INTRODUCTION

1,4-Dihydropyridyl compounds (1,4-DHPs) are valuable heterocyclic compounds in view of pharmaceuticals and drugs development [1]. 1,4-DHPs possess a wide range of biological activities such as anti-atherosclerotic, vasodilator, anti-diabetic, geroprotective, hepatoprotective and the treatment of hypertention and cardiovascular diseases [2-6]. 4H-Pyran compounds present a broad range of biological and pharmacological properties such as anticancer, anti-HIV, anti-inflammatory, anti-microbial, anti-malarial, anti-hyperglycaemic and anti-dyslipidemic activity [7-13]. Realizing the importance of 1,4-dihydropyridyl compounds and 4H-pyran derivatives, increasing interest on synthetic methods of these compounds is ongoing. The classical method for polyhydroquinoline synthesis involves a three-component coupling of an aldehyde, dicarbonyl compounds, and ammonia in acetic acid or in refluxing ethanol for long reaction times which typically leads to low yields [14-16]. Traditional processes for 4H-pyran synthesis were reported as the reaction of active methylene compounds with an aldehyde or ketone in the presence of an organic base such as piperidine or triethylamine under reflux and multiple-step conditions [17]. In recent years, several modified synthesis methods to access polyhydroquinolines [18-28], and 4H-benzo[b]pyrans [29-36], have been developed. Although some reactions are satisfactory in terms of yield, but the use of high temperatures, expensive metal precursors, catalysts that are harmful to the environment, long reaction times, harsh reaction conditions, effluent pollution and tedious workup procedures are drawbacks of these methods.

Nanotechnology is beginning to allow scientists, engineers, chemists, and physicians to work at the molecular and cellular levels to produce important advances in the life sciences and healthcare. The uses of synthetic nanomaterials have increased the scope of their application in areas of medical diagnostics, areas of material modification, degradation of environmental pollutants, chemical reaction catalysis and biotechnology [37, 38]. These widespread applications results to the necessity of nanomaterials modification into different structure with desirable features.

Organic-inorganic hybrid materials are of great interest as heterogeneous catalysts in organic synthesis, due to the functional diversity merged with thermal and mechanical stability of inorganic solids [39]. Their large surface area per unit volume, makes them interesting in the heterogeneous catalysis area; Heterogeneous catalysis in the nano-scale takes advantage of a high exposure of the active species leading to a higher efficiency of the catalyst [40]. Nevertheless, the application of heterogeneous nanocatalysts are usually limited by the inevitable loss of catalyst during the tedious separation processes, i.e. filtration or centrifugation. In this vein, easily separable magnetic nanoparticles (MNPs), e.g. Fe₃O₄, have demonstrated high stability, easy synthesis and functionalization alongside with high surface area, low toxicity and cost. These superb properties set magnetic nanoparticles as a target for extensive investigation as inorganic supports in the synthesis of semi-heterogeneous catalysts [41]. These metallic nanoparticles can be coated with silica shell to introduce numerous surface Si–OH groups for further modification and higher chemical and colloidal stability since the magnetically agglomeration will be diminished [42].

For the above reasons and as a part of our works on design and development of novel heterogeneous catalysts and green chemical methods [43-47], we describe the synthesis and characterization of bis-sulfamic acid-grafted magnetic nanoparticles (MNPs-TBSA) to give access to biologically interesting polyhydroquinolines and 4H-benzo[b]pyrans as a new eco-friendly method (Fig. 1). This novel designed catalyst provided a heterogeneous system with a green synthetic aspects by avoiding the use of hazardous conditions for accessing target heterocyclic compounds.

MATERIALS AND METHODS

All chemicals were purchased from Merck or Acros chemical companies and used without further purification. Melting points were measured by using capillary tubes on an electro thermal digital apparatus and are uncorrected. Known products were identified by comparison of their spectral data and melting points with those reported in the literature. Thin layer chromatography (TLC) was performed on UV active aluminum backed plates of silica gel (TLC Silica gel 60 F254). ¹H and ¹³C NMR spectra were recorded on a Bruker Avance 300 MHz spectrometer at 300 MHz and 75 MHz, respectively. Coupling constants, J, were reported in hertz unit (Hz). IR spectra were recorded on a Unicom Galaxy Series FT-IR 5030 spectrophotometer using KBr pellets and are expressed in cm^-1. Elemental analyses were performed by Vario EL equipment at Arak University. X-ray diffraction (XRD) was performed on Philips XPert (Cu-K_α radiation, λ = 0.15405 nm) over the range 2θ = 20-80° using 0.04° as the step length. Thermal gravimetric analysis (TGA) and differential thermal gravimetric (DTG) data for MNPs-TBSA were recorded on a Mettler TA4000 System under an N₂atmosphere at a heating rate of 10 °C min^-1. The scanning electron microscope measurement was carried out on a Hitachi S-4700 field emission-scanning electron microscope (FE-SEM).

