Epoxidation of vinyl cyanides by lithium hypochlorite in the presence of Fe3O4@Ag-CTAB as a new eco-friendly catalyst in aqueous medium

To cater to the requirement of environmental protection, cetyltrimethylammonium bromide (CTAB)-coated Fe3O4@Ag Nanoparticles as a new hybrid magnetic catalyst was prepared for the one-pot multicomponent epoxidation of a wide range of alkenes by lithium hypochlorite (LiOCl) in aqueous medium. FT-IR, TEM, XRD, EDS and VSM techniques were employed to characterize the structure of the magnetic catalyst. The results obtained revealed that the CTAB modification effectively decreased the particle size and enhanced the dispersion of the particles in solution. Application of this catalyst was studied efficiently in one-pot multicomponent synthesis of epoxide derivatives. The structure of the synthesized epoxides was confirmed by FT-IR, 1H-NMR, 13C-NMR and elemental analysis. The main advantages of our work are reusability of the catalyst, easy work-up, Short reaction times and use of water as a green solvent.


Introduction
Organic synthesis in aqueous medium is an important challenge in green chemistry [1]. But, the main disadvantage of using water as a green solvent is the low solubility of most organic compounds. In order to overcome this problem, surfactants can be used [2]. Surfactants, including nonionic, cationic, anionic, nano, and zwitterionic forms improve the solubility of materials by forming micelles, easing the dispersion of organic molecules in water and reducing aqueous interfacial tension [3]. In recent years, various surfactants such as cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), cocamidopropyl betaine (CAPB) and didodecyldimethylammonium bromide (DDAB) have been applied in many organic reactions [4][5][6][7][8]. On the other hand, magnetically nanocatalysts have attracted more attention than other nanoparticles in synthesis of organic compounds because of easy separation, their high catalytic activity and improved selectivity [9,10]. For example, lately polymer-supported Fe 3 O 4 11 and Cu 2 O/Fe 3 O 4 @guarana nanocatalysts [12] for the synthesis of imidazole derivatives, Fe 3 O 4 @Cu-β-CD for the synthesis of dihydropyrano [2,3-c]pyrazole derivatives [13], TiO 2 -coated Fe 3 O 4 nanoparticles for the synthesis of pyrroles [14] and Fe 3 O 4 @Ag-β-CD for the synthesis of dihydropyrimidinones [15] have been applied. In this paper, we have decorated a class of usable hybrid nanocomposites in aqueous medium which shown combining the positive properties of CTAB and magnetic nanoparticles and we used it for epoxidation of vinyl cyanide derivatives by lithium hypochlorite.
Epoxides are an important class of organic compounds which are good raw material and important intermediates for making valuable products in organic synthesis [16][17][18]. Furthermore, epoxide derivatives have been found to be a good precursor for the synthesis of bioactive compounds in many reactions [19][20][21][22]. Based on foregoing, several efforts have been focused on epoxidation of alkenes and various methods have been reported in recent decades. For example, synthesis of epoxides from alkenes with NaOCl and urea H 2 O 2 in the presence of chiral macrocyclicsalen Mn (III) complexes [23], by LiOCl in the presence of Mn(III) porphyrins [24], by tert-butyl hydroperoxide in the presence of oxidovanadium (IV) tetradentate schiff base complex [25], by H 2 O 2 in the presence of titaniumsilicalite catalyst [26] and vanadium complex [27], have been reported. Epoxidation of 2-benzylidene malononitrile in the presence of cumyl hydroperoxide (CHP) was also reported in 2015 [28]. However, these methods involve some disadvantages such as long reaction times and use of organic solvents. Table 1 represents the comparative methods for the epoxidation of some alkene derivatives.

Materials
The FeCl 2 ·4H 2 O, FeCl 3 ·6H 2 O, benzaldehyd derivatives, malononitrile, ethyl cyanoacrylate, lithium hypochlorite, solvents and the other reagents were purchased from Merck, Aldrich or Fluka, and were used as received without further purification.

Instruments
The FTIR spectra were run on a Bruker Equinox (model 55, Germany), and the NMR spectra measured by a Bruker AC 400 Avance DPX spectrophotometer, Germany at 500 MHz for 1 H and at 125 MHz for 13 C NMR in CDCl 3 solution. Transmission electron microscopy (TEM) was performed using a Zeiss EM10C with accelerating voltage of 100 kV. EDS analysis was carried out using ZEISS (model SIGMA VP-500, Germany) with Oxford Instrument detector (England). XRD patterns were recorded on a PAN alytical (model X'Pert PRO) X-ray diffractometer. A vibrating-sample magnetometer (VSM) (model LBKFB, Meghnatis Daghigh Kavir Co, Iran) was used for hysteresis loop determinations. The melting points were determined by a Buchi melting point B-540 B. V.CHI apparatus.

