Rapid and Simple Microwave-Assisted Synthesis of Benzoxazoles Catalyzed by [CholineCl][Oxalic Acid]

Abstract

Microwave irradiation has been used to enhance the reaction yields and selectivities for organic transformation. In this paper, microwave irradiation (MW) was investigated for the environmentally benign synthesis of benzoxazoles through the cyclization of 2-aminophenols and benzaldehydes using deep eutectic solvent (DES) as a catalyst. The [CholineCl][oxalic acid] was easily synthesized from choline chloride with oxalic acid and used without further purification. [CholineCl][oxalic acid] catalyzed the synthesis of benzoxazoles to produce the desired product in a good to excellent conversion and selectivity under MW irradiation. The presence of [CholineCl][oxalic acid] helps to promote the rapid heating transfer from microwave irradiation into the reaction mixture. The [CholineCl][oxalic acid] can be recovered and reused several times without a considerable degradation in catalytic activity.

1. Introduction

In the past ten years, benzoxazoles have received considerable attention due to a broad range of biological activities, including antitumor, antiviral, anti-inflammatory, antihypertensive, antihistaminic, antimicrobial, and antifungal activities [1,2,3,4,5,6,7]. The preparation of 2-substituted benzoxazoles involves the condensation of 2-aminophenol with aldehydes, carboxylic acids, primary alcohols, benzoyl chlorides in the presence of several catalysts such as halogen metals [8], magnetic nanoparticles [9], triflate metals [10], oxide metals [11], and ionic liquids [12]. Recently, there have been several efforts to synthesize benzoxazoles under milder conditions. Yang and co-workers improved the pathway for synthesizing 2-arylbenzoxazole through the desulfinative arylation of azoles with arylsulfonyl hydrazides catalyzed by using NHC–Pd complexes for 12 h at 100 °C in the presence of dioxane [13]. Tang and co-workers used Cu(OAc)₂ and 1-[2-(N-(3-diphenylphosphinopropyl))aminoethyl]pyrrolidine as the catalyst for the preparation of benzoxazoles in water via the intramolecular O-arylation.[14] Sahoo and co-workers introduced a new method for the synthesis of 2-phenylbenzoxazole from 2-aminophenol and aryl iodide to form C4-aryl benzoxazoles mediated by Pd(OAc)₂, Ag₂O, and MsOH catalysis [15].

Deep eutectic solvents (DESs) have been known as sustainable catalysts and/or solvents for organic synthesis [16]. DESs exhibit special physicochemical properties such as low vapor pressure, wide liquid range, low toxicity, and simplicity of the preparation process [17,18]. Choline chloride (known as vitamin B4) is one of the most popular components for preparing DESs [19]. DESs can be easily synthesized by mixing hydrogen bond acceptors (HBA) and hydrogen bond donors (HBD) without any special covalent bond forming a reaction. This simple synthesis of DESs is 100% atom- economic, requiring no further purification [20]. Thus, the low-cost preparation and biodegradability are prominent features of DESs promising application in industrial processes [21]. Recently, DESs have been used for many applications such as electrochemistry [22], material preparation [23,24], biotransformation [25,26], biomass valorization [27], green catalysts/solvents [28,29], extraction technology [30], and sample preparation [31]. In particular, the DES’s many applications for heterocyclic synthesis using multicomponent reactions by claiming the green chemistry concept have been published in the literature [32,33,34].

The synthesis of benzoxazoles has been studied extensively under conventional heating. Recently, infrared irradiation [35], ultrasound sonication [36,37], and mechanical milling [38] have attracted attention as alternatives to thermal heating, leading to a minimized reaction time, high yield, and high selectivity [8]. Recently, MW irradiation has been demonstrated as a crucial tool for reducing the reaction time, increasing the yield and selectivity, and decreasing energy consumption [39]. MW-assisted organic synthesis could be considered an environmentally friendly method with small amounts of, or without, organic solvents and with less energy consumption [40]. Thus, MW irradiation provides a powerful tool for organic transformations [41]. Although a number of MW-promoted cyclization reactions toward five-membered rings have been developed, the synthesis of 2-phenylbenzenxazoles using DES as a catalyst has not been reported under MW irradiation.

As part of our ongoing research to develop a green and efficient approach for benzoxazole synthesis [42,43,44], we report an improved method for synthesizing benzoxazole compounds via the reaction of different 2-aminophenols and benzaldehydes using DESs under solvent-free MW irradiation. The catalyst was easily prepared from simple and cheap materials such as choline chloride and oxalic acid. Under microwave irradiation, DES provided the 2-arylbenzoxazoles in good to excellent yields. Interestingly, the DES could be recovered and reused without significant loss of catalytic activity.

