Ligand-Free HKUST-1 Metal-Organic-Framework-Catalyzed O-Arylation link w88 Phenols by Nitroarenes

ABSTRACT:

This research is to examine the application link w88HKUST-1(MOF-199) as catalyst for novel organic transformation. The coupling reactions link w88 phenols with nitroarenes forming unsymmetrical diaryl ethers were conducted under heterogeneous HKUST-1 catalysis. The optimal conditions employed 5 % mol HKUST-1, inexpensive K₂CO₃base 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 link w88 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 link w88 large quantities link w88 hazardous transition-metal-based waste produced from these homogeneous processes represent a major problem [3]. Furthermore, in term link w88 environmental issues, formation link w88 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 link w88 metal-organic frameworks (MOFs) in catalysis has increasingly attracted remarkable interests [6]. The use link w88 MOFs catalysts or catalyst supports has been performed in a variety link w88 organic transformations [7, 8], including the three-component coupling reaction link w88 aldehyde, alkyne, and amine [8], the Biginelli reaction [10], the Friedlander reaction [11], asymmetric alkylation link w88 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 link w88 1,3-ketones reactions could be performed under HKUST-1 catalysis [19]. Herein, we report the implementation link w88 HKUST-1 as an efficient heterogeneous catalyst for the coupling reactions link w88 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 link w88 HKUST-1

The HKUST-1  was synthesized following a previously reported method [20]. The synthesis involved the heating link w88 a mixture link w88 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 link w88 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 link w88 catalyst was calculated based on the molar ratio link w88 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 ton-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 link w88 copper, 29.5 %, is close to theoretical value link w88 29.8 % indicating the purity link w88 synthesized catalyst. Langmuir surface areas link w88 1970 m²/g were observed for the HKUST-1, which is significantly higher than those link w88 conventional microporous and mesoporous inorganic materials [21].

Figure1. X-ray powerder diffractogram link w88 the HKUST-1

Fig.2. TGA link w88 the HKUST-1

Our initial efforts were directed toward the use link w88 HKUST-1 for theO-arylation link w88 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 link w88 catalyst was reduced to 1 % or 3 % (entries 4 and 5). The effect link w88 different solvents including dimethylacetamide (DMA),N-methylpyrolidone (NMP), andp-xylene on reaction conversions was investigated. It was found that DMF was the optimal solvent for theO-arylation link w88 phenol under our conditions. Our results are in agreement with previous reports [22].

Table 1. Optimization link w88 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 link w88 K₂CO₃was described in previous studies [23]. The use link w88 inexpensive and easily accessible base broadens the application link w88 the protocol. The effect link w88 molar ration link w88 coupling reagents was also studied. It was found that optimal conversion was obtained with 4-nitrobenzaldehyde/phenol molar ratio link w88 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 link w88 different bases on the reaction  conversion

Fig. 4. Effect link w88 4-nitrobenzaldehyde/ phenol ratios

on the reation conversion

We extended our study to the coupling reaction between 4- nitrobenzaldehyde with a variety link w88 phenol derivatives (Table 2). TheO-arylation reaction was performed at 100^oC in DMF with a 4-nitrobenzaldehyde/phenol molar ratio link w88 1:2 and two equivalents link w88 K₂CO₃ base in the presence link w88 5 mol% HKUST-1 catalyst. Interestingly, theO-arylation link w88 4-methoxyphenol with 4-nitrobenzaldehyde afforded 99 % conversion after 2 h. Other cresol derivatives,o-cresol andm-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 link w88 4-chlorophenol afforded 67 % while 4-hydroxy benzonitrile gave 30 % conversion after 2h. This is consistent with the study link w88 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 link w88 electron-deficient phenols proceeded with difficulty [24].

Table 2. Reaction scope with respect to phenols^a

^aVolume link w88 solvent 4 mL, 2.0 mmol scale, 2 h. Conversion by GC analysis. Number in parenthesis was isolated yield.

To further investigate the generality link w88 the protocol in regard link w88 nitroarenes, several nitroarenes including 4-nitrobenzaldehyde, 4-nitroacetophenone, 1-fluoro-4-nitrobenzene, 1-bromo-4-nitrobenzene, and 1-methyl-4-nitrobenzene were employed. TheO-arylation reaction was performed at 100^oC in DMF with a nitroarene/phenol molar ratio link w88 1:2 and two equivalents link w88 K₂CO₃ in the presence link w88 5 mol% HKUST-1 catalyst. The result showed that only trace amounts link w88 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 link w88 active sites could be possibly dissolved into the liquid phase during the course link w88 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 link w88 phenol with4-nitrobenzaldehyde, an experiment was performed to filtrate the solid catalyst during the course link w88 the reaction. If the transformation continued after the solid catalyst was separated from the liquid phase, theO-arylation possibly occurred either under homogeneous or partially homogeneous catalysis conditions. TheO-arylation reaction was performed at 100^oC in DMF with a 4-nitrobenzaldehyde/phenol molar ratio link w88 1: 2 and two equivalents link w88 K₂CO₃as the base in the presence link w88 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 link w88 phenol with 4-nitrobenzaldehyde could only proceed in the presence link w88 solid HKUST-1. It is unlikely that there is contribution from leached active Cu species in the solution phase.

In the view link w88 green chemistry, important issues that should be seriously considered for organic reactions using heterogeneous catalysts are the ease link w88 separation as well as reusability link w88 the catalysts. It was therefore decided to investigate the recoverability and reusability link w88 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 link w88 two equivalents  K₂CO₃, at the 4-nitrobenzaldehyde:phenol molar ratio link w88 1:2. After each run, the catalyst was separated from the reaction mixture by simple filtration and washed with copious amounts link w88 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 link w88 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 link w88 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 link w88 phenols with nitroarenes to form diaryl ethers in the absence link w88 ligands or additives. The catalyst could be reused several times in the coupling reaction without significant degradation in catalytic activity. To the best link w88 our knowledge, the HKUST-1-catalyzedO-arylation link w88phenolswith nitroarenes to form diaryl ethers has not been reported in the literature. This work contributes to development link w88 applications link w88 MOFs in the field link w88 catalysis, which is link w88 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 tácHKUST-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 K₂CO₃trong 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.