Introduction

Fe₃O₄ nanoparticles are heterogeneous catalysts. Among all iron oxides, magnetite (Fe₃O₄) has the most interesting properties due to the presence of divalent and trivalent Fe⁺² and Fe⁺³ cations in the form of inverted spinel structure [1-3]. Fe₃O₄ nanoparticles are reactive and show high vulnerability in acidic environments. Therefore, to protect against the destruction of acidic environments, these particles are surrounded by various functional ligands [4]. In general, these ligands have terminal functional groups such as phosphoric, carboxylic acid, polyethylene glycol, polylactide, thiol, or silyl oximes that can establish strong bonds with the surface of iron oxide nanoparticles [5]. Some of the reactions in which Fe₃O₄ nanoparticles are used as catalysts are synthesis of pyranopyrazoles in the presence of Fe₃O₄ nanoparticles, the reaction between hydrazine hydrate, ethyl acetoacetate, aldehyde, acetone, and malonitrile in water at room temperature leads to the synthesis of pyranopyrazolamide (Figure 1) [6].

Synthesis of Hydantoins in the presence of Fe₃O₄ nanoparticles: the reaction between carbonyl compounds (aldehydes or ketones), potassium cyanide, and ammonium carbonatein was performed in the condition without the solution produces hydantoins [7]. The mechanism of synthesis reaction of hydantoin derivatives with Fe₃O₄ nanoparticles (Figure 2) is as follows:

γ-Fe₂O₃ nanoparticles as a heterogeneous catalyst: iron (III) oxide in crystalline state has alpha, gamma, and delta structures. Alpha structure: it is the most stable form of iron at room temperature. Iron (III) oxide at room temperature is in the crystalline state of the central cube. Gamma structure. When the iron of the alpha structure is heated, it turns into the face-centered cubic fuzzy structure, which is called gamma [8-10]. This form of iron is used in stainless steel. Delta structure: When molten iron is cooled at 1538 C, it crystallizes into the delta allotrope, which has a central cubic crystal structure. If it cools more, it will turn into a cubic shape with the central face of gamma at 201 C [11]. Because iron oxides are reactive and show high vulnerability in acidic environments, to protect against the destruction of acidic environments, these particles are surrounded by various functional ligands, or supported on the surface of a solid phase [12-14]. In general, these ligands have terminal functional groups such as phosphoric, carboxylic acid, polyethylene glycol, polylactic, thiol, or silyloxy which can establish strong bonds with the surface of iron oxide nanoparticles [15-17].

X-ray diffraction

XRD has been used extensively for the examination of materials and thin films. Its effective use depends upon having a crystalline material. The technique is a bulk-sensitive analytical method, but can be used to provide information relevant to surface changes in suitable circumstances. For example, XRD can provide useful information on the extent to which surface treatment of a carbon system has affected the bulk of the material [18-20]. It is possible to use XRD in a thin-film mode, employing very small take-off angles, to derive some surface information, but generally speaking it should be considered as a bulk structural technique. Examples of the XRD application to the study of carbon fibers can be found in the author’s own work [21].

X-Ray diffraction for structural analysis

X-ray diffraction (XRD) is used for the primary characterization of material properties like crystal structure, crystallite size, and strain. The XRD use in pharmaceutical research is extensively increasing due to its wide application. XRD works on the principle of Bragg’s equation, which can be described in terms of reflection of collimated X-ray beam incidence on a crystal plane of the sample that to be characterized [22]. XRD is based on the wide-angle elastic scattering and is generally used for ordered material (specifically long-range order crystalline material), and is not preferred for the disordered material. A beam of X-rays is passed through the specimen and is scattered, or diffracted, by the atoms in the path of the X-rays investigated. The interference occurring due to scattering of X-rays with each other is observed applying Bragg’s law and a suitably positioned detector, and crystalline structure characteristics of the material are determined. All measurements are carried out in Angstroms (1 Å=0.1 nm or 10−10m). To confirm the results obtained using XRD, they may be compared with microscopy techniques or other solid-state characterization techniques. However, XRD can be time-consuming and may require large amounts of sample [23].

