Fe3O4@Nano-Walnut Shell/BIII as a New Natural Based Catalyst for Synthesis of Tetrahydrobenzo[a]Xanthene-11-One Derivatives

Abstract In this work, Fe3O4@nano-walnut shell/BIII (FNWSB), as a reusable and new catalyst was prepared and then characterized by FT-IR, XRD, FESEM, EDS-MAP, and TGA. This catalyst was used for synthesis of tetrahydrobenzo[a]xanthen-11-ones via three-component, one-pot condensation reaction of dimedone, β-naphthol, and aromatic aldehydes. An uncomplicated work-up, high yields of product, short reaction times, and use of low-cost catalyst are considerable advantages of this procedure. Graphical Abstract


General
FT-IR spectra were run on a Bruker, Equinox 55 spectrometer.A Bruker (DRX-400 Avance) NMR was used to record the 1 H-NMR spectra.Melting points were determined by Buchi melting point B-540 B.V.CHI apparatus and were uncorrected.X-ray diffraction (XRD) pattern was obtained by a BRUKER, D8, and AVANCE.Field Emission Scanning Electron Microscopy (FESEM) was obtained on a TESCAN, Mira III.VSM measurements were performed by using a vibrating sample magnetometer (VSM; Meghnatis Daghigh Kavir Co. Kashan Kavir, Iran).The EDX-MAP analysis were done with FESEM instrument from TESCAN company (Czech Republic), model MIRA II with SAMX detector.Thermal gravimetric analysis (TGA) was conducted using the 'STA 503' from BAHR company (Luhden, Germany).Thin-layer chromatography (TLC) was done on silicagel 60 F 254 plates (Merck).

General procedure for the preparation of THBXO derivatives
The condensation of b-naphthol (1 mmol), dimedone (1.2 mmol), and aldehyde (1 mmol) was occurred in the presence of FNWSB (0.02 g) at 80 C under solvent-free conditions.The progress of reaction was monitored by TLC (ethyl acetate: n-hexane, 1:4).After completion of the reaction, the reaction mixture was cooled to room temperature, dissolved in dichloromethane, and the mixture stirred for 10 min.The magnetic catalyst was removed from the suspended solution by an external magnet.The dichloromethane solvent of obtained mixture was evaporated and the remained crude product was purified by recrystallization in ethanol.

Preparation of nano-walnut shell
First, the walnut shell was boiled in water for 15 min, filtered and dried.Then treated with a 17.5 w/v NaOH solution at 100 C for 12 h under mechanical stirring.The walnut shell was then filtered and washed with distilled water to completely remove the alkali.Then, It was bleached with 100 mL of 1:1 aqueous dilution of 3.5% w/v sodium hypochlorite at 80C for 3 h under mechanical stirring.The resulting alpha cellulose was hydrolysis partially using 45% sulfuric acid aqueous solution with a walnut shell to acid weight ratio of 1-10 at 45 C.After 1 h, the resulting suspension was diluted with water, 5-fold, to stop the hydrolysis reaction.The suspension was centrifuged at 4000 rpm to separate the nano-walnut shell from acid solution.Wash with water and centrifuge four to five times to remove residual free acid.The yield of obtained nano-walnut shell is 70%.
Preparation of nano-Fe 3 O 4 @walnut shell 1 G of nano-walnut shell was mixed in 100 mL of 0.05 M acetic acid solution, then, FeCl 3 Á6H 2 O (3.51 g, 0.013 mol) and FeCl 2 Á4H 2 O (1.29 g, 0.0065 mol) were added to mixture.The mixture was stirred for 4 h at 80 C. As a result, 6.5 mL of 25% NH 4 OH was added dropwise to the reaction mixture with permanent stirring.After 30 min, the mixture was cooled to room temperature and then by using an external magnet, magnetic nano walnut shell nanoparticles were separated.Then magnetic precipitate first was washed with distilled water and then ethanol, and finally dried at 80 C for 4 h.The final weight is obtained nano-Fe 3 O 4 @walnut shell is 1.6 g.

