Supported l-tryptophan on Fe3O4@SiO2 as an efficient and magnetically separable catalyst for one-pot construction of spiro[indene-2,2′-naphthalene]-4′-carbonitrile derivatives

In this work, l-tryptophan functionalized silica-coated magnetic nanoparticles were readily prepared and evaluated as a recyclable magnetic nanocatalyst for the synthesis of spiro[indene-2,2′-naphthalene]-4′-carbonitrile derivatives through the one-pot four-component reaction of malononitrile, cyclohexanone, aromatic aldehydes, and 1,3-indandione. This novel magnetic nanocatalyst was confirmed to be effective and provide products in moderate to excellent yields under reflux conditions. The structure of obtained nanoparticles was characterized using FT-IR, XRD, VSM, EDX, elemental mapping, FE-SEM, and TGA. This synthetic protocol provides several benefits such as excellent yields in short reaction times (64–91%), saving costs, reusability of the catalyst using an external magnet (seven runs), and low catalyst loading.


Introduction
Multicomponent reactions (MCRs) and associated one-pot synthesis such as domino, tandem and cascade reactions have always been one of the most signicant scopes in the development of new methodologies in organic synthesis and catalysis due to their great applications, particularly in pharmaceutical studies. [1][2][3][4] MCRs oen produce highly complex molecules from simple precursors, thus, avoiding complicated purication steps and saving solvents, reagents and times. [5][6][7] Spiro compounds show a broad range of effective performance in pharmacology. They have a large number of pharmacological and biological properties such as anti-cancer, 8 antimicrobial, 9 anti-diabetic, 10 antitumor, 11 and anti-hypertensive 12 activities due to their fascinating structures (Scheme 1). Among the spiro family members, spirocarbocycles are imperative scaffolds known for their extensive pharmacological signicance that are found in numerous products and bioactive molecules. It is worth pointing out that, MCRs are one of the most potent techniques for the production of spiro compounds. 13,14 Furthermore, vinylogous Michael addition was reported as a key step in the preparation of many spiro compounds. [15][16][17] In recent years, a few synthesis methods for the synthesis of spiro[indene-2,2 0 -naphthalene]-4 0 -carbonitriles have been reported in the literature. L-Proline, 18 ethylene glycol 19 and quaternary ammonium surfactant [C [Br] 20 have been utilized in these methods. Nevertheless, there are drawbacks with these reported strategies, for instance, prolonged reaction time, catalyst toxicity, and more importantly the catalyst reusability is difficult. However, further studies are still essential for the efficient, environmental and economical multicomponent methodology for the synthesis of these spiro compounds, even though each of the known methods for the synthesis of spiro [indene-2,2 0 -naphthalene]-4 0 -carbonitrile compounds has its competency.
In general, magnetic nanoparticles have been applied for a range of biomedical applications, particularly in the areas of medical imaging, diagnostics, and treatment. [21][22][23][24][25] Additionally, they have been commonly employed in many organic reactions because of their easy removal and convenient separation using an external magnetic eld. [26][27][28][29] Among all magnetic nanoparticle types, Fe 3 O 4 nanoparticles (NPs) have received more attention owing to their exclusive features such as superparamagnetic nature, non-toxic, biocompatibility, and facile synthesis process from accessible precursors (e.g., ferrous and ferric salts). 30 Also, Fe 3 O 4 nanoparticles possess plenty of hydroxyl groups (OH) on their surfaces, thus, they are normally hydrophilic. However, the naked Fe 3 O 4 is not stable since it can be easily oxidized into the other substances in the air, therefore, coating materials or modication is critical in order to prevent them from oxidation. For this purpose, silica (SiO 2 ) has been widely considered as one of the best protection shells since it is inexpensive, has a high specic surface area, and has high resistance under catalytic conditions. It is noteworthy that the functional groups containing oxygen, which are distributed on the surface of Fe 3 -O 4 @SiO 2 can act as active sites.
L-Tryptophan is known as a chiral molecule. Environmentalfriendliness, less-expensive, and easy availability are some of the benets of L-tryptophan. Moreover, this a-amino acid can participate in many organic reactions as a catalyst because it possesses active site groups (amino group and carboxylic group). In other words, L-tryptophan can be broadly used in base-catalyzed organic reactions. 31 However, tough recovery and prolonged reaction times are prominent drawbacks of this catalyst. Several strategies can be used to overcome the abovementioned problems such as co-catalysts, varying types of supports such as ionic liquids, 32 polymers, 33 and silica. 34 In this study, we introduce a straightforward approach for the fabrication of Fe 3 O 4 @SiO 2 -L-tryptophan as a robust inorganic-organic hybrid catalyst. This heterogeneous, environmentally benign, and highly reusable catalyst represents high catalytic activity to synthesize spiro[indene-2,2 0 -naphthalene]-4 0 -carbonitrile derivatives via a one-pot, four-component condensation reaction of malononitrile (1 mmol), cyclohexanone (1 mmol), various aromatic aldehydes (1 mmol) and 1,3indandione (1 mmol) under reux conditions in EtOH.

