Synthesis of Ibuprofen Using Silica-Supported Preyssler Nanoparticles as an Eco-Friendly, Inexpensive, and Efficient Catalyst

Abstract

This paper describes an alternative and simple procedure for the synthesis of Ibuprofen using Silica-Supported Preyssler Nanoparticles (H₁₄[NaP₅W₃₀O₁₁₀]/SiO₂) (SPNPs), as an eco-friendly, inexpensive, and efficient catalyst. High yields, simplicity of operation, and easy work-up procedure are some advantages of this protocol. Silica-Supported Preyssler Nanoparticles (H₁₄[NaP₅W₃₀O₁₁₀]/SiO₂) (SPNPs) offer the advantages of a higher hydrolytic and thermal stability. The salient features of Preyssler’s anion are availability, nontoxicity and reusability. We believe this methodology can find usefulness in organic synthesis.

1. Introduction

Ibuprofen was developed by the Boots Pure Chemical Company and then patented in 1961. It is a nonsteroidal anti-inflammatory drug (NSAID) and is marketed under a wide variety of trade names including Advil and Motrin. Ibuprofen is one of several 2-aryl propanoic acids that are currently on the market. Others include ketoprofen, flurbiprofen, and naproxen.

The name “ibuprofen” originally came from the name isobutylpropanoicphenolic acid, but this nomenclature has not been used for many years and, in fact, virtually all chemists today are unfamiliar with it. Fortunately, however, the name is still a reasonably good match for the currently accepted name 2-(4-isobutylphenyl)propanoic acid. Ibuprofen ((+/−)2-(4-isobutylphenyl)propionic acid, Figure 1(a)) is one of the most commonly used anti-inflammatory agents. It is considered to be the prototype for the family of synthetic 2-arylpropionic acids, profens, a subclass of the nonsteroidal anti-inflammatory drugs (NSAIDs). In recent years, the profens have come to dominate this therapeutic class. Ibuprofen, for example, is used to treat arthritis, muscular strain, cephalalgia, and so forth.

(a)

(b)

(c)

(d)

Figure 1 

The profens have an asymmetric carbon centre which is attached to a carboxylic acid, a methyl, and an aryl group of varying structures. Some of the available profen drugs are depicted in Figure 1: ibuprofen (a), naproxen (b), ketoprofen (c), and flurbiprofen (d). Ibuprofen is distributed over the counter and naproxen belongs to the top ten of drugs marketed worldwide in 1989 [1]. Ibuprofen is used to relieve the symptoms of a wide range of illnesses including headaches, backache, period pain, dental pain, neuralgia, rheumatic pain, muscular pain, migraine, cold and flu symptoms, and arthritis. NSAIDs exert their pharmacological and toxicological effects primarily by specifically inhibiting the binding of arachidonic acid to the cyclooxygenase subunit of prostaglandin synthetase, thereby preventing the formation of various prostaglandins [2]. In the last two decades, heteropolyacids (HPAs) have found numerous applications as useful and versatile acid catalysts for a range of acid-catalyzed reactions. Heteropolyacids are several times more active than inorganic and organic acids and their molar catalytic activity is 100–1000 times more active than H₂SO₄. They can also be used in low concentrations [3]. The synthesis and characterization of catalysts with lower dimensions have become the most interesting topic of research. We know that as the particle size decreases, the relative number of surface atoms increases and thus the activity increases. Moreover, due to quantum size effects, nanometer-sized particles may exhibit unique properties for a wide range of applications [4]. A Preyssler acid is a highly acidic catalyst from the heteropolyacid family with excellent catalytic activity in a variety of acid-catalyzed reactions [5]. Therefore, we hoped to further improve this catalyst by using it in the form of nanoparticles in organic reactions and synthesizes. Catalysts based on heteropolyacids have many advantages over liquid acid catalysts. They are not corrosive, environmentally benign, presenting fewer disposal problems. Solid heteropolyacids have attracted much attention in organic synthesis owing to easy work-up procedures, easy filtration, and minimization of cost and waste generation due to reuse and recycling of the catalysts [6]. There have been many attempts to optimize these catalysts. Recently, Keggin nanocatalysts have been synthesized [7]. In our attempt to use heteropolyacids as catalysts in organic reactions, we reported that nano-SiO₂-supported Preyssler heteropolyacid (HPA) and Preyssler type heteropolyacids, H₁₄[NaP₅W₃₀O₁₁₀], show strong catalytic action [8, 9]. We also have demonstrated an alternative and simple procedure for the synthesis of (S)-Naproxen using Preyssler catalyst, H₁₄[NaP₅W₃₀O₁₁₀], as an eco-friendly, environment friendly, reusable, inexpensive, noncorrosive, and efficient catalyst [8].

