MgFe2O4@SiO2–SO3H: an efficient, reusable

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MgFe2O4@SiO2–SO3H: an efficient, reusable catalyst for the microwaveassisted synthesis of benzoxazinone and benzthioxazinone via multicomponent reaction under solvent free conditi... Article  in  Research on Chemical Intermediates · January 2018 DOI: 10.1007/s11164-017-3108-z

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Chandrashekhar Arunrao Ladole

Sant Gadge Baba Amravati University

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Res Chem Intermed DOI 10.1007/s11164-017-3108-z

MgFe2O4@SiO2–SO3H: an efficient, reusable catalyst for the microwave-assisted synthesis of benzoxazinone and benzthioxazinone via multicomponent reaction under solvent free condition Nilesh G. Salunkhe1 • Chandrashekhar A. Ladole1 Nikita V. Thakare1 • Anand S. Aswar1

•

Received: 27 February 2017 / Accepted: 11 August 2017  Springer Science+Business Media B.V. 2017

Abstract We report the synthesis of sulfuric acid-functionalized silica-coated magnetic nanoparticles (MgFe2O4@SiO2–SO3H) as a catalyst for the microwave-assisted synthesis of pharmacologically active benzoxazinone and benzthioxazinone via multicomponent reactions under solvent-free conditions. The synthesized catalyst was characterized by FT-IR, XRD, SEM, TEM, EDS, TGA and BET analyses which showed that it had spherical shape, uniform morphology and moderate surface area with superparamagnetic behavior. The catalyst was easily separated by an external magnet and the main accomplishment of the present work is that the catalyst recycled for five cycles while lacking any major loss of its activity. Graphical Abstract

Electronic supplementary material The online version of this article (doi:10.1007/s11164-017-3108z) contains supplementary material, which is available to authorized users. & Anand S. Aswar [email protected] 1

Department of Chemistry, Sant Gadge Baba Amravati University, Amravati 444602, India

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Keywords Sulfuric acid-functionalized MgFe2O4@SiO2
Benzoxazinones
Benzthioxazinones
Multi-component reaction
Microwave

Introduction Sulfuric acid is an important and proficient catalyst for the manufacture of industrial vital compounds. Over 1 billion gallons of sulfuric acid is used annually as a ‘‘homogeneous catalyst’’ which involves difficult procedures for the bifurcation of catalysts from homogenized reactions [1]. Acid catalysis is a very important part of catalysis working by the chemical industries. Magnetically separable catalysts have assisted in conjoining necessary norms for the planning of several contemporary catalytic methods [2–13]. The use of magnetic nanoparticles for catalysis not only copes with the above-mentioned difficulty but also satisfies the declaration of the additional ‘‘green chemistry’’ precept of ‘‘environmental remediation and development of alternative energy sources’’ [14–20]. The undoubted advantages of magnetically separable catalysts is their ability for convenient and rapid separation rather than an excessive reaction size by applying an outside magnetic field to the reaction mixture. Therefore, the utilization of magnetically separable catalysts is possibly a ‘‘double greener goal’’, with not only reduced time but also staving off difficulties such as a deficit of the catalyst, its leaching and the necessity for extra solvent in the formation of reaction side products [21]. Silica supplies stability to magnetic nanoparticles by maintaining a steady state among the repelling and attracting forces [22]. Hence, silica capping allows the generation of new catalysts which are steady under any critical condition such as thermal and chemical conditions. A surface of silica-capped nanospheres has been converted by suitable functionalizing material to produce more reactive sites for the catalyst, and hence a better performance in different chemical transformations [23]. Also, a surface-functionalized magnetic catalyst is an elegant way to bridge the gap between homogenous and heterogeneous catalysis. Recently, acid-functionalized magnetic particles have been introduced as solid acid catalysts and have revealed outstanding catalytic properties for various organic synthesis reactions [24–29]. Therefore, it was thought interesting to develop sulfuric acid-functionalized silica-coated magnetic nanoparticles as a solid acid catalyst towards the synthesis of important organic compounds. Multicomponent reactions (MCRs) are a significant class of organic reactions. In one-pot synthesis, three or more reactants react with each other to get the single desired compound and it is essential that all the atoms of the reactants interact with each other [30]. Multicomponent reactions are very resilient, atom-efficient and chemoselective developments of high exploratory power [31–35]. They can be applicable to the preparation of benzthioxazinones and benzoxazinone compounds. Compounds containing the benzthioxazinones and benzoxazinone moiety have earned significant attention due to their pharmacological activities such as antifungal [36], antihypertensive [37], anti-ulcer [38], anti-inflammatory [39] and

