Role of (3-aminopropyl)tri alkoxysilanes in grafting of chlorosulphonic acid immobilized magnetic nanoparticles and their application as heterogeneous catalysts for the green synthesis of α-aminonitriles

Abstract

The surface modification of SiO₂ coated Fe₃O₄ nanoparticles by grafting silane coupling agents is a highly significant approach to enhancing the interface interaction between the inorganic magnetic core and the organic functionality to be anchored. However, the effect of the grafted molecular structure of silane, utilizing different silane alkoxy groups, has been seldom investigated on the organic–inorganic hybrid magnetic heterogeneous catalysts. The silanization was carried out on the surface of SiO₂ coated Fe₃O₄ nanoparticles using two different trialkoxy silanes, i.e., 3-aminopropyltriethoxysilane (APTES) and 3-aminopropyltrimethoxysilane (APMES). The effect of the alkoxy silane structure anchored to the surface of silica functionalized Fe₃O₄ nanoparticles on their surface area and pore volume has been analyzed by BET analyzer. The silane functionalization using APTES leads to higher surface area and pore volume than APTMS. Further, chlorosulphonic acid was immobilized on both the APTES and APTMS functionalized SiO₂@Fe₃O₄ nanoparticles named (AE and AM), and their role as magnetically separable heterogeneous catalyst for the one pot synthesis of α-aminonitriles using water as solvent was successfully carried out. The chlorofunctionalized nanoparticles, which utilized APTES as the silanizing agent, showed better catalytic activity than APTMS in terms of reaction time and yield. The presented protocol is a water based synthesis involving room temperature conditions, along with reusability and recyclability of the catalyst. The characterization of the synthesized catalysts was carried out by using different techniques, such as FT-IR, XRD, TEM, BET, XPS, VSM and TGA-DTA.

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1. Introduction

Magnetic nanoparticles have received much attention in academics, research and industrial fields, due to their properties, which include high surface area, high degree of chemical stability, high catalytic activity, non-swelling in organic solvents,^1–3 easy recovery using an external magnet, reusability,⁴ regular shape, uniform size, low cost of production, non toxicity and biocompatibility.⁵ They are also used in other fields, such as sealing, oscillation damping, information storage, electronic devices, etc.⁵ Under biomedical applications they are used as biosensors,⁶ contrast agents in magnetic resonance imaging (MRI),⁷ localizers in therapeutic hyperthermia,⁸ drug delivery systems, etc.⁹ A number of methods of preparation of magnetic nanoparticles have been reported, including co-precipitation, solvothermal/hydrothermal synthesis, micro emulsion and thermal decomposition. Due to its simplicity and the formation of monodisperse nanoparticles, the co-precipitation method is declared one of the best methods of synthesis.^10a

Under the catalytic system, a large number of nanocatalysts are used as heterogeneous catalysts in many organic synthetic transformation reactions. These have many advantages over the conventional catalyst, like their nanoscale size, possessing a high surface area of active components, which in turn increases the contact between the reactants and the catalyst, which results in a high reaction rate, and their high surface to volume ratio increases the catalytic activity and accessibility of the catalyst. However, the nanoscale size of heterogeneous catalysts makes their recovery from the reaction mixture difficult; sometimes it is more difficult to isolate or recycle the catalyst by conventional filtration and centrifugation methods. This limitation obstructs the economic and sustainable growth of these kinds of the protocols. Therefore, there is a demand for some alternatives to overcome these problems and reuse or recycle the catalysts in a highly efficient manner. Magnetic nanoparticles are the suitable alternatives and are extensively explored. Their paramagnetic nature allows for their easy separation from the reaction mixture by using an external magnet, which in turn increases the recyclability and reusability. The remarkable catalyst recovery, their non solubility in the reaction medium, green synthesis protocol, minimum waste production and minimum work up procedure are the important characteristics that make them more sustainable.^10b–e

Further application of the magnetic nanoparticles as support has been reported. Super-paramagnetic iron oxide nanoparticles have been studied as magnetic support for the nano catalytic system. Silica coating on magnetic nanoparticles enhances their dispersibility in liquid medium by stopping the dipole attractions between the nanoparticles, improves chemical stability, enhances their performance, protects from leaching in an acidic environment, provides high hydrothermal stability, protects from the further oxidation of bare magnetic nanoparticles, enhances the textural properties of magnetic nanoparticles and protects against toxicity. The presence of a large number of silanol groups on their surface facilitates easy further functionalization with different organic moieties, provides high surface area, which enhances the catalytic loading and activity, and also prevents the unwanted interaction between the core, magnetic nanoparticles and the additional agents linked to the silica surface. All these properties render the inert coating of silica as a solid support for many organic and inorganic moieties.^10f–i (3-Aminopropyl)-trialkoxysilanes are widely used as silanizing agents for the surface modification of the nanoparticles, as their terminal amine groups can facilitate the further functionalization of the nanoparticles.

The reaction between aldehyde, amine and cyanide sources gives rise to the formation of α-aminonitrile,¹¹ and is known as the Strecker reaction;^12,13 its reliability, versatility of the resulting products and easy availability of the starting materials are some reasons behind its use in the formation of the large scale production of amino acids, herbicides and chelating agents.¹⁴ Further, α-aminonitriles are the important intermediates for the formation of amino acids,¹⁵ and important synthons of organic chemistry¹¹ such as nitrogen containing heterocycles¹⁶ like imidazole and thiadiazole, other biologically important molecules like saframycin A, having anti tumor activity, or phthalascidi,¹⁷ manzacidin A, prasugrel, clopidogrel and short acting opioid analgesics.¹⁸ They are also considered as valuable intermediates in the formation of different diamines, amides and pharmaceuticals.¹¹ α-Amino acids have immense biological and economic importance due to their widespread applications in the fields of chemistry and biology. They are also useful as the chiral building blocks in the pharmaceutical industry¹⁹ and as key precursors for the preparation of proteins.¹¹ Recently, by using this strategy, the synthesis of the hepatitis C virus NS3 serine protease inhibitors,²⁰ (±)-phthalascidin 622 ²¹ and novel boron containing retinoids²² have been reported.

In general, the Strecker reaction involves the nucleophilic addition of the cyanide source to the imine.¹² The common cyanide sources that have been used so far are HCN,¹¹ KCN,^23a (EtO)₂POCN,^23b Et₂AlCN,^23c BuSnCN.^23d All these mentioned cyanide sources are hazardous, toxic in nature, require harsh reaction conditions, and special handling and care is required during their use.²⁴ It has been reported that TMSCN, due to its properties like easy handling, effective and safe use, is one of the most promising cyanide sources.²⁵ So far, many Lewis acids or Bronsted acids like Fe(Cp)₂PF₆,^26a InCl₃,^26b RhI₃,^26c Cu(OTf)₂,¹⁴ I₂,^26d BiCl₃,^26e La(NO₃)₃·6H₂O, GdCl₃·6H₂O,^26f NiCl₂,^26g Ga(OTf)₃,^26h GaCl₃,²⁶ⁱ CeCl₃,^26j Pr(OTf)₃,^26k RuCl₃,^26l FeCl₃ ^26m catalysts, etc. have been reported, which homogeneously catalyze this reaction, but leave behind the limitations of costly, toxic catalytic systems, longer reaction times and difficult separation work up.²⁶ⁿ Therefore, there arises the use of heterogeneous catalysts that overcome these problems. Further, supported heterogeneous catalysts have advantages such as simple separation method, minimization of waste, simple recovery, recyclability, reusability and lower contamination of the products.^27–29 Chlorosulphonic acid has been used in a wide variety of applications like alkylation, cyclization, polymerization, halogenations and rearrangement reactions;^29b however, its direct use in water is not feasible, as it causes the formation of HCl and H₂SO₄. Therefore, this encouraged us to immobilize it on another support, which promotes its use in various promising applications of sulphamic acid as an acidic catalyst in reactions like acetalization, esterification, acetylation of phenols and alcohols, nitrile formation as a chemoselective catalyst in the transesterification of β-ketoesters, Beckmann rearrangement, etc.^29c

