Scholarly article on topic 'Nanocopper-Catalyzed Cross-Coupling Reaction for the Synthesis of Diarylethers'

Nanocopper-Catalyzed Cross-Coupling Reaction for the Synthesis of Diarylethers Academic research paper on "Materials engineering"

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Abstract of research paper on Materials engineering, author of scientific article — Duangruthai Phithak, Siripong Sremethean, Jakkrapan Arjsalee, Pranorm Saejueng

Abstract Nanocopper-catalyzed cross-coupling reaction of phenol and aryl halides for the synthesis of diarylethers is reported. With the optimum conditions of using nanocopper as catalyst, 1,10-phenanthroline as a ligand, cesium carbonate as a base, in DMF at 110°C under nitrogen atmosphere, varieties of diarylethers were synthesized at moderate to good yields. This method avoids the use of palladium and/or expensive additives.

Academic research paper on topic "Nanocopper-Catalyzed Cross-Coupling Reaction for the Synthesis of Diarylethers"

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ELSEVIER APCBEE Procedía 3 (2012) 161 - 166 -

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ICCP 2012: 5-6 May 2012, Kuala Lumper, Malaysia

Nanocopper-Catalyzed Cross-Coupling Reaction for the Synthesis of Diarylethers

Duangruthai Phithaka, Siripong Sremetheana, Jakkrapan Arjsaleea,

Pranorm Saejuenga*

aDepartment of Chemistry, Faculty of Science, Ubon Ratchathani University, Warinchamrap, Ubon Ratchathani, 34190, Thailand

Abstract

Nanocopper-catalyzed cross-coupling reaction of phenol and aryl halides for the synthesis of diarylethers is reported. With the optimum conditions of using nanocopper as catalyst, 1,10-phenanthroline as a ligand, cesium carbonate as a base, in DMF at 110oC under nitrogen atmosphere, varieties of diarylethers were synthesized at moderate to good yields. This method avoids the use of palladium and/or expensive additives.

© 2012 Published bb Elsevier B.V. Selection and/or peer review under responsibility of Asia-Pacific Chemical, Biological & Environmental Engineering Society

Keywords: cross-coupling; nanocopper; diarylethers

1. Introduction

Diarylethers are important moieties found in many biologically active compounds such as vancomycin and perrottetines[1]. Palladium-catalyzed cross-coupling reaction of phenols to aryl halides is effective method that offers good to excellent yields, tolerates various function groups on substrate, and could easily couple low reactivity substrates[2-3]. However, for the large industrial scale applications, the use of expensive and airsensitive palladium catalysts and phosphine ligands limit the attractiveness of this method.

Copper is one of the attractive catalysts of choice as an alternative to palladium due to its economic point of view. A large number of methods have been developed for the formation of carbon-carbon and carbonheteroatom bonds[4-5]. Since the use of homogeneous catalysts has some drawbacks such as the need to

* Corresponding author. Tel.: +66-45-353401-0 ext 4107; Fax: +66-45-288379 E-mail address: spranorm@hotmail.com

2212-6708 © 2012 Published by Elsevier B.V. Selection and/or peer review under responsibility of Asia-Pacific Chemical,

Biological & Environmental Engineering Society

doi:10.1016/j.apcbee.2012.06.063

separate heavy metal catalyst and the contamination of the metal in product especially pharmaceutical materials, therefore the search for effective heterogeneous catalysts is still important[6]. Many heterogeneous catalysts based on copper have been developed for applications in synthesis of diarylethers such as nanocopper and nanocopper oxide [7-12]. In this work, we report the use of copper nanoparticle to catalyze cross-coupling reaction of phenol and aryl halides for the synthesis of diarylethers.

2. Results and Discussion

Copper nanoparticle used in this experiment was prepared by chemical reduction of copper(II) chloride with sodium borohydride in water/methanol mixed solvent having sodium stearate as the protecting group at 70oC under under ambient atmosphere. Through the optimizations as shown in Table 1, we found that copper catalyst is necessary for the cross-coupling of iodobenzene with phenol to synthesize the diphenylether (entry 1). A comparison between nanocopper(I) oxide (Cu2O-np) and nanocopper (Cu-np) as catalyst have shown that nanocopper is a better catalyst (entry 2 and 3) but without cesium carbonate as a base the reaction underwent with very low yield (entry 4). 1,10-Phenanthroline was chosen as the most suitable ligand (entry 3 and 5-7) and N,N-dimethylformamide (DMF) gave the higher yield than toluene (entry 9) under nitrogen atmosphere. Therefore, the optimum conditions to synthesize diphenylether are nanocopper as catalyst, 1,10-phenanthroline as a ligand, cesium carbonate as a base, in DMF at 110oC under nitrogen atmosphere.