2. Experimental

2.1. Chemicals

All the chemicals were obtained from Merck (Darmstadt, Germany) and used as received. All solvents were purchased from commercial sources.

2.2. Instruments

Melting points (uncorrected) were measured using Electrothermal IA 9100 digital melting point apparatus. Yields are based on GC/mass analysis using an Agilent (Denver, CO, USA) 6890 GC system Hp-5 capillary 30 m × 530 μm × 1.5 μm nominal. The IR spectra were recorded on a Shimadzu model impact 400D FT-IR spectrophotometer using KBr pellets. ¹H NMR were recorded on a Bruker AC-300F 400 MHz spectrometer in CDCl₃ using tetramethylsilane (TMS) as an internal standard with ¹H resonant frequency of 400 MHz. Optical rotation values were considered on Bellingham Stanly polarimeter.

2.3. Catalyst Synthesis Procedure

Silica-Supported Preyssler Nanoparticles (SPNPs) catalyst was synthesized according to the literature [10]. Thus a solution of surfactant, sodium bis(2-ethylhexyl) sulphosuccinate, in cyclohexane (0.2 M), was treated with a solution of Preyssler acid in a specified amount of water was added. The molar ratio of water to surfactant selected was 3, 5, and 7. Tetraethoxysilane was then added into the microemulsion phase. After mixing for various times (8, 12, 18, 25, and 30 h) at room temperature, dispersed Preyssler acid/SiO₂ nanostructures were centrifuged (1500 rpm) and the particles were rinsed with acetone (4 times) and dried in a vacuum oven. The optimum ratio of water to surfactant was 3 : 1 and the optimum time was 30 h.

2.4. Synthesis of Ethyl Lactate (2)

Lactic acid (10 mL) was dehydrated by vacuum distillation and then refluxed for 3 h in presence of Silica-Supported Preyssler Nanoparticles (H₁₄[NaP₅W₃₀O₁₁₀]/SiO₂) (SPNPs) (0.05 g), ethanol (12 mL), and pyridine (20 mL). The reaction progress was monitored by TLC. After completion of the reaction, the mixture was cooled to room temperature and solidified within an hour. The reaction mixture was filtered to separate out the catalyst, and, following evaporation of the solvent under reduced pressure, a crude product was obtained, which was purified by recrystallization.

2.5. Synthesis of Ethyl-2-(methylsulphonyloxy) Propanoate (3)

To a solution of ethyl lactate (1 g, 8.47 mmol) and triethylamine (1.77 mL, 12.71 mmol) in 15 mL of dry pyridine was added portionwise MsCl (0.7723 mL, 9.32 mmol) at 0°C. The resultant mixture was stirred at 0°C for 1 h and then at room temperature for 2 h. After completion of the reaction, as indicated by TLC, the reaction mixture was diluted with ethyl acetate. The organic layer was washed with H₂O, dried over anhydrous Na₂SO₄, and concentrated under reduced pressure to give the crude compound. Purification of the crude compound by column chromatography over silica gel (100–200 mesh) using hexane as an eluent yielded the pure product ethyl-2-(methylsulphonyloxy) propanoate 3. ¹H NMR (400 MHz, CDCl₃) 5.15 (q, J = 7 Hz, 1H), 4.33 (q, J = 7.5 Hz, 2H), 3.15 (s, 3H), 1.62 (d, J = 7 Hz, 3H), 1.35 (t, J = 7.5 Hz, 3H). ¹³C NMR (100 MHz, CDCl₃): 37.90, 61.60, 70.22, 170.83.

