Magnetically Recyclable Pd Catalyst for C

Article Cite This: Org. Process Res. Dev. 2019, 23, 1321−1332

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A Novel Water-Dispersible/Magnetically Recyclable Pd Catalyst for C−C Cross-Coupling Reactions in Pure Water Sara Sobhani,* Azam Habibollahi, and Zohre Zeraatkar Department of Chemistry, College of Sciences, University of Birjand, Birjand, Iran

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ABSTRACT: A novel water-dispersible magnetically recyclable Pd heterogeneous catalyst, denoted as Pd-γ-Fe2O3-2-ATPTEG-MME, was initially synthesized and then characterized by diverse methods such as FT-IR, TEM, TGA, XPS, VSM, ICP, and elemental analysis. The new catalyst was utilized as a water-dispersible/magnetically separable Pd heterogeneous catalyst for C−C cross-coupling reactions including cyanation of aryl halides, fluoride-free Hiyama and Suzuki reactions in neat water. By using this approach numerous arylcyanides and biaryls were synthesized in good to high yields via the reaction of aryl iodides, bromides, and chlorides (far more extensively available and cheaper than aryl iodides and bromides) with K4[Fe(CN)6]·3H2O, triethoxyphenylsilane, or phenyl boronic acid, respectively. The presence of triethylene glycol tags with hydrophilic character on the Pd-complex supported on magnetic nanoparticles provides dispersion of the catalyst particles in water, which leads to both higher catalytic performance and also facile catalyst recovery and reuse by successive extraction and final magnetic separation. Using water as a green solvent, high turnover number (TON), facile catalyst recovery and reuse, simple workup, and not requiring any additive make this method an ecofriendly protocol for the C−C cross-coupling reactions. KEYWORDS: palladium, heterogeneous catalyst, water, cyanation reaction, Hiyama reaction, Suzuki reaction

1. INTRODUCTION Coupling reactions with the capability of new C−C bond formation catalyzed by transition metals are fundamental tools to promote the synthesis of natural products and structurally complex compounds.1 A number of transition metals such as palladium, nickel, and copper have been developed to catalyze coupling reactions. Since the pioneering study, homogeneous palladium catalysts have attracted a great deal of attention owing to their better functional group tolerance, as well as high catalytic activity. Generally, homogeneous catalysis can provide excellent yield, while heterogeneous catalysis offers the advantages of simplified product separation and recycling of the catalyst.2,3 These properties make heterogeneous catalysts as suitable candidates for green chemistry catalysis due to their great stability, durability, cost effectiveness, and diminishing heavy metal environmental contamination.4 In this regard, a variety of heterogeneous Pd catalysts have been prepared through the immobilization of homogeneous Pd catalysts on the surface of different solid substances such as silica,5 zeolite,6 carbon,7 graghene,8 graphene oxide,9 mesoporous materials,10 and polymers.11 Nevertheless, the mentioned catalysts still demand the tedious and time-consuming chemical or physical procedures, such as filtration or centrifugation for their isolation. Consequently, the introduction of the simply isolable heterogeneous Pd catalysts without much difficulty of metal contamination in the final products while preserving the high accessibility and catalytic activity is particularly desirable. Magnetic separation based method is developed as an appropriate strategy, which complies with these requirements.12 It has emerged as a robust, highly efficient and fast separation tool compared with conventional product/catalyst isolation. Depositing of the catalysts on magnetic nanoparticles (MNPs) as supporting material provides an opportunity to © 2019 American Chemical Society

recover the catalyst from the reaction mixture simply using a magnetic field.13 Moreover, MNPs as a nanosupporting material act as a semi-heterogeneous catalyst, which cover the gap between homogeneous and heterogeneous catalytic systems in terms of the activity owing to their high surface area-to-volume ratio and excellent accessibility.14 However, in most cases, the effectiveness and selectivity of heterogeneous catalysts supported on MNPs are not desirable in water as an eco-benign solvent. To conquer this problem, additives including the phase transfer agents,15 polymers,16 surfactants,17 cyclodextrins,18 organic cosolvents,19 or ionic liquids20 have been used. A more desirable approach, which avoids using any additives, is designing water-dispersible MNPs. This allows that reactions can be accomplished in pure water under near homogeneous conditions and also allows catalyst separation from the reaction mixture easily using an external magnetic field. Although the fabrication of water dispersible MNPs by the surface modification with hydrophilic groups have been followed by numerous chemists for biorelated usages,21−24 there are few articles on the synthesis of magnetically recyclable water-dispersible catalysts and their applications in organic transformations in neat water.25−27 Therefore, there is still much demand for developing new magnetically isolable water-dispersible catalysts in pure water. In the line of our recent efforts toward the development of the greener catalyzed reactions,28−30 herein, we have synthesized a novel waterdispersible/magnetically recyclable Pd heterogeneous catalyst (Pd-γ-Fe2O3-2-ATP-TEG-MME, Scheme 1) and characterized it by different methods. We have used this catalyst as a waterdispersible/magnetically recyclable Pd catalyst for C−C Received: December 6, 2018 Published: June 3, 2019 1321