In the present study, SiO₂@Fe₃O₄ nanoparticles have been synthesized by Stober's method. Further silanization on the surface of hydroxylated SiO₂@Fe₃O₄ nanoparticles was carried out, utilizing two different trialkoxy silanes, i.e., 3-aminopropyltriethoxysilane (APTES) and 3-aminopropyltrimethoxysilane (APTMS). The APTES functionalized SiO₂@Fe₃O₄ nanoparticles have obtained higher surface area and pore volume (6.3891 m² g⁻¹ and 6.03 × 10⁻³ cm³ g⁻¹) than APTMS (1.3220 m² g⁻¹ and 1.4 × 10⁻³ cm³ g⁻¹). Further, the chlorosulphonic acid was immobilized on both NH₂-Pr-SiO₂@Fe₃O₄ nanoparticles generated by two different alkoxy silanes, i.e., APTES and APTMS obtained as Fe₃O₄@SiO₂-Pr-NH-SO₃H (AE) and Fe₃O₄@SiO₂-Pr-NH-SO₃H (AM) nanoparticles. Further, they have been used in the synthesis of α-aminonitriles via coupling of three components, aldehyde, aniline and trimethylsilylcyanide, by using water (as a green solvent) as the reaction medium. Comparison of reaction time and yield of α-aminonitriles has also been carried out. Best results were obtained with AE, which encouraged us to focus on AE nanoparticles. Therefore, in continuation of our previous work on catalysis,^10h a simple and environmentally friendly work up procedure for the formation of product in high yields has been developed. All these steps make this protocol eco friendly, cost effective, simple and efficient in nature.

2. Results and discussion

2.1 Characterization of the as prepared catalyst

FT-IR. In Fig. 1 the peaks of Fe₃O₄ nanoparticles were found to be below 700 cm⁻¹, the peak at 600 cm⁻¹ was attributed to the Fe–O vibrations; O–H deformed vibrations were found near 1646 cm⁻¹ and O–H stretching vibrations at 3132 cm⁻¹. The FT-IR spectra of Fe₃O₄@SiO₂, consist of the peaks at 447 cm⁻¹, 954 cm⁻¹ and a sharp peak at 1061 cm⁻¹, which are due to the asymmetric stretching vibration, symmetric vibration and bending vibration of the all Si–O–Si groups. Similarly the O–H stretching vibration near 3100–3400 cm⁻¹ and the O–H deformed vibration near 1630–1646 cm⁻¹ were present in both cases.³⁰ The decrease in the intensity of the peak at around 600 cm⁻¹ confirmed the coating of silica on the Fe₃O₄ nanoparticles. In the FT-IR spectra of Fe₃O₄@SiO₂-Pr-NH₂ (APTES) and Fe₃O₄@SiO₂-Pr-NH₂ (APTMS), the introduction of APTES and APTMS on Fe₃O₄@SiO₂ was confirmed by the Si–O peak at 1117 cm⁻¹ and 1124 cm⁻¹ and the broad N–H peak at around 3245 cm⁻¹ and 3358 cm⁻¹, respectively. Further, the presence of the propyl group was confirmed by the C–H peaks at 2955 cm⁻¹, 2822 cm⁻¹ and 2904 cm⁻¹, 2890 cm⁻¹ for APTES and APTMS, respectively.³⁵ In the FT-IR spectra of AE, the introduction of the sulphonyl moieties was confirmed by the bands at 1125 cm⁻¹ and 1250 cm⁻¹ and for AM, at 1147 cm⁻¹ and 1280 cm⁻¹.

Fig. 1 Comparative study of Fourier transform infrared (FT-IR) spectra of Fe₃O₄ nanoparticles (MNPs), Fe₃O₄@SiO₂, Fe₃O₄@SiO₂-Pr-NH₂ (APTMS), Fe₃O₄@SiO₂-Pr-NH₂ (APTES), AM and AE.

Surface area and pore volume analysis. BET methodology was used extensively for the estimation of surface area and pore volume of the materials, as both are the important parameters used for the determination of catalytic activity of the catalyst, as shown in Fig. 2.

Fig. 2 (a) Surface area plot of AM, (b) pore volume plot of AM, (c) surface area plot of AE, (d) pore volume plot of AE.

Here, we studied the effect of surface area due to the grafting of APTES and APTMS on Fe₃O₄@SiO₂ nanoparticles, followed by chlorosulphonic acid immobilization. The surface area was calculated by using the BET equation.^31a In the absence of shells, the surface area of bare Fe₃O₄ magnetic nanoparticles was found to be 118.07 m² g⁻¹; the high surface area was attributed to the highly porous structure.^31b However, when the immobilization of different moieties was carried out, the surface area decreased significantly, i.e., 7.2464 m² g⁻¹, 8.5458 m² g⁻¹, 13.43 m² g⁻¹, 1.3220 m² g⁻¹, 6.3891 m² g⁻¹ for Fe₃O₄@SiO₂, Fe₃O₄@SiO₂-Pr-NH₂ (APTMS), Fe₃O₄@SiO₂-Pr-NH₂ (APTES), AM and AE, respectively. This further confirmed the formation of shells on the surface of the bare Fe₃O₄ nanoparticles. The total pore volume was reduced from 0.30473 cm³ g⁻¹ for Fe₃O₄ magnetic nanoparticles to 0.0118 cm³ g⁻¹, 0.0299 cm³ g⁻¹, 0.0433 cm³ g⁻¹, 0.0014 cm³ g⁻¹, 0.0060 cm³ g⁻¹ for Fe₃O₄@SiO₂, Fe₃O₄@SiO₂-Pr-NH₂ (APTMS), Fe₃O₄@SiO-Pr-NH₂ (APTES), AM and AE, respectively, as shown in Table 1. This observation further shows that these shells are of a relatively more dense structure and possess few pores, which aid in protecting the Fe₃O₄ magnetic nanoparticles.^31c The surface area and pore volume of AE were obtained higher than AM, which further enhanced the catalytic properties of AE compared to AM.