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MgFe2O4@SiO2–SO3H: an efficient, reusable catalyst for…

antipyretic [40], and also they can show calmodulin antagonism [41], inhibition of the transforming growth factor b (TGF-b) signaling pathway [42], DP receptor antagonism [43], platelet fibrinogen receptor antagonism [44] and integrin antagonism [45]. The synthesis of benzoxazinones and benzthioxazinones is carried out by multicomponent condensation of aldehydes, 2-naphthol and urea/thiourea, in the presence of various catalysts such as cellulose sulfuric acid [46], p-toluenesulfonic acid (PTSA) [47], iodine [48], phosphomolybdic acid [49], cyanuric chloride [50], thiamine hydrochloride [51], pyridinium-based ionic liquid [52], montmorillonite K10 [53], zinc triflate [54], zinc oxide [55], TMSCl/NaI [56], H14[NaP5W30O110]/ SiO2 [57], RuCl2(PPh3)3 [58], guanidine hydrochloride [59], and FeCl3–SiO2 nanoparticles [60]. However, some of these methods are not environmentally friendly, and have longer reaction times, lower yields, multistep reactions, toxic side products and difficult recovery and reusability of the catalysts. Therefore, developments of clean processes by utilizing eco-friendly and green catalysts which can be simply recyclable at the end of the reactions have been under permanent attention. The demand for environmentally benign procedures with heterogeneous catalysts promoted us to develop a safe alternative method for the preparation of benzoxazinones and benzthioxazinones. Considering the importance allied to this type of reaction, we have highlighted the synergistic effects of multicomponent reactions under solvent-free conditions by using magnetically separable catalysts for the synthesis of benzoxazinones and benzthioxazinones. For that, we choose to investigate a straightforward synthesis of benzoxazinones and benzthioxazinones through the one-pot multicomponent reaction of b-naphthol, urea or thiourea and aldehydes under solvent-free conditions by using using MgFe2O4@SiO2–SO3H as a non-toxic magnetically recoverable catalyst (Scheme 1). According to our best knowledge, there has been no previous report on the synthesis of benzoxazinones and benzthioxazinones by using a magnetically recoverable acid catalyst.

Experimental General FT-IR analysis was recorded on a Shimadzu FTIR-8400 spectrometer equipped with a KBr beam splitter. Thermogravimetric analysis (TGA) was carried using a STA 6000 device manufactured by Perkin Elmer. X-ray diffraction (XRD) was performed using a Rigaku miniflex II X-ray diffractometer. 1H-NMR and 13CNMR analysis was carried on a Bruker spectrometer. Scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX) for surface topology were performed by a JEOL Model JSM-6390LV and ZEISS ultra. The size and morphology of the particles were studied by a Philips transmission electron microscopy (TEM; CM200). Room-temperature magnetic hysteresis measurements were carried out by a conventional induction technique at 50 Hz used to obtain the coercivity (Hc). N2-sorption was carried out in a Quantachrome Novawin at 273 K.

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Scheme 1 MNPs-MgFe2O4@SiO2–SO3H as a catalyst for the synthesis of benzoxazinones and benzthioxazinones