2.6. Synthesis of Ethyl-2-(tosyloxy) Propanoate (4)

To a solution of ethyl lactate (1 g, 8.47 mmol) in 15 mL of dry pyridine was added portionwise TsCl (1.77 g, 9.32 mmol) at 0°C. The resultant mixture was stirred at 0°C for 1 h and then at room temperature for 2 h. After completion of the reaction, as indicated by TLC, the reaction mixture was diluted with C₂H₅OAc. The organic layer was washed with H₂O, dried over anhydrous Na₂SO₄, and concentrated under reduced pressure to give the crude compound. Purification of the crude compound by column chromatography over silica gel (100–200 mesh) using hexane as an eluent yielded the pure product 4 ethyl-2-(tosyloxy) propanoate 4. ¹H NMR (400 MHz, CDCl₃) 7.78 (d, 2H), 7.30 (d, 2H), 4.10 (q, 2H), 2.46 (s, 3H), 1.50 (d, 3H), 1.21 (t, 3H). ¹³C NMR (100 MHz, CDCl₃): 21.31, 61.62, 70.44, 128.30, 130.50, 140.33, 144.41.

2.7. Synthesis of Ethyl-2-(4-isobutylphenyl) Propanoate (5)

(H₁₄[NaP₅W₃₀O₁₁₀])/SiO₂ nanoparticles (SPNPs) (0.05 g) were added to pyridine (15 mL) and isobutylbenzene (2.73 g, 20.41 mmol) at 0°C. Ethyl-2-(methylsulphonyloxy) propanoate was added to the cold solution portionwise and the mixture was warmed to room temperature. This was then heated to 80°C for 8 h (the solution was reflux for 8 h) and then cooled to room temperature. The reaction mixture was quenched with dil. HCl at 0°C and extracted with ethyl acetate. The organic layer was dried over anhydrous Na₂SO₄ and concentrated under reduced pressure to give crude compound. Purification of the crude compound by column chromatography over silica gel using hexane as an eluent yielded the pure product 5.¹H NMR (400 MHz, CDCl₃) 7.12-7.63 (m, 4H), 4.23 (m, 2H), 3.66 (m, 1H), 2.64 (d, J = 8 Hz, 2H), 2.41 (m, 1H), 1.54 (m, 3H), 1.13 (m, 3H), 0.91 (m, 6H). ¹³C NMR (100 MHz, CDCl₃): 22.81, 29.05, 40.44, 44.50, 44.51, 61.62, 128.80, 128.91, 132.22, 140.23.

2.8. Synthesis of Ibuprofen (6)

To a solution of ethyl-2-(4-isobutylphenyl) propanoate (1 g, 4.27 mmol) in 6 mL of CH₃OH a solution of KOH was added (479 mg, 8.55 mmol) in 5 mL of H₂O. The resultant solution was stirred at room temperature for 4 h. Methanol was removed under reduced pressure and the resulting solution was extracted with ethyl acetate and the organic extracts were washed with H₂O, dried over anhydrous Na₂SO₄, and concentrated under reduced pressure to give compound 6. M.P (°C) 130-133, IR (KBr, cm⁻¹): 3100, 2920, 2870, 1716, 1408, 1419, 1321, 1230, 1184, 935, 779, 668, 583. ¹H NMR (400 MHz, CDCl₃) 7.15 (d, J = 8.1 Hz, 2H), 7.02 (d, J = 8.1 Hz, 2H), 3.64 (q, J = 7.2 Hz, 1H), 2.37 (d, J = 7.1 Hz, 2H), 1.75 (m, 1H), 1.43 (d, J = 7.1 Hz, 3H), 0.82 (d, J = 6.6 Hz, 6H). ¹³C NMR (100 MHz, CDCl₃): 22.81, 22.82, 29.07, 42.64, 44.50, 128.80, 128.93, 128.95, 132.22, 140.23, 181.26. Anal. Calcd. for C₁₃H₁₈O₂: C, 75.69; H, 8.80%. Found: C, 75.61; H, 8.70%. HRMS (EI) Calcd. for C₂₆H₂₅FN₄O₆ [M]⁺, 206.1600, Found 206.1009.