2. Results and Discussion

[CholineCl][oxalic acid] DES catalyst was synthesized according to our previously reported studies [28,45,46]. The DES was formed by heating choline chloride (5.0 mmol) and oxalic acid (5.0 mmol) at 100 °C for 60 min until a homogeneous liquid was obtained (Scheme 1). The characteristics of the [CholineCl][oxalic acid] were determined by ¹H, ¹³C NMR (see Supplementary Materials), FT-IR, and TGA.

Initially, to illustrate the formation of new hydrogen bonding in the DES, FT-IR analyses of the synthesized DES, pure oxalic acid, and choline chloride were studied in Figure 1. For pure choline chloride, the signal at 3476 cm⁻¹ was assigned to the OH stretching frequency. The sharp peak around 1481 cm⁻¹ was associated with the CN bond in choline chloride. For oxalic acid, the OH gave a broad signal around 3428 cm⁻¹ that was assigned to typical carboxylic acid forming intermolecular H-bonding between the C-O and O-H groups of two molecules. The vibration at 1691 and 1254 cm⁻¹ indicated the C=O and C-O stretching frequency in free oxalic acid. The broad OH stretching peak of the DES shifted to a lower frequency at 3404 cm⁻¹, indicating the formation of new hydrogen bonds. Additionally, the C=O stretching frequency of the DES moved to a lower frequency at 1683 cm⁻¹, indicating the presence of new hydrogen bondings [47,48].

The thermal stability of oxalic acid, choline chloride, and [CholineCl][oxalic acid] was determined by thermogravimetric analysis (TGA) from 50 °C to 600 °C, and the results were presented in Figure 2. The results revealed that [CholineCl][oxalic acid] began to decompose at around 150 °C. The first weight loss ~10% up to 150 °C is due to the presence of water molecules. The significant weight loss observed over a relatively broad temperature range from 150 to 320 °C was related to the [CholineCl][oxalic acid] combustion. The DES was completely decomposed at 320 °C. The TGA of choline chloride and oxalic acid were also recorded to be compared with the DES and showed the difference in decomposition temperature attributed to the formation of H-bonding interactions between the choline chloride and oxalic acid.

The catalytic activity was assessed through the reaction for the synthesis of 5-chloro-2-phenylbenzoxazole (4c) from 2-amino-4-chlorophenol (1b) and benzaldehyde (2a) under microwave irradiation (Table 1). Initially, various parameters were examined, such as temperature, time, and catalyst amount. The results are demonstrated in Table 1. In order to screen the temperature, the reactions were conducted from 100 to 140 °C. The result showed that the high conversion of (1b) was obtained in 94% at 130 °C for 15 min (entries 1–4, Table 1). The ratio of (3c)/(4c) increased with the increasing reaction temperature. Subsequently, the effect of the reaction time was evaluated by changing the reaction time from 5 to 30 min under microwave irradiation. The results showed that the reaction time had a specific influence on the conversion of the substrate and (3c)/(4c) product ratio (entries 5–9, Table 1). Only 74% conversion of 2-amino-4-chlorophenol and 10/90 of (3c)/(4c) was observed at 120 °C for 5 min. The conversion (95%) and product ratio (1/99) were significantly improved when the reaction time was prolonged to 30 min. The catalytic loading was also investigated under the current method. Only (3c) intermediate was obtained in the absence of the catalyst (entry 10, Table 1). The conversion and product ratio gradually increased when the quantity of [CholineCl][oxalic acid] was increased (entries 11–14, Table 1). The best result was obtained with 10 mol% of [CholineCl][oxalic acid] at 120 °C for 15 min under microwave irradiation. However, the use of a catalyst with a higher concentration led to diminished yields. Control experiments were performed under conventional heating and ultrasound irradiation which provided a lower conversion than MW heating (entries 15–17, Table 1).