Despite some limitations, XRD is widely used to determine the material structure at an atomic level. Another difficulty with XRD is that it is difficult to analyze the growing crystal in real time, and therefore this technique gives results only from single confirmation or binding state [24-26]. The next disadvantage of the XRD technique is the low intensity of diffracted X-rays, specifically for low atomic number material, with a comparison to the electron diffractions. Nevertheless, new approaches are being developed in XRD analysis using femtosecond pulses from a hard X-ray free electron laser, for structure determination of macromolecules, which does not provide sufficient crystal size using conventional radiation sources [27].

Fe₃O₄ and γ-Fe₂O₃ catalyst covered with silica gel: In most cases, silica is used as a support for the reactants and leads to a high surface area and larger pore volume. Among the types of catalysts supported on silica, iron oxides are of particular importance due to their magnetic properties, ease of separation from reaction products, and the ability to recover and reuse. The supported catalysts on the solid phase have different activity compared to the similar unsupported samples and can act more selectively [10]. Some of the reactions in which iron oxides covered with silica are used as catalysts are as follow:

1- Synthesis of 1 and 4-dihydropyridine derivatives with Fe₃O₄@SiO₂ nanoparticles

It is prepared from the multicomponent reaction of Dimedone or 4-hydroxycoumarin, aldehydes, and ammonium acetate in a one pot under reflux conditions and in an aqueous medium in the presence of Fe₃O₄@SiO₂ nanoparticles as a catalyst, 1 and 4-dihydropyridine derivatives [21]. The synthesis mechanism of 1 and 4-dihydropyridine derivatives with Fe₃O₄@SiO₂ nanoparticles is displayed in Figure 3.

2- Synthesis of xanthene derivatives in the presence of γ-Fe₂O₃@SiO₂ nanoparticles

For the synthesis of 1 and 8-dioxooctahydroxanthene derivatives, a mixture of aldehyde and dimedone in a stoichiometric ratio of 1:2 was placed in the presence of γ-Fe₂O₃@SiO₂ nanoparticles in water under reflux conditions and the desired product is produced. For the synthesis of 14-aryl-H14-dibenzo-xanthene derivatives, a combination of butanephthol and aldehydes in a ratio of 2:1 in the presence of γ-Fe₂O₃@SiO₂ nanoparticles in water and under reflux conditions leads to the production of the desired product (Figure 4) [11].  

The mechanism of synthesis reaction of xanthene derivatives in the presence of γ-Fe₂O₃@SiO₂ nanoparticles is depicted in Figure 5.

Materials and methods

Solvents, reagents, and chemicals used in this research were prepared from Merck Chemicals Company, Germany. Melting points are obtained using Moiner tubes in Electro thermal melting point device, IA-9000 model. The IR spectra were also prepared using FT-IR-JASCO-460 plus spectrometer. Thin layer chromatography (TLC) was performed using Silica gel 60 GF254 sheets from Merck Chemical Company, Germany. Preparation of Fe₃O₄ nanoparticles by co-precipitation method:

First, add 2.62 mL of 25% ammonia in a volumetric flask to 50 mL to prepare 0.7 M ammonia. Then dissolve 0.401 grams (2 mmol) of iron (II) tetrahydrochloride (FeCl₂.4H₂O) completely in 1 mL, M₂ HCl, and then dissolve 1.092 g (4 mmol) of iron (III) hexahydratechloride (FeCl₃.6H₂O) in 4 mL, M₂ HCl and after complete dissolution, add these two solutions together. A cm² magnet is placed inside the beaker containing the mixture and placed on a magnetic stirrer. Now the valve of the burette containing the ammonium hydroxide solution is opened and added to the solution drop by drop for 5-10 minutes evenly, making sure that the burette head is closed by a cork to prevent the absorption of carbon dioxide by the ammonium hydroxide solution. 