Table 1 Comparison study of surface area and pore volume

Catalyst type	Surface area (m^2 g^−1)	Total pore volume (cm^3 g^−1)
Fe3O4	118.0789	0.3047
Fe3O4@SiO2	7.2464	0.0118
Fe3O4@SiO2-Pr-NH2 (APTMS)	8.5458	0.0299
Fe3O4@SiO2-Pr-NH2 (APTES)	13.43	0.0433
AM	1.3220	0.0014
AE	6.3891	0.00608

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VSM. For MCT (magnetic carrier technology) applications,^31d the material must show magnetic properties. During this investigation, the magnetic properties of the as synthesized catalyst (AE) were determined by using a vibrating sample magnetometer (VSM). The value of saturation magnetization (M_s) calculated from the plot was found to be 15.42 emu g⁻¹, as shown in Fig. 3, and was compared with the value of 46.7 emu g^−1 31e of uncoated Fe₃O₄ magnetic nanoparticles. The lower value of the saturation magnetization (M_s) confirmed the coating of different shells. This value was sufficient for magnetic separation with an external magnet. Furthermore, there was no hysteresis loss H_c = 0 Oe; i.e., this catalyst was superparamagnetic in nature. It was also clear from the plot that the prepared functionalized magnetic nanoparticles were soft ferrite in nature.

Fig. 3 VSM of AE.

Fig. 4 Digital camera images of (a) dispersed catalyst, (b) after applied magnetic field.

XRD. The XRD patterns of Fe₃O₄, Fe₃O₄@SiO₂, Fe₃O₄@SiO₂-Pr-NH₂ (APTES), AE are shown in Fig. 5. In the case of Fe₃O₄, the relative position and intensities of all the diffraction peaks were matched with the standard XRD pattern (JCPDS card no. 75-1609) and the six characteristic diffraction peaks at 30.0°, 35.4°, 43.1°, 53.7°, 57.2° and 63.2° were observed, which correspond to the (220), (311), (400), (422), (511) and (440) miller indices, respectively. A broad diffraction peak observed in the range 20–25°, indicates the formation of an amorphous silane shell around the Fe₃O₄.³² No other diffraction peak was observed in both the resulting functionalized magnetic nanoparticles; i.e., functionalization with APTES and chlorosulphonic acid on the silica coated and propyl amine coated magnetic nanoparticles, respectively. This revealed the retention of the crystalline spinel ferrite structure during the coating process.³⁰ Furthermore, the intensities of the diffraction peaks in all the four cases were compared and it was observed that there was a large decrease in intensity in the cases of Fe₃O₄@SiO₂-Pr-NH₂ (APTES) and AE, which confirmed the surface coating on the Fe₃O₄@SiO₂ core shell. By using Scherrer's formula, D = kλ/β cos θ, where λ is the wavelength, β is the corrected diffraction line full-width at half-maximum, k is a constant (having a value of 0.94) and θ is Bragg's angle,³³ the average diameter of the AE nanoparticles was calculated and found to be 13.47 nm.

Fig. 5 Comparison study of the XRD analysis of (1) Fe₃O₄ nanoparticles (MNPs), (2) Fe₃O₄@SiO₂, (3) Fe₃O₄@SiO₂-Pr-NH₂ (APTES), (4) AE.

TEM. Information related to the surface morphology and particle size was provided, using TEM analysis of the as synthesized nanoparticles.

The particle size calculated for AE nanoparticles from TEM images was 13.5 nm, and it was in agreement with the XRD analysis. The TEM images also confirmed the coating of silica, amine and chlorosulphonic acid on the bare magnetic nanoparticles by the formation of concentric circles around it, shown in Fig. 6(b)–(d). Furthermore, the spherical functionalized particles were observed.

Fig. 6 TEM of (a) Fe₃O₄ nanoparticles, (b) Fe₃O₄@SiO₂, (c) Fe₃O₄@SiO₂-Pr-NH₂ (APTES), (d) AE.

XPS analysis. This was used for the determination of chemical elements on the surface of the adsorbent. The elements N, S, Si and O were detected on the surface of AE, as shown in Fig. 7.

Fig. 7 XPS analysis of AE.

The binding energy for O 1s was found near 532.38 eV for the SO₃H group, while the Si 2p binding energy at 102.86 eV is due to the overlapping of Si 2p_3/2 and Si 2p_1/2, and that at 155.84 eV is due to the presence of Si(2s). The binding energy of 169–177 eV for S 2p clearly confirmed the formation of the sulphur–oxygen bond.^34a,b The peak near 400.15 eV was observed for N 1s, which further proved the formation of the N–C bond.^34c The peaks corresponding to 710.5 and 724.1 eV are related to Fe 2p_3/2 and Fe 2p_1/2 for mixed oxides,^34c and peaks in the range from 56 to 99 eV for Fe 3p and Fe 3s were absent.^34a This observation confirmed that coating was successful, which also matched the earlier XRD data and further confirmed the formation of shells on the surface of the bare Fe₃O₄ magnetic nanoparticles, the same as in TEM images.

Thermal analysis. TGA curves give us information about the thermal stability and weight loss of the material with temperature. The TGA curves of AM and AE were divided into several parts or regions on the basis of weight loss. In the initial region up to 174.5 °C, the weight loss was 8.1% and 7.0% for AM and AE respectively, which was attributed to the loss of water molecules, removal of physically adsorbed solvent and hydroxyl groups.⁵ From 260–531 °C, weight loss was 4% for AM and 3% for AE, which was attributed to the loss of sulphamic acid moieties from the surface.³⁵ Further loss from 531.7 °C to 605 °C might be due to the loss of aminopropyl groups, which were found to be 2.3% for AM and 0.02% for AE. From 605 °C to 758 °C, the weight loss may be due the decomposition of silica shell, which was 0.9% and 0.68% for AM and AE, respectively.⁵ From the above discussion, it is clear that AE was more stable than AM in every region of the thermal analysis; also at higher temperatures after decomposition, AE remained at about 87.7% and AM was found to be 82.6%, as shown in Fig. 8. The above results demonstrate that our catalyst is stable and could be used in organic reactions occurring at higher temperatures.

Fig. 8 TGA of AE and AM.

The formation of synthesized catalyst is represented diagrammatically, as shown in Scheme 1.

Scheme 1 Synthesis of Fe₃O₄ and their functionalization with silica, amine and chlorosulphonic acid moieties.

3. Optimization of various conditions of the model reaction

Various reaction conditions, such as different types of catalysts, amount of catalysts, different solvents, etc. were employed in the model reaction of benzaldehyde (1 mmol), aniline (1 mmol) and TMSCN (1.3 mmol).

3.1 Effect of different catalysts

Here, the study of the effect of different kinds of coatings on the bare Fe₃O₄ magnetic nanoparticles is elaborated. It was investigated by fixing the amount of catalyst up to 5 mg, and trying the reaction with different kinds of catalysts, prepared step by step. Firstly, the reaction was carried out with bare Fe₃O₄ magnetic nanoparticles, giving rise to a 72% yield, and took 155 minutes for its completion. Further, silica coated Fe₃O₄@SiO₂ magnetic nanoparticles were used, which completed the reaction in 250 minutes, giving a 5% increment in yield, i.e., 77%, as compared to bare Fe₃O₄ magnetic nanoparticles. Then, the amine coated catalysts prepared with two different amines, APTES and APTMS, were used, i.e., Fe₃O₄@SiO₂-Pr-NH₂ (APTES) and Fe₃O₄@SiO₂-Pr-NH₂ (APTMS), and the yields were 90% in 240 minutes and 85% in 84 minutes, respectively. A further coating of chlorosulphonic acid was employed on these two different kinds of amine coated catalysts and the resultant catalysts were successfully used in the reaction. The yields obtained using AE and AM were 99% in 2 minutes and 87% in 40 minutes, respectively (Table 2). Catalytic activities of AE and AM were compared and better results were found (Fig. 9) with AE nanoparticles.