The catalytic activity was tested by various DESs, and the results are shown in Table 2. The model reaction of 2-amino-4-chlorophenol (1b) and benzaldehyde (2a) was evaluated with several types of DESs such as [CholineCl][oxalic acid], [CholineCl][succinic acid], [CholineCl][Urea]₂, [CholineCl][ethyleneglycol]₂, [CholineCl]₂[glucose], [CholineCl]₂[fructose] under microwave irradiation to form 5-chloro-2-phenylbenzoxazole (4c). In general, [CholineCl][succinic acid] and [CholineCl][ethyleneglycol]₂ were favourable catalysts with a conversion above 70% (entries 2–4, Table 2) with high selectivity (4c). [CholineCl][Urea]₂ provided the intermediate imine (3c) as the major product (4c). However, the conversion strongly decreased in the presence of [CholineCl]₂[glucose] or [CholineCl]₂[fructose] (entries 5–6, Table 2). Among these, [CholineCl][oxalic acid] showed the best activity with a conversion of up to 99%, and a ratio of (3c)/(4c) was 1/99. The large-scale procedure was carried out with 5 mmol of the substrate. The conversion was obtained in 85% with a decrease of 10% in the selective ratio (entry 1, Table 2).

Table 3 presents the comparison of the catalytic activity of [CholineCl][oxalic acid] with the other reported catalysts. We carried out the synthesis of 5-chloro-2-phenylbenzoxazole (4c) from 2-amino-4-chlorophenol (1b) and benzaldehyde (2a) at 120 °C for 15 min under MW irradiation. Zhou and co-workers reported that the [BMIm]₂[WO₄] catalyst exhibited good activity for the synthesis of the benzoxazoles product in 1,4-dioxane [49]. Next, Gorepatil and co-workers developed a simple, green, and efficient method for synthesizing benzoxazoles using samarium triflate as a reusable acid catalyst under mild reaction conditions in an aqueous medium [50]. Cho and co-workers reported a cyanide-catalyzed synthesis of 2-substituted benzoxazoles from Schiff bases via aerobic oxidation [51]. Sirgamalla and co-workers developed the synthesis of 2-arylbenzoxazoles by using a copper catalyst at room temperature [7]. Although the previous experiment was carried out at a lower temperature than the current method, our work exhibited a higher conversion (99%) and shorter reaction time (15 min) than other reported catalysts.

To explore the scope and generality of the current method, various 2-aminophenols and aromatic aldehydes were evaluated for the synthesis of benzoxazoles in the presence of [CholineCl][oxalic acid] (10 mol%) under MW irradiation. The results are shown in Table 4. Initially, the cyclization was carried out between 2-aminophenol (1a) and two different substituted aryl aldehydes at the para position, including benzaldehyde (2a), and 4-methoxybenzaldehyde (2b) to deliver the desired 2-arylbenzoxazoles (4a–4b) in a moderate to good conversion (81–84%) with high selectivity (Entries 1 and 2, Table 4). Next, a series of 2-aminophenol derivatives with different substituents involving both donating and withdrawing on 4-position of the benzene-ring such as 4-chloro-2-aminophenol (1b), 2-amino-4-methylphenol (1c), and 2-amino-4-nitrophenol (1d) were tested with various 4-substituted benzaldehydes (4-H, 4-Me, 4-F, 4-Cl, 4-Br) under optimized conditions. The 2-aminophenol bearing 4-chloro group (1b) was converted into the products 4c–4f (Entries 3–6, Table 4) in an excellent conversion (89–99%) and the ratio of major products and intermediate imine recorded high percentages. Similarly, when 2-amino-4-methylphenol (1c) was used as a substrate in this route, the desired products (4g–4k) were obtained in a high conversion (87–99.5%) with high selectivity (Entries 7–11, Table 4). Unfortunately, 2-amino-4-nitrophenol (1d) was treated with aryl aldehydes (2a, 2c, and 2d) and produced the corresponding products (4l-n) in the lower conversions (30–42%) with lower selectivity than the electron-donating groups on the 2-aminophenol (Entries 12–14, Table 4). These results show that the electron-withdrawing group on the 2-aminophenol was the unfavorable reactant in the current method.

To gain insight into the mechanism, the catalytic role of [CholineCl][oxalic acid] was investigated in the reaction between 4-chloro-2-aminophenol (1b) and benzaldehyde (2a) to give 5-chloro-2-phenylbenzoxazole (4c) (Scheme 2). In the presence of [CholineCl][oxalic acid], the C=O group of the benzaldehyde was activated, followed by the nucleophilic attack of nitrogen atom of amino group in 4-chloro-2-aminophenol (1b) to form intermediate (A). Afterward, the migration of the proton of A took place to form intermediates (B) and (C). Next, the imine (3c) was formed through a dehydration reaction of (C) observed by HRMS-ESI (m/z 232.0494 [M+H]⁺ cacld m/z 232.0529) or GCMS. The intramolecular cyclization of -OH group in (3c) with imine group in (3c) led to the formation of the intermediate (3d), confirmed by HRMS-ESI (m/z 233.0554 [(C)+H]⁺ cacld m/z 233.0607). Finally, the desired product was formed via oxidation with oxygen in the air, which was observed by HRMS-ESI (m/z 230.0369 [M+H]⁺ cacld m/z 230.0372).

