The design of environmentally friendly catalysts could be advantages in green chemistry. Since the progress of catalysts, heterogeneous catalysts have been received specific attention because of particular benefits like comfort of separation[1-5].Among the numerous kinds of heterogeneous catalysis systems, scientists have attentive typically on nanoscale particles due to their informal operational process, effective catalytic activity, reusability, high stability, huge surface area and superb functionalization ability[6-10]. Metal nano-catalysis has developed as an efficient approach for the syntheses of various molecules, and its use in numerous transformations as a heterogeneous catalyst is significant .
Imidazole derivatives, one of the most substantial heterocyclic compounds, have been establish in many natural products [12-14] and broadly used in functional materials [15-18]. Especially, they have appropriate pharmacological activities [19-21] for example anti-tumor [22, 23], anti-plasmodium , anti-bacterial[25-27], anti-fungal[28-30] and anti-inflammatory[31, 32]. Furthermore, they have worthy photo physical properties [33, 34] and are applied as ligands in metal catalyzed reactions [35-37]. Various synthetic processes and catalysts have been known for the synthesis of imidazoles because of the worth of this type of compounds such as [MIMPS]3PW12O40 and [TEAPS]3PW12O40 , alumina-methane sulfonic acid, copper nanoparticles on activated carbon, MCM-41, glycerol as green solvent, scolecite , oxone , activated carbon , SDS  and [2,6-DMPy-NO2]C(NO2)3 .
In our continuous attempt to progress approaches for the synthesis of heterocyclic compounds and ongoing study on the metal nano oxide catalyst [48-54], herein, we report an efficient and benign process for the synthesis of the 2-substituted benzimidazoles and 1,2-disubstituted benzimidazoles via the cyclo-condensation reaction between 1,2-phenylenediamine and aldehydes by using 1 mol% catalytic amount of SnO2 nanoparticles in ethanol solvent at room temperature.
MATERIALS AND METHODS
General procedure for the synthesis of 2-substituted benzimidazoles(3) and 1,2-disubstituted benzimidazoles(4) by using SnO2 nanoparticles
According to Fig. 1, SnO2 nanoparticles (1mol%) was added to a ethanolicsolution (2 mL) of 1,2-phenylenediamine (1 mmol; 0.108 g) and aldehydes(1mmol for the synthesis of 3 and 2 mmol for the synthesis of 4) at room temperature for the appropriate period of time as showed in Table 2. The progress of the reaction was checked by TLC (n-hexane/ethyl acetate; 5:2). After end of the reaction, the reaction mixture was heated in ethanol. SnO2 nanoparticles was filtered (the product was soluble in hot ethanol and the catalyst was insoluble). The corresponding products 3 and 4 were attained via simple filtering and recrystallized from ethanol.
General procedure for the synthesis of 2,2’-Bis-1H-benzimidazole (10) by using SnO2 nanoparticles
To a ethanolic solution (2 mL) of 1,2-phenylenediamine (2mmol; 0.216 g) and oxalic acid (1 mmol; 0.09 g; 0.05 mL), SnO2 nanoparticles (1 mol%) was added. The mixture solution was stirred at room temperature for 24 hours. The progress of the reaction was observed by TLC (n-hexane/ethyl acetate; 5:2). After completion of the reaction, the reaction mixture was heated in ethanol. SnO2 nanoparticles was filtered and the corresponding product 10 was achieved via simple filtering and recrystallized from ethanol.
Selected characterization data for the products
2-(4-Nitrophenyl)-1H-benzo[d]imidazole (Table 2, 3a): Dark orange solid, M.p.: 319-321 oC; Yield: 98%; FT-IR (KBr)(υmax, cm-1): 3482, 1598, 1513, 1438, 1336, 1106, 856; 1H NMR (400 MHz, DMSO-d6): δppm = 13.28 (s, 1H, –NH), 8.40-8.44 (dd, 4H, J1= 16.0 Hz and J2= 8.0 Hz, Ar–H), 7.72-7.74(d, 1H, J=8.0 Hz, Ar–H), 7.58-7.60 (d, 1H, J=8.0 Hz, Ar–H), 7.25-7.29 (dd, 2H, J1= 16.0 Hz and J2= 8.0 Hz, Ar–H).