The microwave-assisted reactions were carried out in a Ragatech microwave oven (2450 MHz) with a maximum power output of 700 W. This system is equipped with a power and temperature feedback control switch and measures the temperature via a highly sensitive IR sensor. Preparation of sulfuric group-functionalized nanoparticles immobilized on silica-capped magnetic particles Preparation of the magnetic MgFe2O4 nanoparticles MgFe2O4 was prepared using a simple chemical co-precipitation method [61]. A stoichiometric amount of 0.0078 mol of ferric nitrate [Fe(NO3)3
9H2O] and 0.0039 mol of magnesium nitrate [Mg(NO3)2
6H2O] were dissolved separately in 20 mL deionized water and mixed, and the resulting mixture was stirred for 1 h. To this mixture, 0.1 M sodium hydroxide solution was added dropwise until its pH was adjusted to 9–10. After adjusting the pH, a black precipitate was separated out, which was filtered and washed with deionized water until the pH of filtrate had become neutral. Finally, the precipitate was dried at 120 C in an oven and subsequently calcined at 500 C for 5 h, after which a brown-colored solid, MgFe2O4 (2.3 g), was formed. The synthesized nano-magnesium ferrite was characterized by XRD and FT-IR. Preparation of silica coated magnetic nanoparticles(MgFe2O4@SiO2) The MgFe2O4@SiO2 core–shell particles were prepared according to the Sto¨ber method [62]. The synthesized MgFe2O4 (1 g) was added in deionized water (150 mL) and ethanol (400 mL). The suspension was homogeneously dispersed by probe ultrasonication for 45 min. The NH3 solution (20 mL) was added and further stirred for 15 min, followed by the slow addition of 0.5 mL of tetraethyl orthosilicate (TEOS) under an inert atmosphere. After the addition was completed, the mixture was vigorously stirred at 40 C for 14 h, when a silica coating formed on the surface of the MgFe2O4 magnetic nanoparticles. The core–shell MgFe2O4@SiO2 magnetic nanoparticles were isolated from the liquid by applying an external magnetic field, then further washed with water followed by ethanol. The synthesized material was dried at 70 C for 10 h.

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General procedure for the synthesis of sulfuric group-immobilized silicacapped magnesium ferrite particles (MgFe2O4@SiO2–SO3H) A 500-mL suction flask was equipped with a pressure-equalizing dropping funnel. The gas outlet was connected to a vacuum system through an adsorbing solution in an alkali trap. MgFe2O4@SiO2 (1.5 g) was dispersed in chloroform (50 mL). Chlorosulfonic acid (0.5 mL) was added through the dropping funnel at 10–15 C. After completion of the addition, the mixture was stirred for 90 min during which the residual HCl gas was eliminated by suction. MgFe2O4@SiO2–SO3H was separated from the mixture by an external magnet, washed with chloroform and dried under vacuum at 60 C (Scheme 2). General synthesis for the preparation of benzoxazinones and benzthioxazinones derivatives using MgFe2O4@SiO2–SO3H A mixture of b-naphthol (1.0 mol), aldehyde (1.0 mol), urea/thiourea (1.2 mol) and MgFe2O4@SiO2–SO3H (25 wt% with respect to limiting the reactant b-naphthol) was taken in 50 mL RBF and the reaction mixture was exposed in the microwave oven (120 C at 350 W) for the required time. The progress of the reaction was checked by thin layer chromatography. After completion of the reaction, the reaction mixture was diluted with ethanol (20 mL) and the catalyst was separated by an external magnet. The decanted solution was concentrated under reduced pressure to acquire a solid product which was washed by water and recrystallized by ethanol. The recovered catalyst was washed by ethanol followed by chloroform, dried at 60 C and reused for the next cycle. All purified products were characterized by IR, 1 H-NMR, and 13C-NMR and compared with literature data. Spectral data for the synthesized compounds 1-phenyl-1H-naphtho[1,2-e] [1, 3] oxazin-3(2H)-one (4a) IR (cm-1): 3324, 1692, 1436, 1436, 1221, 831, 743;1H-NMR (400 MHz, DMSO-d6): d 6.07 (d, 1H), 7.26 (m, 6H), 7.40 (m, 2H), 7.55 (m, 1H), 7.82 (m, 2H), 9.54 (s, 1H); 13C NMR (100 MHz, DMSO-d6): d 53.2, 113.2, 119.9, 124.5, 124.9, 126.0, 126.9, 127.8, 128.1, 128.8, 131.2, 132.3, 141.1, 147.9, 158.0; MS: m/z 275(M).

Scheme 2 Synthetic scheme of sulfuric group functionalized MgFe2O4@SiO2 nanoparticles

