The synthesis of aminonaphtols and β-amino carbonyls in the presence of a magnetic recyclable Fe₃O₄@MCM-48–NaHSO₄ nano catalyst

Abstract

In this study, a magnetic recyclable Fe₃O₄@MCM-48–NaHSO₄ nano catalyst was used for the synthesis of aminonaphtols with heteroaromatic amines and β-amino carbonyls during the Mannich reaction. For this purpose, magnetite nanoparticles (MNPs) and MCM-48 mesoporous-coated MNPs with a particle size lower than 11 nm were synthesized via chemical precipitation methods. Then NaHSO₄ was adsorbed on the surface of mesoporous MNPs. The prepared inorganic magnetic catalyst was characterized by FT-IR, XRD, UV-DRS, TEM, VSM and titration. Finally, the applicability of the synthesized solid acid catalyst for the catalysis of Mannich reactions was investigated.

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Introduction

Consume minimal energy, reagents or auxiliaries and minimize waste are the principles of “Green Chemistry” for catalyzed organic reactions. Many unrecyclable catalysts are ecologically harmful and do not follow these principles. Hence, the development of new catalysts for synthetic methods has become an important research area, aiming to make the synthesis simpler, to save energy and to prevent toxicity in chemical processes. Consequently, we need a catalyst system that not only shows a high activity and selectivity (like a homogeneous system) but also possesses the ease of catalyst separation and recovery (like a heterogeneous system). These goals can be achieved by nanocatalysis based on metal nanoparticles. Nanocatalysis can bridge the gap between homogeneous and heterogeneous catalysis, preserving the desirable attributes of both systems. Clearly, the development of metal nanoparticles with a tunable catalytic activity is of great significance for both academia and industry.

However, the recycling problem must be addressed before the nanocatalytic processes can be scaled-up, due to the fact that nanoparticles, which include nano-scaled metal catalysts and supports are difficult to separate from the reaction mixture, which can lead to the blocking of filters and valves by the nanoparticle catalyst.¹ Currently, the use of magnetic nanoparticles can solve this problem. Catalysts supported on magnetic nanoparticles, usually iron oxides, can be quickly and easily recovered in the presence of an external magnetic field for reuse.

In the last decade, significant research efforts have been devoted to obtaining materials with well defined nanoparticles for the synthesis of new catalysts. Nanocatalysts based on mesoporous silica materials like MCM-n, SBA-n and FSM, among others, are a fairly new type of material that has pores in the mesoscopic range of 2–50 nm.² The performance of these materials as a catalyst depends directly on the silica network porosity, the high surface area, the large pores, the high hydrothermal stability, the easy preparation, etc. These materials with these particular properties can be used as coatings for MNPs. Thereby, the synthesis of MNPs functionalized with mesoporous silica leads to an increase in the surface area and an enhancement in the textural properties of the MNPs, which allows their usage as strong and stable supports for very organic and inorganic catalysts.

Sulfuric acid is one of the most important catalysts for the production of industrial chemicals. Over 15 million tons of sulfuric acid are annually consumed as an unrecyclable catalyst that does not follow the principles of “Green Chemistry” because this process is costly, produces high waste and the separation of the catalyst from the homogeneous reaction mixture is inefficient. These drawbacks can be overcome by the synthesis of mesoporous MNPs-based solid acid with a high density of sulfonic acid groups (–OSO₃H), which can be easily removed from the reaction mixture applying an external magnetic field.³

The synthesis of natural molecules, pharmaceuticals and other nitrogenous biologically active compounds for a long time has been a significant branch of organic synthesis.⁴ The Mannich reaction provides one of the most basic and useful methods for the synthesis of such compounds. Due to the drastic reaction conditions, severe side-reactions, substrate limitations and the long reaction time, the classical intermolecular Mannich reaction is plagued. To overcome the drawbacks of the classic method, the Lewis acid-catalyzed condensation between silyl enol ethers or silyl ketene acetals and imines has been developed. Recently, some Bronsted acid or Lewis acid-catalyzed one-pot Mannich reactions of unmodified aldehydes, ketones and amines have been catalyzed by HCl,⁵ proline,⁶ p-dodecyl benzene sulfonic acid (DBSA),⁷ polymer-support sulfonic acid (PS-SO₃H)⁸ and Lewis acids⁹ as well as silica–AlCl₃.¹⁰ However, the long reaction time, costly catalysts and the requirement of special efforts for the catalyst preparation cannot be avoided. Therefore, there has been continuous interest to develop easier methods for the synthesis of β-amino carbonyl compounds.