It is necessary to study the recyclability of the [CholineCl][oxalic acid] to be reused for consecutive runs. The recyclability of [CholineCl][oxalic acid] was carried out under the optimized condition of a 2-amino-4-chlorophenol (1b) and benzaldehyde (2a). Upon completion of the reaction, the recovered catalyst was easily separated by liquid–liquid extraction. The recovered catalyst was collected and reused for three cycles. A slight loss of catalytic activity in the recycling test was observed after each cycle due to the minor loss of the catalyst in the recovery process (Figure 3). We believe that the increase of (3c)/(4c) ratios could be explained by the decrease in catalytic concentration. When the catalyst concentration was decreased in the reaction mixture, the imine intermediate could not convert completely to the final product. Furthermore, Fourier transform infrared (FT-IR) corresponding to the fresh and reused [CholineCl][oxalic acid] after three times was observed without a changed structure (Figure 4).

3. Experimental Section

3.1. Chemicals and Equipment

All chemicals including choline chloride (99%), oxalic acid (99%), 2-aminophenol (98%), 2-amino-4-chlorophenol (≥97%), 2-amino-4-methylphenol (≥97%), 4-nitro-2-aminophenol (≥96%), benzaldehyde, 4-methoxybenzaldehyde (≥98%), 4-fluorobenzaldehyde (98%), 4-chlorobenzaldehyde (≥97%) were obtained from Sigma-Aldrich (St. Louis, MO, USA). Succinic acid (99%), fructose (≥99%), urea, ethylene glycol (99.8%), and glucose (99%) were supplied by Xilong Chemical (Guangdong, China). Solvents such as ethyl acetate, n-hexane, and acetone were obtained from the ChemSol Co., Ltd. (Ho Chi Minh, Vietnam).

Microwave reactions were carried out on a CEM Discover monomode microwave (Matthews, NC, USA). Fourier Transform Infrared spectra were performed on a Bruker Vertex 70 (Rheinstetten, Germany) apparatus using KBr pellets. Thermal gravimetric analysis (TGA) was recorded on a TA Q500 thermal analysis system (platinum pan, continuous airflow). Gas chromatography-mass spectrometry (GC-MS) was recorded on an Agilent 7890 (Santa Clara, CA, USA) equipped with a mass triple-axis detector and a capillary column (DB-5MS, 30 m × 250 μm × 0.25 μm) and MS spectra were compared with the spectra gathered in the NIST library (NIST 20).

3.2. Synthesis of [CholineCl][Oxalic Acid]

In this work, DES was synthesized following the procedure described in the previous report [52]. A mixture of choline chloride (10 mmol, 1.39 g) and oxalic acid (10 mmol, 0.90 g) was heated to 100 °C with constant stirring in an oil bath until a clear colorless liquid was observed. The structure of [CholineCl][oxalic acid] was determined by FTIR, ¹H NMR, and TGA.

The other DESs were prepared according to the previously reported literature [53,54,55,56,57].

3.3. Synthesis of Benzoxazole Derivatives

A microwave tube containing a mixture of 2-aminophenols (1–4, 1.0 mmol), aromatic aldehydes (2a–f, 1.0 mmol), and [CholineCl][oxalic acid] (10 mol%) was irradiated. After completion of the reaction (checked by TLC and GCMS), the mixture was extracted in ethyl acetate (3 × 5 mL). The ethyl acetate layer was then washed with distilled water (3 × 10 mL) and dried with Na₂SO₄. The solvent was removed in a vacuum to obtain the crude product. The conversion based on the consumption of aldehydes and selectivity was determined by using the GC-MS method. The temperature program for the GC-MS analysis was set up as follows: initial 50 °C for 2 min, 50 to 300 °C with ramping at 10 °C/min, and held for 5 min. The inlet temperature was set at 300 °C. The identification of compounds was performed using a NIST 20 library (using a percent matching higher than 95% as the threshold value for acceptance).

3.4. Recycling of [CholineCl][Oxalic Acid]

To recover the [CholineCl][oxalic acid] after the completion of the reaction, the reaction mixture was washed several times with ethyl acetate to extract both the starting materials and products entirely from [CholineCl][oxalic acid]. The residue was dried in a vacuum at 80 °C for 1 h, and the [CholineCl][oxalic acid] was recovered and reused for the consecutive run.