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1-(4-methoxyphenyl)-1H-naphtho[1,2-e] [1, 3] oxazin-3(2H)-one (4e) IR (cm-1): 3147, 2953, 1732, 1511, 1385, 1025, 836, 756; 1H-NMR (400 MHz, DMSO-d6): d 3.70 (s,3H), 6.06 (d, 1H), 6.81 (m, 2H), 7.19 (m, 2H), 7.23 (d, 1H), 7.37 (m, 2H), 7.68 (d, 1H), 7.85 (m, 2H), 8.70 (s, 1H); 13C NMR (100 MHz, DMSO-d6): d 53.5, 54.5, 78.5, 78.9, 113.8, 113.9, 116.6, 122.7, 124.6, 126.99, 128.0, 128.3, 128.9, 129.8, 130.3, 134.7, 147.3, 149.3, 158.8; MS: m/z 305 (M). 1-(4-ethoxyphenyl)-1H-naphtho[1,2-e] [1, 3] oxazin-3(2H)-one (4f) 1H-NMR (400 MHz, DMSO-d6): d 1.32 (t, 3H), 3.90 (q, 2H), 6.01 (d, 1H), 6.64 (s, 1H), 6.78 (d, 2H), 7.14 (t, 2H), 7.25 (s, 1H), 7.32 (d, 2H), 7.37-7.44 (m, 1H), 7.81–7.85 (m, 2H); 13C NMR (100 MHz, DMSO-d6): d 14.7, 55.3, 63.4, 76.7, 77.3, 112.9, 115.1, 117.0, 122.9, 125.1, 127.3, 128.2, 128.7, 129.3, 130.3, 131.0, 133.8, 147.5, 150.6, 159.0; MS: m/z 319 (M). 1-(3-ethoxy-4-hydroxyphenyl)-1H-naphtho[1,2-e] [1, 3] oxazin-3(2H)-one (4i) IR (cm-1): 3241, 3137, 2986, 1713,1516, 1279,1040, 813,749; 1H-NMR (400 MHz, DMSO-d6): d 1.31 (t, 3H), 3.95 (q, 2H), 5.99 (d, 1H), 6.56 (m, 1H), 6.68 (d, 1H), 6.91 (s, 1H), 7.27 (d, 1H), 7.38 (m, 2H), 7.7 (d, 1H), 7.85 (m, 2H), 8.63 (s, 1H), 8.77 (s, 1H); 13C NMR (100 MHz, DMSO-d6): d 14.5, 53.9, 63.9, 112.5, 113.8, 115.4, 116.5, 119.2, 122.8, 124.6, 126.9, 128.3, 129.0, 129.7, 130.2, 133.5, 146.5, 146.6, 147.3, 149.4; MS: m/z 336 (M ? 1). 1-(3,4-dimethoxyphenyl)-1H-naphtho[1,2-e] [1, 3] oxazine-3(2H)-thione (4o) IR (cm-1): 3149, 1736, 1519, 1223, 1138, 1025, 880, 743; 1H-NMR (400 MHz, DMSO-d6): d 3.74 (s, 3H), 3.77 (s, 3H), 6.04 (d, 1H), 6.65 (m, 1H), 6.76 (d, 1H), 6.98 (d, 1H), 7.28 (d, 1H), 7.39 (m, 2H), 7.68 (d, 1H), 7.86 (m, 2H), 8.66 (s, 1H); 13 C NMR (100 MHz, DMSO-d6): d 50.0, 55.9, 56.0, 111.6, 112.4, 114.5, 117.3, 119.3, 123.7, 125.5, 127.7, 129.0, 129.5, 130.6, 130.8, 135.7, 147.9, 148.9, 149.3, 149.8; MS: m/z 351(M).

Results and discussion Characterization of MgFe2O4@SiO2–SO3H catalyst A modified surface of MgFe2O4 particles was prepared by the capping of silica onto a magnesium ferrite surface followed by the hydrolysis of TEOS using ammonia to provide reaction sites for functionalization and thermal stability. The MgFe2O4@SiO2 served as the support for the functionalization of the sulfuric group by adding chlorosulfonic acid in chloroform. A number of acid sites of MgFe2O4@SiO2– SO3H were found by simple acid–base titration and discovered to be 0.28 mmol/g of the catalyst. The FT-IR spectrum (Fig. 1) shows a Fe–O band at 578 cm-1 and –OH vibration at 1616 cm-1 for both MgFe2O4 and MgFe2O4@SiO2. The broad and strong peak at 3400 cm-1 can be ascribed to Si–OH. The peaks at 476, 794, 977, 1093 cm-1 are ascribable to symmetrical and unsymmetrical vibrations of the terminal Si–O–Si group. The broad peak at about 3400 cm-1 indicates some free Si–OH groups. In

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MgFe2O4@SiO2–SO3H: an efficient, reusable catalyst for…