Preparation and characterization of Fe 3 O 4 @SiO 2 -Ltryptophan catalyst
Magnetic nanoparticles (MNPs) were prepared using Fe 2+ and Fe 3+ ions by the co-precipitation method. Then, Fe 3 O 4 NPs coated by the silica network provided reaction sites for further functionalization and thermal stability. In the end, Fe 3 O 4 @SiO 2 was reacted with L-tryptophan in the presence of H 2 SO 4 in order to produce Fe 3 O 4 @SiO 2 -L-tryptophan nanoparticles (Scheme 2). Of note, the L-tryptophan layer is linked on the surface of Fe 3 -O 4 @SiO 2 by chemical adsorption. 35 Fig . 1 illustrates FT-IR spectra of Fe 3 O 4 , Fe 3 O 4 @SiO 2 , Fe 3 -O 4 @SiO 2 -L-tryptophan, and L-tryptophan. It can be observed that the band in the region of 578 cm À1 is attributed to vibrations of the Fe-O bond and bands at 3400 and 1626 cm À1 are ascribed to O-H stretching and bending vibrations, respectively (Fig. 1a). In the Fe 3 O 4 @SiO 2 spectrum, the strong peak at 1089 cm À1 could be assigned to asymmetric stretching vibrations of Si-O-Si bonds. This peak veries that the silica layer was coated on the surface of Fe 3 O 4 very well (Fig. 1b). For the Ltryptophan functional group, the absorption band at 1162 cm À1 belongs to the C-N stretching vibration and the peak at 3409 cm À1 corresponds to O-H stretching vibrations (Fig. 1c).
The powder X-ray diffraction (XRD) patterns of Fe 3 O 4 and Fe 3 O 4 @SiO 2 -L-tryptophan are displayed in Fig. 2. XRD data clearly demonstrated 6 diffraction angles (2q) at 30.2 , 35.5 , 43.2 , 53.6 , 57.1 , and 62.8 , following standard Fe 3 O 4 XRD patterns (JCPDS No. 75-0449) (Fig. 2a). From Fig. 2b, it can be clearly observed at about 2q ¼ 24 , which is assigned to the amorphous organic group. The diameter (D) of the Fe 3 O 4 @SiO 2 -L-tryptophan NPs was calculated using the Scherrer equation (D ¼ Kl/(b cos q)), where q is the Bragg angle of the maximum of the diffraction peak and b is the line broadening at half the maximum, while l is the X-ray wavelength (0.154 nm for CuKa). K is a dimensionless shape factor, which usually takes a value of about 0.9.
Magnetic properties of Fe 3 O 4 , Fe 3 O 4 @SiO 2 , and Fe 3 O 4 @-SiO 2 -L-tryptophan nanoparticles were measured with the help of a vibrating sample magnetometer (VSM) (Fig. 3). According to these results, all three samples are superparamagnetic at 60.7 emu g À1 , which belongs to Fe 3 O 4 NPs showing the highest value of saturation magnetization (Ms). Additionally, the magnitude of saturation magnetization values for Fe 3 O 4 @-SiO 2 and Fe 3 O 4 @SiO 2 -L-tryptophan are 48.23 emu g À1 and 29.64 emu g À1 , respectively. These results illustrate that the magnetization property decreases aer coating and functionalization.
The presenting elements can be clearly seen in the structure of Fe 3 O 4 @SiO 2 -L-tryptophan using energy-dispersive X-ray spectroscopy (EDX) (Fig. 4). Besides, all elements are well distributed throughout the Fe 3 O 4 @SiO 2 -L-tryptophan, which is revealed by elemental mapping images (Fig. 5).
In order to study the morphology, uniformity and size of the catalyst, FE-SEM was used. As it is obvious from Fig. 6, SEM images of the sample indicate that the prepared nanoparticles have a uniform size, spherical shape, and disordered mesosphere. The average size of Fe 3 O 4 @SiO 2 -L-tryptophan MNPs was calculated to be about 37 nm.
The thermal behavior of Fe 3 O 4 @SiO 2 -L-tryptophan nanoparticles was estimated by TGA analysis (Fig. 7). Results depict appropriate thermal stability with no signicant decline in the weight. The weight loss at low temperatures (<$100 C) can be either due to the removal of surface -OH groups or physically adsorbed solvent molecules trapped in the SiO 2 layer. According to the curve, the observed weight loss of about 11.1 (%) above 250 C can be because of the decomposition of Ltryptophan group, which is attached to the silica layer. Therefore, the TGA curve reveals that Fe 3 O 4 @SiO 2 -L-tryptophan NPs is stable up to 250 C and is certainly t for the synthesis of spiro[indene-2,2 0 -naphthalene]-4 0 -carbonitrile compounds.
In order to calculate the extent of the functionalization per SiO 2 group, weight loss values were employed together with the molecular weight of diverse moieties, and eqn (1) was used for the calculation. 36 In this equation, X stands for the number of sample SiO 2 groups per each covalent functional group (L-tryptophan), R (%) is the residual mass at 800 C in the TGA plot, L (%) is the weight loss in the range of 100-800 C, and M w is the molecular weight of the desorbed functional groups. Taking into account that the covalently L-tryptophan measurements   depicted one functionality every $25 SiO 2 groups in the Fe 3 -O 4 @SiO 2 -L-tryptophan.
2.2 Comparison of the efficiency of solvents, amount of Fe 3 O 4 @SiO 2 -L-tryptophan NPs, and other catalysts on the synthesis of spiro[indene-2,2 0 -naphthalene]-4 0 -carbonitriles The tandem one-pot reaction of malononitrile, cyclohexanone, 4-nitro benzaldehyde, and 1,3-indandione was chiey selected as a model reaction in order to observe the catalytic activity of the prepared Fe 3 O 4 @SiO 2 -L-tryptophan. To optimize the reaction, we utilized different conditions. At rst, the effect of the various electron-pair donor (EPD) and electron-pair acceptor (EPA) solvents were studied to assess the best reaction conditions. According to the results displayed in Table 1, EPD and protic solvents (acting as Lewis bases) showed better performance in the reaction. Based on the reaction mechanism, protic solvents surrounded carboanion and carbocations, hence, the activation energy was decreased. 37 Satisfactory result were observed in terms of the yield and reaction time in the EtOH solvent. Besides, when the reaction was carried out using Fe 3 -O 4 @SiO 2 -L-tryptophan NPs as the catalyst in EtOH, the product was obtained in excellent yields in a very short time (Table 1,  entry 7). As a remarkable point, when we utilized a strong base catalyst, the obtained product had a low yield (Table 1, entry 9). In addition, in the presence of the acid catalyst, no product was observed in the period of 180 min (Table 1, entry 10). According to the data, conducting the model reaction without any catalyst gave only 21% aer 180 min ( Table 2, entry 1). We also investigated the catalytic activity of Fe 3 O 4 @SiO 2 , L-tryptophan and Fe 3 O 4 @SiO 2 -L-tryptophan. Based on our empirical experiments, L-tryptophan, which was modied on the surface of Fe 3 O 4 @SiO 2 indicated the best catalytic performance in the reaction. Moreover, it was discovered that in the presence of Fe 3 O 4 @SiO 2 -Ltryptophan at 20 mg, the reaction rate and yield were considerably increased (Table 2, entry 5).
To investigate the scope of this protocol, various aromatic aldehydes (4), possessing both electron-donating and withdrawing groups were reacted with 1,3-indandione, malononitrile, and cyclohexanone ( Table 3). The results were ascertained that aromatic aldehydes with electron-withdrawing groups reacted faster compared to those with electron-donating groups. Furthermore, the electron-withdrawing groups at the para position of benzaldehyde (5a, 5d, and 5j) resulted in excellent yields (Table 3).

