The prepared MCM-41-APS-PMDA-SO3H nanomaterial was analyzed by FT-IR, XRD, FESEM, EDX and TGA analysis. The FT-IR spectra of MCM-41 (a), MCM-41-APS (b) MCM-41-APS-PMDA (c) and MCM-41-APS-PMDA-SO3H have been compared in Fig. 1. The nano ordered MCM-41 showed a band in the area of 3443 cm− 1 which is due to the OH of Si–OH and adsorbed water molecules on the surface (Fig. 1a) 61. The band due to Si–O–Si for MCM-41 and all subsequent added layers are observed around 1062–1228 cm− 1. The signals appeared around 1600 cm− 1 and 2883 cm− 1 are attributed to the symmetric bending vibration of NH2 and aliphatic C-H stretching of 3-APTES, respectively (Fig. 1b). These observed characteristic bands and decrease intensity of OH bond of MCM-41 surface confirms that MCM-41 substrate covalently modified by linker. Moreover, the signals appeared at around 2923–3604, 1716 and 1569 cm− 1 are attributed to the stretching vibrations of the pyromellitic dianhydride (Fig. 1c). Also, the special peaks at 1228 cm− 1 are assigned to the S = O stretching vibration of the SO3H group (Fig. 1d).
The morphology and tissue structure of MCM-41and MCM-41-APS-PMDA-SO3H (1) were examined using field emission scanning electron microscopy (FESEM). As shown in Fig. 2, the morphological distinction between the pure MCM-41 (a,b and c images) and MCM-41-APS-PMDA-SO3H (1) (d,e and f images) are a very important proof of the immobilization of the sulfonated pyromellitic dianhydride-aminopropyl silane complex on the external surface of the MCM-41 matrix.
The thermogravimetric analysis (TGA) of the MCM-41-APS-PMDA-SO3H (1) are shown in Fig. 3. The TGA curve of MCM-41-APS-PMDA-SO3H shows three steps of weight loss. In the first step, 10% weight loss between room temperature and 150 °C belongs to absorbed water held in the pores of nanomaterial. While the second weight loss between 150–350 ºC is due to decomposition of the organically modified framework. Also, the third weight loss (17%) between 380–600 ºC was related to decomposition of silanol groups. These results also indicate that pyromellitic dianhydride has been conjuncted onto the surface of MCM-41.
As shown in Fig. 4, the EDX analysis of the MCM-41-APS-PMDA-SO3H (1) verified the presence of Si (11.61 %), C (14.89 %), O (57.63 %), N (12.90 %), and S (2.98 %), respectively.
The powder XRD pattern of MCM-41 (Fig. 5, part a) shows low angle reflections of (d100), (d110) and (d200) at 2θ = 2.77, 4.67 and 5.13, respectively. These plates confirm the formation of mesoporous with hexagonal structure of pore particles and regular, a marker of MCM-41 of type mesoporous. Furthermore, other peaks of the XRD pattern can be belongs to other components of components of nanocatalyst in the wide angle area. The diffraction signals (2θ) at 14°, 19°, 23°, 25°, 26°, 29.28° illustrates the formation of MCM-41-APTES-PMDA-SO3H (Fig. 5, part b).
Figure 6 demonstrates the N2 adsorption/desorption isotherms related to MCM-41, MCM-41-APS-PMDA-SO3H. Isotherm type V was recognizable for MCM-41 with hysteresis. In fact, grafting of PMDA-SO3H groups through (3-aminopropyl) triethoxysilane linker onto the surface of MCM-41 can close them, cause to broad pore distribution and the decrement of the surface. The table shows the parameters such as pore volume as well as average pore diameter in MCM-41 and synthesized nanocomposite
Figure 6. Adsorption/desorption isotherm of the MCM-41-APS-PMDA-SO3H (1).
General procedure for the synthesis of 3,4-dihydropyrimidinones catalyzed by the MCM-41-APS-PMDA-SO3H (1)
To evaluate the effect of the MCM-41-APS-PMDA-SO3H (1) as an introduced catalyst under different reaction conditions, the reaction of ethyl acetoacetate (2, 1 mmol), 4-chlorobenzaldehyde (3a, 1 mmol) and urea (4, 1.2 mmol) was investigated as the model reaction to afford 3,4-dihydropyrimidin-2(1H)-ones. The systematic reaction parameters like solvent, catalyst loading and temperature were optimized. The results are summarized in Table 1. The results of using different protic and aprotic solvents showed that the reaction in solvent-free conditions at 80 ᵒC proceeded with acceptable efficiency (95 % yield) in 35 min, which encouraged us to perform this reaction in solvent-free conditions (Table 1, entry 11). Remarkably, the model reaction was investigated in the presence of 15 mg of catalyst 1 in solvent-free conditions at 80 °C, the favorable product was gained with quantitative efficiency (Entry 11). Subsequently, by further reducing the amount of catalyst, a lower yield of the favorable product was gained under similar conditions even in over a longer time. To our delight, it was found that 15 mg of catalyst 1 in solvent-free conditions at 80 ᵒC as optimizing conditions were sufficient to promote the model reaction efficiently. Indeed, the results strongly confirmed the role of MCM-41-APS-PMDA-SO3H (1) to promote the synthesis of 3,4-dihydropyrimidin-2(1H)-ones. Therefore, optimized conditions have been developed using different aromatic aldehydes to synthesize other derivatives of favorable products 5. The results are given in Table 2.