DOI: 10.1021/acs.oprd.8b00426 Org. Process Res. Dev. 2019, 23, 1321−1332

Organic Process Research & Development

Article

Scheme 1. Pd-γ-Fe2O3-2-ATP-TEG-MME

Scheme 2. Synthesis of Pd-γ-Fe2O3-2-ATP-TEG-MME

2.1.2. Synthesis of 2-ATP-TEG-MME. A mixture of 2aminothiophenol (2-ATP) (0.1 mL, 1 mmol), TsO-TEGMME (0.47 g, 1.5 mmol) and K2CO3 (0.30 g, 2.2 mmol) in CH3CN (5 mL) was stirred under argon at 60 °C for overnight. The reaction mixture was extracted with CH2Cl2. The organic phase was dried over MgSO4 and concentrated under reduced pressure. The oily residue was purified by column chromatography with n-hexane:ethyl acetate (1:1) as eluent to afford 2-ATP-TEG-MME. 2.1.3. Synthesis of γ-Fe2O3-2-ATP-TEG-MME. Chlorofunctionalized γ-Fe2O332 (2 g) was sonicated in dry DMF (20 mL) for 30 min. A solution of 2-ATP-TEG-MME (0.5 mmol, 0.143 g) in DMF (10 mL) and NaOH (0.5 mmol, 0.02 g) was added dropwise to the chloro-functionalized γ-Fe2O3 dispersed in DMF and was refluxed for 48 h under Ar atmosphere. The resulting light-brown solid was separated using an external magnet, repeatedly washed with DMF, EtOH, and MeOH (3 × 20 mL), and finally dried in an oven at 90 °C under a vacuum. 2.1.4. Synthesis of Pd-γ-Fe2O3-2-ATP-TEG-MME. γ-Fe2O32-ATP-TEG-MME (2 g) was sonicated in MeOH (20 mL) for 30 min. A solution of Pd(OAc)2 (0.3 mmol, 0.067 g) in MeOH (10 mL) was added to the dispersed γ-Fe2O3-2-ATPTEG-MME in MeOH and stirred under reflux for 10 h. The reaction mixture was cooled to room temperature. The solid was separated using an external magnet, washed several times

(arylcyanation, Hiyama, and Suzuki) cross-coupling reactions in neat water. Incorporation of triethylene glycol (TEG) functionality with hydrophilic character into Pd-complex supported on MNPs provides dispersion of catalyst particles in water, which leads to higher catalytic performance and also facile catalyst recovery and reuse.

2. EXPERIMENTAL SECTION 2.1. Synthesis of the Pd-γ-Fe2O3-2-ATP-TEG-MME. 2.1.1. Synthesis of TsO-TEG-MME.31 A solution of NaOH (0.58 g, 15 mmol) in water (3 mL) was added to a solution of triethylene glycol monomethyl ether (TEG-MME) (1.64 g, 10 mmol) in THF (3 mL), and the whole mixture was cooled to 0−5 °C by means of an ice−water bath. A solution of tosyl chloride (1.9 g, 10 mmol) in THF (30 mL) was added dropwise to the stirring mixture over 2 h. After completion of the addition, the reaction mixture was stirred at the same temperature for an additional 2 h. The reaction mixture was then poured into a large volume of ice cold water and extracted with CH2Cl2. The organic layer was separated, washed several times with water, dried over anhydrous MgSO4, and evaporated in a rotary evaporator. The obtained product was purified by column chromatography with n-hexane:ethyl acetate (1:1) as eluent. 1322



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Figure 1. TEM images of γ-Fe2O3 (a,b) and Pd-γ-Fe2O3-2-ATP-TEG-MME (c,d), and particle size distribution histogram for Pd-γ-Fe2O3-2-ATPTEG-MME (e).

with MeOH (3 × 20 mL), and dried in an oven at 80 °C overnight under a vacuum. 2.2. General Method for the Cyanation Reaction of Aryl Halides by K4[Fe(CN)6]·3H2O. Pd-γ-Fe2O3-2-ATPTEG-MME (0.2 mol %) was added to the stirred mixture of aryl halide (1 mmol), K4[Fe(CN)6]·3H2O (1 mmol), and Et3N (6 mmol) in H2O (10 mL) and heated at 80 °C. The reaction was monitored using TLC. After a proper time (see Table 2), the reaction mixture was allowed to be cooled to ambient temperature. The organic compound was extracted with EtOAc (3 × 5 mL) from aqueous layer while the catalyst was left in the aqueous phase. The organic phase was dried by anhydrous Na2SO4 and filtered. The organic solvent was evaporated by vacuum distillation to obtain a crude organic mixture, which was purified using column chromatography packed by silica gel (n-hexane:EtOAc = 6:1) to afford the desired product. The aqueous phase which contains the catalyst was used for another run by adding aryl halide (1 mmol), K4[Fe(CN)6]·3H2O (1 mmol) and Et3N (6 mmol) and heating at 80 °C. 2.3. General Method for the Hiyama Coupling Reaction of Aryl Halides with Triethoxyphenylsilane. Pd-γ-Fe2O3-2-ATP-TEG-MME (0.1 mol %) was added to the stirred mixture of aryl halide (1 mmol), triethoxyphenylsilane (1 mmol), and Et3N (2 mmol) in H2O (10 mL) and heated at 80 °C. The reaction was monitored using TLC. After a suitable time (see Table 3), the reaction mixture was allowed to be cooled to ambient temperature. The organic compound was extracted with EtOAc (3 × 5 mL) from aqueous phase while the catalyst was left in the aqueous phase. The organic layer was concentrated in vacuum after drying over Na2SO4. The crude product was then purified using column chromatography packed by silica gel (n-hexane:EtOAc = 30:1) to afford the desired product. 2.4. General Method for the Suzuki Coupling Reaction of Aryl Halides with Phenyl Boronic Acid. Pd-γ-Fe2O3-2-ATP-TEG-MME (0.01 mol %) was added to the stirred mixture of aryl halide (1 mmol), phenyl boronic acid (1 mmol) and Et3N (2 mmol) in H2O (10 mL) and heated at 80 °C. The reaction was monitored using TLC. After passing the