In the present study, the synthesis of new mesoporous magnetic nanoparticles as a new solid acid catalyst with a high density of sulfonic acid groups (SO₃H) are reported and their performance as novel, strong and stable catalysts is discussed. This strategy involves Fe₃O₄ nanoparticle as the magnetic core coated by MCM-48 mesoporous silica as a thin layer and functional groups of sulfonic acid.

The application of MCM-48 as a support for sulfonic acid has been reported in previous works,^11–13 but this study focused on the possibility of applying NaHSO₄ and nanotechnology for the design of a novel, active and recyclable, sulfonic acid derivative for the first time. Also, the current work shows unique advantages, such as the simple usage of the catalyst system in solvent-free conditions, the easy separation of the catalyst with a permanent magnet and the application of inexpensive and available precursors.

Result and discussion

The characterization of the synthesized MNPs

Magnetite nanoparticles (Fe₃O₄) with 9.0 nm average diameter were prepared by the chemical coprecipitation technique. The sulfonated mesoporous MNPs were synthesized in two steps as follows: (i) the preparation of colloidal iron oxide magnetic nanoparticles and (ii) the development of an MCM-48 mesoporous structure within the MNP surface. NaHSO₄ was used as the sulfonating agent for the synthesis of the Fe₃O₄@MCM-48–NaHSO₄ nanoparticles. The new synthesized nanoparticles and solid acid catalysts were characterized via FT-IR, XRD, DRS, TEM, VSM and titration.

The FT-IR spectra of the MNPs, MCM-48, mesoporous-coated MNPs and Fe₃O₄@MCM-48–NaHSO₄ MNPs confirm the structure of the synthesized particles (see ESI†). For the bare MNPs, the peak at ∼575 cm⁻¹ is attributed to the Fe–O band vibration of Fe₃O₄. In the case of the MCM-48 coated nanoporous particle the band at 1085 cm⁻¹ corresponds to the Si–O–Si anti symmetric stretching vibrations, which are indicative of the existence of SiO₂ in the nanoparticles. The FT-IR spectra of the mesoporous-coated MNPs without and with calcination confirm the removal of the surfactant template. According to the previous works the stretching and bending vibrations of the O–H bonds can be observed at ∼3500 and 1647 cm⁻¹; respectively. Also, the external vibrations of the SiO₄ chains can be observed at ∼1222 and 789 cm⁻¹; at 962 cm⁻¹ due to the asymmetric Si–O vibrations adjacent to the sylanol groups; at 580 cm⁻¹ due to the presence of double ring vibrations and at 454 cm⁻¹ due to the angular bending of the Si–O units.^14,15 Fe₃O₄ usually presents bands at ∼570 and 430 cm⁻¹, due to Fe–O vibrations in tetrahedral and octahedral sites, respectively.¹⁶ In the case of Fe₃O₄@MCM-48–NaHSO₄, the sulfonic acid bonds can be observed at ∼1200–1250, 1010–1100 and 650 cm⁻¹, which are attributed to the O=S=O asymmetric and symmetric stretching vibrations and the S–O stretching vibration of the sulfonic groups (–SO₃H), respectively. However, in the FT-IR spectra of the synthesized nanoparticles, such bands could not be observed because they are probably overlapped by the bands of SiO₂. The increase in the intensities of the bands at 3000–3500 cm⁻¹ suggests that there are more OH groups under the mesoporous MNPs surface after the sulfonation. On the other hand, the band at ∼3360 cm⁻¹ became much broader.