1-(4-Nitrobenzyl)-2-(4-nitrophenyl)-1H-benzo[d]imidazole (Table 2, 4a): Dark orange solid, M.p.: 195-197oC; Yield: 98%; FT-IR (KBr)(υmax, cm-1): 3104, 1594, 1515, 1342, 1103, 964; 1H NMR (400 MHz, DMSO-d6): δppm = 8.56 (s, 2H, –CH2), 8.32-8.34 (dd, 4H, J1= 8.0 Hz and J2= 4.0 Hz, Ar–H), 8.06-8.08 (dd, 4H, J1= 8.0 Hz and J2= 4.0 Hz, Ar–H), 7.36-7.38(dd, 2H, J1= 8.0 Hz and J2= 4.0 Hz, Ar–H), 7.24-7.26 (dd, 2H, J1= 8.0 Hz and J2= 4.0 Hz, Ar–H).
1H,1’H-2,2’-bibenzo[d]imidazole (Fig. 3, 10): Orange solid, M.p.: 250oC; Yield: 85%; FT-IR (KBr)(υmax, cm-1): 3423, 3344, 3187, 1554, 1315, 1155, 742; 1H NMR (400 MHz, DMSO-d6): δppm = 6.57-6.61 (dd, 4H, J1= 8.0 Hz and J2= 4.0 Hz, Ar–H), 6.46-6.50 (dd, 4H, J1= 16.0 Hz and J2= 8.0 Hz, Ar–H), 5.71 (brs, 2H, –NH); 1H NMR (400 MHz, D2O): δppm = 6.58-6.60 (dd, 4H, J1= 8.0 Hz and J2= 4.0 Hz, Ar–H), 6.46-6.48 (dd, 4H, J1= 8.0 Hz and J2= 4.0 Hz, Ar–H);13C NMR (100 MHz, DMSO-d6): δppm = 163.1, 134.2, 118.9, 116.2.
RESULTS AND DISCUSSION
Characterization of SnO2 nanoparticles
SnO2 nanoparticles was purchased from commercial centers and then characterized by X-ray diffraction patterns (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) analyses (Figs. 2-5) [50, 55] and EDS spectra (Fig. 6).
For study of the crystal structure of the SnO2 nanoparticles, the XRD pattern was attained (Fig. 2). The crystalline peaks at diffraction angles indicate the structure of SnO2 nanoparticles (JCPDS card no. 41-1445), and are showed by Miller indices in the spectrum. All the diffraction peaks prove the tetragonal structure of SnO2 nanoparticles. The average crystallite size was calculated to be 65 nm. Also, no peaks of impurities are identified, signifying that the SnO2 nanoparticles are pure and good crystallized.
The sizes, morphology, and uniformity of the SnO2 nanoparticlesare obviously showed in the SEM images (Fig. 3). This figure describes that the SnO2 nanoparticles have a homogeneous size distribution and grain have spherical in shape. The SEM images display approximately uniform and spherical nanoparticles with sizes about 65 nm.
Surface morphology of the SnO2 nanoparticles was characterized by TEM images (Fig. 4). It was showed an approximately spherical shape of the particle size ranged from 60 to 65 nm which was in worthy agreement with that calculated from XRD and SEM analyses.
The morphology and size of the recycled SnO2 nanoparticles was studied by SEM images, and (Fig. 5) shows that the dimensions of recycled SnO2 nanoparticles is between 85 and 90 nm. According to SEM images of recycled SnO2 nanoparticles, its structure is stable.
The presence of Sn and O were conﬁrmed by EDS analysis from nanoparticle (Fig. 6).
Application of SnO2 nanoparticles for the synthesis of mono- and di-substituted benzimidazoles
To investigation the effect of catalyst loading on the synthesis of corresponding mono- and di-substituted benzimidazoles, the reaction of 1,2-phenylenediamine and 4-nitrobenzaldehyde was selected as a model in ethanol at room temperature (Table 1). To prove the requirement of SnO2 nanoparticles as a catalyst for the synthesis of 3a and 4a, we studied the model reaction in the absence of catalyst. The results display obviously that SnO2 nanoparticles is an effective catalyst for this synthesis and without the catalyst the reaction did not occur, even after 60 minutes. As showed in Table 1, the best results have been attained with 1 mol% of SnO2 nanoparticles (Table 1, entry 2).Extending the reaction time did not improve the yield. Nonetheless, the yield was shrinking when the catalyst loading was decreased to 0.5 mol% (Table 1, entry 1), while the yield remained unaffected when the catalyst loading was increased to 5 mol% (Table 1, entries 3 and 4).