Table 1. Optimizations of reaction

CT+^O Cu-np, 10% Ligand 2.0 eq Base, solvent 110 oC, 24 h ax 3

Entry Catalyst Ligand Base Solvent % Yielda

1 - Phen Cs2CO3 DMF 0

2 Cu2O-np Phen Cs2CO3 DMF 58

3 Cu-np Phen Cs2CO3 DMF 66

4 Cu-np Phen - DMF 2

5 Cu-np Bipy Cs2CO3 DMF 44

6 Cu-np PPh3 Cs2CO3 DMF 41

7 Cu-np Phen/PPh3 Cs2CO3 DMF 67

8b Cu-np Phen Cs2CO3 DMF 0

9 Cu-np Phen Cs2CO3 Toluene 24

1 GC yield with dodecane as internal standard, Under air

To determine the scope of the reaction, we then examined the cross-coupling reactions of phenol with variety of aryl halides as shown in Table 2. Both electron rich and electron poor aryl iodides were coupled with phenol to provide the corresponding diarylethers in moderate to good yields. When electron rich substrates such as iodoanisole and iodotoluene (entries 2-4) were used, the reactions gave moderate yields after 24 hours. The poor yield was observed in the coupling of 4-iodonitrobenzene (entry 5) due to the homocoupling of substrate. However, using electron poor substrate as 4-acetyliodobenzene, the cross-coupling reaction afforded considerably higher yield after 15 hours (entry 6) and sterically hindered 2-acetyliodobenzene could be coupled in moderate yield (entry 7). Base-sensitive functional groups such as methyl ester and ethyl ester (entries 8 and 12) gave relatively low yields due to the hydrolysis. Furthermore, with this protocol, we found that various aryl bromides with electron poor could also successfully coupled in

good yields (entries 8-11). These results are comparable with other groups that Cu, Cu2O, or CuO were used as catalysts to couple aryl halides with electron rich phenols [7-10].

Table 2. Nanocopper-catalyzed cross-coupling reaction of phenol and aryl halides

aIsolated yield

3. Conclusion

In conclusion, we developed a method for cross-coupling of phenol and aryl halides for the synthesis of diarylethers. Varieties of diarylethers were synthesized at moderate to good yields. This protocol tolerates a wide range of functional groups and the reaction avoids the use of palladium and/or expensive additives.

4. Experimental Section

All of the reactions reported herein were conducted under nitrogen atmosphere. All reagents and solvents were obtained from commercial suppliers and were used without further purification. Purification was performed by flash chromatography using silica gel. The yields given refer to isolated yields of the characterized compounds, :H NMR and 13C NMR. NMR spectra were recorded on a Bruker AVANCE 300 MHz spectrometer. Chemical shifts were reported in parts per million (5) and TMS was used as the internal reference.

Preparation of copper nanoparticle

Copper nanoparticle was synthesized under ambient atmosphere by the following procedure: dissolved copper(II) chloride dihydrate 20 mmol in 100 mL distilled water. Sodium stearate 3.75 mmol used as the protecting group was prepared by vigorously stirring in 50 mL methanol at 70 oC for 1 h. Then transferred the solution into copper salt solution and continued stirring at 70 oC for 1 h. Next, slowly added sodium borohydride solution (60 mmol) dropwise while stirring for 2-2.5 h until the reduction was complete. Cooled down the mixture and the nanoparticles were separated by centrifugation. Washed the solid with 15 mL methanol twice and separated by centrifugation. The resulting precipitates were transferred to Buchner funnel and washed with excess hot water to remove residual protecting group and then dried at 70-80 oC.

General procedure for the synthesis of diphenylether

A flamed-dried Pyrex glass tube (2.5 cm in diameter) equipped with a stir bar was charged with copper nanoparticle (20 mg), phenol (1.1 mmol), cesium carbonate (1.5 mmol), 1,10-phenanthroline (10%), and the appropriate aryl halide (1.0 mmol). The tube was then sealed with a rubber septum and evacuated and backfilled with nitrogen gas 5 times. N,N-dimethyl formamide 3 mL was injected through the septum. The contents were then stirred at 110 °C for 15 h. Cooled down the reaction mixture to room temperature, added dodecane (1.0 mmol) as an internal standard, extracted with ethyl acetate, and then injected to gas chromatography. To determine the isolated yield, the reaction mixture was filtered through celite to remove any insoluble residues. The filtrate was then extracted with ethyl acetate (3x10 mL), washed with brine, dried with sodium sulfate anh., filtered and concentrated in vacuo. The residue was purified by column chromatography on silica gel to obtain the analytically pure product.

Diphenylether: 'H NMR (300 MHz, CDCl3) 8 7.34 (t, J = 7.9 Hz, 4H), 7.11 (t, J = 7.4 Hz, 2H), 7.02 (d, J = 7.8 Hz, 4H). 13C NMR (75 MHz, CDCl3) 8 157.21, 129.71, 123.18, 118.85.