The X-ray diffraction (XRD) patterns are shown in Fig. 1 for the bare MNPs and Fe₃O₄@MCM-48. The MNPs have peaks with 2θ at 29.72, 35.57, 43.17, 57.15 and 62.77 which are characteristic peaks of Fe₃O₄, indicating the purity of the synthesized nanoparticles of Fe₃O₄. The XRD pattern of the mesoporous MNPs showed peaks that could be indexed either to the mesoporous structure or the MNPs, (i) the siliceous mesoporous structure indicated four peaks with 2θ at 1.5–10 or reflection from the 211, 220, 420 and 332, which are the characteristic peaks of MCM-48; (ii) the same peaks with MNPs, which indicated the presence of magnetite in the cave of the synthesized composite. The same peaks were observed in both bare and mesoporous coated MNPs, which indicated the accurate synthesis of the Fe₃O₄@MCM-48 MNPs.

Fig. 1 The X-ray diffraction patterns of the Fe₃O₄ MNPs (a) and Fe₃O₄@MCM-48 (b).

Fig. 2 shows the diffuse reflectance spectroscopy (UV-DRS) of the synthesized bare MNPs, MCM-48 NPs, mesoporous-coated MNPs and Fe₃O₄@MCM-48–NaHSO₄ MNPs, respectively. The MNPs indicated peaks at 220 nm, which indicating the purity of the synthesized Fe₃O₄. The same peaks were observed in all particles. The UV-DRS patterns thus indicate the existence of the magnetic core during the all synthesized MNPs.

Fig. 2 The UV-DRS spectra of (a) Fe₃O₄; (b) Fe₃O₄@MCM-48; (c) Fe₃O₄@MCM-48–NaHSO₄; (d) MCM-48.

The TEM images of the prepared MNPs are shown in Fig. 3. Based on the TEM images, the analysis of the Fe₃O₄ and Fe₃O₄@MCM-48 surface morphology demonstrated the agglomeration of many ultrafine particles with a diameter of about 9.0 and 12 nm, respectively.

Fig. 3 The TEM image of (a) Fe₃O₄; (b) Fe₃O₄@MCM-48.

It is most important that the MNPs and Fe₃O₄@MCM-48–NaHSO₄ should possess sufficient magnetic and super paramagnetism properties for magnetic carrier technology (MCT) practical applications. The magnetic hysteresis curves of the MNPs are shown in Fig. 4. The bare MNPs and MCM-48 mesoporous MNPs exhibited a typical superparamagnetic behavior due to not exhibiting hysteresis, remanence and coercively. The large saturation magnetization values of the bare MNPs and MCM-48 mesoporous MNPs were 82 emu g⁻¹ and 50 emu g⁻¹ respectively, which is sufficient for magnetic separation with a conventional magnet.

Fig. 4 The magnetic hysteresis curves of the Fe₃O₄ and Fe₃O₄@MCM-48.

The amount of NaHSO₄ adsorbed on Fe₃O₄@MCM-48 was 3.19–3.22 mmol g⁻¹, which was determined through the neutralization titration of six synthesized samples. We can suggest that the –SO₃H group in NaHSO₄ can act as Bronsted acid sites in the catalytic mechanism. To exhibit the magnetic properties of the synthesized solid acid, the sulfonic acid-loaded mesoporous MNPs were dispersed in water, resulting in a dark dispersion. In the presence of an external magnetic field nanoparticles of Fe₃O₄@MCM-48–NaHSO₄ were completely gathered onto one side of the cuvette wall steadily (Fig. 5).

Fig. 5 Photographs of aqueous suspension of Fe₃O₄@MCM-48–NaHSO₄ before (a) and after (b) magnetic capture.

The catalytic properties of Fe₃O₄@MCM-48–NaHSO₄

The catalytic properties of Fe₃O₄@MCM-48–NaHSO₄ were investigated in the Mannich reaction for the synthesis of aminonaphtol derivatives and the stereoselective synthesis of β-amino carbonyl compounds.

