showed high thermal ability from TG analysis. Nanopalladiums sites on the surface of the
274
cellulose support were very well distributed, as demonstrated by TEM images. Moreover, 17 Page 17 of 26
this catalyst efficiently catalyzed Suzuki cross-coupling reactions and gave the
276
corresponding products in good to excellent yields. Of particular note is that the
277
Cell-NHC-Pd catalyst can be recycled by simple filtration following a Suzuki cross-coupling
278
reaction. Further applications of cellulose-based N-methylimidazole in other organic
279
reactions are currently being investigated in our laboratory.
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Acknowledgment
282
This work was supported by National Natural Science Foundation of China (No. 20972109).
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Highlights: NHC ligand was introduced onto cellulose backbone to explore an avenue for polysaccharide-based catalysts. Cellulose-supported NHC-Pd complex can efficiently promote Suzuki coupling reactions. Catalysts can be recovered by easy filtration and can be used several times.
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S0144-8617(14)00799-1 http://dx.doi.org/doi:10.1016/j.carbpol.2014.08.030 CARP 9167
To appear in: Received date: Revised date: Accepted date:
16-3-2014 23-7-2014 6-8-2014
Please cite this article as: Wang, X., Hu, P., Xue, F., and Wei, Y.,Cellulosesupported N-Heterocyclic Carbene-palladium Catalyst: Synthesis and its applications in the Suzuki Cross-coupling Reaction, Carbohydrate Polymers (2014), http://dx.doi.org/10.1016/j.carbpol.2014.08.030 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1
Cellulose-supported N-Heterocyclic Carbene-palladium Catalyst:
2
Synthesis and its applications in the Suzuki Cross-coupling
3
Reaction
5
Xiaoxia Wang1, Peibo Hu1, Fengjun Xue, Yuping Weia,b,*
6
a
7
China
8
b
9
300072, P.R. China
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Department of Chemistry, School of Science, Tianjin University, Tianjin 300072, P.R.
an
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin
10
1
11
*Corresponding Author. Tel and Fax: +86 22 27403475; E-mail: [email protected]
12
14
Abstract
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Both authors contributed equally to this work.
A cellulose-supported
N-methylimidazole-palladium catalyst
(Cell-NHC-Pd)
was
15
synthesized and used for Suzuki cross-coupling reactions between aryl halides and
16
phenylboronic acids to create the corresponding coupling products in good to excellent
17
yields. Moreover, the catalyst is easily recovered using only a few cycles of simple filtration.
18
Keywords: Cellulose, N-methylimidazole, Palladium, Suzuki cross-coupling reaction
19 20
1. Introduction
21
Palladium-catalyzed Suzuki cross-coupling reactions of aryl halides with arylboronic
22
acids are among the most efficient methods to selectively construct biaryl compounds in
1 Page 1 of 26
organic synthesis (Littke & Fu, 2002; Martin & Buchwald, 2008; Yin & Liebscher, 2007;
24
Old et al., 1998; Zhang et al., 1999; Lemo et al., 2005; Li et al., 2000). To date, Suzuki
25
reactions have been extensively used for the synthesis of natural products, herbicides,
26
pharmaceuticals, and other products (Corbet & Mignani, 2006; Hassan et al., 2002; Suzuki,
27
1999; Miyaura & Suzuki, 1995; Meijere & Diederich, 2004). Commonly, phosphine ligands
28
are employed to facilitate the corresponding transformations. However, a critical setback for
29
phosphine ligands is the fact that they are toxic and air-sensitive (Birkholz et al., 2009; Fu,
30
2008). Recently, N-heterocyclic carbenes (NHCs) have attracted many chemists’ attention as
31
transition metal ligands (Wang et al., 2012; Gao et al., 2012; Fleckenstein et al., 2007) after
32
they were first reported by Herrmann and co-workers (Herrmann et al., 1995; Herrmann et
33
al., 1998). Compared with phosphine ligands, NHCs have the same σ-donor and low
34
π-acceptor ability while they can form stronger bonds with the majority of metals. More
35
importantly, they are air and moisture stable. Nowadays, a wide range of NHC ligands are
36
commercially available which exhibit high activities in various important organic
37
transformations when combined with metallic pre-catalysts (Drge & Glorius, 2010; Dorta et
38
al., 2005; Marion & Nolan, 2008). Despite the significant progress that has been made in the
39
design of these NHC ligands, the desired products are always inevitably contaminated by
40
catalyst residues during these homogeneous reactions (Blaser, 2002; Cole-Hamilton, 2003),
41
which greatly limits the use of NHCs-related organic transformations in industrial
42
applications.