During these stages, the color of the solution changes from green to yellow, orange, purple, light brown, dark brown, and black. The orange and yellow particles are also observed on the surface. After the ammonium hydroxide solution is finished the reaction container is removed from the stirrer and it is taken out of beaker by a bar magnet. After the sediment is completely settled by an external magnet that is placed under the body, the watery sediment is poured into the centrifuge tubes and centrifuged 3 times at a speed of 1000 rpm to remove impurities such as ammonium chloride and iron hydroxides.

Now a weight boat is placed on the magnet and the liquid is poured on the sediment in this way 1-2 mL of tetramethylammonium hydroxide solution (surfactant) is added to the sediment by a dropper and quickly stirred with a glass stirrer at a uniform speed until the sediment is completely stirred. By keeping the external magnet steady, we emptify the water on the sediment again and by moving the magnet, we see the pins’ growth. Finally, the obtained black sediment was placed in a dark place to dry completely and the FT-IR spectrum was taken from the resulting sediment (Figure 6).

Preparation of γ-Fe₂O₃ nanoparticles

To prepare γ-Fe₂O₃ nanoparticles, Fe₃O₄ nanoparticles produced by co-precipitation method were heated for 2 hours at 300 °C until γ-Fe₂O₃ nanoparticles were produced and FT-IR spectrum was taken from it (Figure 7) [12].

Preparation of γ-Fe₂O₃@SiO₂ nanoparticles

Pure γ-Fe₂O₃ was heated in 40 mL of ethanol at 40 °C for 1 hour, and then 10 mL of tetraethylorthosilicate was added to the reaction mixture and the mixture was stirred for 24 hours at 40 °C. In this way, the γ-Fe₂O₃ coated basilic nanoparticles obtained were collected with a magnet and washed 5 times with ethanol and diethyl ether and it was placed in a vacuum oven for 12 hours at 100 °C until it dried, and the FT-IR spectrum was taken from it (Figure 8).

Synthesis of chlorinated γ-Fe₂O₃@SiO₂

Three grams of dry γ-Fe₂O₃@SiO₂ was stirred in 40 mL of toluene for 45 minutes (Sonicite), 1 mL of 3-chloropropyltrimethoxysilane is added to the reaction mixture and the temperature was slowly raised to 105 °C and was stirred in 24 hours. The obtained product was separated with an external magnet and washed 3 times with diethyl ether and CH₂Cl₂ and dried under vacuum and FT-IR spectrum was taken from it (Figure 9).

Synthesis of γ-Fe₂O₃@SiO₂-(L-leucine) catalyst

One gram of chlorinated γ-Fe₂O₃@SiO₂ was dissolved in 20 mL of distilled water and 1 gram of leucine was dissolved in 50 mL of water-methanol (1:1); they were added to each other and stirred for 24 hours at room temperature. The obtained product was separated with an external magnet and washed twice with 10 mL of water and 2 mL of methanol respectively and dried at 60 °C in a vacuum for 24 hours [13]. After drying, the reaction product which is the deisred catalyst FT-IR and EDX spectra were taken. The graphic design of the γ-Fe₂O₃@SiO₂-(L-leucine) catalyst is as follows (Figure 10):

Determining the amount of H+ catalyst γ-Fe₂O₃@SiO^2- (L-leucine)

0/05 g of γ-Fe₂O₃@SiO₂-(L-leucine) catalyst was stirred in 5 mL of 0.1 M sodium for 10 minutes. Two drops of phenol-phthalein indicator were added to this mixture (in the base environment, this indicator has a purple color). Then, the reverse titration was titrated by titrating the unreacted additional amount of soda in the mixture with the standard solution of 0.1 M HCl  titration continued until the mixture got colorless, based on the amount of consumed HCl, we found out how much of the soda has reacted with the catalyst according to the calculations.

The total volume of HCl used for 4.7 mL titration.