Fig. 1 Comparative FT-IR spectra for a MgFe2O4; b MgFe2O4@SiO2; c MgFe2O4@SiO2–SO3H

the case of MgFe2O4@SiO2–SO3H, sulfonic acid bands can be observed at 1288, 1170–1074 and 610 cm-1 which are attributed to the O=S=O and S–O stretching vibration of the sulfonic groups (–SO3H), respectively. The increase in the intensities of the band at 3000–3400 cm-1 indicates that there are more –OH under the magnetic nanoparticle surface after sulfonation. All observations confirm that the sulfonic groups have functionalized the surface of the MgFe2O4@SiO2 core– shell magnetic nanoparticles. The crystallographic structure and composition of MgFe2O4, MgFe2O4@SiO2 and MgFe2O4@SiO2–SO3H were characterized by XRD. Figure 2 shows that the

Fig. 2 Powder X-ray diffraction of a MgFe2O4; b MgFe2O4@SiO2; c MgFe2O4@SiO2–SO3H

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position and relative intensity of all the characteristic peaks of MgFe2O4 at 2h = 18.3, 30.1, 35.5, 37.1, 43.2, 53.5, 56.5, 62.6 and 75.3 could be indexed to the spinel structure of the MgFe2O4 powder diffraction data (JCPD card no:- 36-0398). The same peaks were observed in both MgFe2O4 and MgFe2O4@SiO2 nanoparticle XRD patterns, suggesting that the crystalline spinel MgFe2O4 has been encapsulated by the thinner amorphous silica layer without affecting the original crystallinity of the MgFe2O4 structure. The peak at 2h = 21.9 was consistent with an amorphous silica phase in the shell of the MgFe2O4@SiO2 magnetic nanoparticles. The characteristic peaks of MgFe2O4 can be detected in the XRD pattern of MgFe2O4@SiO2–SO3H, but with a slight decrease in intensity. The thermogram of MgFe2O4@SiO2–SO3H shows several regions corresponding to different mass losses (Fig. 3). The first region stage, below 140 C, displayed a mass loss due to adsorbed solvent or trapped water from the catalyst. The weight loss observed in the range 220–350 C corresponds to the to loss of the –SO3H group. The further mass losses at higher temperature resulted from the decomposition of the silica shell [29]. Thus, the catalyst was stable up to 200 C, confirming that it could be safely used in organic reactions at a temperature around 180 C. The components of the MgFe2O4@SiO2 and MgFe2O4@SiO2–SO3H magnetic nanoparticles were analyzed by energy dispersive spectrometer (EDS) (Fig. 4a, b). The presence of Si, Mg, Fe and O signals indicate that the magnesium ferrite particles were loaded into the silica, while the higher intensity of the Si peak compared with the Fe and Mg peaks indicates that MgFe2O4 nanoparticles were trapped by SiO2 (Fig. 4a). The characteristic peak of S and the intensity of the O peak indicates that the MgFe2O4@SiO2nanoparticles have been coated by the – SO3H group (Fig. 4b). The morphology and particle size distribution of the MgFe2O4@SiO2 and MgFe2O4@SiO2–SO3H nanostructures were performed by SEM. The SEM images shown in Fig. 5a demonstrate that most of the MgFe2O4@SiO2 nanoparticles are spherical with particle sizes in the range of 75–110 nm. Figure 5b shows tnat MgFe2O4@SiO2–SO3H nanoparticles are

Fig. 3 Combined TGA and DSC curves of MgFe2O4@SiO2–SO3H nanoparticles

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MgFe2O4@SiO2–SO3H: an efficient, reusable catalyst for…

Fig. 4 The energy dispersive X-ray spectroscopy spectra of a MgFe2O4@SiO2, and b MgFe2O4@SiO2– SO3H nanoparticles

spherical with nano-dimensions in the range of 120–150 nm, having a smoother surface. However, no significant change was found in the spherical shape. The morphology of the MgFe2O4@SiO2–SO3H nanoparticles was observed using transmission electron microscopy (TEM). Figure 6 displays a spherical morphology with dark MgFe2O4 nanoparticles core surrounded by lighter amorphous silica

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Fig. 5 Scanning electron microscopy images of a MgFe2O4@SiO2, and b MgFe2O4@SiO2–SO3H nanoparticles

Fig. 6 Transmission electron microscopy image of MgFe2O4@SiO2–SO3H nanoparticles

shells about 25–30 nm thick. The average diameter of the MgFe2O4@SiO2–SO3H nanoparticleswas determined from TEM images to be in the range of 80–150 nm. The nitrogen adsorption–desorption isotherms, specific surface area, pore volume, and pore radius were analyzed by the Brunauer–Emmett–Teller (BET)