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The immobilization of metal catalysts on solid supports has been considered an efficient
44
approach to address the aforementioned issues. Numerous solid supports have been utilized
2 Page 2 of 26
as anchors for NHC and their metal complex such as polystyrene (Kang et al., 2005; Lo &
46
Luo, 2012), polysilylethers (Lázaro et al., 2013) and mesoporous silica (Pozo et al., 2011).
47
However, the progress in the quality of catalysts on solid support has been relatively slow
48
compared to soluble homogeneous complexes because of the deactivation that results from
49
particle aggregation, metal leaching and matrix degradation (Molnár & Papp, 2014). Another
50
major problem is the environmental concern arising from the use of poisonous and often
51
costly materials applied in catalyst synthesis. In this regards, cellulose is an attractive and
52
promising candidate that could be used as an efficient, renewable, and biodegradable support
53
for catalysts (Jamwal et al., 2011). In addition, cellulose is stable in most organic solvents,
54
inexpensive, and has a high surface area. Typically, functional groups present in cellulose
55
backbone participate in bonding with metals. Compared with widely used PS support,
56
cellulose can easily be functionalized due to its abundant hydroxyl groups and can create
57
either soluble or insoluble supports through chemical modification. Cellulose and its
58
derivatives have also been widely used for coatings, optical films, pharmaceuticals, and
59
consumables (Klemm et al., 2005). The use of cellulose as an efficient support for Pd
60
nanoparticles in cross-coupling reactions has also been widely reported (Jamwal et al., 2011;
61
Reddy et al., 2006; Cirtiu et al., 2011; Xu et al., 2008). As demonstrated in a recently
62
accepted review publication (Molnár & Papp, 2014), the most studied catalysts are still
63
cellulose deposited Pd particles. In contrast, the use of immobilized Pd complexes has not
64
been fully exploited. According to the literature, only three examples for such support
65
materials have been synthesized: cellulose-tagged diphenylphosphinate (Du & Li, 2012; Du
66
& Li, 2011; Keshipour et al., 2013), triphenylphosphine (Xu et al., 2014) and
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67
ethylenediamine (Zheng et al., 2009). More versatile functionalization techniques should be
68
developed to achieve stable, immobilized cellulose metal complexes, allowing for the
69
improvement of catalytic activities. With all these considerations in mind, in this paper, we present the anchoring of N-
71
methylimidazole onto a cellulose backbone as a precursor, followed by coordination with
72
Pd(OAc)2 to prepare a novel cellulose-NHC-palladium catalyst (Cell-NHC-Pd). In addition,
73
we explore its catalytic activity and recycling performance using the Suzuki reaction as an
74
example.
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2. Experimental
77
2.1. Materials
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Microcrystalline cellulose AVICEL PH-101 (Microcrystalline cellulose, degree of
79
polymerization = 180) was purchased from Shandong Liaocheng A Hua Pharmaceutical Co.,
80
Ltd. (Shandong Province, P.R. China). Triethylamine (Et3N), anhydrous lithium chloride
81
(LiCl), potassium carbonate (K2CO3), dimethylformamide (DMF), all of which are analytical
82
grade (AR), were obtained from Tianjin Guangfu Fine Chemical Research Institute (Tianjin,
83
P.R. China). N-methylimidazole was purchased from Sinopharm Chemical Reagent Beijing
84
Co., Ltd. Acetone was obtained from Tianjin Jiangtian Chemical Technology Co., Ltd. The
85
cellulose, anhydrous lithium chloride, and potassium carbonate were vacuum dried for 48 h
86
at 50 °C, 40 °C and 30 °C respectively, before use. Dimethylacetamide (DMAc),
87
dimethylsulfoxide (DMSO), dichloromethane (CH2Cl2) and DMF were desiccated over
88
CaH2 overnight, then filtered through silica gel; they were then vacuumed distilled and
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89
stored in brown bottles with 4A molecular sieves under N2. Other reagents were only
90
redistilled.