The Mannich reaction is one of the most important multicomponent reactions that is used for carbon–carbon bond formation. This reaction is a nucleophilic addition of an amine to a carbonyl group followed by dehydration to the Schiff base.

An attractive feature of this method is the use of Fe₃O₄@MCM-48–NaHSO₄ as the solid acid catalyst for the synthesis of aminonaphtols with a high yield in a short time and the synthesis of β-amino ketones which favors the anti-isomer.

(I) Aminonaphtols. The prepared Fe₃O₄@MCM-48–NaHSO₄ magnetic particles have been used as catalysts in the three-component, one-pot Mannich reaction for the synthesis of aminonaphtol derivatives with heteroaromatic amines. For the initial optimization of the reaction conditions and the identification of the best amount catalyst (Fe₃O₄@MCM-48–NaHSO₄), benzaldehyde, 3-amino pyridine and naphthalen-2-ol were chosen as model substrates (Fig. 6).

Fig. 6 The synthesis of aminonaphtols catalyzed by Fe₃O₄@MCM-48–NaHSO₄.

The amount of the catalyst is a main factor affecting the synthesis procedure. Thus, after screening different amounts of the synthesized catalyst (Table 1), the results show that the product 4 could be obtained in a yield range 81–95%. Hence, 0.025 g of Fe₃O₄@MCM-48–NaHSO₄ can catalyze the Mannich reaction for the synthesis of aminonaphtols.

Table 1 The influence of the amount of the synthesized catalyst on the Mannich synthesis of aminonaphtol^a

Entry	Fe3O4@MCM-48–NaHSO4 (g)	Time (min)	Yield^b (%)
a Benzaldehyde–3-amino pyridine–2-naphtol = 1 : 1 : 1, solvent-free, room temperature.b Refers to the isolated yield.
1	0.0125	60	95
2	0.025	18	97
3	0.05	35	90
4	0.1	47	81

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Afterwards, we investigated the effect of the temperature on the reaction rate as well as the yield of the products. According to the obtained results, 25 °C was selected for the synthesis procedure (Table 2, entry 2).

Table 2 The influence of temperature on the Fe₃O₄@MCM-48–NaHSO₄ catalyzed synthesis of aminonaphtol^a

Entry	Temperature (°C)	Time (min)	Yield^b (%)
a Benzaldehyde–3-amino pyridine–2-naphtol = 1 : 1 : 1, solvent-free, 0.025 g of Fe3O4@MCM-48–NaHSO4.b Refers to the isolated yield.
1	10	95	85
2	25	13	97
3	50	13	97
4	100	13	95

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In summary, the optimal conditions for the Fe₃O₄@MCM-48–NaHSO₄ catalyzed Mannich reaction (for the synthesis of aminonaphtols) involved the combination of Fe₃O₄@MCM-48–NaHSO₄ (0.025 g), naphthalen-2-ol 1 (1 mmol), 3-amino pyridine 2a (1 mmol) and benzaldehyde 3 (1 mmol) at room temperature under solvent-free conditions. In view of the obtained results, the optimized reaction conditions were selected to determine the scope of this Fe₃O₄@MCM-48–NaHSO₄ catalyzed reaction. A wide range of aromatic aldehydes and heteroaromatic amines 2a–d were subjected to react with naphthalen-2-ol in the presence of 0.025 g Fe₃O₄@MCM-48–NaHSO₄ to generate 4–7 (Fig. 6) and the results are summarized in Table 3.