3. Results and Discussion

This experiment was carried out for the first time by inexpensive, recyclable Silica-Supported Preyssler Nanoparticles (H₁₄[NaP₅W₃₀O₁₁₀]/SiO₂) (SPNPs) nanocatalyst. We used Silica-Supported Preyssler Nanoparticles (H₁₄[NaP₅W₃₀O₁₁₀]/SiO₂) (SPNPs) heteropolyacid for the synthesis of Ibuprofen. Friedel-Crafts type alkylation of isobutylbenzene with various lactic acid derivatives was explored for the synthesis of Ibuprofen. Silica-Supported Preyssler Nanoparticles (H₁₄[NaP₅W₃₀O₁₁₀]/SiO₂) (SPNPs) were employed as catalysts for Friedel-Crafts alkylation. Mesylate and tosylate of ethyl lactate were the lactic acid derivatives used for the reaction. The scheme for the synthesis of Ibuprofen employing lactic acid derivatives is shown below (Scheme 1).

Scheme 1 

Synthesis of ibuprofen using Silica-Supported Preyssler Nanoparticles (H14[NaP5W30O110]/SiO2) (SPNPs) using ethanol and pyridine in their reactions.

Lactic acid (90%) was dehydrated by vacuum distillation and immediately refluxed with ethanol in presence of Silica-Supported Preyssler Nanoparticles (H₁₄[NaP₅W₃₀O₁₁₀]/SiO₂) (SPNPs) and with azeotropic removal of water to afford ethyl lactate (2) in 82 % yield, (Scheme 1). Mesylation and tosylation of ethyl lactate in the presence of triethylamine or pyridine at room temperature afforded the corresponding ethyl-2-(methylsulphonyloxy) propanoate (3) and ethyl-2-(tosyloxy) propanoate (4). Friedel-Crafts alkylation of mesylate or tosylate with isobutylbenzene for single step synthesis of ethyl-2-(4-isobutylphenyl) propanoate was carried out by heating with Silica-Supported Preyssler Nanoparticles (H₁₄[NaP₅W₃₀O₁₁₀]/SiO₂) (SPNPs) under neat reaction conditions. Ethyl-2-(4-isobutylphenyl) propanoate (5) formed in 65% yield was hydrolysed with KOH in methanol to afford the racemic Ibuprofen (6) in 97% yield. The nanocatalyst was found to be ineffective for the Friedel-Crafts alkylation using mesylates and tosylates with isobutylbenzene.

The yields of the synthesis of Ibuprofen ethyl ester with Silica-Supported Preyssler Nanoparticles (H₁₄[NaP₅W₃₀O₁₁₀]/SiO₂) (SPNPs) and other acid catalysts are given (Scheme 1, Table 1). The result shows that the Silica-Supported Preyssler Nanoparticles (H₁₄[NaP₅W₃₀O₁₁₀]/SiO₂) (SPNPs) catalyst has higher activity and performance in Ibuprofen synthesis compared with other heteropolyacids such as Keggin as well as the standard method using H₂SO₄ (Table 1). The best yield of Ibuprofen (97%) with good selectivity was attained with H₁₄[NaP₅W₃₀O₁₁₀]/SiO₂ (SPNPs) nanocatalyst after 4 h at room temperature (Table 1, entry 1). Keggin (H₃[PW₁₂O₄₀], H₄[SiW₁₂O₄₀], H₃[PMo₁₂O₄₀], H₄[SiMo₁₂O₄₀]) heteropolyacid has also lower activity than H₁₄[NaP₅W₃₀O₁₁₀]/SiO₂ (SPNPs) nanocatalyst. They lead to order 97, 91, 88, 86% Ibuprofen with good selectivity after 4 h of reaction at room temperature (Table 1, entries 1, 2, 5, 10).

Table 1 

Synthesis of Ibuprofen (6) in the presence of various heteropolyacids catalysts at room temperature conditions.

3.1. Effect of the Catalyst Type

Initially, we compared the catalytic performance of Silica-Supported Preyssler Nanoparticles (H₁₄[NaP₅W₃₀O₁₁₀]/SiO₂) (SPNPs) catalyst, Keggin, H₃[PW₁₂O₄₀], H₄[SiW₁₂O₄₀], H₃[PMo₁₂O₄₀], H₄[SiMo₁₂O₄₀], H₁₄[NaP₅W₂₉MoO₁₁₀], H₁₄[NaP₅W₃₀O₁₁₀]/SiO₂(50%), H₃[PW₁₂O₄₀]/SiO₂(50%), H₂SO₄, and without catalyst in the synthesis of Ibuprofen. The results are shown in Table 1.