91
2.2. Instruments NMR spectra were recorded on a Bruker Avance 400 MHz spectrometer in CDCl3 using
93
TMS as an internal standard. The particle sizes of the samples were observed using a Tecnai
94
G2 F20 transmission electron microscope (TEM). The elemental content of palladium was
95
determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) using a
96
Thermo Jarrell-Ash corporation, ICP-9000 (N+M) instrument. X-ray photoelectron
97
spectrometry (XPS) was conducted using a PHI1600 ESCA System (Perkin-Elmer, United
98
States). The content of sulfur and nitrogen were determined by a Vario Micro cube elemental
99
analyzer. Thermogravimetric analysis (TGA) was measured with a STA 409 PC thermal
100
analyzer (NETZSCH, Germany). Fourier transform infrared spectroscopy (FTIR) analysis
101
was performed on a BIO-RAD FTS3000 IR Spectrum Scanner.
102
2.3. Preparation of Cell-NHC-Cl
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Cellulose p-toluenesulfonate (Cell-OTs) was prepared according to methods reported in
104
the literature (Rahn et al., 1996). Cell-OTs (1.91 g, 10.0 mmol) was then added to DMSO
105
(10 mL) in a 25 mL flask, with degree of substitution (DS) of 0.14, and the mixture was
106
stirred for 24 h at 60 °C followed by overnight stirring at room temperature to dissolve the
107
Cell-OTs completely. N-methylimidazole (0.86 g, 10.0 mmol) was then added to the reaction
108
mixture under nitrogen and the viscous black mixture was stirred for 12 h at 100 °C. After
109
cooling to room temperature, the mixture was added dropwise into acetone in a beaker and
110
stirred vigorously for approximately 6 h. The reaction mixture was filtered and the resulting
5 Page 5 of 26
[Cell-NHC][OTs]- was obtained. [Cell-NHC][OTs]- was then dissolved in a solution of NaCl
112
(1.15 g, 20.0 mmol) in acetone (10 mL) to replace OTs- with Cl-. The precipitate
113
(Cell-NHC-Cl) was collected via filtration. The obtained brown powder was vacuum dried
114
for 24 h at 40 °C. The DS of Cell-NHC was 0.08 determined on the base of nitrogen content.
115
2.4. Preparation of Cell-NHC-Pd complex
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Under nitrogen, Cell-NHC-Cl (0.50 g) was added to a solution of Pd(OAc)2 (0.10 g, 0.45
117
mmol) in DMF (10 mL) and the mixture was stirred for 24 h at 60 °C. After cooling to room
118
temperature, the reaction mixture was filtered. The obtained solid black product was washed
119
carefully with distilled water (3×25 mL), absolute ether (2×25 mL) and absolute ethyl
120
alcohol (2×25 mL) successively then dried under reduced pressure at room temperature to
121
give the dark palladium complex (Cell-NHC-Pd).
122
2.5 General procedure for the Suzuki reaction of aryl halides with arylboronic acids
d
M
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cr
116
The aryl halide (0.5 mmol), arylboronic acid (0.75 mmol), Cell-NHC-Pd (0.75 mol%),
124
K2CO3 (1 mmol) and DMF : H2O (1:1) (2.5 mL) were successively added into a 25 mL
125
round bottom flask. The mixture was stirred vigorously at 80 °C for the varying specific
126
length of time based on each different substrate. After reaction completion, the solid catalyst
127
was filtered off. The filtrate was extracted with ethyl acetate (3×5 mL), washed with water
128
and brine three times, and then dried over anhydrous MgSO4. The MgSO4 was then filtered
129
off. After removal of the solvent, the crude product was purified by preparative TLC (eluent:
130
petroleum ether/ethyl acetate, 20/1) to give the desired product and calculate the yield.