5 mL NaOH – 4.7 mL HCl = 0.3 mL soda neutralized with catalyst

0.3 mL NaOH 0.00003 mol = 0.03 mmol NaOH

0.03 mmol NaOH   0.03 mmol H⁺ for 0.05 g of catalyst

20  0.03 mmol H⁺  0.6 mmol H⁺ for 1 gram of catalyst

The reaction for the preparation of thiazole derivatives in the presence of γ-Fe2O3@SiO2-(L-leucine) as a catalys (Figure 11):

Optimizing the temperature and amount of γ-Fe₂O₃@SiO₂-(L-leucine) catalyst in the preparation of 3-(2,4-diphenylthiazol-5-yl)-4-hydroxy-H2-chromen-2-one:

To get the right amount of catalyst and temperature, to a test tube containing 0.5 mmol (0.0761 g) Phenylglyoxal monohydrate, 0.5 mmol (0.0811 g) 4-hydroxycoumarin and 0.5 mmol (0.0685 g) thiobenzamide at different temperatures and in the presence of 1 mL of water as solvent were added to various amounts of γ-Fe₂O₃@SiO₂-(L-leucine) catalyst while stirring. The completion time of the reaction was monitored and measured by TLC. Then, the catalyst was separated from the reaction mixture and the sediment resulted from the reaction was poured on filter paper and after drying, the sediment was recrystallized with ethanol and ethyl acetate, and after purifying and drying the resulting sediment, the reaction yield was calculated and the amount of the appropriate catalyst was determined. Optimizing the amount of γ-Fe₂O₃@SiO₂-(L-leucine) catalyst at 50 C in the presence of water as a solvent for the reaction of preparing 3-(2,4-diphenyl-thiazol-5-yl)-4-hydroxy-H2- Chromen-2-onjdol (1) was performed (Table 1).

The mentioned yield is related to the product obtained after the purification process. According to the results obtained in Table 1, the optimal amount of catalyst consumption γ-Fe₂O₃@SiO₂-(L-leucine) (0.0.20 g) at 50 C and in the presence of water as a solvent was selected for the reaction of preparing 3-(2,4-diphenylthiazol-5-yl)-4-hydroxy-H2-chromen-2-one.

Optimization of the amount of γ-Fe₂O₃@SiO₂-(L-leucine) catalyst at 70 C in the presence of water as a solvent for the reaction of preparation of 3-(2,4-diphenylthiazol-5-yl)-4-hydroxy-H2-chromene-2-one is presented Table 2.

The mentioned yield is related to the product obtained after the purification process. According to the results obtained in Table 2, the optimal amount of γ-Fe₂O₃@SiO₂-(L-leucine) catalyst consumption (0.02 g) at 70 C and in the presence of water as a solvent was selected for the preparation of 3-(2,4-diphenyl-thiazol-5-yl)-4-hydroxy-H2-chromen-2-one. Optimizing the amount Catalyst γ-Fe₂O₃@SiO₂-(L-leucine) at 80 C in the presence of water as a solvent for the preparation reaction of 3-(2,4-diphenylthiazol-5-yl)-4-hydroxy-H2-chroman-2-one is demonstrated in Table 3.

The yield mentioned is related to the obtained product after the purification process. According to the results obtained in Table 3, the optimal amount of catalyst consumption γ-Fe₂O₃@SiO₂-(-(L-leucine)) 0.02 g) at 80 C and in the presence of water as a solvent was selected for the preparing 3-(2,4-diphenyl-thiazol-5-yl)-4-hydroxy-H2-chromen-2-one. Optimizing the amount Catalyst γ-Fe₂O₃@SiO₂-(L-leucine) at 90 C in the presence of water as a solvent for the preparation reaction of 3-(2,4-diphenylthiazol-5-yl)-4-hydroxy-H2-chromen-2-one is indicated in Table 4.