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MgFe2O4@SiO2–SO3H: an efficient, reusable catalyst for…

and the Barrett–Joyner–Halenda (BJH) methods. The results show the N2 adsorption–desorption isotherm of the MgFe2O4@SiO2–SO3H core–shell with surface area 21.906 m2/g. pore volume 0.027 cc/g and pore radius 25.733 A. MgFe2O4@SiO2–SO3H nanoparticles exhibit a type II nitrogen adsorption–desorption isotherm curve (Fig. 7). The magnetic properties of the catalyst were characterized by a conventional technique at 50 Hz used to obtain coercivity (Hc). Figure 8 shows the typical room-temperature magnetization curves of MgFe2O4, MgFe2O4@SiO2 and MgFe2 O4@SiO2–SO3H nanoparticles. All materials show a typical superparamagnetic behavior. The coercivity (Hc) value of MgFe2O4@SiO2–SO3H was 23092.7 A/m, which is lower than that of the MgFe2O4 magnetic nanoparticles. This may be due to the formation of the silica shell around the MgFe2O4 core. In order to check the magnetic properties, the sulfonic acid-loaded magnetic particles were dispersed in ethyl acetate, resulting in a dark dispersion (Fig. 9a).

Fig. 7 N2 adsorption–desorption isotherms by the BET method (a) and pore size by the BJH method (b) of MgFe2O4@SiO2–SO3H nanoparticles

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Fig. 8 Room-temperature magnetization curves of a MgFe2O4; b MgFe2O4@SiO2; c MgFe2O4@SiO2– SO3H nanoparticles Fig. 9 Isolation of the dispersed MgFe2O4@SiO2– SO3H nanoparticles before (a) and after (b) application of an external magnet to the reaction mixture

Within 5–8 s in the presence of an external magnet –SO3H-functionalized MNPs were completely gathered at one side of the cuvette wall and the dispersion become clear and transparent (Fig. 9b). This suggests that the MgFe2O4@SiO2–SO3H magnetic nanoparticles have been successfully synthesized. Evaluation of the catalytic activity of MgFe2O4@SiO2–SO3H for one-pot synthesis of benzoxazinones and benzthioxazinones First, the reaction between b-naphthol (1 mol), benzaldehyde (1 mol) and urea (1.2 mol) was used as a model reaction under solvent-free conditions in 100 C irradiation in the absence and presence of MgFe2O4@SiO2 and MgFe2O4@SiO2 sulfuric acid and the results are reported in Table 1. It was found that, in the absence of catalyst, only a trace of the desired product was observed on the TLC plate even after microwave irradiation for 30 min with recovery of the starting material (Table 1, entry 1). In the presence of MgFe2O4@SiO2 (5 wt%), the reaction is slightly enhanced with a 13% yield within 30 min (Table 1, entry 2), whereas, in the

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MgFe2O4@SiO2–SO3H: an efficient, reusable catalyst for… Table 1 Optimization of the amount of catalyst and the reaction temperature for the synthesis of benzoxazinones Entry

Catalyst (Wt%)

Temperature (C)/condition

Time (min)

Yielda(%)

1

–

100 C/solvent free

30

Trace

2

MgFe2O4@SiO2 (5)

100 C/solvent free

30

13

3

MgFe2O4@SiO2–SO3H (5)

100 C/solvent free

15

32

4

MgFe2O4@SiO2–SO3H (10)

100 C/solvent free

10

58

5

MgFe2O4@SiO2–SO3H (15)

100 C/solvent free

10

69

6

MgFe2O4@SiO2–SO3H (20)

100 C/solvent free

10

76

7

MgFe2O4@SiO2–SO3H (25)

100 C/solvent free

5

84

8

MgFe2O4@SiO2–SO3H (25)

110 C/solvent free

5

91

9

MgFe2O4@SiO2–SO3H (25)

120 C/solvent free

3

97

10

MgFe2O4@SiO2–SO3H (25)

140 C/solvent free

3

97

Reaction conditions: b-naphthol: benzaldehyde: urea (1:1:1.2) in Microwave and MgFe2O4@SiO2 and MgFe2O4@SiO2–SO3H as catalyst a