131
2.6 Separation of the catalyst and recycling tests
132
Ac ce pt e
123
After completion of the Suzuki–Miyaura reaction, the liquid phase was removed by
6 Page 6 of 26
133
centrifugation. The solid catalyst was simply recovered by filtration and washing. Without
134
drying, the recovered catalyst was directly used in next run with new portions of reactants.
136
3. Results and Discussion
137
3.1. Synthesis and characterization of Cell-NHC-Pd
ip t
135
Cellulose-supported NHC (Cell-NHC) was synthesized via the nucleophilic substitution of
139
Cell-OTs with N-methylimidazole as shown in Scheme 1. The tosylate acted as an effective
140
leaving group, allowing the SN2 reaction with N-methylimidazole to create Cell-NHC-OTs.
141
In order to improve the catalytic activity, OTs- was exchanged by Cl- (Kim et al., 2005). The
142
resulting Cell-NHC-Cl was then coordinated with palladium acetate in DMF to give the final
143
catalyst Cell-NHC-Pd complex. The Pd loading was determined to be 0.24 mmol/g by
144
inductively coupled plasma-atomic emission spectroscopy (ICP-AES), which was higher
145
than that of other polymer-supported NHC palladium complex reported in previous literature
146
(Lo & Luo, 2012; Kim et al., 2006).
Ac ce pt e
d
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cr
138
147 148
Scheme 1. Reaction scheme for the formation of catalyst (Cell-NHC-Pd).
149
The FTIR spectra of cellulose, cellulose derivatives Cell-OTs, Cell-NHC-Cl, and
150
Cell-NHC-Pd are shown in Fig. 1. The bands at 1359 cm-1 and 1179 cm-1 corresponds to the
7 Page 7 of 26
O=S asymmetric and symmetric stretching vibration of Cell-OTs, respectively, which are
152
greatly weakened after substitution with N-methylimidazole as shown in Fig. 1b-c.
153
Correspondingly, the N-methylimidazole graft is visible from appearance of a new band at
154
1580 cm−1, which corresponds to the stretching vibration of N–sp2 C bond (Zhao et al., 1995;
155
Kurt et al., 2000). In addition, the absorption in the 1220 cm−1 region is interpreted as the
156
vibration of N–sp3 C bond (Kurt et al., 2000; Rodil et al., 2000). As shown in Fig. 1c-d, the
157
band at 632 cm−1 region corresponds to the vibration of the imidazole ring, and the intensity
158
of its absorption is reduced after reaction with palladium acetate, suggesting the formation of
159
the NHC-Pd complex (He & Cai, 2011).
160 161
Ac ce pt e
d
M
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cr
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151
Fig. 1. FT-IR spectra of Cell (a), Cell-OTs (b), Cell-NHC-Cl (c), and Cell-NHC-Pd (d).
162
As the Suzuki reaction usually requires heating, thermogravimetric analysis (TGA) was
163
performed to evaluate the thermal stability of the cellulose-supported catalyst in air and high
164
temperature. As shown in Fig. 2, TGA of the catalyst system showed high thermal stability
165
with decomposition at around 225 °C in air. The result demonstrated that cellulose was
8 Page 8 of 26
considerably stable in an oxygen-containing atmosphere as a catalyst support in
167
experimental conditions.
an
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166
169
M
168
Fig. 2. TGA traces of Cell-NHC and Cell-NHC-Pd catalyst under air flow. XPS spectroscopy was employed to investigate the elemental coordination state of Pd
171
species. As shown in Fig. 3a, the binding energies of Pd3d5/2 in the Cell-NHC-Pd catalyst
172
were 335.3 eV and 337.4 eV, respectively (Xu et al., 2008), which revealed that both Pd (0)
173
and Pd (II) were present. Based on previous literature reports (Wu et al., 2013; Xiong et al.,
174
2013), parts of Pd (II) species were likely reduced in the existence of cellulose backbone.
Ac ce pt e
d
170
9 Page 9 of 26
ip t cr us an Ac ce pt e
d
M
175
176
10 Page 10 of 26
ip t cr us an
177
Fig. 3. XPS spectrum of fresh Cell-NHC-Pd catalyst: Pd 3d (a), C 1s (b), N 1s (c).