The mentioned yield is related to the product obtained after the purification process. According to the results obtained in Table 4, the optimal amount of γ-Fe₂O₃@SiO₂-(L-leucine) catalyst consumption (0.020 g) at 90 C and in the presence of water as a solvent was selected for the preparation of 3-(2,4-diphenyl-thiazol-5-yl)-4-hydroxy-H2-chromen-2-one. Optimizing the amount Catalyst γ-Fe₂O₃@SiO₂-(L-leucine) at 100 C in the presence of water as a solvent for the reaction of preparing 3-(2,4-diphenylthiazol-5-yl)-4-hydroxy-H2-chroman-2-one is reported in Table 5.

The mentioned yield is related to the product obtained after the purification process. According to the results obtained in Table 5, the optimal amount of γ Fe₂O₃@SiO₂-(L-leucine) catalyst consumption (0.015 g) at 100 ‎°C and in the presence of water as a solvent was selected for the preparation of 3-(2,4-diphenyl-thiazol-5-yl)-4-hydroxy-H2-chromen-2-one. Optimizing the amount Catalyst γ-Fe₂O₃@SiO₂-(L-leucine) at 120 ºC in the presence of water as a solvent for the reaction of preparing 3-(2,4-diphenylthiazol-5-yl)-4-hydroxy-H2-chromen-2-one (Table 6).

The mentioned yield is related to the product obtained after the purification process. According to the results obtained in Table 6, the optimal amount of catalyst consumption γ-Fe₂O₃@SiO₂-(L-leucine), (0.015 g) at 120 C and in the presence of water as a solvent was selected for the reaction of preparing 3-(2,4-diphenyl-thiazol-5-yl)-4-hydroxy-H2-chromen-2-one. After the end of the reactions, the optimal value was determined from each optimization table in different amounts of catalyst at constant temperature and its results are given in Table 7.

The mentioned yield is related to the product obtained after the purification process. According to the results obtained in Table 7, the optimal amount of γ-Fe₂O₃@SiO₂-(L-leucine) catalyst consumption (0.015 g) at 120 C and in the presence of water as a solvent was selected for the reaction of preparing 3-(2,4-diphenyl-thiazol-5-yl)-4-hydroxy-H2-chromen-2-one. After conducting the reactions, the catalyst/product was easily separated by an external magnetic field; more than 95% of the catalyst is usually recovered. The catalyst was used at least five times without losing the high activity of the catalyst. The general procedure for the thiazoles preparation in the presence of the γ-Fe₂O₃@SiO₂-(L-leucine) catalyst at 120 C in the presence of water as a solvent was as follow:

One mL water was added to a test tube containing 1 mmol of arylglyoxal monohydrate (4-methoxyphenylglyoxal monohydrate, 4-nitrophenylglyoxal monohydrate and phenylglyoxal monohydrate), 1 mmol Cyclic 1 and 3-dicarbonyls (4-hydroxy-coumarin, 4-hydroxy-1-methyl-coumarin and dimedone), and 1 millimol of thiobenzamide derivatives (benzothioamide, 4 chlorobenzothioamido, 4-methoxybenzothioamide), and then 0.015 g of γ-Fe₂O₃@SiO₂-(L-leucine) catalyst was added and the reaction was stirred at 120 ºC by placing a magnet in the reaction container. The completion time of the reaction was monitored and measured by TLC. Then the catalyst was separated from the reaction mixture (the catalyst was used at least five times without losing the high activity of the catalyst) and the resulting precipitate was poured on filter paper and after drying, the precipitate was recrystallized with ethanol and ethyl acetate. And after purifying and drying the precipitate, the yield of the reaction was calculated in Table 8.

The mentioned yield is related to the product obtained after the purification process. Recovery of γ-Fe₂O₃@SiO₂-(L-leucine) catalyst in the preparation of thiazole derivatives:

To recover the γ-Fe₂O₃@SiO₂-(L-leucine) catalyst, the reaction of preparing 3-(2,4-diphenyl-thiazol-5-yl)-4-hydroxy-H2-chromen-2-one was used and the catalyst  was recoverd five times and used it again in the reaction model, the following diagram represents the recycling of the catalyst (Figure 13).