Table 2 Effect of different as prepared catalysts on reaction yield and time^a

Entry	Catalyst type	Amount (mg)	Time (min)	Yield^b (%)
a Reaction conditions: benzaldehyde (1 mmol), aniline (1 mmol), TMSCN (1.3 mmol), catalyst (5 mg), water (1.5 ml), room temperature.b Isolated yield.
1.	Fe3O4 MNPs	5	155	72
2.	Fe3O4@SiO2 MNPs	5	250	77
3.	Fe3O4@SiO2@-Pr-NH2 (APTES)	5	240	90
4.	Fe3O4@SiO2@-Pr-NH2 (APTMS)	5	84	85
5.	AM	5	40	87
6.	AE	5	2	99

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Fig. 9 Yields from catalysts AE and AM.

The reaction was carried out without catalyst, which took 210 minutes, with 62% yield. These results encouraged us to further optimize the amount of catalyst, i.e., AE nanoparticles. Variable amounts of AE and AM (2.5, 5.0, 7.5 and 10.0 mg) were used and yields of 78, 99, 71 and 78% for AE and 58, 87, 67 and 67 for AM were obtained, respectively, as in Fig. 9. The difference in yields may be attributed to the difference in surface area of AE and AM. The surface area and pore volume obtained by BET plots were 6.3891 m² g⁻¹, 6.03 × 10⁻³ cm³ g⁻¹ and 1.3220 m² g⁻¹, 1.4 × 10⁻³ cm³ g⁻¹ for AE and AM, respectively, as shown in Fig. 4. Both the higher and lower amounts of the catalyst were unable to increase the yield and decrease the reaction time. The present study clearly indicates that the best yield was obtained with 5 mg of the catalyst AE, with 99% yield of product; thus, it was considered as the optimized amount for the reaction, as shown in Table 3.

Table 3 Optimization of the catalytic amount of AE^a

Entry	AE (mg)	Time (min)	Yield^b (%)
a Reaction conditions: benzaldehyde (1 mmol), aniline (1 mmol), TMSCN (1.3 mmol), AE (mg), water (1.5 ml), room temperature.b Isolated yield.
1	0	210	62
2.	2.5	22	78
3.	5	2	99
4.	7.5	19	71
5.	10	13	78

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3.2 Screening of the solvent

Different solvents were used in the model reaction for investigation of the appropriate solvent for the reaction. The reaction was first tried with pure water as green solvent and it was found that the reaction was completed within 2 minutes, with 99% yield. Other solvents, ethanol and a mixture of H₂O : EtOH (1 : 1) were also used, giving 82% and 35% yields, respectively. The high yield and faster reaction in water may be attributed to the high amphoteric nature and high polarizability of water molecules.³⁶ Further, the reaction was tried in pure water and the H₂O : EtOH (1 : 1) mixture without catalyst; it was completed in 210 minutes and 29 minutes with 62% and 36% yield, respectively. The reaction was also tried in solvent free conditions, but it took a long time (12 h) for its completion, with 66% yield (Table 4).

Table 4 Optimization of the solvent^a

Entry	Solvent (ml)	Catalyst (mg)	Time (min)	Yield^b (%)
a Reaction conditions: benzaldehyde (1 mmol), aniline (1 mmol), TMSCN (1.3 mmol), AE (mg), solvent (1.5 ml), room temperature.b Isolated yield.
1	H2O	5	2	99
2	H2O	—	210	62
3	EtOH	5	4	82
4	H2O : EtOH	5	6	35
5	H2O : EtOH	—	29	36
6	—	5	12 h	66

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During the investigation of the formation of different α-aminonitriles, different types of aldehydes and amines substituted at ortho, meta and para positions were used to react with TMSCN in water to afford the desired product by using AE (Table 5). The reaction was successfully completed with electron withdrawing, as well as electron releasing groups, on the aldehydes and amines. No undesired side products, such as cyanohydrins, trimethylsilyl ether, etc., were reported for the reaction of aldehyde and TMSCN, which may be due to the formation of the imine intermediate. The products formed were higher in yield and highly selective in nature. No further additives were required to promote the reaction.

Table 5 Formation of a series of derivatives of α-aminonitriles by using aldehydes and amines^a

Sr. No.	Aldehyde	Amine	Product	Time (min)	Yield^b (%)	M.P./reported M.P. (°C)
a Reaction conditions: aldehyde (1 mmol), amine (1 mmol), TMSCN (1.3 mmol), AE (5 mg), water (1.5 ml), room temperature.b Isolated yield.
1				2	99	76–78/75–76 (ref. 37)
2.	1a			10	87	66–70
3.	1a			15	98	106–108/108–110 (ref. 40)
4.	1a			2.5	73	72–73/80–82 (ref. 38)
5.	1a			25	87	60–62
6.	1a			2	65	88–90
7.	1a			9	99	64–66
8.	1a			20	78	96–100
9.		2a		7	95	100–105/108–110 (ref. 39)
10.		2a		30	69	94–96/94–97 (ref. 26a)
11.		2a		12	65	120–126/134–136 (ref. 40)
12.		2a		11	94	102–105/108–110 (ref. 41)
13.		2a		35	91	121–122/120–122 (ref. 40)
14.	1f			40	90	148–152
15.	1b	2i		120	80	80–82
16.	1b	2e		180	67	98–100

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The catalytic system was also reacted with acid sensitive 5-methylfurfuraldehyde, giving rise to a better product yield, i.e., 94% (entry 12, Table 5). The effects on yield and reaction time of different electron withdrawing and releasing groups depend on the formation of the imine intermediate, which plays the key role in the formation of products.²⁷

3.3 Recyclability of the catalyst

Due to the magnetic character, the catalyst was recovered from the reaction mixture after the completion of the reaction by using an external magnet. The obtained catalyst was washed with EtOH and dried in an oven at 50 °C. After this, it was recycled up to five cycles with very little loss in activity and percentage yield, as shown in Fig. 10. Furthermore, to check the stability of –SO₃H on the surface of the catalyst, which is the active site of the catalyst, the resulting mixture was filtered and washed with distilled water to extract the sulfate ions possibly leached from the catalyst surface; the BaCl₂ precipitation test was performed⁴² and no white precipitation occurred, which clearly suggests that there was not any kind of leaching of the catalyst. This test was also applied to the reaction mixture for 24 h and the same result was observed. In Fig. 11, peaks belonging to sulphonyl moieties in AE after the fifth cycle have almost the same position as fresh AE; no significant change occurred. This observation also further confirmed that there was not any kind of leaching from the surface of the catalyst after being reused. This clearly indicates the stability and reusability of the catalyst. Fig. 12 shows the TEM of recovered AE after the fifth cycle, and particle size observed after recyclability was slightly increased, which could be attributed to the agglomeration of magnetic nanoparticles.

Fig. 10 Recyclability of AE, up to five cycles.

Fig. 11 Comparison study of Fourier transform infrared (FT-IR) spectra of fresh AE and reused AE after the fifth cycle.

Fig. 12 TEM of AE after the fifth cycle.