Table 3 The Fe₃O₄@MCM-48–NaHSO₄ catalyzed Mannich synthesis of aminonaphtols 4–7^a

Amine	R	Product	Time (min)	Yield^b (%)
a All products were characterized by FT-IR, ^1H NMR and ^13C NMR spectroscopy.b Refers to the isolated yield.
2a	H	4a	13	97
2a	3-NO2	4b	20	94
2a	4-NO2	4c	17	97
2a	4-Br	4d	15	93
2a	3-Cl	4e	20	93
2a	4-Cl	4f	17	95
2b	H	5a	3	95
2b	4-Me	5b	2	97
2b	4-Br	5c	2	91
2b	4-Cl	5d	1	93
2c	H	6a	2	98
2c	4-Me	6b	2	92
2c	4-Cl	6c	2	92
2d	H	7a	1	94
2d	4-Me	7b	2	97
2d	4-Cl	7c	1	95

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(II) β-amino carbonyl. After reporting the synthesis of β-amino carbonyl in our previous work,¹⁷ herein we report an efficient and operationally convenient solid acid catalyst for the synthesis of β-amino carbonyl compounds with high yields at room temperature and solvent-free conditions.

The one-pot synthesis of β-amino carbonyl compounds was achieved by the three-component condensation of cyclohexanone, aromatic aldehydes and aromatic amines in the presence of Fe₃O₄@MCM-48–NaHSO₄ MNPs as a heterogeneous catalyst (Fig. 7).

Fig. 7 The one-pot three-component direct Mannich reaction for the synthesis of β-amino carbonyl.

In the reaction, benzaldehyde, cyclohexanone and aniline were chosen as the model substrates for the optimization of the conditions. The reaction was carried out by stirring a mixture of cyclohexanone (3.0 mmol), benzaldehyde (2.5 mmol) and aniline (2.5 mmol) in the presence of various amounts of Fe₃O₄@MCM-48–NaHSO₄ MNPs as the catalyst in solvent-free conditions. The efficiency of the reaction is affected mainly by the amount of the Fe₃O₄@MCM-48–NaHSO₄ MNPs (Table 4). According to the obtained results, to give the product the catalyst is necessary for the reaction. Increasing the amount of the catalyst increased the yield of the product. The optimal amount of the Fe₃O₄@MCM-48–NaHSO₄ MNPs was 0.05 g (entry 4); increasing the amount of the catalyst beyond this value did not increase the yield noticeably (entry 5).

Table 4 The influence of the amount of the synthesized catalyst on the Mannich synthesis of β-amino carbonyl^a

Entry	Fe3O4@MCM-48–NaHSO4 (g)	Time (min)	Yield^b (%)
a Solvent-free and room temperature conditions.b Refers to the isolated yield.
1	None	240	None
2	0.0125	120	87
3	0.025	80	92
4	0.05	30	97
5	0.1	55	97

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The anti- and syn-isomers of the products were identified by the coupling constants (J) of the vicinal protons adjacent to C=O and NH in ¹H NMR spectra.¹⁷ The coupling constants for the anti- isomers are reported to be bigger than those of the syn-isomers.¹⁸ Probably, the interaction between the catalyst and the transition state in this reaction conduces to the formation of the anti- or syn-isomer.^19,20

Under optimum conditions, to show the generality and scope of this new protocol, a wide range of aromatic aldehydes and aromatic amines were used as bearing electron-withdrawing and electron-donating groups in addition to cyclohexanone, which gave the product in good to high yields with an excellent anti selectivity (Table 5).

Table 5 The Fe₃O₄@MCM-48–NaHSO₄ catalyzed Mannich synthesis of β-amino carbonyl

Entry	R^1	R^2	Yield^a (%)	Time (min)	anti/syn^b
a Isolated yields, the products were confirmed by ^1H NMR.b The anti/syn ratio was determined by ^1H NMR.
1	H	H	95	30	99/1
2	4-Me	H	80	25	99/1
4	4-Br	H	75	40	70/30
5	4-Cl	H	75	35	99/1
6	4-NO2	H	90	25	99/1
7	2-Cl	H	90	35	99/1
8	H	4-Br	95	40	99/1
9	H	4-Me	90	20	98/2

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The different group substitution of the aldehydes and aniline with the same groups located at different positions of the aromatic ring has been shown not to have much effect on the formation of the final product and affords the expected anti-isomer of the products. The products were characterized by IR and ¹H NMR spectroscopy.

