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Magnetically separable modified sulfuric acid (CuFe2O4@SiO2- OSO3H): Preparation, characterization and catalytic application for the synthesis of 1, 8-dioxo-octahydroxanthenes

A nanomagnetic acidic catalyst (CuFe2O4@SiO2-OSO3H) was prepared by the chemical anchoring of sulfuric acid onto the surface of modified CuFe2O4 magnetic nanoparticles and characterized using FT-IR, SEM, EDX, and VSM techniques. The results confirmed that the sulfuric acid is well dispersed on the surface of the nanomagnetic support. The catalytic activity of CuFe2O4@SiO2-OSO3H was evaluated in the synthesis of 1, 8-dioxo-octahydroxanthenes under solvent-free conditions. The reactions using this nanomagnetic acidic catalyst could be carried out in lower than 12 min with excellent yields. Also, the catalyst was easily isolated from the reaction mixture by an external magnet and used at least four times without significant loss of activity.

Catalysts and catalytic reactions have attracted great attention in industrial applications and basic researches [22][23][24][25]. With increasing environmental concerns, the development of clean synthetic procedures has been widely studied. Most of the homogeneous catalysts have high activity and selectivity [26,27], however, heterogeneous ones can be easily handled, separated and reused. The potential advantage of heterogeneous catalysts allows the development of environmentally benign processes in both academic and industrial environment [28,29]. In recent years, among the various heterogeneous catalysts, magnetic nanoparticles (MNPs) with high surface area and their unique magnetic properties [30] have been widely applied in various chemical reactions [31][32][33][34][35][36][37]. They are separated from the reaction medium by an external permanent magnet. The MNPs containing acidic functional groups, especially SO3H-functionalized, have become crucial and demanding researches and are suitable substitute catalysts for conventional acids such as H2SO4, HF and AlCl3 in chemical processes [38][39][40].
In this view and in line with our interest in the application of reusable catalysts in organic reactions [41][42][43][44][45][46][47] and in continuation of our previous works in the synthesis of new MNPs [48,49], we report the preparation of SO3Hfunctionalized magnetic core-shell nanoparticles, CuFe2O4@SiO2-OSO3H, by coating a SiO2 shell around CuFe2O4 MNPs followed by immobilization of sulfuric acid (Scheme 1). The catalytic activity of the prepared heterogeneous and green acidic magnetic nanocatalyst was tested in the synthesis of 1,8-dioxooctahydroxanthenes 3a-j by the reaction of aromatic aldehydes 2a-j with dimedone 1 under solvent-free conditions (Scheme 2).

Preparation of CuFe2O4 MNPs
CuFe2O4 MNPs were precipitated in water in the presence of sodium hydroxide solution of Cu(NO3)2 and Fe(NO3)3 [50]. Typically, Fe(NO3)3⋅9H2O (2.02 g, 5 mmol) and Cu(NO3)2⋅3H2O (0.60 g, 2.5 mmol) were dissolved in water (10 mL), and aqueous NaOH (4 M, 30 mmol, 7.5 mL) was added at room temperature over a period of 10 min to form a reddish-black precipitate. The reaction continued for 2 h at 90 o C. After cooling to room temperature, magnetic particles were collected by a magnetic separator, washed with water (3 × 10 mL) and kept in an air oven overnight at 80 °C. The resulting particles were finally ground with a pestle and mortar and kept in a furnace at 800 °C for 4 h and then slowly cooled to room temperature to form CuFe2O4 MNPs.