Isolated yield

Table 2 Evaluation of the results of MgFe2O4@SiO2–SO3H with literature-reported catalysts Entry

Catalyst

Reaction condition

Product

Time (min)

Yielda (%)

References

1

Cellulose sulfuric acid

80 C/water ? SDS

4a

180

92

[46]

2

p-toluenesulfonic acid (PTSA)

160 C/solvent free

4a

120

58

[47]

3

Iodine

150 C/solvent free

4a

5

80

[48]

4

Phosphomolybdic acid

100 C/DMF

4a

180

87

[49]

5

Cyanuric chloride

150 C/solvent free

4a

12

87

[50]

6

Thiamine hydrochloride

150 C/solvent free

4a

30

92

[51]

7

Pyridinium-based ionic liquid

150 C/solvent free

4a

60

85

[52] [53]

8

Montmorillonite K10

160 C/solvent free

4a

30

89

9

Zinc triflate

Reflux/CH3CN

4a

300

82

[54]

10

Zinc oxide

150 C/solvent free

4b

90

85

[55]

11

TMSCl/NaI

140 C/DMF ? CH3CN

4a

120

78

[56]

12

H14[NaP5W30O110]/ SiO2

Reflux/EtOH

4a

60

91

[57]

13

RuCl2(PPh3)3

Reflux/Toluene

4a

900

93

[58]

14

Guanidine hydrochloride

140 C/solvent free

4a

70

88

[59]

15

FeCl3–SiO2 NPs

150 C/solvent free

4a

10

85

[60]

16

MgFe2O4@SiO2–SO3H

Microwave(120 C)/solvent free

4a

3

97

This work

a

Isolated yield

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presence of MgFe2O4@SiO2–SO3H (5 wt%) under the same experimental conditions, the yield increased to 32% (Table 1, entry 3). It was found that, with further increasing the amount of catalyst (25 wt%) of MgFe2O4@SiO2–SO3H, the yield significantly increased to 84% within 5 min (Table 1, entry 7). The rate of reaction was also increased with increasing temperature in the presence of MgFe2O4@SiO2–SO3H (25 wt%) at 120 C, and the reaction was complete within 3 min with 97% yield (Table 1, entry 9). Furthermore, no increase in yield was observed when the mixture was heated up to 140 C for 3 min in the microwave oven (Table 1, entry 10). With microwave irradiation below 350 W, the reaction proceeded slowly giving a relatively low yield and no improvement was observed above 350 W. All further studies were carried out under solvent-free conditions with 25 wt% catalyst at 120 C with microwave irradiation (350 W). The efficiency of the synthesized catalyst MgFe2O4@SiO2–SO3H along with various reported catalysts for the synthesis of benzoxazinone was estimated and Table 3 Synthesis of benzoxazinones and benzthioxazinones in the presence of MNPs MgFe2O4@SiO2– SO3H without solvent Entry

Producta

Substrate/product

Time (min)

R

X

Yield (%)

m.p. (C) Found

Reported (references)

1

4a

C6H5

O

3

97

221–223

219–222 [46]

2

4b

4-CH3–C6H4

O

5

91

164–166

163–165 [60]

3

4c

3-Cl–C6H4

O

8

85

193–195

192–195 [53]

4

4d

4-Cl–C6H4

O

8

87

207–209

209–212 [46]

5

4e

4-OCH3–C6H4

O

5

93

185–187

185–187 [46]

6

4f

4-OC2H5–C6H4

O

4

90

215–218

216–218 [55]

7

4g

3,4-OCH3–C6H3

O

6

94

200–203

199–200 [48]

8

4h

3,4,5-OCH3–C6H3

O

6

91

225–227

225–226 [59]

9

4i

3-OC2H5,4–OH– C6H3

O

9

89

258–260

–

10

4j

4-(CH3)2CH–C6H4

O

5

88

170–172

171–172 [56]

11

4k

2-Furyl

O

7

87

220–222

221-223 [63]

12

4l

2-Thiophene

O

8

89

208–209

209–211 [60]

13

4m

3-Indole

O

8

81

191–193

192–193 [63]

14

4n

C6H5

S

5

92

207–209

208–210 [58]

15

4o

3,4-OCH3–C6H3

S

7

88

208–210

–

16

4p

4-OCH3–C6H4

S

7

90

191–193

190–192 [58]

17

4q

4-NO2–C6H4

O

8

82

172–174

170–174 [58]