3.5. Spectroscopy Data

[CholineCl][oxalic acid] [58]

¹H-NMR (500 MHz, DMSO-d₆) δ ppm 3.81–3.80 (m, 2H, –CH₂–N), 3.42–3.40 (m, 2H, –CH₂–O), 4.71(s, 3H, –OH), 3.11 (s, 9H, –N(CH₃)₃)

¹³C-NMR (125 MHz, DMSO-d₆) δ ppm 161.80, 67.42 (t, J = 2.8 Hz), 55.5, 53.6 (t, J = 3.8 Hz).

2-Phenylbenzo[d]oxazole (4a) [59,60,61]

White solid, isolated yield: 70%, melting point: 101–103 °C.

¹H-NMR (500 MHz, CD₃OD-d₄) δ ppm 8.28–8.26 (m, 2H), 7.78–7.76 (m, 1H), 7.72–7.70 (m, 1H), 7.64–7.59 (m, 3H), 7.47–7.41 (m, 2H).

¹³C-NMR (125 MHz, CD₃OD-d₄) δ ppm 113.9, 111.1, 109.5, 107.6, 106.9,101.5, 92.7.

EI-MS m/z 195.

2-(4-Methoxyphenyl)benzo[d]oxazole (4b) [60,61]

White solid, isolated yield: 72%, melting point: 102–104 °C.

¹H-NMR (500 MHz, (CD₃)₂CO-d₆) δ ppm 8.21 (d, J = 9.0 Hz, 2H), 7.74–7.72 (m, 1H), 7.68–7.66 (m, 1H), 7.41–7.37 (m, 2H), 7.16 (d, J = 8.5 Hz, 2H), 3.93 (s, 3H).

¹³C-NMR (125 MHz, (CD₃)₂CO-d₆) δ ppm 162.9, 162.7, 150.7, 142.4, 129.2, 124.8, 124.5, 119.5, 114.5, 110.4, 55.1.

EI-MS m/z 225.

5-Chloro-2-phenylbenzo[d]oxazole (4c) [60,61]

White solid, isolated yield: 85%, melting point: 147–149 °C.

¹H-NMR (500 MHz, CD₃OD-d₄) δ ppm 8.24–8.22 (m, 2H), 7.73 (d, J = 2.0 Hz, 1H), 7.67–7.57 (m, 4H), 7.41 (dd, J = 2.0 Hz, 1H),

¹³C-NMR (125 MHz, CD₃OD-d₄) δ ppm 166.0, 150.7, 144.2, 133.4, 131.4, 130.3, 128.8, 127.6, 126.8, 120.5, 112.8.

EI-MS m/z 229.

5-Chloro-2-(p-tolyl)benzo[d]oxazole (4d) [59]

White solid, isolated yield: 81%, melting point: 147–148 °C.

¹H-NMR (500 MHz, CD₃OD-d₄) δ ppm 8.13 (d, J = 8.0 Hz, 2H), 7.73 (d, J = 2.0 Hz, 1H), 7.66 (d, J = 9.0 Hz, 1H), 7.43–7.40 (m, 3H), 2.45 (s, 3H).

¹³C-NMR (125 MHz, CD₃OD-d₄) δ ppm 164.9, 149.3, 143.1, 142.8, 130.0, 129.5, 127.4, 125.2, 123.4, 118.9, 111.4, 20.2.

EI-MS m/z 243.

5-Chloro-2-(4-fluorophenyl)benzo[d]oxazole (4e) [62]

White solid, isolated yield: 75%, melting point: 155.5–156.5 °C.

¹H-NMR (500 MHz, CD₃OD-d₄) δ ppm 8.31–8.27 (m, 2H), 7.75 (d, J = 2.0 Hz, 1H), 7.67 (d, J = 8.5 Hz, 1H), 7.43 (dd, J = 2.0 Hz, 1H), 7.35 (t, J = 8.5 Hz, 17.5 Hz, 2H).

¹³C-NMR (125 MHz, CD₃OD-d₄) δ ppm 165.0 (d, J = 250 Hz), 163.7, 149.4, 142.8, 130.1, 129.9 (d, J = 9.1 Hz), 125.4, 122.8 (d, J = 3.3 Hz), 119.1, 116.0 (d, J = 22.5 Hz), 111.5.

EI-MS m/z 247.

