A new catalyst was prepared as nano-SiO2/DBN in two steps. At first, a mixture of thionyl chloride and commercial nano-silica gel was stirred for 48 h under reflux condition to carry out nano-silica chloride. In this reaction, OH functional groups of silica gel were replaced by Cl atoms of thionyl chloride. Then, nano-silica chloride ,which is dried, reacted with DBN in n-hexane under reflux condition. The chlorine atoms in nano-silica chloride were replaced with N-nucleophiles in DBN (Scheme 1- see Supplemental Files for Scheme 1).
Fig. 2 (a-c) shows the FT-IR spectra of the synthesized materials. Fig.2d shows absorption band at 3397 cm-1 which is due to the SiO–H stretching vibration, 1652 cm-1 for the SiO–H bending vibration and 1056 cm-1 for Si–O stretching vibration and 796 cm-1 due to the Si–O–Si bending vibrational mode.
Energy-dispersive X-ray spectroscopy (EDS) was used to determine the percentage of elements in nano-SiO2/DBN (Fig. 3). The percentage of C, N, O, Si and Cl in nano-SiO2/DBN was 10.75, 4.88, 47.38, 36.48 and 0.25 respectively.
The EDX-map of elements in the structure of nano-SiO2/DBN (Fig. 4) displays homogenous distribution of elements in catalyst.
The particle size of nano-SiO2/DBN was studied using field emission scanning electron microscopy (FESEM) and found to be less than 50 nm (Fig. 5).
TGA analysis is shown in Fig. 6(A), which exhibits the stability of the nano-SiO2/DBN as nano-catalyst which can be used up to 120 °C. The weight loss (1%) below 100 °C is likely due to the loss of surface hydroxyl and solvent molecules. However, the main decomposition occurs at 450 °C. The thermal stability of nano-SiO2/DBN was noticeably improved, which may be due to the strong interaction between DBN and nano-SiO2. range of 10-80°. A broad peak (Fig. 7(a)) is observed at 2θ = 22°, showing the SiO2 is amorphous. While, the diffraction pattern of the nano-SiO2/DBN (Fig.6B (b)) indicated peak at 2θ = 25°.
Fig. 7 shows (A) BJH plot, (B) BET (Brunauer–Emmett–Teller) plot, (C) t- plot, (D) Langmuir plot and (E) Adsorption / desorption isotherm of nano-SiO2/DBN. The obtained data of BET, Langmuir, t and BJH plots were summerized in table 1.
To optimize the reaction conditions in the synthesis of tetrahydrobenzo[b]pyran, the one-pot three-component condensation reaction of 4-chlorobenzaldehyde, dimedone and malononitrile was investigated, as model reaction, for various factors such as the amount of nano-SiO2/DBN, time, temperature and solvent (Table 2). Therefore, the best reaction condition was performed using 0.03 g of catalyst in various solvents such as H2O, CHCl3, MeOH, EtOH and H2O/EtOH (Table 2, entries 1‒5). The use of H2O/EtOH (1:1) as solvent at 60 ° C is the most efficient condition for the model reaction with high yield and short time (Table 2, entry 10). The reaction performed under solvent free conditions, gave a lower yield in comparison with those performed in the solvent (Table 2, entries 6, 7).
After determining the optimized condition, the reaction between different aldehydes with dimedone and malononitrile was investigated (Table 3). In result, tetrahydrobenzo[b]pyrans were synthesized in good to high yields and short reaction times.
As shown in Table 4, performance of synthesized catalyst compared to previously reported catalysts. Nano-SiO2/DBN can be presented as an efficacious one, among others. catalyst in terms of reaction time and yields. There are many privileges in this regard simple procedure, nontoxic solvent and mild reaction conditions.
A suggested mechanism for synthesis of tetrahydrobenzo[b]pyran derivatives by using nano-SiO2/DBN is illustrated in scheme 2. Initially, the nano-SiO2/DBN catalyst activates both the methylene group 5 and the carbonyl group 1. After, the knoevenagal condensation reaction between the malononitrile and aldehyde in existence of basic catalyst forms the intermediate 6. Then, the Michael addition of enol 4 and intermediate 6 is performed to produce the intermediate 7. Finally, the product is formed by cyclization and tautomerization of the intermediate 8.
The reusability of the nano-SiO2/DBN was investigated. After completion of the reaction, the nanocatalyst was separated and washed with some EtOH, then dried at 70 °C. The catalyst was regained in good yields and catalyst was used in the synthesis of tetrahydrobenzo[b]pyran for five times (Fig .8).