M
178
As shown in Fig. 3b, the status of C in Cell-NHC-Pd can be also explored by XPS
180
analysis of C 1s. The C 1s peak was divided into four components at binding energies of
181
282.8, 284.8, 286.5 and 288.0 eV, which were attributed to C=C, C–C, C–O and carbene
182
carbon to Pd bonds, respectively (Khabashesku et al., 2000; Mao et al., 2011).
183
Correspondingly, XPS of N 1s (Fig. 3c) showed 401.3 eV as the major peak and 397.4 eV as
184
a minor peak, which were assigned to N–sp2C and N–sp3C bonds (Gammon et al., 2003; Li
185
et al., 2009). Combining these results with those obtained from FTIR, it can be speculated
186
that N-methylimidazole on cellulose backbone captured the Pd species successfully, as
187
shown by the model in scheme 1.
Ac ce pt e
d
179
188
Surface morphology of the cellulose fibers was investigated with transmission electron
189
microscopy (TEM). TEM images (Fig. 4) revealed the presence of palladium nanoparticles
190
of about 9 nm in size, which proved the excellent dispersion of palladium sites on the surface
11 Page 11 of 26
of cellulose.
193 194
Fig. 4. TEM images of the fresh Cell-NHC-Pd (a and c) and recovered (b and d).
Ac ce pt e
192
d
M
an
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cr
ip t
191
195
3.2. Applying the catalyst to Suzuki cross-coupling reactions
196
3.2.1 Evaluation of its catalytic activity
197
Initial examination was carried out using p-bromoanisole (0.250 mmol, 1.0 equiv) and
198
phenylboronic acid (0.375 mmol, 1.5 equiv) as a model reaction in the presence of
199
Cell-NHC-Pd. Several variables such as the amount of catalyst, temperature, solvent and
200
base were screened to optimize the reaction conditions. Results were summarized in Table 1.
201
As shown in Table 1, the product yield increased with the increase of the amount of
202
catalyst (Table 1, entries 1-3). However, the yield decreased when the amount of catalyst
12 Page 12 of 26
increased from 0.75 to 1.25 mol% (Table 1, entries 3-5). Therefore, 0.75 mol% of the
204
catalyst (Table 1, entry 3) was sufficient to efficiently catalyze the reaction. The coupling
205
yield increased slightly from 60 °C to 80 °C (Table 1, entries 6 and 7). However, the yield
206
decreased when the temperature rose to 100 °C (Table 1, entry 8). We tried a variety of
207
solvent systems, going from pure water (Table 1, entry 12) to a mixture of an organic solvent
208
with water. The organic solvents tried were dimethylformamide (DMF), dimethylacetamide
209
(DMAc) and Ethanol (EtOH) (Table 1, entries 6-11, 13-17). Results showed that DMF : H2O
210
(1:1) was the most favorable system. Using potassium carbonate and potassium tert-butoxide
211
(Table 1, entries 7 and 16) both gave excellent yields. However, due to the hygroscopic
212
nature of potassium tert-butoxide and considering its higher cost, we opted for potassium
213
carbonate. When we increased the amount of K2CO3 to 3 equivalents, the yield rose to 98%
214
(Table 1, entry 13).
215
Ac ce pt e
d
M
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203
216
Table 1
217
Cell-NHC-Pd catalyzed Suzuki cross-coupling reaction.
218 Entry a
Amount of catalyst (mol%)
Temperature (°C)
Solvent
Base
Yield (%) b
0.25
80
DMF : H2O (1:1)
K2CO3
79
0.50
80
DMF : H2O (1:1)
K2CO3
85
3
0.75
80
DMF : H2O (1:1)
K2CO3
94
4
1.00
80
DMF : H2O (1:1)
K2CO3
92
5
1.25
80
DMF : H2O (1:1)
K2CO3
85
1 2
13 Page 13 of 26
222
60
DMF : H2O (1:1)
K2CO3
89
7
0.75
80
DMF : H2O (1:1)
K2CO3
93
8
0.75
100
DMF : H2O (1:1)
K2CO3
84
9
0.75
80
DMAc : H2O (1:1)
K2CO3
91
10
0.75
80
DMF : H2O (1:3)
K2CO3
76
11
0.75
80
EtOH : H2O (1:1)
K2CO3
86
12
0.75
80
H2O
K2CO3
22
13 c
0.75
80
DMF : H2O (1:1)
K2CO3
98
14
0.75
80
DMF : H2O (1:1)
Cs2CO3
88
15
0.75
80
DMF : H2O (1:1)
K3PO4
90
16
0.75
80
DMF : H2O (1:1)
t-BuOK
92
17
0.75
80
DMF : H2O (1:1)
Et3N
52
cr
us
an M
d
a
ip t
0.75
Reaction conditions: 4-bromoanisole (0.250 mmol), phenylboronic acid (0.375 mmol), base (0.5 mmol),
solvent (2.5 mL), heating in air for 2 h, detected by TLC. b Isolated yield. c K2CO3 was used for 3 equiv (0.75 mmol).