Noticeably, the desired products were gained in high to excellent yields. Actually, electron-withdrawing groups of the aromatic ring of aldehydes 3 generally react faster compared to the electron-donating groups. These results clearly confirm the suitable catalytic activity of the MCM-41-APS-PMDA-SO3H (1) hybrid nanomaterials to promote the biginelli condensation of a wide range of aldehydes with ethyl acetoacetate and urea. According to above standpoints, the following mechanism can be proposed for the synthesis of 3,4-dihydropyrimidin-2(1H)-ones synthesis condensation (Scheme 3). First, MCM-41-APTES-PMDA-SO3H (1) activates the carbonyl functional group of aromatic aldehydes 3 followed by addition of urea 4 forming intermediate (I). Followed by dehydration, to form the corresponding imminium intermediate (II) from an equivalent of urea 4 and aromatic aldehydes 3. Subsequent addition of the ethyl acetoacetate compound forming intermediate (III), followed by cyclization and dehydration, would afford the 3,4-dihydropyrimidinone 5.
After completing the reaction, the introduced catalyst was separated, washed several times, and dried then reused in the reaction without any reduction inactivity. The catalyst was used for four cycles in reaction with very little activity reduction (approx. 10%). The results are shown in Fig. 6.
To illustrate the catalytic activity of the new MCM-41-APS-PMDA-SO3H organosilica nanomaterials as a heterogenous nanocatalyst, its efficiency has been compared with some of the previously reported catalysts for the preparation of 5a (Table 3). The results illustrate that this study is actually superior to other cases in terms of reaction time, product performance, amount of catalyst, non-toxicity, non-use of intermediate and expensive transition metals, and the obtained efficiency for the catalyst recycling.
Experimental Section
General Information
All chemicals are purchased from Merck or Aldrich. Melting points were specified using an Electrothermal 9100 device and are unmodified. Characterization of new hybrid nanocatalyst 1 was performed by FESEM TESCAN-MIRA3, EDX Numerix DXP-X10P, Shimadzu FT-IR-8400S and TGA Bahr Company STA 504. The XRD pattern of the catalyst was obtained using TW 1800 diffractometer with Cu Ka radiation (λ = 1.54050 Å). The analytical thin layer chromatography (TLC) experiments was performed using Merck 0.2 mm silica gel 60F-254Al-plates. All compounds well characterized by IR and 1H NMR (500 MHz, Bruker DRX-500 Avance spectrometers in DMSO).
General procedure for preparation of the MCM-41 Nano ordered mesoporous silica MCM-41 were prepared by hydrothermal synthesis conforming to the known method. 33. 2.70 g of diethyl amine was dissolved in 42 mL deionized water at room temperature. The mixture was stirred for 10 min, then 1.47 g of cetyltrimethylammonium bromide (CTAB) was added and the surfactant solution was stirred for 30 min until a clear solution was gained. Next, 2.10 g tetraethyl orthosilicate (TEOS) was gently added and by drop wise addition of HCl solution (1 M), the pH of the mixture was fixed at 8.5 to gain the final precipitate. The resulting mixture was stirred for 2 h, next the resulting white precipitate was filtered and washed with 100 ml of water. Then it was dried at 45 ° C for 12 h and finally, the sample was calcined at 550 °C with the rate of 2 °C/min for 5 h.
General procedure for preparation of the MCM-41-APS-PMDA-SO 3 H (1) In a 200-mL round button flask, (3-aminopropyl) triethoxysilane 0.15 mmol, d = 0.946 g/mL) was added to a 0.15 g MCM-41 in 15 mL dry toluene. After 8 h, the residue white solid MCM-41-(SiCH2CH2CH2NH2)x was filtered, and washed with toluene and chloroform several times to remove any excess of linker. Then, dried solid was dehydrated at 120 ᵒC for 1 h under nitrogen atmosphere Next, 0.15 g white solids and 0.15 g of pyromellitic dianhydride were disperse in dry THF (30 ml) for 1 h. Following this, 0.10 g of triethylamine (TEA) was added to the obtained mixture. The mixture was stirred at room temperature for 24 h, under inert atmosphere. Then, the obtained solid was filtered off and washed with toluene and EtOH for several times, respectively. For sulfonation of prepare solid, 0.10 g of triethylamine was added to 0.10 g of sulfamic acid and stirred for 1 h. Following, the obtained solid dissolved in dry toluene (20 mL) and added to the sulfamic acid solution. The mixture was stirred at reflux condition for 36 under inert atmosphere. Finally, the residues were filtered, washed several times and dried in vacuum drying oven at 60 oC for 8 h. The preparation schematic route of the MCM-41-APS-PMDA-SO3H (1) has been shown in Scheme 1.
General procedure for the synthesis of 3,4-dihydropyrimidinones catalyzed by the MCM-41-APS-PMDA-SO 3 H (1) In a 5 mL round-bottom flask, a mixture of ethyl acetoacetate (2, 1 mmol), aldehydes (3, 1 mmol), urea (4, 1.2 mmol) and MCM-41-APS-PMDA-SO3H (1, 15 mg) were heated to 80 ᵒC under solvent-free conditions for times indicated in Table 2. The advance of the reactions was checked out by TLC (Eluent: EtOAc: n-hexane, 1:3). At the end of the reaction, 96% EtOH (5 mL) was added to the mixture. The heterogeneous catalyst was then separated by filtration and allowed to cool filtrate over time to give pure crystals of the desired 3,4-dihydropyrimidinones. The separated catalyst was suspended in ethanol (1 mL), for 30 min and filtered off, heated in an oven at 60 °C for 1.5 h and then reused for successive runs.