Importantly, note that the superparamagnetic properties of Fe₃O₄@MCM-48–NaHSO₄ made the isolation and reuse of this catalyst very easy. After the completion of the reaction, the products which were connected to the catalyst were separated with a permanent magnet. Finally, the reaction product was eluted from the catalyst. The Fe₃O₄@MCM-48–NaHSO₄, after washing and drying in air, can be directly reused without any deactivation even after ten rounds of the synthesis of the product.

A comparison between the results of the proposed catalyst and some of the recently used catalysts for the Mannich reaction is summarized in Table 6. This table shows that the heterogeneous catalyst of Fe₃O₄@MCM-48–NaHSO₄ is the best in comparison to other mentioned catalysts. The proposed new catalyst has some advantages in comparison with the other catalyst including a shorter reaction time, activity as a stereoselective catalyst, easy separation via an external magnetic field, a low consumption of organic solvents and the ability to perform reactions in solvent free conditions. It is a stable solid acid catalyst with high densities of sulfuric acid groups that can be easily synthesized in the lab and can be reused several times. Also, it is more stable with respect to the non-functionalized or bare magnetite nanoparticles that were synthesized and used in our previous work.¹⁷

Table 6 A comparison of the characteristics of the newly synthesized catalyst with some recently used catalysts

Catalyst	Time (min)	Temperature (°C)	Solvent	Stereoselective	Ref.
a Nanoparticle.b 2,4,6-Trichloro-1,3,5-triazine.c Cellulose sulfuric acid.d Carbon-based solid acid.e Bare magnetite nanoparticle.f Polyethyleneglycol.
MgO/ZrO2	480	80	Acetonitrile	No	21
Cu/C Np^a	180	room temperature	Water–EtOH	No	22
TCT^b	150	room temperature	PEG^f	Yes	23
TCT	420	room temperature	EtOH	No	24
CSA^c	420	room temperature	EtOH	Yes	25
SiO2–OSbCl2	300	room temperature	EtOH	No	26
ZnO NP	600	60	H2O	Yes	27
CBSA^d	270	room temperature	EtOH	No	28
Fe3O4^e	45	room temperature	EtOH	Yes	17
Proposed catalyst	60	room temperature	Solvent free	Yes	Proposed method

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Experimental

Materials and apparatus

All chemicals including FeCl₃· 6H₂O, FeCl₂· 4H₂O, tetraethyl orthosilicate (TEOS), NaOH, NaF and NaHSO₄· H₂O were purchased with a high purity from Fluka and Merck (Darmstadt, Germany). The crystal phases and crystallinity of the synthesized MNPs were analyzed by X-PERTPRO X-ray diffraction (PANalitical) and measured with Cu-Kα radiations in the range of 1.5–70 (2θ). The quality and composition of the synthesized nanoparticles were characterized by a Shimadzo Fourier transform infrared (FTIR-470) spectrometer in the range of 400–4000 cm⁻¹. The size and morphology of the particles were studying using a PHILIPS transmission electron microscope (CM10 HT 100 kV). The UV-Vis absorption behavior of the synthesized MNPs were characterized using a Sinco diffuse reflectance spectrophotometer (DRS, S-4100).

Preparation of the MNPs

The MNPs were prepared by the addition of an aqueous solution of ammonia to an aqueous mixture of ferrous and ferric salts, according to our previous method with a little modification.¹⁷ Briefly, 6.3 g FeCl₃· 6H₂O, 4.0 g FeCl₂· 4H₂O and 1.7 mL HCl (12 mol L⁻¹) were dissolved in 50 mL of deionized water in a beaker in order to prepare the stock solution of ferrous and ferric chloride. After that, the solution was degassed with argon gas and heated to 80 °C in a reactor. Simultaneously, 250 mL of a 1.5 mol L⁻¹ ammonia solution was slowly added to the solution under argon gas protection and vigorous stirring (1000 rpm). During the process, the solution temperature was kept constant at 80 °C and argon gas was purged to prevent the intrusion of oxygen. After the completion of the reaction, the obtained precipitate of the magnetite (Fe₃O₄) nanoparticles was separated from the reaction medium by the magnetic field, and then was washed four times with 500 mL doubly distilled water. Finally, the obtained MNPs were resuspended in 500 mL of degassed deionized water. The concentration of the obtained MNPs was obtained as 6.2 mg mL⁻¹.