time mentioned in Table 4, the reaction mixture was allowed to be cooled to ambient temperature. The organic compound was extracted with EtOAc (3 × 5 mL) from aqueous layer while the catalyst was left in the aqueous phase. The organic layer was dried by anhydrous Na2SO4 and filtered. The organic solvent was evaporated in vacuum to produce the crude product. At the end, the pure product was obtained by column chromatography using silica gel (n-hexane:EtOAc = 30:1).

3. RESULTS AND DISCUSSION 3.1. Synthesis of Pd-γ-Fe2O3-2-ATP-TEG-MME. MNPsupported water-dispersible Pd-complex was synthesized by

Figure 2. FT-IR spectrum of Pd-γ-Fe2O3-2-ATP-TEG-MME.

following the procedure that is shown in Scheme 2. Initially, 2aminothiophenol (2-ATP) was reacted with tosylated triethylene glycol monomethyl ether (TsO-TEG-MME) to generate 2-aminothiophenol that contains triethylene glycol moiety (2-ATP-TEG-MME). Grafting 2-ATP-TEG-MME onto the surface of γ-Fe2O3 was performed by the reaction 1323



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Figure 3. TGA diagram of γ-Fe2O3-2-ATP-TEG-MME.

Figure 5. Magnetization curves of γ-Fe2O3 and Pd-γ-Fe2O3-2-ATPTEG-MME.

Table 1. Cyanation Reaction of Iodobenzene with K4[Fe(CN)6]·3H2O in Pure Water under Different Conditions entry

catalyst (mol %)

base

T (°C)

time (h)

isolated yielda (%)

1 2 3 4 5 6 7 8 9 10b 11c 12 13 14 15d

0.2 0.2 0.2 0.2 0.2 0.3 0.4 0.5 0.1 0.2 0.2 0 0.2 0.2 0.2

Pyridine K2CO3 KOH Et3N − Et3N Et3N Et3N Et3N Et3N Et3N Et3N Et3N Et3N Et3N

80 80 80 80 80 80 80 80 80 80 80 80 50 r.t. 80

10 8 7 6 24 6 6 6 11 10 10 24 7 10 17

50 61 68 99 Trace 99 99 99 81 50 72 0 59 10 11

a K4[Fe(CN)6]·3H2O = 1 equiv (except for entry 11); base = 6 equiv (except for entry 10); Pd-γ-Fe2O3-2-ATP-TEG-MME (except for entries 9 and 12); Mean ± (1−3)% standard deviation (number of replicates = 3). bBase = 3 equiv. cK4[Fe(CN)6]·3H2O = 0.5 equiv. d Pd-γ-Fe2O3-2-ATP.

the structure of the synthesized materials was performed by different techniques including TEM, FT-IR, TGA, ICP, XPS, VSM, and elemental analysis. The TEM images of γ-Fe2O3 and Pd-γ-Fe2O3-2-ATP-TEGMME are presented in Figure 1. By comparison of the TEM images of Pd-γ-Fe2O3-2-ATP-TEG-MME (Figure 1c,d) with those of γ-Fe2O3 (Figure 1a,b), it is seen that γ-Fe2O3 modified by hydrophilic Pd complex was highly dispersed without any agglomeration. It is noted that the shape and particle size of Pd-γ-Fe2O3-2-ATP-TEG-MME did not change after functionalization. The size distribution histogram of Pd-γ-Fe2O3-2ATP-TEG-MME (Figure 1e) shows the size uniformity of spherical MNPs with a mean diameter of ∼17 nm. The surface modification of γ-Fe2O3-2-ATP-TEG-MME was characterized by FT-IR spectroscopy (Figure 2). In the FT-IR

Figure 4. XPS of Pd-γ-Fe2O3-2-ATP-TEG-MME: overall elemental survey spectrum (a), Pd core level spectrum (b).