The Yield of Product Decreases in the Following Order. Silica-Supported Preyssler Nanoparticles (H₁₄[NaP₅W₃₀O₁₁₀]/SiO₂) (SPNPs) nanocatalyst > H₁₄[NaP₅W₃₀O₁₁₀] > H₁₄[NaP₅W₂₉MoO₁₁₀] > H₆[P₂W₁₈O₆₂] > H₃[PW₁₂O₄₀] > Zeolite > H₃[PW₁₂O₄₀]/SiO₂(50%) > H₄[SiW₁₂O₄₀] > p-TSA > H₃[PMo₁₂O₄₀] > H₂SO₄ > H₄[SiMo₁₂O₄₀] catalyst > free catalyst. The results are summarized in Table 1.

As could be seen 30-tungstophosphoric acid, H₁₄[NaP₅W₃₀O₁₁₀]/SiO₂ nanocatalyst, is more effective than the other heteropolyanions and in the presence of this catalyst the highest yields of products are obtained. The interesting feature of this polyanion compared the other heteropolyacids is its hydrolytic stability (pH 0–12), which is very important in catalytic processes. It is clear from these reactions that the efficiency of H₁₄[NaP₅W₃₀O₁₁₀]/SiO₂ (SPNPs) nanocatalyst is higher than these of Keggin heteropolyacids. The reason is likely to be due to higher surface area available. Also, as the particle size of nanomaterial decreases, the relative number of surface atoms will be increased and thus the catalyst activity will be enhanced. Silica-Supported Preyssler Nanoparticles, H₁₄[NaP₅W₃₀O₁₁₀]/SiO₂, (SPNPs) were found to be the best catalyst in terms of reaction time, yield, and recyclability. Silica-Supported Preyssler Nanostructures were obtained through a microemulsion method. A shape change of the particles has been observed in other synthetic methods of nanoparticle preparation [11]. The reason for this can be attributed to metastable states, which can spontaneously change under equilibrium reaction conditions, which is in agreement with previous observations [11]. Silica nanostructures were obtained through a sol-gel method. All of the conditions are shown in Experimental section. The BET surface area, pore volume, and average pore size of nanosized SiO₂ were obtained as 287 m²/g, 0.28 cm³/g, and 0.25 nm, respectively. After the impregnation of HPA (with 30% being the best loading), the BET surface area, pore volume, and average pore size were obtained as 201 m²/g, 0.10 cm³/g, and 0.21 nm, respectively. The BET surface area and pore volume decreased, indicating that the pores of nanosized silica are being filled and the supported HPA blocked some pores of the support. Scanning electron microscopy (SEM) pictures of samples and X-ray diffraction (XRD) patterns of the synthesized samples were taken. The obtained nanostructures were characterized by TEM as shown in Figure 2. This figure shows 40 nm spheres (Figure 2) [9].

Figure 2 

TEM image of the synthesized nano-SiO2 (SPNPs).

Figure 3 shows powder X-ray diffraction (XRD) patterns of the synthesized samples. The patterns of the spherical products confirm the SiO₂ structure. The XRD pattern of nano-SiO₂ with sharp peaks in the 2θ range from 7° to 36° confirmed the crystalline nature of SiO₂. In addition, the lack of an XRD peak centered at 2θ angle 22° (typical for amorphous SiO₂) supports the crystallinity. The patterns of the spherical products confirm the SiO₂ structure [9].

Figure 3 

X-ray diffraction (XRD) pattern.

Analogous diffraction patterns have been observed for other synthesized samples. The heteropolyacid (H₁₄[NaP₅W₃₀O₁₁₀]) on SiO₂ nanoparticles (SPNPs) was characterised by infrared (IR) spectroscopy (Figure 4) [9]. IR spectroscopy demonstrates that (H₁₄[NaP₅W₃₀O₁₁₀]) is preserved in the HPA/SiO₂ nanoparticles. The antisymmetric stretching wavenumber of the terminal oxygen-containing group is observed at 960 cm⁻¹ and the antisymmetric P-O stretching wavenumber is noted at 1080 and 1165 cm⁻¹. The prominent P-O bands at 960, 1080, and 1165 cm⁻¹ are consistent with a symmetry anion [9]. It could therefore be concluded that the heteropolyacid (H₁₄[NaP₅W₃₀O₁₁₀]) was successfully immobilized onto the SiO₂ nanoparticles. TEM and IR studies showed that the heteropolyacid stayed intact on the nanoparticles after it was recycled several times in the reaction reported below. Bleeding of the heteropolyacid was found to be negligible by weighing the catalyst again after it was recycled five times.