3.4 Comparison of catalytic activity with reported catalysts

The catalytic activity of AE was compared with other existing catalysts and methods for the formation of α-aminonitriles. To calculate the number of catalytic active sites (–SO₃H), the acid base titration method was used.⁴² In a typical experiment, 5 mg of the catalyst was dissolved in 8.33 ml of an aqueous solution of NaHCO₃ (5 × 10⁻³ mol L⁻¹) and this mixture was stirred for 24 h. After this, the catalyst was separated by using external magnet and the resultant solution was titrated against HCl (0.1 M) by using phenolphthalein as an indicator. The number of acid sites was investigated by the amount of NaHCO₃ consumed and it was found to be 0.01 mmol g⁻¹. Reaction time, yield, very low catalytic amount, use of water as the reaction medium and mild reaction conditions make this method superior to the other existing methods, as shown in Table 6. The proposed mechanism for the synthesis of α-aminonitriles is shown in Scheme 2.

Table 6 One pot synthesis of α-aminonitriles by using different catalysts (comparison table)

Catalyst	Amount	Solvent	Conditions	Time (min)	% yield	Ref.
Silica-based scandium(III)	5–6 mol%	CH2Cl2	RT	840	94	36
Montmorillonite	1 g mmol^−1	CH2Cl2	RT	330	87	43
MCM-41-SO3H	1.5 mg	MeOH	RT	70	97	27
InI3	10 mol%	H2O	RT	30	95	26b
Fe(Cp)2PF6	5 mol%	Neat	RT	20	94	26a
TiCl3·4H2O	1 mol%	Neat	RT	15	94	37
K2PdCl4	10 mol%	H2O	RT	12	95	40
SA-MNPs	20 mg	H2O	RT	10	97	35
SnHBeta	2.4 mol%/10 mg	Neat	RT	10	93	44
AE	5μ mol%/5 mg	H2O	RT	2	99	This work

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Scheme 2 Proposed mechanism of the reaction.

4. Experimental section

4.1 Materials

3-Aminopropyltriethoxy silane (APTES) with 99% purity and 3-aminopropyltriethoxy silane (APTMS) with 95% purity were supplied by Acros Organics. Trimethylsilyl cyanide (TMSCN) with 90% purity and chlorosulphonic acid (98%) were obtained from Avra Synthesis. Ammonia solution 30% was obtained from Merck, dichloromethane (DCM) of 99.0% purity from SDFCL, and tetraethylorthosilicate (TEOS), 98% from HiMedia. All these were used directly without any further purification.

4.2 Methods

The crystal structure of the prepared magnetic nanoparticles catalyst was investigated by X-ray diffraction (XRD), using a Panalytical X'PERT PRO (NPD) X-ray diffractometer with Cu Kα radiation (λ = 0.154 nm), in the 2θ range 10–80°. Fourier transform IR (FT-IR) spectroscopy was carried out by using an Agilent Cary 600 instrument in the range 400–4000 cm⁻¹. The morphology was studied using transmission electron microscopy (TEM). The sample was prepared by the dispersion of nanoparticles in ethanol, which were deposited on a carbon-coated Cu grid, and after drying, analysis was performed using a Hitachi S7500 instrument. The magnetic properties were measured using a vibrating sample magnetometer (VSM, Princeton Applied Research model 155) at room temperature, with a maximum magnetic field range of +1 T to −1 T. Thermogravimetric analysis (TGA) was performed on the catalyst using an EXSTAR 6000 TG/DTA 6300 instrument in the temperature range 50–700 °C, at a heating rate of 10 °C min⁻¹ under nitrogen atmosphere. Surface area and pore characteristics were characterized using a Quanta Chrome Nova-1000 surface analyzer instrument under liquid nitrogen, with vacuum degassing at 110 °C for 3 h. The specific surface area was calculated using the BET equation. Surface properties were studied by using X-ray photoelectron spectroscopy (XPS, PHI 5000 Versa Prob II, FEI Inc.). The organic products were characterized by ¹H nuclear magnetic resonance spectroscopy at 400 MHz with the aid of an Advance 400 spectrophotometer, using tetramethylsilane as the internal standard and CDCl₃ solvent. Software used for NMR was MNOVA, and that used for drawing chemical structures was ChemDraw ultra 8.0.

4.3 Synthesis of Fe₃O₄ magnetic nanoparticles

The Fe₃O₄ magnetic nanoparticles were prepared by using the co-precipitation method, with some alteration in the reported method. In a typical synthesis, 0.013 moles of Fe₂(SO₄)₃ were dissolved in 10 ml of distilled water and 0.013 moles of FeSO₄ were also dissolved in 10 ml of distilled water. Both solutions were mixed together and the mixture was sonicated up to 15 minutes to get a clear solution. The pH of the mixture was adjusted to 10 by the slow, drop-wise addition of ammonium hydroxide solution. The mixture was then stirred vigorously at 60 °C for 1 hour. The resultant mixture was washed 3–4 times with distilled water and finally centrifuged to get the black precipitate of Fe₃O₄ magnetic nanoparticles.

4.4 Synthesis of Fe₃O₄@SiO₂

The precursor was prepared by suspending of 1 g of Fe₃O₄ MNPs with 60 ml of EtOH, followed by the addition of 20 ml distilled water, and sonicated up to 45 minutes. After mixing, NH₄OH (5 ml) and TEOS (31.36 mmol) were added to the mixture and again sonicated for 10 minutes. The mixture was magnetically stirred at 35 °C for 5.5 h. The resultant mixture was then washed 4 times with distilled water and centrifuged. Finally, the brown precipitates were dried in an oven at 50 °C.⁴⁵

4.5 Synthesis of Fe₃O₄@SiO₂-Pr-NH₂

Firstly, dilution of 0.5 g of Fe₃O₄@SiO₂ was done with the addition of 125 ml of distilled water. The mixture was then sonicated for up to 1 hour. The pH was adjusted to 10.5 by the slow, drop-wise addition of the ammonium hydroxide solution. Then, 53.4 mmol of (3-aminopropyl)triethoxysilane (APTES)/(3-aminopropyl)trimethoxysilane (APTMS) were added to the above solution separately, and again sonicated for up to 15 minutes more. The resultant mixture was stirred at 50 °C with 300 rpm for 3.5 h. Finally, it was washed 4 times with distilled water, centrifuged and dried in an oven at 50 °C.⁴⁶

4.6 Synthesis of Fe₃O₄@SiO₂-Pr-NH-SO₃H

The obtained Fe₃O₄@SiO₂@NH₂ powder (0.07272 g) was dispersed in DCM (5 ml) using the ultrasonic bath for 30 minutes. Then, 1.74 mmol (0.116 ml) of chlorosulphonic acid were added drop-wise, with constant shaking by hand for 15 minutes in an ice bath; there was an evolution of hydrogen gas from the reaction mixture. Finally, the mixture was washed 3 times with dry DCM to remove the unreacted parts, and kept at room temperature for drying.³⁶

4.7 Synthesis of α-aminonitriles

Aldehyde (1 mmol), aniline (1 mmol) and TMSCN (1.3 mmol, 0.162 ml) were mixed in water (1.5 ml) as solvent at room temperature. The appropriate amount of the catalyst was then added to the reaction and the mixture was stirred. The progress of the reaction was indicated by TLC. The solid product was separated at the end of the reaction. After this, the reaction product was dissolved in hot EtOH and the catalyst was removed by using an external magnet. Recrystallization was carried out using EtOH to afford the pure product.