General procedure for the synthesis of polyhydroquinolines

A mixture of dimedone (1 mmol), ethylaceto-acetate (1 mmol), aldehyde (1 mmol), ammonium acetate (1.2 mmol) and MNPs-TBSA (30 mg) as catalyst in 5 mL EtOH was heated at 70 °C and were stirred for appropriate time. After completion of the reaction as followed by TLC, the resulting solidified mixture was diluted with hot EtOH (15 mL). Then, the catalyst was separated using an external magnet, the solvent was evaporated, and the product was recrystallized with EtOH/H₂O (4:1), and dried in an oven at 90 °C (Table 2).

General procedure for the synthesis of 4H-benzo[b]pyrans

A mixture of an aromatic aldehyde (1 mmol), malononitrile (1 mmol), dimedone (1 mmol) and MNPs-TBSA (30 mg) as catalyst in 5 ml EtOH was heated at 70 °C with stirring for an appropriate time. The resulting solidified mixture was diluted with hot EtOH (15 mL). Then, the catalyst was separated using an external magnet, the solvent was evaporated, and the product was recrystallized with EtOH/H₂O (4:1), and dried in an oven at 90 °C (Table 4).

Selected data for desired products

6a: IR (KBr) (ν_max): 3288, 2962, 1699, 1610, 1485, 1381,1211,1072 cm^-1; ¹H NMR (300 MHz, DMSO-d₆): δ_H: 0.84 (3H, s, CH₃), 1.00 (3H, s, CH₃), 1.13 (3H, t, J = 7.1 Hz, CH₃), 2.14–2.50 (4H, m, CH₂), 3.97 (2H, q, J = 7.1 Hz, CH₂), 4.84 (1H, s, CH), 7.03–7.20 (5H, m, H_Ar), 9.07 (1H, s, NH) ppm. Anal. Calcd for C₂₁H₂₅NO₃: C, 74.31; H, 7.42; N, 4.13. Found C, 74.63; H, 7.67; N, 4.27.

6f: IR (KBr) (ν_max): 3280, 3217, 3065, 2951, 1695, 1637, 1608, 1489, 1381, 1263, 1210, 1092 cm^-1; ¹H NMR (300 MHz, DMSO-d₆): δ_H: 0.82 (3H, s, CH₃), 1.00 (3H, s, CH₃), 1.12 (3H, t, J = 7.0 Hz, CH₃), 1.94–2.44 (4H, m, CH₂), 2.29 (3H, s, CH₃), 3.96 (2H, q, J = 7.0 Hz, CH₂), 4.82 (1H, s, CH), 7.08–7.39 (4H, m, H_Ar), 9.12 (1H, s, NH) ppm. Anal. Calcd for C₂₁H₂₄BrNO₃: C, 60.29; H, 5.78; N, 3.35. Found C, 60.47; H, 5.93; N, 3.47.

7a: IR (KBr) (ν_max): 3393, 3317, 3185, 2958, 2196, 1687, 1652, 1367 cm^-1. ¹H NMR (300 MHz, DMSO–d₆) δ_H: 0.94 (3H, s, CH₃), 1.04 (3H, s, CH₃), 2.08 (1H, d, J = 16.0 Hz, H(CH₂)), 2.23 (1H, d, J = 16.0 Hz, H(CH₂)), 2.50 (2H, m, CH₂), 4.11 (1H, s, CH), 7.06 (2H, br s, NH₂), 7.19 (3H, m, H_Ar), 7.33 (2H, m, H_Ar) ppm. Anal. Calcd for C₁₈H₁₈N₂O₂: C, 73.45; H, 6.16; N, 9.52. Found C, 73.83; H, 6.43; N, 9.41.

