One-pot synthesis of naphtho[1,2-e][1,3]oxazines in the presence of FNAOSiPAMP*/CuII as an almond shell based nanocatalyst

In the present research work, a novel catalyst based on natural material, namely, Fe3O4@nano-almondshell@OSi(CH2)3/NHCH2pyridine/CuII abbreviated (FNAOSiPAMP/CuII) was designed and prepared. The properties of the catalyst was identified by Fourier-transform infrared spectroscopy (FT-IR), Thermogravimetry ananlysis (TG), X-ray diffraction (XRD), Energy-dispersive X-ray spectroscopy (EDS), Field emission scanning electron microscopy (FESEM), Transmission electron microscopy (TEM), and Mapping. Furthermore, the evaluation of catalytic activity was done in the course of naphtho-1,3-oxazines synthesis. Solvent-free conditions, simplicity of operation, easy work-up and use of an eco-friendly catalyst are some of advantages of this protocol.


Experimental
Synthesis of nano-almond shell. Firstly, 5 g of almond shell was well powdered and reacted with 80 ml of 17.5% NaOH solution under reflux conditions for 24 h. Then, almond shell was filtered and washed with distilled water. It was then bleached with 20 ml of sodium hypochlorite solution and 60 ml of distilled water under reflux conditions for 2 h. Subsequently, the almond shell was filtered and washed well with distilled water. The obtained almond shell powder was added to 80 ml of the 35% sulfuric acid aqueous solution and heated under reflux condition for 6-7 h. The resulting suspension was diluted with water and centrifuged many times to obtain the resulting nano-almond shell.

Synthesis of naphtho[1,2-e][1,3] oxazines.
In an electrical mortar-heater vessel, amine 1° (1.0 mmol), formaldehyde 37% (2.0 mmol), β-naphthol (1.0 mmol) and FNAOSiPAMP/Cu II (0.04 g) were charged and ground at room temperature for determined time. Finally, the obtained mixture was poured in hot ethanol (3 mL) and the catalyst was separated by using an external magnet. Then, cold water was added to residue and the obtained solid product was filtered, washed with water and dried at room temperature.

Test of hot filtration and metal leaching.
To study the leaching of (FNAOSiPAMP/Cu II ), a hot filtration test was done. For hot filtration test, we have run a model reaction in the presence of catalyst. After ten minutes, the catalyst was removed from reaction mixture by an external magnet. The remained mixture was stirred for 15 min. The progress of reaction was not of observed and shown no leaching of catalyst in this protocol.  Fig. S17-S19). (Table 2, Fig. S31, S32).

Results and discussion
In this research, we have prepared FNAOSiPAMP/Cu II catalyst via a simple procedure that shown in Fig. 1     www.nature.com/scientificreports/ A comparison between FTIR of fresh and used catalyst shows no any leaching of catalyst under reaction condition (Fig. 3.). Figure 4 shows the XRD patterns of Fe 3 O 4 and FNAOSiPAMP/Cu II , in 10°-80°. In XRD spectrum of FNAOSiPAMP/Cu II , in addition of Fe 3 O 4 signals (2θ = 30°, 35°, 43°, 53°, 57°and 63°), 2θ = 21° shown the existence of almondshell.

XRD analysis.
FESEM and TEM imaging. Figure 5 represents the result of field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) of FNAOSiPAMP/Cu II to investigate its particle size and surface morphology. This images indicates that FNAOSiPAMP/Cu II nanoparticles have average size below 30 nm. The presence of spherical particles with nano dimensions in the catalyst increases the contact surface between the catalyst and the raw materials and increases the speed of the reaction. VSM analysis. Vibrating sample magnetometer (VSM) was used for the study magnetic property of catalyst at 300 °K (Fig. 6). This experiment approves the superparamagnetic property of catalyst which can be efficiently separated from reaction medium with an external magnet.
EDX and ICP analysis. Energy-dispersive X-ray spectroscopy EDS (EDX) analysis was applied for the identification of elements in FNAOSiPAMP/Cu II (Fig. 7). The EDX data confirmed the existence of C, N, O, Fe, ele-  www.nature.com/scientificreports/ ments in the catalyst. For approve no any leaching of catalyst under reaction condition, the EDX of fresh and used catalyst were shown in Fig. 7.
To finding the weight percentages of Cu and Fe in catalyst structure we have used ICP-MS. According to obtained data, the weight percentages of Cu and Fe are 3% and 15%, respectively.
In conclusion, the best condition of this reaction was obtained using 0.04 g of FNAOSiPAMP/Cu II , solvent-free and room temperature conditions (Table 1, entry 10). According to above modified condition, we have synthesized various derivatives of naphtho[1,2-e] [1,3]oxazine using different amines, β-naphthol and formaldehyde with good to excellent yields in short reaction times ( Table 2).
According to ICP data, the amount of Cu II in catalyst is 3%. We have used 0.04 g of catalyst for 1 mmol of substrate (benzaldehyde). Thus 0.04 g of catalyst is containing of 1.2 × 10 -3 g of Cu II and equal to 1.88 × 10 -5 mol of Cu II . Thus, TON and TOF of catalyst are equal to 52.6 and 2.10638 min −1 , respectively.
The reusability of the magnetic nano-catalyst was examined in model reaction for four times. The result showed no considerable decrease of catalytic activity (Fig. 10). Meanwhile, no decomposition of catalyst in the reaction medium was confirmed by study of the reused catalyst FT-IR spectrum. Figure 11 shows the our proposed mechanism for synthesis of naphtho[1,2-e] [1,3]oxazines in the presence of FNAOSiPAMP/Cu II . Cu II activates the carbonyl group in formaldehyde and then Mannich type condensation of the amine (1) and the formaldehyde (2) gives imine (3). In the next step, the β-naphthol attackes to the imine (3) to form intermediate (4) which condenses with the second molecule of formaldehyde to form intermediate (5). Then, by an intramolecular cyclization, the naphtho[1,2-e] [1,3]oxazine is synthesized.

Conclusions
In summary, this work introduces that FNAOSiPAMP/Cu II as an effective novel catalyst promote a versatile, simple and environmentally benign protocol for the synthesis of naphtho[1,2-e] [1,3]oxazines. This novel ecofriendly procedure for synthesis of naphtho[1,2-e] [1,3]oxazine has many advantages such as simplicity, easy workup, reusability of catalyst, high yields and solvent-free reaction conditions of which turn it into a suitable alternative for the naphtho[1,2-e][1,3]oxazines synthesis.