Preparation of FNWSB
In a 50 mL flask containing 10 mL of dichloromethane, nano-Fe 3 O 4 @walnut shell (0.5 g) was added with stirring.Then, 7.5 mL of BF 3 .OEt 2 was added.A dark brown mixture was obtained immediately that was stirred at room temperature.After 6 h, FNWSB was removed from solution by an external magnet.The catalyst washed with dichloromethane, two times, and dried at an oven at 80 C.

Catalyst leaching experiment
A mixture of b-naphthol (1 mmol), dimedone (1.2 mmol), 4-nitrobenzaldehyde (1 mmol), and FNWSB (0.02 g) was stirred at 80 C under solvent-free conditions.After 10 min, the progress of reaction was 45% and the catalyst was removed from reaction mixture by external magnet.The remained catalyst-free reaction mixture was stirred for 15 min at 80 C. Any addition of progress and leaching of B was not observed in reaction.

Results and discussion
The sequential step for the preparation of FNWSB has been shown in Scheme 1.At first, nanowalnut shell was prepared from walnut shell using the previously reported method for preparation of nano-cellulose.Then, magnetic core-shell nanoparticles, nano-Fe 3 O 4 @walnut shell, were obtained simply through in situ co-precipitation of ferric and ferrous ions with ammonium hydroxide in an aqueous solution containing nano-walnut shell.At the end, the nano-Fe 3 O 4 @walnut shell served as a magnetic support for the immobilization of B III by simple grinding with BF 3 at room temperature (Scheme 1).The characterization of FNWSB was performed by Fourier transform infrared (FT-IR) spectroscopy, XRD, VSM, FESEM, energy-dispersive X-ray spectroscopy (EDS), and thermo-gravimetric analysis (TGA).
Figure 1 shows the FT-IR spectra of nano-walnut shell, nano-Fe 3 O 4 @walnut shell, and nano-FNWSB.The FT-IR spectrum of nano-walnut shell (Figure 1(a)), has shown a broad band at 3441 cm À1 which corresponds to the stretching vibrations of OH groups.The absorption bands at 1045-1143 cm À1 display the stretching vibrations of the C-O bonds.For nano-Fe 3 O 4 @walnut shell (Figure 1(b)), in addition to the walnut shell absorptions bands, stretching vibrations of Fe/ O groups at 518 cm À1 are appeared which is indicated that the magnetic Fe 3 O 4 nano particles are coated by nano-walnut shell.The FT-IR spectrum of FNWSB (Figure 1(c)) has shown a characteristic absorption band under 1354 cm À1 that may be attributed to B-O band for B bonded to walnut shell.
Figure 2 presents the result of FESEM of FNWSB to investigate its particle size and surface morphology.This image indicates that FNWSB nanoparticles have a quasi-spherical shape with an average size about 40 nm.
The magnetic properties of Fe 3 O 4 and FNWSB were characterized at RT (300 K) by a VSM and their hysteresis curves are presented in Figure 3.According to this image, the zero coercivity and remanence of the hysteresis loops of these magnetic nanoparticles confirm super paramagnetic property of them at room temperature.The amount of specific saturation magnetization (Ms) for Fe 3 O 4 nanoparticles was about 48 emu g À1 , which decreased to 31 emu g À1 after coating  the Fe 3 O 4 with walnut shell and to 8 emu g À1 after the immobilization of B III on the surface of nano-Fe 3 O 4 @walnut shell.Despite this significant decrease, the saturated magnetization of these magnetic nanoparticles is sufficient for magnetic separation.
EDS spectrum of FNWSB (Figure 4) provided the presence of the expected elements in the structure of this catalyst.The elemental compositions of FNWSB were found to be 4.21, 3.92, 37.9, 27.20, and 26.76% for B, C, O, F, and Fe, respectively.Elemental maps show homogenous distribution of elements in the structure of catalyst (Figure 5).TGA-DTA analysis was performed to study thermal stability of the nano-Fe 3 O 4 @walnut shell/ B III in the temperature range of 50-820 C (Figure 6).The first decrease of weight was assigned to the catalyst moisture removal (endothermic effect at 50-100 C, 5% weight loss).Subsequently, the main weight loss step in the temperature ranges 100-200 C (28%) is attributed to the decomposition of walnut shell.The char yield of the catalyst in 820 C is 67%.