18

4r

4-CN–C6H4

O

12

62

212–214

–

19

4s

4-NH2–C6H4

O

30

No reaction

–

–

Reaction conditions: aldehyde (1 mol); b-naphthol (1 mol); urea or thiourea (1.2 mol); MgFe2O4@SiO2– SO3H (25 Wt%) a

All products are known and were identified by their melting point, IR and 1H and according to the literature

123

13

C NMR spectra

MgFe2O4@SiO2–SO3H: an efficient, reusable catalyst for…

comparative data are represented in Table 2. Compared with cellulose sulfuric acid, p-toluenesulfonic acid(PTSA), iodine, phosphomolybdic acid, cyanuric chloride, thiamine hydrochloride, pyridinium-based ionic liquid, montmorillonite K10, zinc triflate, zinc oxide, TMSCl/NaI, H14[NaP5W30O110]/SiO2, RuCl2(PPh3)3, guanidine hydrochloride, FeCl3–SiO2 NPs, our catalyst, MgFe2O4@SiO2–SO3H, was found to

Scheme 3 Proposed mechanism for the synthesis of benzoxazinone using MgFe2O4@SiO2–SO3H

Fig. 10 Recyclability of MNPs MgFe2O4@SiO2–SO3H

123

N. G. Salunkhe et al. Table 4 Reusability of catalyst Entry

Number of cycles run

Catalyst recoverya (%)

Yieldb (%)

1

Fresh

–

97

2

First cycle

98

95

3

Second cycle

96

92

4

Third cycle

94

90

5

Forth cycle

92

88

6

Fifth cycle

90

83

a

Catalyst recovered by magnetic separation

b

Isolated yield

be better. Also, it was observed that, in addition to having the general advantages attributed to the inherent magnetic property of nanocatalysts, MgFe2O4@SiO2– SO3H is a more efficient catalyst for this three-component reaction. After successful optimization of the reaction conditions, the series of benzoxazinone and benzthioxazinones derivatives (4a–4p) were synthesized (Table 3). Under these reaction conditions, the reaction took place between b-naphthol, urea and various aldehydes such as aromatic and some heterocyclic aldehydes in the presence of MgFe2O4@SiO2–SO3H as the catalyst for the synthesis of benzoxazinone derivatives. Thiourea has also been used to synthesize the corresponding benzthioxazinones in high yields (products 4n–4p; Table 3). It is well known that the synthesis of benzoxazinones in aromatic aldehydes works very well. On the basis of all our experimental findings along with literature reports [46], we have hypothesized the mechanism of our reaction in the presence of MgFe2O4@SiO2–SO3H as shown in Scheme 3. The reaction is believed to proceed through the formation of an N-acylimine intermediate formed in situ by the reaction of an aldehyde with urea or thiourea. The subsequent activation of naphthol by the conjugate base of functionalized sulfuric acid thus favors nucleophilic attack to provide the intermediate (5). The cyclization afforded due to ammonia elimination leads to the formation of the final compound (4). The possibility of the magnetic recycling of the catalyst was examined. For this reason, the reaction of benzaldehyde, urea and b-naphthol in the presence of MgFe2O4@SiO2–SO3H under solvent-free conditions was studied. Upon completion of the reaction, the solid product was dissolved in chloroform?methanol (50%) and the catalyst was isolated and recovered from a reaction mixture by applying an external magnetic field to the vessel. The recovered catalyst was dried and could be reused five times. After the fifth cycle, the catalyst was examined by EDAX, SEM, XRD and FT-IR and was closely similar to the freshly prepared catalyst, showing no significant loss of activity, which clearly demonstrates the stability of the catalyst for these conditions in the reaction. The relationship between the number of cycles of reaction and the catalytic activity in terms of the yield of products is presented in Fig. 10 and Table 4.

123

MgFe2O4@SiO2–SO3H: an efficient, reusable catalyst for…

Conclusions In the present work, we have designed, prepared and characterized sulfonic acidfunctionalized silica-coated magnetic nanoparticles with acid functionality. The activity of the catalyst was tested by an efficient one-pot method for the synthesis of benzoxazine and benzthioxazine derivatives under microwave irradiation. This method is significant for its mild reaction conditions, high activity, easy work-up, high yield and reusability of a catalyst, thus making the present protocol a green alternative. Acknowledgement We acknowledged SGB Amravati University for partial support of this work.

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