7d: IR (KBr) (ν_max): 3533, 3364, 3153, 2966, 2193, 1685, 1658, 1367 cm^-1. ¹H NMR (300 MHz, DMSO–d₆) δH: 1.00 (3H, s, CH₃), 1.06 (3H, s, CH₃), 2.11 (1H, d, J = 16.0 Hz, H(CH₂)), 2.27 (1H, d, J = 16.0 Hz, H(CH₂)), 2.47–2.61 (2H, m, CH₂), 4.70 (1H, s, CH), 7.15 (2H, br s, NH₂), 7.25 (1H, d, J = 8.4 Hz, H_Ar), 7.39 (1H, d, J = 8.4 Hz, H_Ar), 7.56 (1H, s, H_Ar) ppm. Anal. Calcd for C₁₈H₁₆Cl₂N₂O₂: C, 59.52; H, 4.44; N, 7.71. Found C, 59.81; H, 4.58; N, 7.61.

RESULTS AND DISCUSSION

Preparation and characterization of the catalyst

The magnetic nanoparticle supported bis sulfamic acid catalyst (MNPs-TBSA) was prepared via sequential reactions as shown in Fig. 2. Magnetite (Fe₃O₄) nanoparticles were easily prepared via the chemical co-precipitation of Fe²⁺ and Fe³⁺ ions in basic solution. These were subsequently coated with silica layer (Fe₃O₄@SiO₂) through the well-known Stober method [48]. The Fe₃O₄@SiO₂core-shell structures were treated with 3-aminopropyltriethoxysilane (APTS), which can bind covalently to the free-OH groups at the particles surface (Fe₃O₄@SiO₂-NH₂). Triazine-functionalized silica-coated magnetite nanoparticles (MNPs-TDCl) prepared with the reaction of the 3-aminopropyl-functionalized silica-coated magnetic nanoparticles (Fe₃O₄@SiO₂-NH₂) and triazine trichloride. Reaction of the Triazine-functionalized silica-coated magnetite nanoparticles with ammonia gives the triazine diamine-functionalized silica-coated Fe₃O₄ nanoparticles (Fe₃O₄@SiO₂-TDA). The supported bis-sulfamic acid catalyst (MNPs–TBSA) was prepared via the reaction of MNPs-TDA with chlorosulfonic acid.

The FT-IR spectrum of Fe₃O₄, Fe₃O₄@SiO₂, Fe₃O₄@SiO₂-NH₂, Fe₃O₄@SiO₂-TDCl, Fe₃O₄@SiO₂-TDA and MNPs-TBSA nanoparticles in the wavenumber range of 4000-400 cm^-1 is shown in Fig. 3. The magnetic Fe₃O₄ nanoparticles FT-IR spectra (Fig. 3a) showed the characteristic Fe−O absorption near 574 cm^-1. FT-IR spectrum of Fe₃O₄@SiO₂ displays bands at about 1084 (asymmetric stretching), 951 (symmetric stretching), 808 (in plane bending) and 453 cm^-1 (rocking mode) of the Si–O–Si group and confirm the formation of SiO₂ shell. The broad peaks in the range 3200–3500 cm^-1 (stretching vibration mode Si–OH) and the weak peak at 1610 cm^-1 (twisting vibration mode of H–O–H adsorbed in the silica shell) are obvious in the spectrum. The weak aliphatic vibrations at 2930 and 2955 cm^-1 (Fig. 3c, d, e and f) related to C–H symmetric and asymmetric stretching and confirmed the presence of the attached alkyl groups. From reaction of cyanuric chloride with Fe₃O₄@SiO₂-NH₂, the peaks correspond to C=N and C=C in heterocyclic rings are appeared at 1500-1600 cm^-1 and 1450 cm^-1 respectively (Fig. 3d). NH₂ bending of triazine diamine-functionalized MNPs appeared at 1625 cm^-1 (Fig. 3e) [49]. The peaks at 1400 cm^-1 have been assigned to the stretching vibrations of S=O acid sulfonic groups. (Fig. 3f) [50]. Therefore the above results prove that the functional groups were successfully grafted on to the surface of the magnetic Fe₃O₄@SiO₂ nanoparticles.

The nanoparticle size and morphology of MNPs-TBSA catalyst were investigated by field emission scanning electron microscopy (FE-SEM) (Fig. 4). As can be seen from Fig. 4, MNPs-TBSA particles have a mean diameter of 25-35 nm and a nearly spherical shape.