Synthesis of Fe 3 O 4 @Ag-CTAB MNPs
In the first step, the prepared Fe 3 O 4 nanoparticles [29] (0.2 g) were dispersed in deionized water (5 mL) and then AgNO 3 solution (5 mL of 0.01 M) was added under ultrasound treatment. Thereafter, NaBH 4 solution (30 mL of 0.02 M) was added dropwise. The reaction mixture was stirred vigorously on a magnetic plate for 20 min. The obtained product was separated with external magnet and then washed

Typical procedure for the preparation of 3-Phenyloxirane-2,2-dicarbonitrile
Benzaldehyde (1 mmol), malononitrile (1 mmol), lithium hypochlorite (1 mmol), Fe 3 O 4 @Ag-CTAB (0.05 g) and 10 mL H 2 O were introduced into a test tube equipped with a mechanical stirrer. The mixture constantly stirred at room temperature for 2 h. After completion of the reaction, the catalyst was separated using an external magnet. The solvent of the remained mixture was evaporated and a pure product was obtained by recrystallization from ethanol with yield of 90%. Under optimized conditions, 3-aryloxirane-2-carbonitriles (2a-o) were synthesized. The results are summarized in Table 2.
Reaction of benzaldehyde, malononitrile and oxidant was selected as a model reaction. The model reaction behaviour was studied by adjusting various parameters such as presence of catalyst, type of catalyst, amount of the catalyst and type of oxidant. The results are summarized in Table 3. The model reaction was attempted in the presence of LiOCl without any catalyst and only a 20% yield was obtained ( Table 3, entry 1). To find the true role of metal catalyst in this catalysis, several catalysts such as Fe 3 O 4 , CTAB, Fe 3 O 4 @Ag and Fe 3 O 4 @Ag-CTAB were employed. However, it was noticed that the highest yield was achieved in the presence of Fe 3 O 4 @Ag-CTAB as catalyst (Table 3,    NaOCl, NaIO 3 and NaIO 4 were also used. No reaction occurred with these oxidants under optimized conditions and only a 65% yield was obtained by NaOCl (Table 3,  entries 10-12). Finally, we found that this reaction optimally proceeded with LiOCl in the presence of Fe 3 O 4 @Ag-CTAB as catalyst and completed within 2 h to afford the corresponding product in 90% yields.

Results and discussion
FT-IR study Figure 1 shows the FT-IR spectra of Fe 3 O 4 (Fig. 1a), CTAB (Fig. 1b) and Fe 3 O 4 @Ag-CTAB (Fig. 1c) In FT-IR spectra of Fe 3 O 4 nanoparticles, Peak at 599.24 cm −1 is attributed to the Fe-O band vibration. The broad band around 3000-3500 cm −1 displays the surface hydroxy stretching vibrations (Fig. 1a). In FT-IR spectra of CTAB, The peak at 1486.97 cm −1 is attributed to CN bond and the peaks at 2849.71 and 2918.15 cm −1 are attributed to two different CH bands vibration of CTAB (Fig. 1b). In FT-IR spectra of Fe 3 O 4 @Ag-CTAB, observed band at 586.03 cm −1 corresponds to stretching vibration of Fe-O bond. The peaks at 2850.49 and 2919.02 cm −1 are attributed to CH bands vibration of CTAB (Fig. 1c). Any specific signals are not observed for Ag in the FT-IR spectra because silver is a metal element.

Particle size analysis
Analyzing the size of prepared nanoparticles carried out according to TEM technique, in which the dimensions of them were achieved about 30-90 nm (Fig. 2). Also, the results obtained confirmed that the modification of Fe 3 O 4 @Ag nanoparticles using CTAB as a coating material effectively prevented the aggregation of magnetite nanocomposites and led to a better dispersion of the particles in solution.   Figure 4 shows the results of energy-dispersive X-ray spectroscopy EDS analysis, which confirms the presence of C, N, Fe and Ag species. In EDS spectrum, the intensity of Ag peak is higher than that of Fe, which indicates that Fe particles  Figure 4 also shows the EDX elements mapping images of Fe 3 O 4 @Ag-CTAB, including the contents of C, N, Ag and Fe elements. As shown in Fig. 4, the contents of C (in yellow), N (in blue), Ag (in brown) and Fe (in red) elements for Fe 3 O 4 @Ag-CTAB were 48.23, 40.71, 6.64 and 4.43% respectively. These results confirm the existence of C and N elements in Fe 3 O 4 @ Ag-CTAB.

Mechanism
The positively charged ammonium moiety of CTAB adsorbs on the surface of Fe 3 O 4 @Ag NPs by a strong electrostatic interaction, while the hydrophobic chains point outwards into a solution phase, leading to the formation of hydrophobic hemimicelle. So, organic reactants can be preconcentrated on the surface of Fe 3 O 4 @Ag NPs. First, Fe 3 O 4 @Ag-CTAB promotes the Knoevenagel condensation of the benzaldehyde with malononitrile. It should be noted that some knoevenagel reaction could occur in water without the addition of any base [30,31]. We, therefore, proposed a mechanism involving the ionization of malononitrile

Conclusion
In conclusion, we have developed a convenient procedure for the epoxidation of vinyl cyanide derivatives by LiOCl in the presence of Fe 3 O 4 @Ag-CTAB as a new environmental friendly and reusable catalyst in aqueous medium with good to high yields (70-99%). It seems that CTAB provides a hydrophobic medium to accumulate organic reactants and the Fe 3 O 4 @Ag, as a Lewis acid, catalyzes the reactions.
The remarkable advantages of this method are avoidance of organic solvents, short reaction times, high yields, and easily recoverable heterogeneous magnetic catalyst.