2-(4-Bromophenyl)-5-chlorobenzo[d]oxazole (4f) [61]

White solid, isolated yield: 82%, melting point: 157–158 °C.

¹H-NMR (500 MHz, CD₃OD-d₄) δ ppm 8.18 (d, J = 8.0 Hz, 2H), 7.80 (d, J = 9.5 Hz, 3H), 7.71 (d, J = 8.5 Hz, 1H), 7.47 (d, J = 8.5 Hz, 1H).

¹³C-NMR (125 MHz, CD₃OD-d₄) δ ppm 132.2, 129.0, 125.4, 119.7, 111.1.

EI-MS m/z 306.

5-Methyl-2-phenylbenzo[d]oxazole (4g) [60,61]

White solid, isolated yield: 87%, melting point: 113.5–145 °C.

¹H-NMR (500 MHz, CD₃OD-d₄) δ ppm 8.20–8.18 (m, 2H), 7.59–7.54 (m, 3H), 7.52–7.49 (m, 2H), 7.22 (d, J = 8.5 Hz, 1H), 2.47 (s, 3H).

¹³C-NMR (125 MHz, CD₃OD-d₄) δ ppm 164.6, 150.2, 142.9, 136.2, 132.9, 130.2, 128.5, 128.1, 127.8, 120.3, 111.2, 21.5.

EI-MS m/z 209

5-Methyl-2-(p-tolyl)benzo[d]oxazole (4h) [59,61]

White solid, isolated yield: 83%, melting point: 134.5–135.5 °C.

¹H-NMR (500 MHz, CD₃OD-d₄) δ ppm 8.10 (d, J = 8.5 Hz, 2H), 7.53 (d, J = 9.0 Hz, 2H), 7.40 (d, J = 8.0 Hz, 2H), 7.24 (d, J = 8.5 Hz, 1H), 2.49 (s, 3H), 2.45 (s, 3H).

¹³C-NMR (125 MHz, CD₃OD-d₄) δ ppm 164.6, 149.8, 143.5, 142.6, 135.8, 130.5, 128.2, 127.2, 124.9, 119.8, 110.7, 21.2, 21.1.

EI-MS m/z 223.

2-(4-Fluorophenyl)-5-methylbenzo[d]oxazole (4i) [63]

White solid, isolated yield: 69%, melting point: 169–171 °C.

¹H-NMR (500 MHz, CD₃OD-d₄) δ ppm 8.26–8.22 (m, 2H), 7.52 (d, J = 8.5 Hz, 2H), 7.31 (t, J = 9.0 Hz, 17.5 Hz, 2H), 7.24 (d, J = 8.0 Hz, 1H), 2.48 (s, 3H).

¹³C-NMR (125 MHz, CD₃OD-d₄) δ ppm 165.0 (d, J = 250 Hz), 162.4, 148.9, 141.5, 134.9, 129.6 (d, J = 8.8 Hz), 126.4, 123.2 (d, J = 2.5 Hz), 119.0, 115.9 (d, J = 22.5 Hz), 109.7, 20.1.

EI-MS m/z 227.

2-(4-Chlorophenyl)-5-methylbenzo[d]oxazole (4j) [63]

White solid, isolated yield: 66%, melting point: 124.5–125.5 °C.

¹H-NMR (500 MHz, CD₃OD-d₄) δ ppm 8.20 (d, J = 9.0 Hz, 2H), 7.61 (d, J = 8.5 Hz, 2H), 7.55 (d, J = 8.0 Hz, 2H), 7.27 (d, J = 8.0 Hz, 1H), 2.50 (s, 3H).

¹³C-NMR (125 MHz, CD₃OD-d₄) δ ppm 162.8, 149.0, 141.6, 137.7, 135.0, 129.1, 128.6, 126.6, 125.5, 119.1, 109.8, 20.1.

EI-MS m/z 243.

2-(4-Bromophenyl)-5-methylbenzo[d]oxazole (4k) [61]

White solid, isolated yield: 74%, melting point: 181–182 °C.

¹H-NMR (500 MHz, CD₃OD-d₄) δ ppm 8.27–8.24 (m, 2H), 7.53 (d, J = 9.0 Hz, 2H), 7.33 (t, J = 8.5 Hz, 17.5 Hz, 2H), 7.25 (d, J = 8.5 Hz, 1H), 2.49 (s, 3H).

¹³C-NMR (125 MHz, CD₃OD-d₄) δ ppm 166.0, 164.0, 148.9, 141.6, 134.9, 129.6, 126.4, 123.3, 119.1, 116.0, 109.8, 20.0.