Ac ce pt e
219 220 221
6
Overall, the optimized conditions were as follows: phenylboronic acid (0.375 mmol),
223
4-bromoanisole (0.250 mmol), catalyst (0.75 mol%), K2CO3 (0.75 mmol) in DMF : H2O (1:1)
224
(2.5 mL) at 80 °C under air.
225
To extend the scope of the heterogeneous reaction, the Suzuki cross-coupling reaction of
226
aryl halides with various phenylboronic acids were examined as shown in Table 2. Most of
227
the aryl bromides were converted to the corresponding biaryls with good to excellent yields
228
in reasonable reaction times. Coupling of 4-bromoanisole with sterically hindered
229
phenylboronic acids proceeded with different yields (Table 2, entries 3-5 and 6-8). The
230
reaction of electron-deficient aryl bromides (4-bromobenzonitrile as a model substrate) with 14 Page 14 of 26
phenylboronic acid occurred smoothly to afford the biaryls with excellent yields within 4 h
232
at 80 °C (Table 2, entries 10-18). However, the reaction between benzene halides and
233
4-ethylphenylboronic acid was sluggish and gave only small amounts of products even with
234
prolonged reaction times (Table 2, entries 19-21). The reaction of electron-deficient
235
bromobenzene with 4-fluorine phenylboronic acid went smoothly. On the other hand, the
236
reaction of electron-rich bromobenzene with 4-fluorine phenylboronic acid was not
237
successful, with a yield of only 10%.
us
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231
238 Table 2
240
Suzuki Cross-coupling Reaction of Aryl halide with Phenylboronic acidsa.
241 Aryl halide
1
Br
3 4 5
B(OH)2
OCH 3
Ac ce pt e
2
Phenylboronic acid
Br
OCH 3
F3 C
Br
OCH 3
H3 CO
Br
OCH 3
B(OH)2
B(OH)2
7
OCH 3
Br
OCH 3
F
B(OH) 2
Br
OCH 3
10
Br
CN
78
OCH 3
1.5
85
1.5
80
1.5
66
5.0
91
OCH 3
5.0
81
OCH 3
5.0
45
5.0
93
2.0
99
OCH 3
OCH 3 OCH 3
F
OCH 3
B(OH) 2 F
F
Br
4.5
F
OCH 3
9
OCH 3
H 3 CO
B(OH)2
F
8
97
H 3CO
B(OH)2
OCH 3
Br
2.0
OCH 3
F3C
OCH3
6
Yield (%)c
B(OH)2
H3 CO
Br
Time (h)b
Product
d
Entry
M
an
239
C 2H 5
B(OH)2
C2 H5
B(OH)2
OCH 3
CN
15 Page 15 of 26
11
Br
CN
F3 C
B(OH)2
F3C
12
Br
CN
H3 CO
B(OH)2
H 3CO
13
Br
CN
CN
B(OH)2 H3 CO
CN
B(OH)2 OCH3
16
Br
CN
Br
F
F
B(OH) 2
Br
B(OH)2
19
B(OH) 2
CN
I
C 2H 5
B(OH)2
C2 H5
20
Br
C 2H 5
B(OH)2
C2 H5
21
Cl
C 2H 5
B(OH)2
CN
M
C2H5
C2 H5
F
Br
d
B(OH) 2
OCH 3
F
B(OH) 2
CH 3
242 243 244 245 246
a
96
95
83
2.3
98
4.0
61
4.0
53
4.0
33
3.5
76
6.0
10
OCH 3
F
Br
Ac ce pt e
23
2.6
F
B(OH)2
F
2.0
CN
C 2H 5
22
95
F
CN
CN
1.5 2.0
CN
F
18
97
OCH 3
F
17
4.0
ip t
CN
99
cr
Br
4.0
H3CO
CN
15
80
us
Br
CN
1.5
an
14
CN
CH 3
Unless otherwise specified, all the reactions were carried out using aryl halides (0.250 mmol), phenylboronic
acids (0.375 mmol), K2CO3 (0.75 mmol), Cell-NHC-Pd (0.75 mol%), DMF : H2O (1:1) (2.5 mL), under 80 °C in air. b Detected by TLC. c Isolated yield.