Spectral data for products.
Table 4, entry 1 (4a): 2-phenyl-2-(phenylamino)acetonitrile. FT-IR: 3360, 3024, 2905, 2200, 1600, 1501, 1419, 1321, 1120, 918, 754 cm⁻¹, ¹H NMR (CDCl₃, 400 MHz) δ (ppm): 4.01 (br s, 1H), 5.41 (s, 1H), 6.77–7.00 (d, 3H), 7.26–7.58 (t, 7H).
Table 4, entry 2 (4b): 2-(2-methoxyphenylamino)-2-phenylacetonitrile. FT-IR: 3331, 3069, 2958, 2233, 1598, 1508, 1456, 1232, 1120, 1030, 724 cm⁻¹, ¹H NMR (CDCl₃, 400 MHz) δ (ppm): 3.83 (s, 3H), 5.46 (s, IH), 6.82–6.94 (m, 4H), 7.45–7.48 (m, 3H), 7.61–7.63 (m, 2H), 8.0–8.1 (m, 1H).
Table 4, entry 3 (4c): 2-(4-chlorophenylamino)-2-phenylacetonitrile. FT-IR: 3308, 2882, 2255, 1613, 1516, 1486, 1269, 1112, 1008, 754 cm⁻¹, ¹H NMR (CDCl₃, 400 MHz) δ (ppm): 3.10 (br s, 1H), 5.39 (s, 1H), 6.70–6.72 (m, 2H), 7.22–7.23 (m, 2H), 7.44–7.46 (m, 2H), 7.57–7.59 (m, 2H), 8.08–8.10 (m, 1H).
Table 4, entry 4 (4d): 2-(4-fluorophenylamino)-2-phenylacetonitrile. FT-IR: 3346, 3039, 2240, 1605, 1501, 1456, 1299, 1224, 1075, 776 cm⁻¹, ¹H NMR (CDCl₃, 400 MHz) δ (ppm): 4.25 (br s, 1H), 5.37 (s, 1H), 6.72–6.76 (m, 2H), 6.98 (t, 2H), 7.45–7.48 (m, 3H), 7.58–7.61 (m, 1H), 8.10–8.12 (m, 1H).
Table 4, entry 5 (4e): 2-(o-tolylamino)-2-phenylacetonitrile. FT-IR: 3368, 3025, 2968, 2240, 1605, 1501, 1446, 1286, 1117, 918, 734 cm⁻¹, ¹H NMR (CDCl₃, 400 MHz) δ (ppm): 2.16 (s, 3H), 3.05 (br s, 1H), 5.46 (s, 1H), 6.85–6.87 (m, 2H), 7.14 (d, 1H), 7.21 (t, 1H), 7.47–7.48 (m, 3H), 7.62–7.63 (m, 1H), 8.0–8.1 (m, 1H).
Table 4, entry 6 (4f): 2-(m-tolylamino)-2-phenylacetonitrile. FT-IR: 3341, 3039, 2920, 2232, 1603, 1487, 1455, 1307, 1165, 906, 764 cm⁻¹, ¹H NMR (CDCl₃, 400 MHz) δ (ppm): 2.32 (s, 3H), 3.50 (br s, 1H), 5.43 (s, 1H), 6.60 (d, 2H), 6.74 (d, 1H), 7.16 (t, 1H), 7.43–7.48 (m, 3H), 7.59–7.61 (m, 2H).
Table 4, entry 7 (4g): 2-(2-ethoxyphenylamino)-2-phenylacetonitrile. FT-IR: 3338, 3050, 2977, 2174, 1604, 1507, 1448, 1293, 1128, 914, 730 cm⁻¹, ¹H NMR (CDCl₃, 400 MHz) δ (ppm): 1.39 (t, 3H), 2.40 (br s, 1H), 4.0–4.1 (m, 2H), 5.48 (s, 1H), 6.80–6.92 (m, 4H), 7.44–7.47 (m, 3H), 7.61–7.63 (m, 2H).
Table 4, entry 8 (4h): 2-(3-hydoxyphenylamino)-2-phenylacetonitrile. FT-IR: 3301, 3041, 2233, 1619, 1513, 1455, 1273, 1223, 1177, 903, 735 cm⁻¹, ¹H NMR (CDCl₃, 400 MHz) δ (ppm) 4.43 (br s, 1H), 5.33 (s, 1H), 6.70–6.84 (m, 3H), 7.17–7.19 (m, 1H), 7.46–7.58 (m, 3H), 7.88–7.89 (m, 2H), 8.47 (s, 1H).
Table 4, entry 9 (4i): 2-(4-chlorophenyl)-2-(phenylamino)acetonitrile. FT-IR: 3353, 3039, 2225, 1605, 1493, 1426, 1284, 1127, 1008, 814 cm⁻¹, ¹H NMR (CDCl₃, 400 MHz) δ (ppm): 3.48 (br s, 1H), 5.43 (s, 1H), 6.78 (d, 2H), 6.93 (t, 1H), 7.26–7.30 (m, 2H), 7.42–7.44 (m, 2H), 7.53–7.56 (m, 2H).
Table 4, entry 10 (4j): 2-(4-methoxyphenyl)-2-(phenylamino)acetonitrile. FT-IR: 3331, 3024, 2935, 2233, 1620, 1501, 1434, 1239, 1157, 1023, 754 cm⁻¹, ¹H NMR (CDCl₃, 400 MHz) δ (ppm): 3.82 (s, 3H), 4.00 (br s, 1H), 5.34 (s, 1H), 6.76 (d, 2H), 6.88 (t, 1H), 6.95 (d, 2H), 7.27 (t, 2H), 7.49 (d, 2H).
Table 4, entry 11 (4k): 2-(3, 4-dimethoxyphenyl)-2-(phenylamino)acetonitrile. FT-IR: 3352, 2944, 2233, 1599, 1514, 1307, 1268, 1142, 1023, 755 cm⁻¹, ¹H NMR (CDCl₃, 400 MHz) δ (ppm): 2.02 (br s, 1H), 5.36 (s, 1H), 3.89–3.96 (m, 6H), 6.78 (d, 1H), 6.88–6.92 (m, 1H), 6.97 (d, 1H), 7.05 (d, 1H), 7.14–7.17 (m, 1H), 7.24–7.30 (m, 1H), 7.40 (d, 1H), 7.44–7.46 (m, 1H).
Table 4, entry 12 (4l): 2-(5-methylfuran-2-yl)-2-(phenylamino)acetonitrile. FT-IR: 3333, 3032, 2240, 1600, 1566, 1510, 1438, 1292, 1247, 1097, 1023, 896 cm⁻¹, ¹H NMR (CDCl₃, 400 MHz) δ (ppm): 2.32 (s, 3H), 4.15 (br s, 1H), 5.41 (s, 1H), 6.00 (d, 1H), 6.45 (d, 1H), 6.79 (d, 2H), 6.91 (t, 1H), 7.26–7.29 (m, 2H).
Table 4, entry 13 (4m): 2-(4-hydoxyphenyl)-2-(phenylamino)acetonitrile. FT-IR: 3377, 3040, 2197, 1605, 1515, 1443, 1281, 1240, 1157, 978, 761 cm⁻¹, ¹H NMR (CDCl₃, 400 MHz) δ (ppm): 3.97 (br s, 1H), 5.35 (s, 1H), 6.69–6.78 (m, 2H), 6.89–6.95 (m, 3H), 7.16 (t, 1H), 7.27–7.30 (m, 1H), 7.32–7.46 (m, 1H), 7.78–7.92 (m, 1H), 8.39 (s, 1H).
Table 4, entry 14 (4n): 2-(2-methoxyphenylamino)-2-(4-hydroxyphenyl)acetonitrile. FT-IR: 3282, 3025, 2938, 2254, 1602, 1514, 1442, 1225, 1117, 1024, 737 cm⁻¹, ¹H NMR (CDCl₃, 400 MHz) δ (ppm): 3.87 (s, 3H), 4.61 (br s, 1H), 5.36 (s, 1H), 6.84–6.89 (m, 5H), 7.47 (d, 2H), 7.80 (d, 1H), 8.45 (s, 1H).
Table 4, entry 15 (4o): 2-(2-methoxyphenylamino)-2-(4-chlorophenyl)acetonitrile. FT-IR: 3336, 3206, 2941, 2241, 1605, 1509, 1453, 1221, 1119, 1029, 723 cm⁻¹, ¹H NMR (CDCl₃, 400 MHz) δ (ppm): 2.12 (br s, 1H), 3.83 (s, 3H), 5.43 (s, 1H), 6.76 (t, 1H), 6.84–6.85 (m, 2H), 6.86–6.91 (m, 1H), 7.41–7.43 (m, 2H), 7.54–7.56 (m, 2H).
Table 4, entry 16 (4p): 2-(4-chlorophenyl)-2-(phenylamino)acetonitrile. FT-IR: 3333, 3135, 2921, 2243, 1603, 1497, 1436, 1251, 1093, 1023, 750 cm⁻¹, ¹H NMR (CDCl₃, 400 MHz) δ (ppm): 2.31 (s, 3H), 4.16 (br s, 1H), 5.40 (s, 1H), 5.99 (d, 1H), 6.43 (d, 1H), 6.77 (d, 2H), 6.90 (t, 1H), 7.14 (t, 1H), 7.26 (t, 2H).