Preparation of silica-coated CuFe2O4 MNPs (CuFe2O4@SiO2)
The core/shell CuFe2O4@SiO2 was prepared according to the literature procedure [51]. The CuFe2O4 MNPs (2.0 g, 8.5 mmol) were ultrasonically dispersed in ethanol (25 mL) for 15 min at room temperature and then 25% aqueous ammonia (10 mL) was added to the mixture and stirred at 60 °C for 40 min followed by the addition of tetraethyl orthosilicate (TEOS, 1.0 mL) which was drop wise added to this mixture and stirring was continued at the same temperature for 24 h. The obtained CuFe2O4@SiO2 MNPs were separated by a permanent magnet and washed repeatedly with methanol (3 × 5 mL) and dried in vacuum at 50 °C for 48 h. The resulting CuFe2O4@SiO2 MNPs were then calcined at 800 °C for 4 h.

Preparation of silica sulfuric acid coated CuFe2O4
MNPs (CuFe2O4@SiO2-OSO3H) Chlorosulfonic acid (0.52 g, 4.5 mmol) was added in a drop-wise manner to a cooled (ice-bath) solution of CuFe2O4@SiO2 (1 g) in n-hexane (5 mL) over a 2 h period. Upon completion of the addition, the mixture was stirred for a further 3 h until to allow for the complete dissipation of HCl from the reaction vessel. The resulted MNPs were separated using an external magnet and washed with methanol before being dried in an oven at 60 °C to give CuFe2O4@SiO2-OSO3H as a brown powder. The amount of H + in the CuFe2O4@SiO2-OSO3H determined by acid-base potentiometric titration using NaOH as titrant was 2.81 mmol/g.
was typically performed according to the following procedure. The catalyst, CuFe2O4@SiO2-OSO3H, (0.04 g) was added to a mixture of dimedone 1 (2.0 mmol) and an aromatic aldehyde 2a-j (1.0 mmol). The mixture was heated in an oil bath at 110 °C for 8-12 min and monitored by TLC. Upon completion, the reaction mixture was cooled to room temperature and hot acetone (15 mL) was added. The catalyst was separated by external magnet. The solvent was evaporated in vacuo and the residue was recrystallized from ethanol to afford compounds 3a-j in high yields.

Results and discussion Preparation and characterization of CuFe2O4@SiO2-OSO3H MNPs
The preparation of a magnetically separable modified sulfuric acid (CuFe2O4@SiO2-OSO3H) has been delineated through initial synthesis of CuFe2O4 MNPs by a chemical coprecipitation of Fe(NO3)3. 9H2O and Cu(NO3)2. 3H2O in aqueous NaOH [50] followed by coating with a layer of silica using the sol-gel method by the ammonia-catalyzed hydrolysis of TEOS [51] and finally reaction with chlorosulfonic acid in n-hexane. The prepared CuFe2O4@SiO2-OSO3H catalyst was characterized using FT-IR, SEM, EDX and VSM.
The FT-IR analysis of CuFe2O4, CuFe2O4@SiO2 and CuFe2O4@SiO2-OSO3H are shown in Figure 1. A strong band in the range of 565-593 cm -1 appeared in the spectra of all MNPs (Figure 1, a-c) which can be assigned to the stretching vibration of Fe-O bond. The adsorption bands at 3417-3445 cm -1 and also at 1632-1648 cm -1 are assigned to OH groups and remaining H2O in the samples. The additional peak in the range of 955-1091 cm -1 recorded for MNPs having a SiO2 layer (Figure 1, b and   To determine the morphology and size of the catalyst, SEM image of the CuFe2O4@SiO2-OSO3H MNPs was prepared and compared to CuFe2O4 MNPs ( Figure 2). As shown in Figure  2(b), nanoparticles in the prepared catalyst have spherical shape with an average diameter of approximately 40-50 nm, indicating that the nanocatalyst has a large surface area. The slight agglomeration is due to magnetic dipole interactions between the particles.  The appearance of S along with other elements containing Cu, Fe, Si, and O in EDX spectrum of the CuFe2O4@SiO2-OSO3H catalyst shows the successful immobilization of SO3H on CuFe2O4@SiO2 MNPs ( Figure 4). As can be seen in Figure 4, no additional peak related to other impurities was appeared in the spectrum. Finally, the magnetic properties of CuFe2O4@SiO2-OSO3H nanoparticles were studied using VSM at ambient temperature in an applied magnetic field, with the field sweeping from -10000 to +10,000 Oersted (Oe) ( Figure 5). It could be seen that the hysteresis loop for the catalyst is relatively irreversible. This confirms the ferromagnetic nature of the catalyst [52]. Furthermore, the saturation magnetization (Ms) value of 27.12 emu g −1 shows that the catalyst has still sufficient magnetization for easy magnetic separation from a reaction mixture with a permanent magnet.