The preparation of the Fe₃O₄@MCM-48 and Fe₃O₄@MCM-48–NaHSO₄ MNPs

For the synthesis of Fe₃O₄@MCM-48, the MNPs (1.5 g) and ammonia solution (5 mL) were mixed with 50 mL distilled water in a glass reactor and sonicated for 2 minutes at 40° C. After mixing, TEOS (10 mL), NaOH (0.9 g) and NaF (0.19 g) were added to the mixture and stirred for 2.0 hours. Then, cetyltriammonium bromide (CTAB) (7.0 g) was added to the mixture. The mixture was stirred at 40 °C for 2.0 hours again. At the end of this process, the magnetic composite was hydrothermally treated at 120 °C for 48 hours in an autoclave. After two days, the resultant solids were filtered, washed with distilled water and dried at 60 °C. Finally, the template was removed by calcinations of the synthesized particles for 3 hours at 300 °C.

For the synthesis of the functionalized Fe₃O₄@MCM-48 with SO₃H groups, the method of Azarifar et al. was used.²⁹ Fe₃O₄@MCM-48 (1.5 g) was added to 20 mL magnetically stirred aqueous solution of NaHSO₄·H₂O (0.7 g, 5 mmol) at 25 °C over a 60 min period. The mixture was stirred for a further 30 min allowing the sodium bisulfate to be adsorbed onto the mesoporous MNPs. Finally, the water was removed and the powder was dried in an oven at 90 °C for 2–3 h, after which a brown solid of sulfonic acid functionalized MCM-48 mesoporous MNPs (Fe₃O₄@MCM-48–NaHSO₄) was obtained.

The general procedure for the synthesis of 1-(phenyl(pyridin-3-ylamino)methyl)naphthalen-2-ol

Under the optimum conditions, a mixture of benzaldehyde (1 mmol), 3-amino pyridine (1 mmol), naphthalen-2-ol (1 mmol) and MNPs (0.025 g) was stirred at room temperature and under solvent-free conditions for 13 minutes. The reaction products were monitored by TLC. After the completion of the reaction, the mixture was triturated with ethanol. Afterward, in the presence of a magnetic stirrer bar, Fe₃O₄@MCM-48–NaHSO₄ was separated and the reaction mixture turned clear. Finally the crude product was recrystallized from EtOH to give a pure product.

The general procedure for the synthesis of 2-(phenyl(phenyl amino)methyl)cyclohexanone

Under the optimum conditions, the one-pot, three-component Mannich reaction between benzaldehyde (2.5 mmol), aniline (2.5 mmol) and cyclohexanone (3 mmol) in the presence of Fe₃O₄@MCM-48–NaHSO₄ (0.05 g) was also studied. Hence, the mixture was stirred at room temperature and under solvent-free conditions for 30 minute. After the completion of the reaction (monitored by TLC), the reaction product was eluted from the MNPs with hot EtOH and the catalyst was removed by a permanent magnet. Then, the EtOH solution of the product was kept at room temperature, to crystallize the product. Finally, the collected product was filtrated and washed via EtOH (95%).

Conclusion

The present methodology offers several advantages, such as good yields, short reaction times, solvent-free conditions, room temperature and a recyclable catalyst with a very easy operation. In addition, the obtained results indicated that Fe₃O₄@MCM-48–NaHSO₄ as a solid acid can be used as an effective and inexpensive catalyst for the synthesis of aminonaphtols and the stereoselective synthesis of β-amino carbonyl by one-pot three component reactions.

Acknowledgements

The authors acknowledge the Guilan Science & Technology Park, Guilan, Iran for supporting this work.

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Footnote

† Electronic supplementary information (ESI) available: the FT-IR spectra of the catalyst and the ¹H NMR and ¹³C NMR spectra of the compounds. See DOI: 10.1039/c3ra47768a

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