Reusability of the Fe 3 O 4 @SiO 2 -L-tryptophan MNPs
The reusability of the catalyst is well known as key characteristic properties. Herein, we have investigated the retrievability of Fe 3 O 4 @SiO 2 -L-tryptophan MNPs using the model reaction of malononitrile, cyclohexanone, 4-nitro benzaldehyde, and 1,3indandione. As it can be viewed from Fig. 8, the catalytic activity of Fe 3 O 4 @SiO 2 -L-tryptophan declined from 98% in the fresh run to 80% aer the completion of seven runs.       The prepared nanocatalyst was conveniently separated at the end of the reaction using a strong magnet, washed with EtOH and water, then dried at 60 C, and reused seven times without excess purication.
The heterogeneous process was also checked for the model reaction. For this purpose, the catalyst was separated from the reaction mixture aer 10 minutes and the reaction yield was estimated to be around 64%. Then, the reaction continued without the catalyst for another 20 minutes. The obtained product had no signicant increase in the yield ($67%). According to the above-mentioned results, the presence of the catalyst until at the end of the reaction is necessary to reach the best yield. The amount of the base in the catalyst before and aer the cyclic test was quantitatively evaluated through ion-exchange pH analysis. 39 Based on the acid-base titration measurement, the number of basic sites of the catalyst was approximately 1.250 mmol g À1 aer being recycled for seven cycles, while the amount of basic sites in the fresh Fe 3 O 4 @SiO 2 -L-tryptophan nanocatalyst were estimated at 1.406 mmol g À1 . Further, the chemical structure of the recovered catalyst aer 7 cycles was conrmed using the FT-IR spectrum. As can be observed in Fig. 9, there is no considerable difference between FT-IR spectra of the fresh and reused magnetic nanocatalyst.