of chloro-functionalized γ-Fe2O3 with 2-ATP-TEG-MME (γFe2O3-2-ATP-TEG-MME). Finally, the catalyst (Pd-γ-Fe2O32-ATP-TEG-MME) was produced by the reaction of Pd(OAc)2 with γ-Fe2O3-2-ATP-TEG-MME. Characterization of 1324



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spectrum, the characteristic peaks around 573−637 and 1017− 1038 cm−1 were observed due to the Fe−O and Si−O bonds, respectively. Observed peaks at 3440 and 3484 cm−1 could be related to the N−H stretching frequency. The presence of these bands and also peaks at 1435 and 1541 cm−1 (CC) and 2930 cm−1 (CH2) confirmed the successful immobilization of 2-ATP-TEG-MME on the surface of chloro-functionalized γ-Fe2O3. Moreover, appearance of the peak at 1625 cm−1 is a good affirmation for the presence of Pd(OAc)2 on the catalyst surface. Thermal stability of γ-Fe2O3-2-ATP-TEG-MME was investigated by TG analysis (Figure 3). The TGA curve of γ-Fe2O32-ATP-TEG-MME showed an initial weight loss, which was ascribed to the probable loss of the solvent and water trapped onto the surface. The main weight decrease in the second step was due to the removal of organic moieties on the surface. These results showed that the catalyst has reasonable thermal stability. ICP analysis of the catalyst showed that 0.12 mmol of Pd is immobilized on 1 g of the catalyst. The amount of organic moieties in the catalyst was measured to be 0.12 mmol g−1 based on the elemental analysis (C = 2.14%, N = 0.17%). XPS is performed to investigate the chemical state of the surface of Pd-γ-Fe2O3-2-ATP-TEG-MME. Peaks of the oxygen, carbon, nitrogen, palladium, sulfur, and iron are observed in the XPS curve (Figure 4a). As shown in Figure 4b, the spectrum of the Pd 3d region of the catalyst confirmed the presence of Pd (II) with the peak binding energy of 342.8 and 337.6 eV, which are assigned to Pd 3d3/2 and Pd 3d5/2, respectively. The magnetic properties of Pd-γ-Fe2O3-2-ATP-TEG-MME and γ-Fe2O3 were evaluated by vibrating sample magnetometer (VSM) at ambient temperature (Figure 5). The saturation magnetization amount of Pd-γ-Fe2O3-2-ATP-TEG-MME was 64.1 emu/g, which is like γ-Fe2O3 (68.6 emu/g). The low loss of the saturated magnetization value of Pd-γ-Fe2O3-2-ATPTEG-MME compared to that of γ-Fe2O3 can be related to the small increase in the mass as a result of the supported Pdcomplex on the γ-Fe2O3 surface. The magnetization curves revealed no hysteresis loop which pointed to the superparamagnetic characteristic of the MNPs. The strong magnetic features of the MNPs were adequate for entire and simple magnetic isolation by attraction to an external magnet. 3.2. C−C Coupling Reaction in the Presence of Pd-γFe2O3-2-ATP-TEG-MME as a Water- Dispersible/Magnetically Recoverable Pd Catalyst in Pure Water. 3.2.1. Arylcyanation of Aryl Halides. Aryl nitriles are important structural units of various natural products, polymers, pharmaceuticals, agrochemicals, and dyes.33−35 Nitriles are also key intermediates for the heterocyclic synthesis, since they can be simply converted to a range of functional groups such as carboxylic acids, oximes, amines, amides, and ketones.36,37 Sandmeyer and Rosenmund−von Braun reaction are well-known classic protocols for the aryl nitriles synthesis.38,39 These methods suffered from serious problems, such as requiring stoichiometric amounts of hyper toxic copper(I) cyanide and high temperature. Cross-coupling reaction between aryl halides and metal cyanides such as KCN, NaCN, CuSCN, and Zn(CN)2 catalyzed by transition metals is an efficient alternative for the synthesis of aryl nitriles in rather mild reaction conditions and with high yields.40−44 However, since many of the cyanide sources are highly toxic, the industrial applications of the reaction is limited to some

Table 2. Cyanation Reaction of Aryl Halides with K4[Fe(CN)6]·3H2O Catalyzed by Pd-γ-Fe2O3-2-ATP-TEGMME in Pure Water

entry

Ar

X

time (h)

isolated yielda (%)

obtained mp (°C)

reported mp (°C)

154 255 354

C6H5 4-I-C6H4 4-MeOC6H4 C6H5 4-O2NC6H4 4-MeOC6H4 4-NCC6H4 4-MeC6H4 4-F-C6H4 3-pyridyl C6H5 4-O2NC6H4 4-NCC6H4 4-MeC6H4

I I I

6 12 14

99 63 96

−b 120−122 58−60

−b 121−123 57−58

Br Br

10 9

98 82

−b 140−142

−b 141−143

Br

9

70

58−60

57−58

Br

10

91

200−204

204−206 −b

454 554 654 755 54

8

955 1054 1154 1255 1354 1455

Br

16

65

−

Br Br Cl Cl

17 15 10 16

83 62 84 52

35−37 51−53 −b 140−142

34−35 50−52 −b 141−143

Cl

12

82

200−204

204−206

Cl

10

53

−b

−b

b

a

Reaction conditions: K4[Fe(CN)6] = 1 equiv, Pd-γ-Fe2O3-2-ATPTEG-MME = 0.2 mol %, Et3N = 6 equiv, 80 °C; Mean ± (1−5)% standard deviation (number of replicates = 3). bLiquid.