Discussion

Examining the preparation of Fe3O4 nanoparticles: Compared with other supermagnetic nanoparticles, Fe₃O₄ nanoparticles can be prepared more easily and better and can be used as a heterogeneous catalyst in organic reactions. Fe₃O₄ nanoparticles were produced by chemical co-precipitation technique and due to their magnetic properties, they were easily separated from the reaction mixture with an external magnet, and the FT-IR spectrum was taken from the obtained Fe₃O₄ nanoparticles and compared with the reference spectrum. Its spectral data is as follows.

IR (KBr) (v_max, cm^-1): 3435-1588-1487-958-947-575

Investigating the preparation of Fe₂O₃γ-nanoparticles: Due to their superparamagnetic property, Fe₂O₃γ-nanoparticles can be used as a heterogeneous catalyst or the core of heterogeneous catalysts in organic reactions and provide comfortable and easy conditions for the separation of the catalyst and the product, so that it can be easily separated from the reaction mixture with an external magnetic field. FT-IR spectrum was taken for the production of nanoparticles from Fe₂O₃γ-nanoparticles and compared with the reference spectrum, and its spectral data is as follows.  The obtained Fe₂O₃γ was used as part of the catalyst in the reaction to prepare thiazole derivatives.

IR (KBr) (v_max, cm^-1): 3401-2922-1626-1060-637-562-481

Investigating the γ-Fe₂O₃@SiO₂ preparation: Since iron oxides are reactive and show high vulnerability in acidic environments to protect against the destruction of acidic environments, these particles are surrounded by various functional ligands or supported on the surface of a solid phase. Silica is often used as a support for reactants and leads to a high surface area and larger pore volume and greater stability of the catalyst. Likewise, by having enough sites on the surface of silica gel, it provides suitable conditions to support other reactants on this part of the catalyst. These γ-Fe₂O₃ nanoparticles were supported on silica in the presence of tetraethylorthosilicate in the reaction conditions, and the FT-IR spectrum was taken from the produced γ-Fe₂O₃@SiO₂, compared with the reference spectrum, and was used as a component of catalyst in reaction of preparation pf thiazole derivatives (Equation (1)).

IR (KBr) (v_max, cm^-1): 3400-1656-1052-637-563

Investigating the preparation of γ-Fe₂O₃@SiO^2-(L-leucine) catalyst: due to have acidic and basic factors, amino acids can play a role as catalysts in chemical reactions, so catalytic reactions by amino acids takes place in a lower amount and concentration of catalyst at a more suitable temperature, and due to be supported on a silica substrate and the presence of γ-Fe₂O₃ nanoparticles, they have features such as thermal stability, ease of separation from the reaction medium, and the ability to recover and reuse.

IR (KBr) (vmax, cm^-1): 3435 (NH2), 2333, 1629, 1154, 1061 (Si-O-Si), 637, 559

Determining the amount of H+ of γ-Fe₂O₃@SiO₂-(L-leucine) catalyst: The H+ amount of γ-Fe₂O₃@SiO₂-(L-leucine) catalyst was measured by reverse titration method and according to the obtained result; 1 gram of the desired catalyst has 0.6 mmol of H⁺.

Results

Investigation of FT-IR spectrum and EDX spectrum obtained from the catalyst

γ-Fe₂O₃@SiO^2- (L-leucine): FT-IR spectrum was taken from the catalyst γ-Fe₂O₃@SiO^2-(L-leucine) and compared with the reference spectrum, which showed that the catalyst made is γ-Fe₂O₃@SiO₂- (L-leucine).

Investigating the reaction conditions for the thiazoles preparation: 3-6-1-Optimizing the temperature and amount of γ-Fe₂O₃@SiO₂-(L-leucine) catalyst in the preparation of thiazole derivatives:

To get the right amount of catalyst and temperature, various amounts of γ-Fe₂O₃@SiO₂-(L-leucine) catalyst were added to a test tube containing 0.5 mmol (0.0761 g) Phenylglyoxal monohydrate, 0.5 mmol (0.0811 g) 4-hydroxycoumarin, and 0.5 mmol (0.0685 g) thio-benzamide at different temperatures and in the presence of 1 mL water as a solvent while stirring. The completion time of the reaction was controlled and measured by TLC. Then, the catalyst was separated from the reaction mixture and the precipitate from the reaction was poured on filter paper and after drying, the precipitate was recrystallized with ethanol and ethyl acetate, and after purifying and drying the resulting precipitate, the reaction yield was calculated and the amount of the appropriate catalyst was determined.