General
All solvents and reagents were purchased commercially and were utilized without any further purication. Fourier transform infrared (FT-IR) spectroscopy was performed using a Nicolet Magna-400 spectrometer (KBr pellets). 1 H NMR was recorded in DMSO-d 6 solvent using a Bruker DRX-400 spectrometer with tetramethylsilane (TMS) as the internal reference. XRD patterns were recorded using a Philips diffractometer and monochromatized Cu Ka radiation (k ¼ 1.5406Å). The morphological study of the nanoparticles was conducted using eld emission scanning electron microscopy (FE-SEM) (model MIRA3). The electron dispersive X-ray (EDX) analysis of the catalyst was performed using an Oxford instrument company. Thermogravimetric analysis (TGA) was performed on a Mettler TA4000 system TG-50 at a heating rate of 10 K min À1 under N 2 atmosphere. The magnetic properties of samples were measured using a magnetometer (VSM, PPMS-9T) at 300 K in Iran (Kashan University). Melting points were obtained using a Yanagimoto micromelting point apparatus and are uncorrected. The purity determination of the substrates and reaction monitoring were accomplished using TLC on silica-gel polygram SILG/UV 254 plates (from Merck Company).

General procedure for the synthesis of Fe 3 O 4 nanoparticles
Fe 3 O 4 nanoparticles were prepared using the chemical coprecipitation method according to our previous work with some modications. 38,39 Briey, FeCl 3 $6H 2 O (16 mmol, 4.32 g) and FeCl 2 $4H 2 O (8 mmol, 1.6 g) were dissolved in 100 mL of deionized water under N 2 protection. Then, the reaction temperature was increased to 80 C and 25 mL NH 4 OH (25%) was added to the solution dropwise (pH ¼ 12). Aer adding NH 4 OH to the solution, the color of the solution turned black. The reaction was stirred at 80 C for 1 h under reux conditions. The magnetic nanoparticles were separated from the solution using an external magnet and washed several times with deionized water and ethanol and then dried at 70 C for 12 h in an oven.

Preparation of Fe 3 O 4 @SiO 2
Fe 3 O 4 @SiO 2 nanoparticles were obtained according to the reported method in the literature with some modications. 40,41 Magnetic nanoparticles (1 g) were dispersed in a solution of ethanol and water (40 : 10 mL) in an ultrasonic bath for 30 min. The pH was adjusted to 10 with an ammonia solution and 0.5 mL tetraethylorthosilicate (TEOS), which was added dropwise into the mixture over a period of 1 h. The resulting solution was stirred at 35-40 C for 12 h. Fe 3 O 4 @SiO 2 MNPs were separated from the solution by using an external magnet and washed with ethanol (3 Â 15 mL) and dried at room temperature.

Preparation of Fe 3 O 4 @SiO 2 @L-tryptophan
Firstly, Fe 3 O 4 @SiO 2 MNPs (1 g) were dispersed in dry ethanol (10 mL) using an ultrasonic bath for 30 min. Subsequently, H 2 SO 4 (0.5 mL) and L-tryptophan (1.5 g) were added to the solution and heated under reux conditions at 90 C for 12 h. The resulting MNPs were collected by magnetic separation followed by washing with ethanol and water several times before being dried in an oven at 60 C to give Fe 3 O 4 @SiO 2 @L-tryptophan as a light brown powder.
3.5 A common procedure for the synthesis of (5a-l) Typically, malononitrile (1) (1 mmol) and cyclohexanone (2)  reaction was continued for the right time to achieve the desired product (Scheme 4). Upon completion of the time, the reaction was monitored by TLC (n-hexane/ ethyl acetate: 6 : 4) the crude precipitate was ltered off and puried by recrystallization from ethanol to obtain the pure product. All products were characterized by melting point (m.p.), FT-IR, 1 H NMR, 13 C NMR and elemental microanalysis.

Conclusion
In conclusion, Fe 3 O 4 @SiO 2 -L-tryptophan magnetic nanocatalyst as green, magnetically recyclable, and environmentally friendly, was prepared and fully characterized. We applied this catalyst for the synthesis of spiro[indene-2,2 0 -naphthalene]-4 0 -carbonitrile derivatives via a four-component reaction of malononitrile, cyclohexanone, aromatic aldehydes and 1,3-indandione. Excellent yields in short reaction time, facile, so reaction conditions, and high atom economy are some of the advantages of this procedure.

Conflicts of interest
There are no conicts to declare.