3.2.2 Separation of the catalyst and recycling tests.
247
The recyclability of the Cell-NHC-Pd catalyst was also investigated in the Suzuki
248
cross-coupling reaction of 4-bromoanisole with phenylboronic acid. After the reaction, the
249
liquid phase was removed by centrifugation and the catalyst was washed with ethyl acetate.
250
The catalyst was then used in the four additional reactions and the catalytic activity for the
251
five consecutive cycles was shown in Table 3. For the second and third reactions, only a
252
minor loss of catalytic activity was observed compared to the initial run. However, the 16 Page 16 of 26
reaction at the 4th and 5th recycled runs gave the products in 60% and 58% yields,
254
respectively. In addition, the filtrate resulting from the reaction mixture had no catalytic
255
activity under identical conditions. The total amount of palladium leaching after the 5th cycle
256
was 1.2% as detected by ICP-AES analysis (The amount of palladium is 2.55% before
257
reaction and 2.52% after 5 cycles). Therefore, the decrease of activity may be due to the
258
aggregation of palladium nanoparticles during the reaction as shown in TEM images of Fig.
259
4 (Narayanan & El-Sayed, 2003; Hu & Liu, 2005; Sin et al., 2010).
us
cr
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253
260
266 267
an
Reusability of the Cell-NHC-Pd in Suzuki cross-coupling reactiona. 1st
2nd
3rd
Time (h) b
2.6
3
3
Yield (%) c
91
85
79
a
4th
M
Recycle
5th
3.5
4
60
58
d
263 264 265
Table 3
The reactions conditions: 4-bromoanisole (0.250 mmol), phenylboronic acid (0.375 mmol), K2CO3 (0.75
Ac ce pt e
261 262
mmol), Cell-NHC-Pd (0.75 mol%), DMF : H2O (1:1) (2.5 mL), under 80 °C in air. b Detected by TLC.
c
Isolated yield.
4. Conclusions
268
In conclusion, cellulose-supported N-methylimidazole was prepared by the nucleophilic
269
substitution reaction of N-methylimidazole with cellulose tosylate. The success of the
270
grafting of N-methylimidazole onto the cellulose backbone was confirmed by FT-IR and
271
XPS analysis. Cellulose-supported N-methylimidazole was then coordinated with Pd(OAc)2
272
in DMF to give the final cellulose-tagged N-methylimidazole palladium complex, which
273
showed high thermal ability from TG analysis. Nanopalladiums sites on the surface of the
274
cellulose support were very well distributed, as demonstrated by TEM images. Moreover, 17 Page 17 of 26
this catalyst efficiently catalyzed Suzuki cross-coupling reactions and gave the
276
corresponding products in good to excellent yields. Of particular note is that the
277
Cell-NHC-Pd catalyst can be recycled by simple filtration following a Suzuki cross-coupling
278
reaction. Further applications of cellulose-based N-methylimidazole in other organic
279
reactions are currently being investigated in our laboratory.
ip t
275
cr
280
Acknowledgment
282
This work was supported by National Natural Science Foundation of China (No. 20972109).
us
281
an
283
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Highlights: NHC ligand was introduced onto cellulose backbone to explore an avenue for polysaccharide-based catalysts. Cellulose-supported NHC-Pd complex can efficiently promote Suzuki coupling reactions. Catalysts can be recovered by easy filtration and can be used several times.
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