Figure 6. Dispersion of Pd-γ-Fe2O3-2-ATP-TEG-MME in water (a), distribution of Pd-γ-Fe2O3-2-ATP-TEG-MME in a biphasic water/ CHCl3 system (b), distribution of Pd-γ-Fe2O3-2-ATP-TEG-MME in a biphasic water/EtOAc (c), and easy separation of Pd-γ-Fe2O3-2-ATPTEG-MME using an external magnet (d).

Figure 7. Recycling of the Pd-γ-Fe2O3-2-ATP-TEG-MME in the cyanation reaction of 4-iodobenzene with K4[Fe(CN)6]·3H2O.

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Figure 8. TEM image (a,b) and particle size distribution histogram (c) of the recycled catalyst after 9th reaction run.

solvent of the reaction and reusable catalysts are highly desirable goals. To the best of our knowledge, cyanation reactions as a seminal synthetic transformation is still in its infancy in the case of using green solvent system.49−53 In the limited literature reports in which water was used as a reaction media, additives such as phase transfer catalyst, polyethylene glycol, surfactant, and organic solvents have been used. Moreover, less attention has been paid to applying the heterogeneous catalytic systems in the cyanation reaction of aryl halides. In this paper, we have become interested in developing Pd-γFe2O3-2-ATP-TEG-MME as a novel water-dispersible/magnetically recyclable Pd heterogeneous catalyst for the aromatic cyanation, an unusual coupling reaction in pure water. It is worth mentioning that we are the first to report cyanation procedure for aryl halides with K4[Fe(CN)6]·3H2O as a safe cyanide source mediated in pure water as an eco-benign media. At first, the coupling reaction of iodobenzene with K4[Fe(CN)6]·3H2O (molar ratio = 1:1) in water was evaluated as a model reaction to explore the base, temperature, and the amount of the catalyst effect on the progress of the reaction (Table 1). The model reaction was studied in the presence of different bases (6 equiv) at 80 °C using 0.2 mol % of Pd-γFe2O3-2-ATP-TEG-MME (entries 1−4). A superior yield was obtained when Et3N was used as the base (entry 4). The product was obtained in a trace amount in the absence of the base (entry 5). Any further improvement of the product yield was not detected by increasing the amount of the catalyst (entries 6−8). These observations showed that there is not any mass transport limitation in this reaction. In fact, all active sites of the heterogeneous nanocatalyst are well accessible during

Figure 9. FT-IR spectrum of γ-Fe2O3-2-ATP-TEG-MME after 9th reaction run.

examples. Recently, Beller et al. reported K4[Fe(CN)6]·3H2O as a benign cyanide source for the cyanation reaction of aryl halides.45−48 In comparison with the existing cyanide sources, K4[Fe(CN)6]·3H2O is nontoxic, nonhygroscopic, easy to handle, commercially available, and very economic. The high affinity of metal catalysts to cyanide groups can be pointed as a common drawback of cyanation reactions catalyzed by metals. Notably, the slow release of cyanide ions from K4[Fe(CN)6]· 3H2O improves the metal catalyst efficiency. From environmental and economic viewpoints, both the use of water as the 1326



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Figure 10. Time-dependent correlation of the yield of cyanation reaction in the absence (a) and in the presence (b) of 3-mercaptopropyl functionalized γ-Fe2O3 scavenger.

Table 3. Fluoride-Free Hiyama Cross-Coupling Reaction of Halobenzenes with Triethoxyphenylsilane Catalyzed by Pdγ-Fe2O3-2-ATP-TEG-MME in Pure Water

entry

Ar

X

time (h)

168 269 368

C6H5 4-I-C6H4 4-MeOC6H4 C6H5 4-O2NC6H4 4-MeOC6H4 4-NCC6H4 C6H5 4-O2NC6H4 4-NCC6H4

I I I

0.5 11 14

90 91 96

71−73 107−109 77−79

70−72 108.3−109.2 78−80

Br Br

4 10

92 51

71−73 110−112

70−72 112−114

Br

4

75

77−79

78−80

Br

11

71

81−83

80−82

Cl Cl

5 10

90 51

71−73 110−112

70−72 112−114

Cl

15

72

81−83

80−82

468 568 668 68

7

868 968 1068

isolated yielda (%)

obtained mp (°C)

reported mp (°C)

Table 4. Suzuki Cross-Coupling Reaction of Various Aryl Halides with Phenyl Boronic Acid Catalyzed by Pd-γ-Fe2O32-ATP-TEG-MME in Aqueous Media

entry

Ar

X

time (h)

isolated yielda (%)