According to the results obtained in Table 1, the optimal amount of consumption of the catalyst γ-Fe₂O₃@SiO₂-(L-leucine) (0.020 g) at 50 C and in the presence of water as a solvent for the reaction of preparing 3- (2 and 4- Diphenylthiazol-5-yl)-4-hydroxy-H2-chromen-2-one was selected.

According to the results obtained in Table 2, the optimal amount of consumption of the catalyst γ-Fe₂O₃@SiO₂-(L-leucine) (0.020 g) at 70 C and in the presence of water as a solvent was selected for the reaction of preparing 3- (2 and 4- Diphenylthiazol-5-yl)-4-hydroxy-H2-chromen-2-one.

According to the results obtained in Table 3, the optimal amount of consumption of the catalyst γ-Fe₂O₃@SiO₂-(L-leucine) (0.020 g) at 80 C and in the presence of water as a solvent was selected for the reaction of preparing 3- (2 and 4- Diphenylthiazol-5-yl)-4-hydroxy-H²-chromen-2-one.

According to the results obtained in Table 4, the optimal consumption amount of γ-Fe₂O₃@SiO₂-(L-leucine) catalyst (0.020g) at 90 C and in the presence of water as a solvent was selected for reaction of preparaing 3-(2, 4-diphenylthiazol-5-yl)-4-hydroxy-H2-chromen-2-one.

According to the results obtained in Table 5, the optimal amount of γ Fe₂O₃@SiO₂-(L-leucine) catalyst consumption (0.015 g) at 100 C and in the presence of water was selected as a solvent for the reaction of preparing  3-(2,4-diphenylthiazol-5-yl)-4-hydroxy-H2-chromen-2-one.

According to the results obtained in Table 6, the optimal amount of catalyst consumption γ-Fe₂O₃@SiO₂-(L-leucine) (0.015 g) at 120 C and in the presence of water as a solvent was selected for the reaction of preparing 3-(2,4-diphenylthiazol-5-yl)-4-hydroxy-H2-chromen-2-one.

According to the results obtained in Table 7, which is the result of extracting the optimal values of temperature and catalyst from each optimization table, the optimal amount of catalyst consumption γ-Fe2O3@SiO2-(L-leucine) (0.0150 g) and the optimal temperature 120 C were selected for the reaction of preparing 3-(2,4-diphenylthiazol-5-yl)-4-hydroxy-H2-chromen-2-one  because in this condition of reaction condition in the shortest time and amount of catalyst, it has the highest efficiency compared with other conditions. General procedure for preparing thiazole derivatives in the presence of γ-Fe₂O₃@SiO₂-(L-leucine) catalyst at 120 C and in the presence of water as a solvent:

One mL of water was added to a test tube containing 1 mmol of arylglyoxal monohydrate derivatives, 1 mmol of cyclic 1-3-dicarbonyls and 1 mmol of thiobenzamide derivatives, and 0.015 grams of catalyst

γ-Fe₂O₃@SiO₂-(L-leucine) and the reaction were stirred at a temperature of 120 C by placing a magnet in the reaction container. The completion time of the reaction was monitored and measured by TLC. Then, the catalyst was separated from the reaction mixture, and the resulting precipitate was poured on filter paper, and after drying, the precipitate was recrystallized with ethanol and ethyl acetate, and after purifying and drying the resulting precipitate, the yield of the reaction was calculated (Figures 14 and 15). 