EI-MS m/z 286.

5-Nitro-2-phenylbenzo[d]oxazole (4l) [64]

Orange solid, isolated yield: 29%, melting point: 167–169 °C.

¹H-NMR (500 MHz, CD₃OD-d₄) δ ppm 7.47 (dd, J = 3.0 Hz, 2.5 Hz, 1H), 7.39 (d, J = 7.5 Hz, 2H), 7.33 (t, J = 7.5 Hz, 15.5 Hz, 2H), 7.27 (d, J = 2.5 Hz, 1H), 7.24 (t, J = 7.5 Hz, 14.5 Hz, 1H), 6.75 (d, J = 9.0 Hz, 1H)

¹³C-NMR (125 MHz, CD₃OD-d₄) δ ppm 152.1, 142.6, 140.5, 139.0, 129.6, 128.2, 128.1, 114.5, 112.9, 105.6.

EI-MS m/z 240.

5-Nitro-2-(p-tolyl)benzo[d]oxazole (4m) [59,65]

Orange solid, isolated yield: 35%, melting point: 124.5–126 °C.

¹H-NMR (500 MHz, CD₃OD-d₄) δ ppm 8.30–8.27 (m, 2H), 7.56 (d, J = 8.5 Hz, 2H), 7.35 (t, J = 9.0 Hz, 18.0 Hz, 2H), 7.27 (d, J = 8.5 Hz, 1H), 2.51 (s, 3H).

¹³C-NMR (125 MHz, CD₃OD-d₄) δ ppm 166.0, 164.0, 162.4, 148.9, 141.5, 134.9, 129.7, 129.6, 126.5, 123.2, 119.0, 116.0, 115.9, 109.8, 20.1.

EI-MS m/z 254.

2-(4-Fluorophenyl)-5-nitrobenzo[d]oxazole (4n) [65]

Brown solid, isolated yield: 15%, melting point: 221–222 °C.

¹H-NMR (500 MHz, CD₃OD-d₄) δ ppm 7.47 (dd, J = 3.0 Hz, 1H), 7.41–7.38 (m, 2H), 7.24 (d, J = 2.5 Hz, 1H), 7.05 (t, J = 9.0 Hz, 18.0 Hz, 2H), 6.74 (d, J = 8.5 Hz, 1H).

¹³C-NMR (125 MHz, CD₃OD-d₄) δ ppm 163.0 (d, 250 Hz), 152.2, 142.5, 138.9, 136.5, 130.0 (d, J = 8.0 Hz), 116.20 (d, J = 21.5 Hz), 114.6, 113.0, 105.5.

EI-MS m/z 258.

4. Conclusions

In summary, the selective condensation of 2-aminophenols and benzaldehydes has been successfully demonstrated by employing a cheap [CholineCl][oxalic acid] catalyst under microwave irradiation. The yield and selectivity obtained with [CholineCl][oxalic acid] are better than the other deep eutectic solvents under reaction conditions. The low-cost DES can be synthesized from available raw materials, which can be easily applied on a large scale. The advantageous features of the present method are the elimination of toxic and expensive reagents, mild reaction, high yields, the broad scope of the substrate, and a recyclable catalyst.

Author Contributions

Conceptualization, P.T.P. and H.T.N.; Methodology, P.T.P., H.T.N. and T.T.N.; Formal analysis, L.H.T.N., M.-H.D.D. and T.L.H.D.; Investigation, D.D.P. and C.T.N.; Writing—P.H.T.; Writing—review and editing, P.T.P., H.T.N. and P.H.T. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

In this manuscript, our characterizations were FT-IR, TGA, ¹H NMR, ¹³C NMR, and GC-MS. All data have been reported as images.

Acknowledgments

The authors express their sincere thanks to the Center for Innovative Materials & Architectures (INOMAR), Viet Nam National University, Ho Chi Minh City for their support in using the TGA and FT-IR measurements.

Conflicts of Interest

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures, Schemes and Tables

Scheme 1. Synthesis of [CholineCl][oxalic acid].

Figure 1. FT-IR spectrum of choline chloride, oxalic acid, and [CholineCl][oxalic acid].

Figure 2. TGA analysis of [CholineCl][oxalic acid].

Scheme 2. Proposed mechanism.

Figure 3. Recycling of [CholineCl][oxalic acid].

Figure 4. FT-IR spectra of the fresh and reused [CholineCl][oxalic acid].

Table 1

Optimization of the reaction condition ^[a].