Conclusion

Synthesis of Fe₃O₄@SiO₂-Pr-NH-SO₃H (AE) and Fe₃O₄@SiO₂-Pr-NH-SO₃H (AM) nanoparticles was carried out. The effect of grafting with different alkoxy silanes on the catalytic activity was analyzed. This present methodology offers several advantages, such as excellent yields, environmentally friendly work up procedure, shorter reaction times, ambient conditions in an open atmosphere, use of green solvent (water), and a recyclable catalyst with a very easy separation with an external magnet. In addition, the obtained results indicated that AE can be used as an inexpensive and effective catalyst for the synthesis of a variety of α-aminonitriles by one-pot three component reactions. The catalyst shows high thermal stability, non toxicity, recyclability and can be reused in up to five reaction cycles without considerable loss in activity.

Acknowledgements

We are thankful to IIT Ropar for XRD, NMR, SAIF, Panjab University, Chandigarh for TEM, CIL IIT Roorkee for VSM, BIT Bengaluru for BET, IIT Kanpur for XPS, Department of Chemistry NIT Jalandhar for FT-IR, TGA. Three of the authors (H. S., P. A. and J.) are thankful to UGC, MHRD and NIT Jalandhar for providing the research fellowship.

References

1. K. K. Senapati, S. Roy, C. Borgohain and P. Phukan, J. Mol. Catal. A: Chem., 2012, 352, 128–134.
2. D. Lee, J. Lee, H. Lee, S. Jin, T. Hyeon and B. M. Kim, Adv. Synth. Catal., 2006, 348, 41–46.
3. R. Abu-Reziq, H. Alper, D. Wang and M. L. Post, J. Am. Chem. Soc., 2006, 128, 5279–5282.
4. E. Karaoğlu, A. Baykal, H. Erdemi, L. Alpsoy and H. Sozeri, J. Alloys Compd., 2011, 509, 9218–9225.
5. H. Naeimi and Z. S. Nazifi, J. Nanopart. Res., 2013, 15, 2026–2032.
6. A. Jordan, R. Scholz, P. Wust, H. Fähling and R. Felix, J. Magn. Magn. Mater., 1999, 201, 413–419.
7. J. Cao, Y. Wang, J. Yu, J. Xia, C. Zhang, D. Yin and U. O. Häfeli, J. Magn. Magn. Mater., 2004, 277, 165–174.
8. A. Jordan, R. Scholz, P. Wust, H. Schirra, T. Schiestel, H. Schmidt and R. Felix, J. Magn. Magn. Mater., 1999, 194, 185–196.
9. Q. Li, Y. Xuan and J. Wang, Exp. Therm Fluid Sci., 2005, 30, 109–116.
10. (a) M. Ziegler-Borowska, D. Chełminiak, T. Siódmiak, A. Sikora, M. Piotr Marszałł and H. Kaczmarek, Mater. Lett., 2014, 132, 63–65; (b) T. Cheng, D. Zhang, H. Li and G. Liu, Green Chem., 2014, 16, 3401–3427; (c) V. Polshettiwar, R. Luquet, A. Fihri, H. Zhu, M. Bouhrara and J.-M. Basset, Chem. Rev., 2011, 111, 3036–3075; (d) N. Yan, Y. Yuan and P. J. Dyson, Dalton Trans., 2013, 42, 13294–13304; (e) N. Yan, C. Xiao and Y. Kou, Coord. Chem. Rev., 2010, 254, 1179–1218; (f) R. Ilsu, T. Ahmad, H. Sungwook, C. Yongmin and L. Jaejun, J. Korean Phys. Soc., 2010, 57, 1545–1549; (g) B. Maleki, H. Eshghi, A. Khojastehnezhad, R. Tayebee, S. S. Ashrafi, G. E. Kahoo and F. Moeinpour, RSC Adv., 2015, 5, 64850–64857; (h) P. Arora, J. K. Rajput and H. Singh, RSC Adv., 2015, 5, 97212–97223; (i) H. Naeimi and D. Aghaseyedkarimi, New J. Chem., 2015, 39, 9415–9421.
11. D. Enders and J. P. Shilvock, Chem. Soc. Rev., 2000, 29, 359–373.
12. A. Strecker, Ann. Chem. Pharm., 1850, 75, 27–45.
13. D. J. Ramon and M. Yus, Angew. Chem., Int. Ed., 2005, 44, 1602–1634.
14. A. S. Paraskar and A. Sudalai, Tetrahedron Lett., 2006, 47, 5759–5762.
15. Y. M. Shafran, V. A. Bakulev and V. S. Mokrushin, Russ. Chem. Rev., 1989, 58, 148–162.
16. W. L. Matier, D. A. Owens, W. T. Comer, D. Deitchman, H. C. Ferguson, R. J. Seidehamel and J. R. Young, J. Med. Chem., 1973, 16, 901–908.
17. R. O. Duthaler, Tetrahedron, 1994, 50, 1539–1650.
18. P. L. Feldman, M. K. James, M. F. Brackeen, J. M. Bilotta, S. V. Schuster, A. P. Lahey, M. W. Lutz, M. R. Johnson and H. J. Leighton, J. Med. Chem., 1991, 34, 2202–2208.
19. L. M. Weinstock, P. Davis, B. Handelsman and R. J. Tull, J. Org. Chem., 1967, 32, 2823–2829.
20. A. Arasappan, S. Venkatraman, A. I. Padilla, W. Wu, T. Meng, Y. Jin, J. Wong, A. Prongay, V. Girijavallabhan and F. George Njoroge, Tetrahedron Lett., 2007, 48, 6343–6347.
21. C. R. Razafindrabe, S. Aubry, B. Bourdon, M. Andriantsiferana, S. Pellet-Rostaing and M. Lemaire, Tetrahedron, 2010, 66, 9061–9066.
22. B. C. Das, S. M. Mahalingam and P. Mohapatra, Tetrahedron Lett., 2009, 50, 5860–5863.