Comparison of the results obtained in the reaction of preparing thiazole derivatives in the presence of γ-Fe₂O₃@SiO₂-(L-leucine) with other catalysts mentioned in scientific articles:

By examining and comparing the obtained results from the reaction of preparing thiazole derivatives using γ-Fe₂O₃@SiO₂-(L-leucine) and other catalysts introduced in scientific articles, one can understand the efficiency and benefits of the proposed catalyst. The results of these reactions in the preparation of 3-(2,4-diphenylthiazol-5-yl)-4-hydroxy-H2-chromen-2-one are indicated in Table 9 [14-17].

Examining the results of IR and FT-IR spectra of thiazole derivatives: 3-(2,4-Diphenylthiazol-5-yl)-4-hydroxy-H2-chromen-2-one (Figure 16).

4-hydroxy-3-(4-(4-nitrophenyl)-2-phenylthiazol-5-yl)-2H-chromen-2-one (Figure 17).

Mp:233-235, yield:%81, IR(KBr) (v_max, cm-1):632-1371-1465-1594-1686-2950-3165-3430.

4-hydroxy-3-(4-(4-methoxyphenyl)-2-phenylthiazol-5-yl)-1-methylquinolin-2(1H)-one (Figure 18).

Mp: 269-267, yield: 76%, IR(KBr) (v_max, cm-1): 758-1178-1247-1495-1610-2956-3410.

3-(2,4-bis(4-methoxyphenyl) thiazol-5-yl)-4-hydroxy-2H-chromen-2-one (Figure 19).

Conclusion

The advantage of using supported amino acid, especially supported L-leucine, as compared with homogenous samples, is easier recovery, easier product separation, and recycling after the reaction. The combination of magnetic properties of γ-Fe₂O₃ nanoparticles and special properties of amino acid is a good opportunity to design and manufacture γ-Fe₂O₃@SiO₂^-(L-leucine) catalyst as an efficient nanocatalyst with strong and sufficient acid sites due to being supported on silica gel for reaction and production of products. Catalysts are substances that increase the speed of chemical reactions without undergoing changes. Catalysts are generally divided into two types: homogeneous and heterogeneous catalysts. Homogeneous catalysts: When the catalysts are in the same phase as the reactants, they are called homogeneous. The main problem in the technology of homogeneous catalysts is that after the reaction completion, separating the dissolved catalyst from the final mixture is not an easy task. This problem is a big challenge, especially when the catalyst is used in small amounts. A heterogeneous catalyst is not in the same phase as the reactants. The size and characteristics of the heterogeneous catalyst particles are such that they do not dissolve in the reaction medium, hence its activity is limited. Unlike homogeneous catalysts, the heterogeneous catalysts are easily separated from the reaction mixture and do not cause impurity in the products.

Most importantly, γ-Fe₂O₃@SiO₂^-(L-leucine) nanoparticles are inherently heterogeneous and easily recovered by an external magnetic field. The catalyst was used at least five times without losing the high catalytic activity. The use of acidic catalyst γ-Fe₂O₃@ SiO₂^-(L-leucine) is a suitable method for preparing thiazole derivatives at 120 C and the presence of water as a solvent and has the following advantages:

• Purification of products is very easy and convenient.
• High yield of produced products
• Reducing the reaction time
• Ability to easily separate and reuse the catalyst
• Not using dangerous and harmful solvents for humans and the environment in this method.

Acknowledgements

We would like to thank all the people who helped in preparing and compiling the article and collecting the available data.

Conflict of Interest

There are no conflicts of interests.

Orcid:

Elaheh Abedini‎: https://orcid.org/0000-0001-9496-6225

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How to cite this article: E. Abedini*‎, ‎ H.R. Shaterian‎. L-Leucine supported on silica gel encapsulating γ-Fe₂O₃ nanoparticles: A new recoverable magnetic catalyst for preparation of 1, 3-thiazole derivatives. Eurasian Chemical Communications, 2023, 5(3), 228-245. Link: https://www.echemcom.com/article_160311.html

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Copyright © 2023 by SPC (Sami Publishing Company) + is an open access article distributed under the Creative Commons Attribution License(CC BY)  license  (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.