We subsequent ready an investigation on the effect of solvents in the synthesis of 3a and 4a. The model reaction was performed at room temperature by using 1 mol% catalytic amount of SnO2 nanoparticles. As a general rule, nonpolar solvents for example dichloromethane, ethyl acetate and toluene led to low yields (Table 1, entries 7-9). The optimum result was detected when the reaction was carried out in ethanol (Table 1, entry 2).
The third significant part that could be stimulated from these results is that increasing the reaction temperature from room temperature stepwise to reflux condition did not improve the yields of 3a and 4a. Thus, the best temperature was room temperature.
The optimum yield of the chosen products 3a and 4a were attained by perform the reaction with 1:1 of 1,2-phenylenediamine and 4-nitrobenzaldehyde (for the synthesis of mono-substituted benzimidazoles) and 1:2 of 1,2-phenylenediamine and 4-nitrobenzaldehyde (for the synthesis of di-substituted benzimidazoles) by using 1 mol% catalytic amount of SnO2 nanoparticles at room temperature in ethanol solvent.
The recoverability and reusability of the catalyst was studied in model reaction. After end of the reaction, hot ethanol was added to the reaction mixture and filtered to separate the catalyst. The separated catalyst was applied for additional runs. The activity of the catalyst did not display any substantialdecrease even after three runs. The structure of recycled catalyst was studied by SEM analysis and the results displayed that the catalyst is stable (Fig. S4).
Following, we investigated the scope of this reaction (Table 2). As assessed, this reaction progressed effortlessly and the chosen products were attained in appropriate yields. A series of aldehydes with either electron-releasing or electron-withdrawing groups attaching to aromatic ring were studied. The substitution groups on the aromatic ring had no clear effect on the yield. We also investigated reaction of aromatic heterocyclic aldehydes with 1,2-phenylenediamine and the desired products were achieved in good yields.
The proposed mechanism for the synthesis of title compounds 3 and 4were showed in Fig. 7 (pathway a andb)[56, 57]. Initially, SnO2 nanoparticles activates the carbonyl group of aldehyde 2 to provide intermediate 2’. In pathway a, the nucleophilic attack of 1,2-phenylenediamine 1 on the intermediate 2’ to afford intermediates 5 via elimination of one molecule of water. In the next step, intramolecular cyclization of intermediate 5 via nucleophilic attack of amine group on the imine band to give intermediate 6 which aromatized through air oxidation to 2-substituted benzimidazoles3. In pathway b, the nucleophilic attack of 1,2-phenylenediamine 1 on the two molecule of intermediate 2’ to give intermediates 7 via elimination of two molecule of water. At that time, intramolecular cyclization of intermediate 7 to provide intermediate 8 which aromatized via 1,3-hydide shift to 1,2-disubstituted benzimidazoles4.
In other work, for investigation of catalytic activity of SnO2 nanoparticles, the synthesis of 1H,1’H-2,2’-bibenzo[d]imidazole10was performedvia the reaction between 1,2-phenylenediamine1and oxalic acid 9 by using 1 mol% SnO2 nanoparticles in ethanol at room temperature.
In summary, we have described an efficacious approach for the facile and appropriate synthesis of 2-substituted benzimidazoles and 1,2-disubstituted benzimidazolesin a cyclo-condensation reaction between 1,2-phenylenediamine and aldehydes by using 1 mol% catalytic amount of SnO2 nanoparticles under mild conditions. Application of informal reaction conditions, isolation and purification makes this process very interesting from a cost-effective viewpoint.
The authors are thankful for the facilities provided to carry out research in chemistry research laboratory at Ayatollah Amoli Branch, Islamic Azad University.
CONFLICT OF INTEREST
The authors declare that there are no conflicts of interest regarding the publication of this manuscript.