The XRD patterns of Fe 3 O 4 , nano-Fe 3 O 4 @walnut shell, and FNWSB, in a range of 10-80 , is shown in Figure 7.In nano-Fe 3 O 4 @walnut shell XRD pattern, Figure 7(b), the peaks in 2h ¼ 30 , 35 , 43 , 54 , 57 , and 64 show the presence of Fe 3 O 4 , and additional broad peak in 2h ¼ 23 shows the presence of walnut shell in it.The comparison XRD patterns of nano-Fe 3 O 4 @walnut shell with FNWSB shows the additional weak diffraction peaks at 2h ¼ 16 , 22 , 27 , 29 , 32 ,  48 , 52 , and 70 in it, which seems B III to be linked on the surface of nano-Fe 3 O 4 @walnut shell (Figure 7(c)).
We have investigated the efficacy of various solvents and also solvent-free conditions on the three-component coupling of 4-nitrobenzaldehyde (1 mmol), b-naphthol (1 mmol), and dimedone (1.2 mmol) as model reaction, using 0.02 g of FNWSB nanoparticles (Table 1).According to obtained data, the solvent-free condition at 80 C was the best choice for the preparation of tetrahydrobenzo[a]xanthen-11-one.
To optimize the amount of catalyst and time of reaction, the model reaction was carried out using different amount of catalyst (Table 2).The best conditions were obtained by using 0.02 g of FNWSB.We have examined the reaction of b-naphthol and dimedone with different aldehydes under optimized conditions (Table 3).
According to the obtained results in Table 3, aromatic aldehydes bearing both electron-donating and electron-withdrawing groups can successfully produce of THBXO in high yields and very   short reaction times.However, aldehydes containing electron-withdrawing groups, for example Cl and NO 2 (Table 3, Entries 1 and 2), have reacted faster than those bearing electron-donating groups, such as OMe and OH (Table 3, Entries 6 and 7).In order to determine the catalytic behavior of Fe 3 O 4 @walnut shell/B III for the synthesis of THBXO, a plausible reaction mechanism for the reaction of b-naphthol, aldehydes and dimedone is shown in Scheme 2. We have proposed that Fe 3 O 4 @walnut shell/B III nanoparticles behave as a Lewis acid and coordinate to the carbonyl groups of dimedone and aldehydes that makes them susceptible to nucleophilic attack of other reactants.Finally, the product 4 was obtained and Fe 3 O 4 @walnut shell/B III nanoparticles isolated for further reactions.
The reusability of the catalyst is one of the most important benefits and makes it useful for commercial applications.To examine the reusability of the catalyst, the condensation reaction of 4-nitrobenzaldehyde, b-naphthol, and dimedone was studied.After completion of reaction, catalyst was separated and washed with CH 2 Cl 2 , dried at 50 C under vacuum for 1 h and reused for the same reaction.This process was performed over five runs and all reactions led to the pure product with high yield (Figure 8).
The comparison between FTIR spectra of the fresh and used catalyst shows the high stability of catalyst in reaction condition (Figure 9).
According to Table 4, the efficacy of the present catalyst was compared with some reported catalysts for the synthesis of THBXOs.
Fe 3 O 4 @walnut shell/B III is comparable with most of reported catalysts in viewpoints of yield and reaction time.

Conclusion
In summary, we have successfully shown catalytic activity of Fe 3 O 4 @walnut shell/B III in the synthesis of THBXOs by multicomponent reaction of dimedone, aldehydes, and b-naphthol under solvent-free condition.The present protocol is simple, eco-friendly and efficient.The products were obtained in short reaction times and excellent yields.Also, Fe 3 O 4 @walnut shell/B III has considerable benefits, such as reusability and stability.Scheme 2. A proposed mechanism for synthesis of THBXO derivatives using Fe 3 O 4 @walnut shell/B III .
b Isolated yields.
b Isolated yields.

Table 3 .
Synthesis of THBXO derivatives using FNWSB at 80 C under solvent-free condition a .

Table 4 .
Synthesis of THBXO in the presence of previously reported catalysts.