1 2 3 4 5 6 7 8 9 10

C6H5 4−I-C6H4 4-MeO-C6H4 C6H5 4-O2N−C6H4 4-MeO-C6H4 4-NC-C6H4 C6H5 4-O2N−C6H4 4-NC-C6H4

I I I Br Br Br Br Cl Cl Cl

15 min 4 2.5 45 min 1 3.5 4.5 2.5 0.5 5

98 95 98 91 98 98 92 90 98 80

a

Aryl halide (1 mmol), phenyl boronic acid (1 mmol), catalyst (0.01 mol %), Et3N (2 mmol), 80 °C; Mean ± (1−3)% standard deviation (number of replicates = 3).

with K4[Fe(CN)6]·3H2O in pure water (Table 2). As depicted in Table 2, different aryl iodides, bromides and chlorides (far extensively available and cheaper than aryl iodides and bromides) containing electron-withdrawing and electronreleasing groups underwent cynanation reaction to afford the corresponding products in good to high yields. From both the practical and environmental viewpoints, the recovery and recycling of supported catalysts are very important issues. Since the main goal of this study was the development of a water-dispersible/magnetically recyclable Pd heterogeneous catalyst, we investigated the catalyst reusability in the model reaction of iodobenzene with K4[Fe(CN)6]· 3H2O under optimized reaction conditions. Due to the presence of hydrophilic TEG tag in the catalyst, Pd-γ-Fe2O32-ATP-TEG-MME was dispersed in the aqueous phase without any affinity to the organic phase (Figure 6a−c). Therefore, after first run, the product was easily extracted by EtOAc without any isolation of the catalyst from aqueous phase (Figure 6c). The aqueous phase was recharged with iodobenzene, K4[Fe(CN)6]·3H2O, and Et3N for the next run. As shown in Figure 7, the catalyst was reused for nine consecutive runs without any significant decrease of the catalyst activity. The average yield of the product, and also average TON (mmol of the product per mmol of the catalyst) of the catalyst were 96% and 480, respectively. Finally, the

a

Aryl halide (1 mmol), triethoxyphenylsilane (1 mmol), catalyst (0.1 mol %), Et3N (2 mmol), 80 °C; Mean ± (1−4)% standard deviation (number of replicates = 3).

the reaction. Subsequently, the reaction was tried with the lower amounts of the catalyst, Et3N as the base or K4[Fe(CN)6]·3H2O as the cyanide source. Under these conditions the desired product was produced in lower yields (entries 9−11). The reaction showed no appreciable conversion in the absence of the catalyst, which proves the pivotal role of the catalyst for this reaction process (entry 12). Fascinatingly, an intense drop in the catalyst activity was observed by lowering the reaction temperature (entries 13 and 14). We also evaluated the impact of TEG tag in the catalytic activity of Pd-γ-Fe2O3-2-ATP-TEG-MME by employing Pd-γFe2O3-2-ATP as a catalyst with no TEG tag in the model reaction (entry 15). The catalytic activity dramatically was decreased in the absence of TEG. In fact, the hydrophilic character of TEG allowed a good dispersion of Pd-γ-Fe2O3-2ATP-TEG-MME in the aqueous medium and enhanced the catalyst efficiency. Having optimized reaction conditions, the scope of cyanation reaction catalyzed by Pd-γ-Fe2O3-2-ATP-TEGMME was studied for the reaction of various aryl halides 1327



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Figure 11. Reusability of Pd-γ-Fe2O3-2-ATP-TEG-MME in Hiyama and Suzuki reactions.

Scheme 3. Structure of Some Drugs with Biaryl Moiety Core

these compounds, which leads to the production of undesirable homocoupling products. However, due to the weak polarization of the carbon−silicon bond in organosilicon derivatives, they have been known as poor cross-coupling partners. Therefore, the nucleophilicity of the organosilicon reagent needs to be enhanced. Fluoride anions are the most commonly employed activators for this purpose.58,59 However, since the fluoride ion is a strong base, functional groups such as base-sensitive protecting groups and acidic protons cannot tolerate the presence of fluoride ions. This limitation has been overcome by replacement of fluoride ions with inorganic bases in water. The reported procedures in water suffered from one or more following drawbacks such as using organic solvents, phase transfer catalyst, surfactant, or large amount of the catalyst.60−63 Following our research on the introduction of new catalytic methods to perform cross-coupling reactions by environmentally friendly procedures,64−67 we investigated the catalytic activity of Pd-γ-Fe2O3-2-ATP-TEG-MME in fluoridefree Hiyama coupling reaction in pure water. The results of this study are depicted in Table 3. Table 3 clearly shows that various halobenzenes underwent the cross-coupling reaction with triethoxyphenylsilane and the desired products produced in good to high yields without requiring any fluoride ion. It is worth mentioning that any homocoupling reaction did not occur. Promising results obtained from Hiyama reaction encouraged us to investigate the catalytic activity of Pd-γ-Fe2O3-2-ATP-TEG-MME for Suzuki cross-coupling reaction of halobenzenes with phenyl boronic acid in water (Table 4). As the results of Table 4 show, this catalytic method is effective for the coupling reaction of different kinds of iodo, bromo, and chlorobenzenes with phenyl boronic acid. The recyclability of Pd-γ-Fe2O3-2-ATP-TEG-MME was studied in a model reaction of iodobenzene in Hiyama and Suzuki cross-coupling reactions with triethoxyphenylsilane and phenyl boronic acid, respectively under optimized reaction conditions. After 0.5 h, EtOAc was added to the reaction mixtures. The product was simply extracted by EtOAc, while