3-Methoxy-diphenylether: NMR (300 MHz, CDCl3) 8 7.35 - 7.30 (m, 2H), 7.23 - 7.18 (m, 1H), 7.11 - 7.06 (m, 1H), 7.01 - 6.99 (m, 2H), 6.92 - 6.90 (m, 1H), 6.83 - 6.80 (m, 2H), 3.80 (s, 3H). 13C NMR (75 MHz, CDCl3) 8 157.5, 157.4, 140.1, 129.8, 129.6, 124.2, 123.2, 119.7, 119.0, 116.1, 55.50.

4-Methoxy-diphenylether: NMR (300 MHz, CDCl3) 8 7.56 (d, J = 8.9 Hz, 1H), 7.32 - 7.26 (m, 2H), 6.97 - 6.87 (m, 5H), 6.68 (d, J = 8.9 Hz, 1H), 3.81 (s, 3H). 13C NMR (75 MHz, CDCl3) 8 138.09, 129.58, 122.39, 120.82, 117.50, 116.33, 55.63.

4-Methyl-diphenylether: NMR (300 MHz, CDCl3) 8 7.34 - 7.25 (m, 2H), 7.15 - 7.07 (m, 3H), 7.01 - 6.86 (m, 4H), 2.33 (s, 3H).13C NMR (75 MHz, CDCl3) 8 130.24, 129.65, 122.79, 119.13, 118.34, 20.71.

4-Nitro-diphenylether: NMR (300 MHz, CDCl3) 8 8.20 (d, J = 9.2 Hz, 2H), 7.44 (t, J = 7.9 Hz, 2H), 7.26 (t, J = 7.4 Hz, 1H), 7.10 (d, J = 7.7 Hz, 2H), 7.01 (d, J = 9.2 Hz, 2H). 13C NMR (75 MHz, CDCl3) 8 163.35, 154.59, 130.29, 125.91, 125.39, 120.52, 117.04.

4-Acetyl-diphenylether: NMR (300 MHz, CDCl3) 8 7.94 (d, J = 8.8 Hz, 2H), 7.40 (t, J = 7.9 Hz, 2H), 7.20 (t, J = 7.4 Hz, 1H), 7.07 (d, J = 7.8 Hz, 2H), 7.00 (d, J = 8.8 Hz, 2H), 2.58 (s, 3H). 13C NMR (75 MHz, CDCl3) 8 130.58, 130.04, 124.60, 120.16, 117.25, 26.44.

2-Acetyl-diphenylether: NMR (300 MHz, CDCl3) 8 7.34 - 7.26 (m, 2H), 7.19 - 7.02 (m, 3H), 6.99 - 6.90 (m, 4H), 2.33 (s, 3H). 13C NMR (75 MHz, CDCl3) 8 162.00, 130.59, 130.04, 129.62, 124.61, 120.16, 117.26, 115.26, 26.44.

Methyl-4-phenoxybenzoate: NMR (300 MHz, CDCl3) 8 8.00 (d, J = 8.8 Hz, 2H), 7.39 (t, J = 7.9 Hz, 2H), 7.19 (t, J = 7.4 Hz, 1H), 7.07 (d, J = 8.6 Hz, 2H), 6.99 (d, J = 8.8 Hz, 2H), 3.90 (s, 3H). 13C NMR (75 MHz, CDCl3) 8 131.65, 130.00, 124.48, 121.45, 120.09, 117.25, 102.45, 29.68.

4-Phenoxybenzaldehyde: NMR (300 MHz, CDCl3) 8 9.93 (s, 1H), 7.85 (d, J = 8.7 Hz, 2H), 7.42 (t, J = 7.9 Hz, 2H), 7.28 - 7.20 (m, 1H), 7.08 (t, J = 8.5 Hz, 4H). 13C NMR (75 MHz, CDCl3) 8 190.84, 132.39, 131.98, 130.17, 124.97, 120.45, 120.29, 117.58, 117.24.

Ethyl-4-phenoxybenzoate: NMR (300 MHz, CDCl3) 8 8.01 (d, J = 8.8 Hz, 2H), 7.39 (t, J = 7.9 Hz, 2H), 7.19 (t, J = 7.4 Hz, 1H), 7.06 (d, J = 7.7 Hz, 2H), 6.99 (d, J = 8.8 Hz, 2H), 4.40 - 4.34 (m, 2H), 1.42 - 1.36 (m, 3H). 13C NMR (75 MHz, CDCl3) 8 131.62, 131.09, 130.01, 124.44, 120.03, 117.30, 60.84, 14.36.

Acknowledgements

This work was supported by National Research Council of Thailand (2552A11702066).

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