[Image omitted. Please see PDF.]
Entry	Catalyst Loading (mol%)	Reaction Condition	Conversion [b] (%)	Ratio of 3c/4c
1	30	100 °C, 15 min, MW	86	29/71
2	30	120 °C, 15 min, MW	90	6/94
3	30	130 °C, 15 min, MW	94	4/96
4	30	140 °C, 15 min, MW	88	2/98
5	30	120 °C, 5 min, MW	74	10/90
6	30	120 °C, 10 min, MW	75	28/72
7	30	120 °C, 20 min, MW	87	3/97
8	30	120 °C, 25 min, MW	90	2/98
9	30	120 °C, 30 min, MW	95	1/99
10	0	120 °C, 15 min, MW	77	100/0
11	10	120 °C, 15 min, MW	99	1/99
12	20	120 °C, 15 min, MW	95	1/99
13	40	120 °C, 15 min, MW	91	17/83
14	50	120 °C, 15 min, MW	72	15/85
15	10	120 °C, 15 min, Stirring	14	1/99
16	10	120 °C, 60 min, Stirring	25	1/99
17	10	80 °C, 15 min, Ultrasound	6	0/100

^[a] Reaction condition: 2-Amino-4-chlorophenol (1b, 1.0 mmol), benzaldehyde (2a, 1.0 mmol). ^[b] Conversion and selectivity of 3c/4c were recorded by GC-MS.

Table 2

Effect of the catalysts ^[a].

Entry	Catalyst	Conversion [b] (%)	Selective Ratio of (3c)/(4c) (%)
1	[CholineCl][oxalic acid]	99 (85) [c]	1/99 (11/89) [c]
2	[CholineCl][succinic acid]	86	7/93
3	[CholineCl][Urea]2	86	94/6
4	[CholineCl][ethyleneglycol]2	72	8/92
5	[CholineCl]2[glucose]	33	41/59
6	[CholineCl]2[fructose]	39	41/59

^[a] Reaction condition: 2-Amino-4-chlorophenol (1b, 1.0 mmol), benzaldehyde (2a, 1.0 mmol), catalysts (10 mol%) under MW at 120 °C for 15 min. ^[b] Conversion and selectivity of 3c/4c were recorded by GC-MS. ^[c] 5 mmol scale.

Table 3

Comparative effectiveness of the synthesis of 2-phenylbenzoxazole (4c) from 2-amino-4-chlorophenol (1b) and benzaldehyde (2a).

Entry	Catalyst	Temperature (°C)	Time (h)	Yield or Conversion (%)
1	([BMIm]2[WO4]) (0.1 mmol), 1,4-dioxane (5 mL)	100	5	81
2	Sm(OTf)3 (10 mol%),EtOH–H2O (4:2 mL)	50–60	2	92
3	NaCN (10 mol%), DMF, air	80	4	71
4	Cu2O/DMSO	r.t.	2	95
5	Present work: [CholineCl][oxalic acid](10 mol%), MW	120	0.25	99

Table 4

[CholineCl][oxalic acid]-catalyzed for the synthesis of benzoxazole derivatives under microwave irradiation ^[a].

[Image omitted. Please see PDF.]
Entry	Structure of Products	[Image omitted. Please see PDF.]	Product	Time (min)	Temperature (°C)	Conversion [b] (%)	Ratio of 3/4 [b] (%)
1	[Image omitted. Please see PDF.]	–H	4a	15	120	81	25/75
2		–OMe	4b	15	160	84	21/79
3	[Image omitted. Please see PDF.]	–H	4c	15	120	99	1/99
4		–Me	4d	15	160	99	4/96
5		–F	4e	15	160	89	1/99
6		–Br	4f	15	160	92	1/99
7	[Image omitted. Please see PDF.]	–H	4g	30	130	99	11/89
8		–Me	4h	30	140	99.5	6.5/93.5
9		–F	4i	15	160	88	10/90
10		–Cl	4j	15	160	87	9/91
11		–Br	4k	15	160	87	9/91
12	[Image omitted. Please see PDF.]	–H	4l	30	130	38	18/82
13		–Me	4m	30	140	42	1/99
14		–F	4n	15	160	30	43/57

^[a] Reaction condition: Phenol derivatives (1, 1.0 mmol), aldehydes (2, 1.0 mmol), and [CholineCl][oxalic acid] (10 mol%) under MW. ^[b] Conversion and selectivity of 3/4 products were recorded by GC-MS.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal12111394/s1, ¹H NMR, ¹³C NMR, and GC-MS.

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