Ligand-Free HKUST-1 Metal-Organic-Framework-Catalyzed O-Arylation of Phenols by Nitroarenes

ABSTRACT:

This research is to examine the application of HKUST-1 (MOF-199) as catalyst for novel organic transformation. The coupling reactions of phenols with nitroarenes forming unsymmetrical diaryl ethers were conducted under heterogeneous HKUST-1 catalysis. The optimal conditions employed 5 % mol HKUST-1, inexpensive K₂CO₃ base in DMF at 100 ^oC. The reaction offered several environmental and economic advantages in comparison with the conventional C-O coupling reaction and previous approaches. The HKUST-1 could be facilely separated and reused several times without a significant degradation in catalytic activity.

Keywords: Metal-Organic-Framework, MOF-199, C-O coupling, heterogeneous, nitroarenes.

1. INTRODUCTION

Unsymmetrical diaryl ethers have been known as important structural units that have been frequently found in a variety of biologically active natural products, polymer, and materials [1]. Traditional routes to access these molecules often suffer from disadvantages such as harsh conditions, low yields, and limited reaction scope [2]. Ullmann coupling is an example in which a stoichiometric reductant and high temperature are needed to obtain reasonable yields [2]. In addition, the separation and handling of large quantities of hazardous transition-metal-based waste produced from these homogeneous processes represent a major problem [3]. Furthermore, in term of environmental issues, formation of halide by-product pollutants in Ullmann coupling limits their applications. Recently, elegant methods to synthesize unsymmetrical diaryl ethers without using aryl halides were reported [4, 5]. In particular, nitroarenes were employed as coupling components in Cu-catalyzed homogeneous cross coupling reaction with phenols and aryl boronic acids. However, protocols for environmentally and economically more efficient should be targeted.

Application of metal-organic frameworks (MOFs) in catalysis has increasingly attracted remarkable interests [6]. The use of MOFs catalysts or catalyst supports has been performed in a variety of organic transformations [7, 8], including the three-component coupling reaction of aldehyde, alkyne, and amine [8], the Biginelli reaction [10], the Friedlander reaction [11], asymmetric alkylation of aldehydes [12], oxidation [13], and cross-coupling reaction [14]. Among MOFs containing metal sites with potential coordinative unsaturation[7, 15], HKUST-1 (MOF-199) has recently been found to possess catalytic activity in various noble transformations [16-19]. In particular, aza-Michael, Friedlaender [16], CO oxidation [17], esterification [18], cross coupling between phenols and iodobenzenes, and α-arylation of 1,3-ketones reactions could be performed under HKUST-1 catalysis [19]. Herein, we report the implementation of HKUST-1 as an efficient heterogeneous catalyst for the coupling reactions of phenols with nitroarenes forming diaryl ethers without using any ligands or additives. The method avoids generating toxic halide by-products. Additionally, the catalyst could be separated from the reaction mixture by simple filtration and could be reused several times without a significant loss in catalytic activity.

2. EXPERIMENTAL

2.1. Synthesis of HKUST-1  

The HKUST-1  was synthesized following a previously reported method [20]. The synthesis involved the heating of a mixture of Cu(NO₃)₂ (4,38g; 18,099 mmol) and 1,3,5- benzenetricarboxylic acid (H₃BTC,  2,36 g; 10,925 mmol) in DMF/ethanol/H₂O (100 mL, ratio 4:3:3) at 85 °C for 24 h. The solid crystal was characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FT-IR), atomic absorption spectrophotometry (AAS), and nitrogen physisorption measurements.      

2.2. Catalytic studies 

In a typical procedure, the corresponding amount of HKUST-1 was added to the reaction flask containing 4-nitrobenzaldehyde (0.302 g, 2.0 mmol), phenol (0.376 g, 4.0 mmol), K₂CO₃ (0.552 g, 4.0 mmol), dodecane (0.15 mL), and DMF (4 mL). The percentage of catalyst was calculated based on the molar ratio of copper/4-nitrobenzaldehyde. The reaction mixture was magnetically stirred at 100 ^oC for 2 hours. In kinetic studies, the reaction conversion was monitored by withdrawing aliquots from the reaction mixture at different time intervals. The reaction mixture was quenched with an aqueous NaOH solution (1%, 0.1 mL), dried over anhydrous Na₂SO₄, and analyzed by GC with reference to n-dodecane.