catalyst was isolated from the aqueous phase by the aid of an external magnet (Figure 6d). TEM images of the recycled catalyst showed that the morphology and structure of the catalyst remained intact even after nine recoveries (Figure 8a,b). Significantly, on the basis of the particle size distribution histogram of the ninth reused catalyst (Figure 8c), the size of the catalyst nanoparticles, as in the fresh one, were estimated to be ∼17 nm. Also, the FT-IR spectrum of the catalyst after nine times reuse exhibited that the catalyst structure maintained unchanged during the recycling process (Figure 9). To find out the real heterogeneous nature of the catalyst, hot filtration and poisoning tests were conducted. In the hot filtration test, once the 50% of cyanation reaction was proceeded, the catalyst was isolated and the liquid phase was permitted to react for 24 h. No further conversion proved the absence of any homogeneous catalyst in the reaction mixture. In the poisoning test, 3-mercaptopropyl functionalized γ-Fe2O3 was used as an efficient Pd scavenger. Toward this point, the reaction of iodobenzene (1 mmol), K4[Fe(CN)6]·3H2O (1 mmol), Et3N (6 mmol), and Pd-γ-Fe2O3-2-ATP-TEG-MME (0.2 mol %) was evaluated in the absence and in the presence of 3-mercaptopropyl functionalized γ-Fe2O3 (0.05 g) at 80 °C. The results are illustrated in Figure 10. As it is apparent, the presence of scavenger has no effect on the reaction progress and the desired product was obtained in excellent yield after 6 h. This finding clearly confirmed the real heterogeneous nature of the catalyst. 3.3. Hiyama and Suzuki Reactions. Among the palladium-catalyzed C−C couplings, Hiyama cross-coupling reaction is an efficient tool in organic chemistry for the synthesis of biphenyl derivatives.56 In these coupling reactions organosilicon compounds are used. Such compounds are the best alternative rather than the other organometallic reagents such as organozinc or organomagnesium compounds and tin reagents from an environmental perspective, convenient preparation, cost effectiveness, stability, and availability.57 They are also preferable to organoboron compounds due to the instability, difficult purification, and frequent boron loss of 1328



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Table 5. Comparison of Catalytic Activity of Pd-γ-Fe2O3-2-ATP-TEG-MME with Some Other Reported Pd-Catalysts for the Cyanation, Hiyama, and Suzuki Reactions in Water catalyst

(mol %)

entry

reaction

Ar-X

b

1

Cyanation

Pd-γ-Fe2O3-2-ATP-TEG-MME

0.2

251

Cyanation

Pd/C

10

352

Cyanation

Pd(OAc)2

2

I Br Cl I Br Cl OMs

450

Cyanation

Pd(OAc)2

5

OTs I

552

Cyanation

Pd(dba)2

2

Br Br

6b

Hiyama


0.1

760

Hiyama

Fe3O4@SiO2/ APTMSd/Pd(cdha)e

0.2

862 963

Hiyama Hiyama

Pd NPs/Euphorbia thymifolia Colloidal palladium

1 1

I Br Cl I Br Cl Br Br

10b

Suzuki


0.01

I

reaction conditions H2O/Et3N/80 °C

KI/NaF/H2O:PEG 4000/MW/100−160 °C t-BuOH:H2O/K2CO3/ 80 °C NaF/TBABc/H2O/150 °C/MW KOBut/MeCN:H2O (1:1) /50 °C H2O/Et3N/80 °C

H2O/NaOH/100 °C/SDS

NaOH/H2O/90 °C NaOH/PEG-6000 in water/90 °C H2O/Et3N/80 °C

Br

1175

Suzuki

Pd/chitosan

1276

Suzuki

1377

Suzuki

Pd(OAc)2/ hierarchical MFI zeolitesupported ionic liquid Pd(OAc)2

1478 1579

Suzuki Suzuki

Pd(OAc)2 Pd(OAc)2

0.5

Cl I

0.3

Br Cl Br

0.4

Br

0.25 1

Br I Br

K3PO4/H2O/TBAB/ MW/150 °C

K3PO4/H2O/TBAB/50, 65 °C Na2CO3/H2O/TBAB/ MW/150 °C (i-Pr)2NH/H2O/100 °C Na2CO3/H2O/PEG 2000/50 °C

time (h)

isolated yield (%)