In addition, TEM analysis showed a dark nano-Fe₃O₄ core surrounded by a grey silica shell about 5-10 nm thick and the average size of the obtained particles is 20-35 nm (Fig. 5).

The energy dispersive X-ray spectroscopy (EDS) results, obtained from SEM analysis of MNPs- TBSA, are shown in Fig. 6, and clearly show the presence of S in the MNPs-TBSA catalyst. Moreover, the presence of Si, O, and Fe signals indicates that the iron oxide particles are loaded into silica, and the higher intensity of the Si peak compared with the Fe peaks indicates that the Fe₃O₄nanoparticles were trapped by SiO₂. According to the above analysis, it can be concluded that the MNPs-TBSA have been successfully synthesized.

The presence as well as the degree of crystallinity of magnetic Fe₃O₄ and the MNPs-TBSA catalyst was obtained from XRD measurements (Fig. 7). The same peaks were observed in the both of the magnetic Fe₃O₄ and MNPs-TBSA XRD patterns, indicating retention of the crystalline spinel ferrite core structure during the silica-coating process. The XRD data of the synthesized magnetic nanoparticles show diffraction peaks at 2θ = 30.3° ,35.7°, 43.3°, 53.7°, 57.3°, 62.9°, and 74.5° which can be assigned to the (220), (311), (400), (422), (511), (440) and (533) planes of Fe₃O₄, respectively, indicating that the Fe₃O₄ particles in the nanoparticles were pure Fe₃O₄ with a cubic spinel structure; these match well with the standard Fe₃O₄ sample (JCPDS card no. 85-1436). The broad peak from 2θ = 20° to 27 ° (Fig. 5b) is consistent with an amorphous silica phase in the shell of the silica-coated Fe₃O₄ nanoparticles (Fe₃O₄@SiO₂) [51]. The (311) XRD peak was used to estimate the average crystallite size of the magnetic nanoparticles by Scherrer’s equation (D = 0.9λ/β cos θ), where D is the average crystalline size, λ is the X-ray wave-length (0.154 nm), β denotes the full width in radians subtended by the half maximum intensity width of the (311) powder peak, and θ corresponds to the Bragg angle of the (311) peak in degrees [52]. From the width of the peak at 2θ = 35.7 (311), the crystallite size of the magnetic nanoparticle is calculated to be 15.5 nm using Scherrer’s equation, which is in range the size determined by FE-SEM analysis (Fig. 4).

The stability of the MNPs-TBSA catalyst was determined by thermogravimetric analysis (TGA) and derivative thermogravimetry (DTG) (Fig. 8). The magnetic catalyst shows two step weight loss steps over the temperature range of TG analysis. The first stage, including a low amount of weight loss at T< 200 °C, is due to the removal of physically adsorbed solvent and surface hydroxyl groups, the second stage at about 220 °C to nearly 560 °C is attributed to the decomposition of the organic moiety in the nanocomposite. Therefore, the weight loss between 220-560 °C gives the organic grafting ratios of the magnetic catalyst. The grafted organic moiety on the magnetic Fe₃O₄@SiO₂ nanoparticles was approximately 16 wt%. In accordance with this mass loss, it was calculated that 0.40 mmol of TBSA was loaded on 1 g of MNPs-TBSA catalyst. Therefore, the MNPs-TBSA is stable around or below 250 °C. The agreement between the acid amount of MNPs-TBSA (0.35 mmol g^-1) measured by back titration using HCl and the organic group loading determined by TGA (0.40 mmol g^-1), is clear evidence that the triazinediyl bis sulfamic acid groups are principally located on the surfaces of Fe₃O₄@SiO₂ nanoparticles, where they are accessible for adsorption and catalytic reaction processes.