3. Results and discussion

HKUST-1 was achieved as deep purple crystals with 88% yield (based on 1,3,5-benzenetricarboxylic acid). The material was then characterized by several techniques, including XRD, SEM, TEM, TGA, FT-IR, and nitrogen physisorption measurements (Fig. 1 – Fig. 2). The spectroscopic results are consistent with previous studies [16-20]. The measured percentage of copper, 29.5 %, is close to theoretical value of 29.8 % indicating the purity of synthesized catalyst. Langmuir surface areas of 1970 m²/g were observed for the HKUST-1, which is significantly higher than those of conventional microporous and mesoporous inorganic materials [21].

Figure1. X-ray powerder diffractogram of the HKUST-1

 

Fig.2. TGA of the HKUST-1

Our initial efforts were directed toward the use of HKUST-1 for the O-arylation of phenol with 4-nitrobenzaldehyde (Table 1). In optimization screening, reactions were carried out with respect to catalyst, base, solvent, and temperature. The results indicated that the catalyst was not effective at 60 ^oC and 80 ^oC (entries). In contrast, 82 % conversion was obtained at 100 ^oC in 2 hours (entry 3). Moreover, significant drops in reaction conversion were observed when the amount of catalyst was reduced to 1 % or 3 % (entries 4 and 5). The effect of different solvents including dimethylacetamide (DMA), N-methylpyrolidone (NMP), and p-xylene on reaction conversions was investigated. It was found that DMF was the optimal solvent for the O-arylation of phenol under our conditions. Our results are in agreement with previous reports [22].

Table 1. Optimization of Reaction Conditions^a

Several bases including Na₂CO₃, tBuOK, and CH₃COONa were tested along with the kinetic behavior (Fig.3). In Na₂CO₃, tBuOK, and CH₃COONa, reactions were inactive with less than 20 % conversion in 2 hours. The efficiency of K₂CO₃ was described in previous studies [23]. The use of inexpensive and easily accessible base broadens the application of the protocol. The effect of molar ration of coupling reagents was also studied. It was found that optimal conversion was obtained with 4-nitrobenzaldehyde/phenol molar ratio of 1:2 (Fig. 4). Furthermore, product was isolated by flash chromatography with 73 % yield and structural confirmation was done by GC-MS and NMR. This indicated the correlation between the conversion and isolated yields as well as the excellent reaction selectivity.

Fig. 3.  Effect of different bases on the reaction  conversion

Fig. 4. Effect of 4-nitrobenzaldehyde/ phenol ratios

on the reation conversion

We extended our study to the coupling reaction between 4- nitrobenzaldehyde with a variety of phenol derivatives (Table 2). The O-arylation reaction was performed at 100^oC in DMF with a 4-nitrobenzaldehyde/phenol molar ratio of 1:2 and two equivalents of K₂CO₃ base in the presence of 5 mol% HKUST-1 catalyst. Interestingly, the O-arylation of 4-methoxyphenol with 4-nitrobenzaldehyde afforded 99 % conversion after 2 h. Other cresol derivatives, o-cresol and m-cresol, are also active and excellent conversions were achieved with 96 % and 85 %, respectively. As expected, lower conversions were obtained for phenol structures that contain electron-withdrawing groups. The results showed that the reaction of 4-chlorophenol afforded 67 % while 4-hydroxy benzonitrile gave 30 % conversion after 2h. This is consistent with the study of Chen and co-workers, in which phenols that contained electron-donating groups are more active than those with electron-withdrawing groups [4].  Several other groups have also reported that the Ullmann-type transformations of electron-deficient phenols proceeded with difficulty [24].

Table 2. Reaction scope with respect to phenols^a

^aVolume of solvent 4 mL, 2.0 mmol scale, 2 h. Conversion by GC analysis. Number in parenthesis was isolated yield.

To further investigate the generality of the protocol in regard of nitroarenes, several nitroarenes including 4-nitrobenzaldehyde, 4-nitroacetophenone, 1-fluoro-4-nitrobenzene, 1-bromo-4-nitrobenzene, and 1-methyl-4-nitrobenzene were employed. The O-arylation reaction was performed at 100^oC in DMF with a nitroarene/phenol molar ratio of 1:2 and two equivalents of K₂CO₃ in the presence of 5 mol% HKUST-1 catalyst. The result showed that only trace amounts of the product were detected when 1-methyl-4-nitrobenzene was used as coupling component. Nitroarenes containing electron-withdrawing groups are all active. Indeed, 1-fluoro-4-nitrobenzene, 4-nitrobenzaldehyde, 4-nitroacetophenone, 1-bromo-4-nitrobenzene was afforded 99%, 82%, 78%, and 60% conversion, respectively (entries 7 - 11).