23. (a) S. Kobayashi and H. Ishitani, Chem. Rev., 1999, 99, 1069–1094; (b) S. Harusawa, Y. Hamada and T. Shioiri, Tetrahedron Lett., 1979, 20, 4663–4666; (c) S. Nakamura, N. Sato, M. Sugimoto and T. Toru, Tetrahedron: Asymmetry, 2004, 15, 1513–1516; (d) P. Vachal and E. N. Jacobsen, J. Am. Chem. Soc., 2002, 124, 10012–10014.
24. A. Shaabani and A. Maleki, Appl. Catal., A, 2007, 331, 149–151.
25. P. Vongvilai and O. Ramström, J. Am. Chem. Soc., 2009, 131, 14419–14425.
26. (a) N. H. Khan, S. Agrawal, R. I. Kureshy, S. H. R. Abdi, S. Singh, E. Suresh and R. V Jasra, Tetrahedron Lett., 2008, 49, 640–644; (b) Z.-L. Shen, S.-J. Ji and T.-P. Loh, Tetrahedron, 2008, 64, 8159–8163; (c) A. Majhi, S. S. Kim and S. T. Kadam, Tetrahedron, 2008, 64, 5509–5514; (d) L. Royer, S. K. De and R. A. Gibbs, Tetrahedron Lett., 2005, 46, 4595–4597; (e) S. K. De and R. A. Gibbs, Tetrahedron Lett., 2004, 45, 7407–7408; (f) M. Narasimhulu, T. S. Reddy, K. C. Mahesh, S. M. Reddy, A. V. Reddy and Y. Venkateswarlu, J. Mol. Catal. A: Chem., 2007, 264, 288–292; (g) S. K. De, J. Mol. Catal. A: Chem., 2005, 225, 169–171; (h) G. K. S. Prakash, T. Mathew, C. Panja, S. Alconcel, H. Vaghoo, C. Do and G. A. Olah, Proc. Natl. Acad. Sci. U. S. A., 2007, 104, 3703–3706; (i) D. Kumar, A. Saini and J. S. Sandhu, Rasayan J. Chem., 2008, 1, 639–644; (j) M. A. Pasha, H. M. Nanjundaswamy and V. P. Jayashankara, Synth. Commun., 2007, 37, 4371–4380; (k) S. K. De and R. A. Gibbs, Synth. Commun., 2005, 35, 961–966; (l) S. K. De, Synth. Commun., 2005, 35, 653–656; (m) M. M. Heravi, M. Ebrahimzadeh, H. A. Oskooie and B. Baghernejad, Chin. J. Chem., 2010, 28, 480–482; (n) C. Reddy and M. Raghu, Indian J. Chem., Sect. B: Org. Chem. Incl. Med. Chem., 2008, 47, 1572–1577.
27. M. G. Dekamin and Z. Mokhtari, Tetrahedron, 2012, 68, 922–930.
28. C. W. Lim and I. S. Lee, Nano Today, 2010, 5, 412–434.
29. (a) F.-P. Ma, P.-H. Li, B.-L. Li, L.-P. Mo, N. Liu, H.-J. Kang, Y.-N. Liu and Z.-H. Zhang, Appl. Catal., A, 2013, 457, 34–41; (b) M. Rajeshwai, B. Sammaiah, D. Sumalatha and L. N. Sarda, Indian J. Adv. Chem. Sci., 2013, 1, 236–239; (c) R. R. Nagawade and D. B. Shinde, Chin. J. Chem., 2007, 25, 1710–1714.
30. J. Safari and Z. Zarnegar, J. Mol. Catal. A: Chem., 2013, 379, 269–276.
31. (a) S. J. Gregg and K. S. W. Sing, Adsorption, Surface Area, and Porosity, Academic Press, London, 2nd edn, 1982; (b) W. Sun, Q. Li, S. Gao and J. K. Shang, J. Mater. Chem. A., 2013, 1, 9215–9224; (c) W. Sun, W. Yang, Z. Xu, Q. Li and J. K. Shang, ACS Appl. Mater. Interfaces, 2016, 8, 2035–2047; (d) M. Golshekan, S. Shariati and N. Saadatjoo, RSC Adv., 2014, 4, 16589–16596; (e) H. El Ghandoor, H. M. Zidan, M. M. H. Khalil and M. I. M. Ismail, Int. J. Electrochem. Sci., 2012, 7, 5734–5745.
32. J. Wu, P. Su, J. Huang, S. Wang and Y. Yang, J. Colloid Interface Sci., 2013, 399, 107–114.
33. B. D. Cullity and S. R. Stock, Elements of X-ray Diffraction, Prentice-Hall, Englewood Cliffs, NJ, 3rd edn, 2001.
34. (a) S. Zhang, Y. Zhang, J. Liu, Q. Xu, H. Xiao, X. Wang and H. Xu, Chem. Eng. J., 2013, 226, 30–38; (b) K. M. Hello, H. R. Hasan, M. H. Sauodi and P. Morgen, Appl. Catal., A, 2014, 475, 226–234; (c) M. Kim, H. Kang and K. H. Park, Catal. Commun., 2015, 72, 150–155.
35. M. Z. Kassaee, H. Masrouri and F. Movahedi, Appl. Catal., A, 2011, 395, 28–33.
36. B. Karimi and A. A. Safari, J. Organomet. Chem., 2008, 693, 2967–2970.
37. A. Majhi, S. S. Kim and S. T. Kadam, Appl. Organomet. Chem., 2008, 22, 705–711.
38. M. A. Kumar, M. F. S. Babu, K. Srinivasulu, Y. B. Kiran and C. S. Reddy, J. Mol. Catal. A: Chem., 2007, 265, 268–271.
39. H. Ghafuri and M. Roshani, RSC Adv., 2014, 4, 58280–58286.
40. B. Karmakar and J. Banerji, Tetrahedron Lett., 2010, 51, 2748–2750.
41. S. S. Mansoor, K. Aswin, K. Logaiya and S. P. N. Sudhan, J. Saudi Chem. Soc., 2012 DOI:10.1016/j.jscs.2012.10.009.
42. B. Chang, Y. Tian, W. Shi, J. Liu, F. Xi and X. Dong, RSC Adv., 2013, 3, 20999–21006.
43. J. S. Yadav, B. V. Subba Reddy, B. Eeshwaraiah and M. Srinivas, Tetrahedron, 2004, 60, 1767–1771.
44. A. K. Shah, N. H. Khan, G. Sethia, S. Saravanan, R. I. Kureshy, S. H. R. Abdi and H. C. Bajaj, Appl. Catal., A, 2012, 419–420, 22–30.
45. W. Stober, J. Colloid Interface Sci., 1968, 26, 62–69.
46. Y. Liu, Y. Li, X. M. Li and T. He, Langmuir, 2013, 29, 15275–15282.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra20095h

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