After the characterization of the MNPs-TBSA catalyst, its catalytic activity was evaluated for the synthesis of polyhydroquinoline derivatives (Scheme 1). An important feature of these nano-catalysts is simple separation of them using an external magnet, thereby removing the necessity for filtration or centrifugation. In order to optimize the reaction conditions and obtain the best catalytic activity, the reaction of benzaldehyde, dimedone, ethylacetoacetate and ammonium acetate was used as a model, and was conducted under different reaction parameters (Table 1). As shown in Table 1 (entry 7), the best result was found in EtOH at 70 °C using 30 mg of the catalyst. Moreover, the catalyst is essential and in the absence of the catalyst, only 23% of the corresponding polyhydroquinoline was produced even after prolonged reaction times (Table 1, entry 10). The efficiencies of Fe₃O₄, Fe₃O₄@SiO₂ and MNPs-NH₂, Fe₃O₄@SiO₂-TDA and MNPs-TBSA as the catalyst towards the model reaction were compared and the results are depicted in Table 1. It was observed that the MNPs-TBSA was more efficient than the other ones (entries 7-14). In order to determine the generality and efficacy of the catalyst, various aldehyde carrying either electron-donating or electron-withdrawing groups were reacted under the optimized reaction condition (Table 2). All reactions proceeded efficiently in the presence of catalytic amounts of MNPs-TBSA at 70 °C and the desired products were obtained in good to excellent yields (86–96%) in relatively short reaction times, without formation of side products.

A plausible mechanism for the formation of polyhydroquinolines catalyzed by MNPs-TBSA, is shown in Fig. 9. The MNPs-TBSA catalyst participates in the reaction by activating the carbonyl group of the aldehyde followed by the nucleophilic addition of dimedone anion and H₂O elimination to obtain alkene intermediate. The alkene intermediate is attacked by enolized ethyl acetoacetate. This intermediate reacts with ammonium acetate and an intramolecular cyclization and H₂O elimination afford the desired polyhydroquinoline.

To compare the applicability of our catalyst with other catalysts used for the synthesis of polyhydroquinoline derivatives, the results of these catalysts in the condensation reaction of benzaldehyde, ethylacetoacetate, dimedone and ammonium acetate under optimized conditions have been indicated in Table 3. As can be seen, the catalytic system reported in this paper has benefits in terms of simple conditions, short reaction times and excellent yields and is superior to many other methods.

Encouraged by the obtained results on polyhydroquinolines preparation, the possible synthesis of tetrahydrobenzopyrans was examined in the presence of MNPs-TBSA as a catalyst in the same conditions. Ethanol serves as the best solvent with respect to green nature, polarity and clean workup procedure for this synthesis. For the reaction completion, 30 mg of the catalyst (MNPs-TBSA) is sufficient. Likewise, arylaldehydes with either electron-withdrawing or electrondonating groups were examined using the optimized conditions to afford a wide range of desired 4H-benzo[b]pyran in good to excellent yields (85-95%) in short reaction times (Table 4).

Catalyst recovery and reuse

The recovery and reusability of the catalyst are very important for commercial and industrial applications as well as green process aspects. Thus, the recovery and reusability of MNPs-TBSA (30 mg) was investigated in the sequential reaction of benzaldehdye (1 mmol) with other reactants and MNPs-TBSA (30 mg) as catalyst in EtOH at 70 °C for 25 min. After completion of the reaction, the resulting solidified mixture was diluted with hot EtOH (15 mL). Then, the catalyst was easily separated using an external magnet, washed with hot EtOH, dried under vacuum and reused in a subsequent reaction. Nearly quantitative recovery of catalyst (up to 98%) could be obtained from each run. As seen in Fig. 10, the recycled catalyst could be reused five times without any additional treatment or appreciable reduction in catalytic activity. The recovered catalyst after five runs had no obvious change in structure, as shown by comparison of the FT-IR spectra to that of fresh catalyst (Fig. 11). The consistent structure and activity of recovered and reused MNPs-TBSA catalyst indicates that the reused MNPs-TBSA also shows excellent performance for the synthesis of desired heterocycles.

CONCLUSION

In conclusion, we have described a successful preparation of bissulfamic acid-functionalized magnetic nanoparticles as an efficient, magnetically separable and reusable heterogeneous catalyst. We have considered also convenient synthesis of polyhydroquinoline and 4H-benzo[b]pyran derivatives via one-pot multicomponent reaction in the presence of prepared catalyst on green reaction condition. The method offers several advantages including mild reaction condition, simple work-up procedure, recyclability of catalyst, high purity of products, excellent yields and short reaction time.

ACKNOWLEDGEMENTS

We gratefully acknowledge the financial support of this work by the research council of Arak University.

CONFLICT OF INTERESTS

The authors declare that there is no conflict of interests regarding the publication of this paper.