TONa

6−14 9−17 10−16 2 2−3 2 18

63−99 62−98 52−84 60−97 70−95 0−48 67−95

315−495 310−490 260−420 6−9.7 7−9.5 0−4.8 33.5−47.5

18 20 min

43−96 0−94

21.5−48 0−18.8

20 min 24

40−80 62−96

8−16 31−48

0.5−14 4−11 5−15 5 min 5 min 5 min 3−4 2−3.5

90−96 51−92 51−90 100f 63−100f 68−82f 0−96 88−98

900−960 510−920 510−900 500 310−500 340−410 0−96 88−98

15 min to 4h 45 min to 4.5 h 0.5−5 5−30 min

95−98

9500−9800

91−98

9100−9800

80−98 70−98

8000−9800 140−196

0.5−3

77−98 14−40 51−98

154−196 28−80 170−326.6

5 min

70−96

175−240

5−120 min 10−40

75−99 95−98

300−396 95−98

15−60

85−96

85−96

a

Turnover number. bThis work. cTetrabutyl ammonium bromide. d3-Aminopropyltrimethoxysilane. eBis(2-chloro-3,4-dihydroxyacetophenone). f Conversion.

considering the performing importance of large-scale reactions, in the next experiment we have evaluated the scalability of the Hiyama reaction. To do this, the reaction of aryl iodide and triethoxyphenylsilane in a scaled-up procedure (50 times) in the presence of Pd-γ-Fe2O3-2-ATP-TEG-MME was performed successfully under the optimized reaction conditions. Interestingly, the scaled-up reaction was accompanied by the 88% isolated yield of the desired product. Finally, the activity of Pd-γ-Fe2O3-2-ATP-TEG-MME was compared with some reported Pd catalysts in the cyanation, Hiyama, and Suzuki reactions in water (Table 5). As depicted in Table 5, Pd-γ-Fe2O3-2-ATP-TEG-MME is the most efficient catalyst for the C−C coupling reactions of aryl iodides, bromides, and chlorides (the most challenging aryl halides, which are far more extensively available and cheaper than aryl iodides and bromides) in water in terms of TON. Notably, most of the reported methods suffer from lack of generality for the coupling reactions of aryl chlorides. Water dispersibility of

the catalyst remained in the aqueous layer. Then the aqueous layer was recharged with the starting material and reagents. As shown in Figure 11, the catalyst could be reused for nine consecutive cycles without any substantial loss of its reactivity. The resulting coupling products in the current study have widespread usages as vital building blocks in the preparation of various structurally different molecules in many fields of chemistry such as pharmaceuticals, material sciences, and agrochemicals.70,71 For instance, biaryl segment is the core structure of a number of drugs such as losartan (antihypertensive), felbinac (anti-inflammatory), telmisartan (antihypertensive), and knipholone (antiplasmodial)72 (Scheme 3). It also exists in numerous pesticides.72 Several synthetic protocols have been introduced for the construction of functionalized biaryls, with most of them relying on the C− C cross-coupling reactions.72,73 One of the greatest common processes includes the metal-catalyzed Hiyama coupling reactions of aryl siloxanes with aryl halides.74 So, by 1329



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the catalyst could be considered as the main reason for this observation. This property ensures the better contact between the catalyst and the reactants, and thus improves the catalytic activity and stability of the catalyst as well. Most importantly, Pd-γ-Fe2O3-2-ATP-TEG-MME can be readily isolated from the reaction mixture by means of an external magnet. Moreover, the reported synthetic routes have certain limitations such as requiring an additive or high temperature and most importantly use of unrecyclable catalyst.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.oprd.8b00426.

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REFERENCES

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4. CONCLUSIONS In summary, we have synthesized a novel water-dispersible/ magnetically recyclable Pd heterogeneous catalyst and characterized it using diverse methods such as FT-IR, TEM, TGA, XPS, VSM, ICP, and elemental analysis. The hydrophilic character of triethylene glycol allows dispersion of Pd catalyst in the aqueous medium and ensures the better contact between the catalyst and the reactants like homogeneous systems, which leads to high catalytic performance. We have used this catalyst as the first water-dispersible/magnetically recyclable Pd catalyst for C−C cross-coupling reactions including cyanation, fluoride-free Hiyama and Suzuki reactions in water. By this method various aryl iodides, bromides, and chlorides (far more extensively available and cheaper than aryl iodides and bromides) reacted with K4[Fe(CN)6]·3H2O, triethoxyphenylsilane, and phenyl boronic acid to afford the desired products in good to high yields. Due to the extremely low solubility of the catalyst in organic solvents, the separated aqueous phase which contains the catalyst can be readily recycled for nine sequential runs without a noteworthy loss in activity. Finally, the catalyst can be simply isolated from the aqueous layer using an external magnet. Hot filtration and poisoning tests proved the true heterogeneous nature of the catalyst. Using water as an environmentally benign reaction media, high turnover number (TON), facile catalyst recovery and reuse by successive extraction and final magnetic separation, simple workup, and not requiring any additive or promoter make this method appealing from the environmental and economic viewpoints for C−C coupling reactions.

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General information and spectral data of the products (PDF)

AUTHOR INFORMATION

Corresponding Author

*Fax: +98 56 32202065. Tel: +98 56 32202065. E-mail: [email protected], [email protected]. ORCID

Sara Sobhani: 0000-0002-7764-8847 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support of this project by University of Birjand Research Council and Iran National Science Foundation (INSF) is appreciated. 1330



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Magnetically Recyclable Pd Catalyst for C

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