Catalytic application of CuFe2O4@SiO2-OSO3H MNPs
The performance of CuFe2O4@SiO2-OSO3H as an acidic nanocatalyst was tested in the synthesis of 1,8-dioxooctahydroxanthenes. In order to investigate the optimum amount of the catalyst, the effect of the solvent and influence of temperature, the reaction between dimedone 1 (1 mmol) and 4chlorobenzaldehyde 2a (1 mmol) for the synthesis of compound 3a was selected as the test reaction. A summary of the optimization experiments is provided in Table 1. Because of the several advantages of solvent-free conditions in chemical reactions we firstly decided to investigate the model reaction under solvent-free conditions. As can be seen, the efficiency of the reaction is mainly affected by the amount of the CuFe2O4@SiO2-OSO3H catalyst. No significant yield of the product was obtained in the absence of the catalyst (Table 1, Entry 1) indicating that the catalyst is necessary for the reaction. Raising the amount of the catalyst increased the yield of the product 3a. The best result was conducted in the presence of 0.04 g of the catalyst at 110 °C (Table  1, Entry 11). Higher amount of the catalyst and temperature did not improve the reaction time and yield of the product. Subsequently, the same model reaction in the presence of 0.04 g of catalyst was carried out in different solvents including H2O, MeOH, EtOH, CHCl3 and CH3CN, to assess the effect of solvent on the reaction. As shown, among the solvents tested and also solvent-free conditions, the yields of the reaction under solvent-free conditions were greater and the reaction times were generally shorter than the solvents. Therefore, all subsequent reactions were carried out using 0.04 g of the catalyst at 110 °C under solvent-free conditions.  Next, in order to evaluate the generality of this model reaction, the catalytic activity of the catalyst was tested using different aromatic aldehydes in the reaction with dimeone under optimized reaction conditions. As shown in Table 2, all the reactions on a wide range of aromatic aldehydes bearing both electron-donating (methyl, hydroxyl and methoxy) and electron-withdrawing (halogens and nitro) substituents afforded excellent yields of the corresponding product 3a-j.  Table 3. As can be seen, our reaction conditions showed a shorter reaction time than the other conditions and gave high yields of the desired products The reusability of the catalyst was also investigated. For this purpose, the same model reaction was again studied under optimized conditions. After the completion of the reaction, hot acetone was added to the reaction mixture to dissolve the product. With the aid of an external magnet, the catalyst was held on the sidewall of the reaction vessel, while the solution was decanted. The catalyst was washed with acetone and ethanol, dried at 100 o C under vacuum for 1 h, and reused for a similar reaction. The catalyst could be used at least four times with only a slight reduction in activity (91, 90, 88, and 87% yields for first to fourth use, respectively) which clearly demonstrates the practical reusability of this catalyst.

Conclusion
In summary, we report a new, simple and green catalytic method for the synthesis of 1,8-dioxo-octahydroxanthenes via the reaction of aromatic aldehydes with dimedone using sulfuric acid linked on silica-coated CuFe2O4 MNPs (CuFe2O4@SiO2-SO3H) at 110 o C under solvent-free conditions. The catalyst could simply be recovered with the aid of an external magnet, and used at least four times without significant loss of its catalytic activity. High yields, short reaction times, easy work-up, and the absence of any volatile and hazardous organic solvents are some advantages of this protocol.