As for most liquid-phase organic transformations using solid catalysts, some of active sites could be possibly dissolved into the liquid phase during the course of the reaction. Therefore, leaching should be examined. In several cases, the reaction was partially or totally catalyzed by these leached species, thus implying that the transformation would not proceed under real heterogeneous catalysis conditions [25]. To confirm if Cu species leached from solid HKUST-1 could be significantly active in the coupling reaction of phenol with 4-nitrobenzaldehyde, an experiment was performed to filtrate the solid catalyst during the course of the reaction. If the transformation continued after the solid catalyst was separated from the liquid phase, the O-arylation possibly occurred either under homogeneous or partially homogeneous catalysis conditions. The O-arylation reaction was performed at 100^oC in DMF with a 4-nitrobenzaldehyde/phenol molar ratio of 1: 2 and two equivalents of K₂CO₃ as the base in the presence of 5 mol% HKUST-1 catalyst. After 20 minutes, the DMF phase was separated from the solid HKUST-1 by filtration and transferred to a new reactor vessel. The reaction mixture without the solid catalyst was then stirred for an additional 100 min at 100⁰C, during which aliquots were sampled at different intervals and analyzed by GC. The results showed that no further conversion to form 4-formyldiphenyl ether was observed after the solid HKUST-1 catalyst was separated from the reaction mixture (Fig. 5). It is likely that the coupling reaction of phenol with 4-nitrobenzaldehyde could only proceed in the presence of solid HKUST-1. It is unlikely that there is contribution from leached active Cu species in the solution phase.

In the view of green chemistry, important issues that should be seriously considered for organic reactions using heterogeneous catalysts are the ease of separation as well as reusability of the catalysts. It was therefore decided to investigate the recoverability and reusability of the HKUST-1 catalyst in the C-O coupling reaction by repeatedly separating the HKUST-1 from the reaction mixture, washing and reusing it. The reaction was carried out for 2 h at 100^oC in DMF, using 5 mol% HKUST-1 catalyst, in the presence of two equivalents  K₂CO₃, at the 4-nitrobenzaldehyde:phenol molar ratio of 1:2. After each run, the catalyst was separated from the reaction mixture by simple filtration and washed with copious amounts of methanol. The recovered HKUST-1 was dried under vacuum at room temperature for 6 h, and then reused in further reactions under identical conditions to those of the previous runs. Experimental results showed that the HKUST-1 could be recovered and reused several times without a significant degradation in catalytic activity (Fig. 6). It was observed that a conversion of 77% was still achieved in the 5th run.

4. CONCLUSIONS

     A highly porous metal-organic framework (HKUST-1) was synthesized using solvothermal method and was characterized by using XRD, TEM, SEM, FTIR spectroscopy, TGA, AAS, and N₂ physisorption measurements. HKUST-1 was used as an efficient heterogeneous catalyst for the coupling reaction of phenols with nitroarenes to form diaryl ethers in the absence of ligands or additives. The catalyst could be reused several times in the coupling reaction without significant degradation in catalytic activity. To the best of our knowledge, the HKUST-1-catalyzed O-arylation of phenols with nitroarenes to form diaryl ethers has not been reported in the literature. This work contributes to development of applications of MOFs in the field of catalysis, which is of interest to the chemical industry.

Acknowledgements:

The National Foundation for Science and Technology Development (NAFOSTED) is acknowledged for financial support through project code 104.01-2018.44.

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Nghiên cứu thực hiện phản ứng ghép đôi C-O giữa Phenols

và Nitroarenes sử dụng xúc tác HKUST-1

trong điều kiện không Ligand

Trần Thị Ngọc Tú - TS. Nguyễn Văn Chí - PGS.TS. Trương Vũ Thanh

Khoa Kỹ thuật Hóa học, Trường Đại học Bách khoa,

Đại học Quốc gia TP.Hồ Chí Minh

TÓM TẮT:

Nghiên cứu này được thực hiện nhằm đánh giá tính ứng dụng của vật liệu HKUST-1 (MOF-199) làm chất xúc tác cho phản ứng hữu cơ. Phản ứng ghép đôi giữa hợp chất phenols và nitroarenes tạo thành hợp chất biphenyls bất đối xứng đã được thực hiện với việc sử dụng vật liệu MOF-199 là xúc tác dị thể. Việc sử dụng 5% xúc tác MOF-199, sử dụng base K₂CO₃ trong dung môi DMF tại 100^oC là điều kiện tối ưu. So với các phương pháp ghép C-O truyền thống, phản ứng này có các ưu điểm về môi trường, cũng như về mặt kinh tế. Xúc tác MOF-199 có thể lọc, thu hồi và tái sử dụng nhiều lần mà không bị giảm đáng kể hoạt tính xúc tác.

Từ khoá: Vật liệu MOFs, MOF-199, phản ứng ghép C-O, xúc tác dị thể, nitroarenes.