Figure 4 

IR spectra of Preyssler heteropolyacid in bulk form (1) and nano form (2).

It is clear that in the synthesis of Ibuprofen the efficiency of (H₁₄[NaP₅W₃₀O₁₁₀])/SiO₂ is slightly higher than that of the conventional Preyssler catalyst. Infrared spectroscopy shows that in (H₁₄[NaP₅W₃₀O₁₁₀])/SiO₂ nanoparticles (SPNPs) the heteropolyacid structure (H₁₄[NaP₅W₃₀O₁₁₀]) is preserved (Figure 4). It is therefore expected that (H₁₄[NaP₅W₃₀O₁₁₀])/SiO₂ nanoparticles (SPNPs) will exhibit the same catalytic characteristics of classical Preyssler catalysts. As the particle size of the nanomaterial decreases, the relative number of surface atoms increases, and thus activity increases. Moreover, due to quantum size effects, nanometre-sized particles can exhibit unique properties.

3.2. Reusability of the Catalyst

The catalyst was recovered after the reaction and reused as a catalyst in the synthesis of Ibuprofen. Several recycling of the catalyst had only slightly decreased the catalytic activity, demonstrating the stability and retention capability of this useful polyanion. Recycling of the catalyst at the end of the reaction involved filtering the catalyst, washing with diethyl ether, and drying at 130°C for 1 h. The recycled catalyst was used for three reactions without observation of appreciable loss in its catalytic activities. In Table 2, the comparison of efficiency of Silica-Supported Preyssler Nanoparticles (H₁₄[NaP₅W₃₀O₁₁₀]/SiO₂) (SPNPs) catalyst in the synthesis of Ibuprofen after five times is reported. As it is shown in Table 2 the first, second, third, fourth, and fifth reactions using recovered Silica-Supported Preyssler Nanoparticles (H₁₄[NaP₅W₃₀O₁₁₀]/SiO₂) (SPNPs) afforded similar yields. The reusability of the catalyst was also investigated. At the end of the reaction, the catalyst was recovered by a simple filtration. The recovered catalyst was reused successfully.

Table 2 

Reuse of the Silica-Supported Preyssler Nanoparticles (H₁₄NaP₅W₃₀O₁₁₀/SiO₂) (SPNPs) catalyst for the synthesis of Ibuprofen.

4. Conclusions

In conclusion, we have reported a new catalytic method for the synthesis of Ibuprofen using Silica-Supported Preyssler Nanoparticles (H₁₄[NaP₅W₃₀O₁₁₀]/SiO₂) (SPNPs) as an efficient, reusable, and eco-friendly heterogeneous inorganic catalyst. The advantages of this method are reusability of the catalyst, easy work-up procedure, and high yields. The Ibuprofen synthesis is carried out in three steps with usual reagents. However, the results show that Silica-Supported Preyssler Nanoparticles (H₁₄[NaP₅W₃₀O₁₁₀]/SiO₂) catalyst gives the highest yield. Compared with mineral acids, such as H₂SO₄, Silica-Supported Preyssler Nanoparticles (H₁₄[NaP₅W₃₀O₁₁₀]/SiO₂) (SPNPs) catalyst is more active and shows a higher selectivity and minimized side reactions. Important features of this polyanion are high thermal and hydrolytic stability throughout a wide pH range. Eco-friendly, recyclable, and easily prepared Silica-Supported Preyssler Nanoparticles (H₁₄[NaP₅W₃₀O₁₁₀]/SiO₂) (SPNPs) catalyst is an efficient solid acid catalyst for highly selective synthesis of Ibuprofen. In the Silica-Supported Preyssler Nanoparticles (H₁₄[NaP₅W₃₀O₁₁₀]/SiO₂) (SPNPs) catalyst in both heterogeneous conditions can easily be recovered and reused without loss of structure and activity. The SPNPs catalyst was prepared in the nanoscale and used as a heterogeneous catalyst for the synthesis of Ibuprofen.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.