Pyrazolones demonstrate important biological properties such as being anticancer , anti-bacterial , anti-inflammatory , antioxidant  and analgesic . Indenopyrazoles display antidiabetic , antimicrobial , antitumor , and anticonvulsant  activities. Indeno-fused heterocycles have attracted considerable attention from both medicinal and synthetic chemists in recent years [10-12]. These attributes make pyrazolones a noteworthy target in organic synthesis. A number of ways have been improved for the preparation of pyrazolones in the presence of such catalysts as acetic acid , [HMIM]HSO4 , 3-aminopropylated silica gel , sodium dodecyl sulfate  silica-bonded S-sulfonic acid , and Ce/SiO2 composite . While the development of each of these methods has contributed to moving the field forward, some of them suffer such drawbacks as prolonged reaction times, complicated work-up, low yields, and hazardous reaction conditions. Therefore, in order to avoid these drawbacks, finding an effective way for the preparation of indenopyrazolones is still favored. Microporous materials with regular-pore frameworks are a good choice for immobilizing ligands . An improved understanding of the efficiency of ordered mesoporous silica (OMS) could be obtained by evaluating their potential applications in drug delivery, separation, gas storage, catalysis, and biomolecules . SBA-15 was employed due to its properties such as large pore volume, uniform-sized pores, high specific surface area, and thermal and hydrothermal stability . The anchoring of a wide range of organic functional groups to the pore surface of SBA-15 improves the catalytic ability . A stable attachment of ligand could be reached by post-synthetically available ligand precursors to SBA-15 . We report herein the use of functionalized SBA-15 as an effective catalyst for the preparation of indenopyrazolones by the multi-component reactions of phenylhydrazine, aromatic aldehydes and indan-1,2,3-trione at room temperature in acetonitrile (Scheme 1).
Preparation of Ligand (imidazolidone)
In a 50 ml round-bottom flask, 4 mmol of hydantoin was dissolved in 4 mL water. Then an amount of saturated sodium hydrogen carbonate solution was added and the pH of the reaction mixture was maintained at 7.0. After that, ethanolamine (0.36 mL) was added, and the solution was warmed up gradually to 90 ° C. A solution of terephthaldehyde (4 mmol) in 4 mL of ethanol was added dropwise. Then the mixture was continuously stirred for 48 hrs at 120 °C and kept under reflux. A precipitate was formed by cooling in an ice-salt bath at about 0 °C. It was filtered and washed with H2O/EtOH 5:1. The structure of ligand (imidazolidone) was confirmed by 1H NMR and FT-IR spectrum presented in Figs. 1 and 5, respectively.
Preparation of SBA-15 nanostructure
The hexagonal pore structure of SBA-15 has been produced using pluronic 123 triblock copolymers (EO20–PO70–EO20) by the procedure as reported in the previous paper .
Preparation of 3-chloropropyltriethoxysilane atta-ched to SBA-15 (chlorinated-SBA-15)
In a 50 mL round-bottom flask, 3 mmol of 3-chloropropyltriethoxysilane and 1 g of calcined SBA-15 were suspended in dry toluene and refluxed at 80 °C under an inert atmosphere for 24 hrs. Then, the solid was filtered and washed successively with ethanol. Finally, a white powder was dried under vacuum at 70 °C for 8 hrs to generate the 3-chloropropyltriethoxysilane attached to SBA-15 (Cl-modified SBA-15).
Preparation of modified SBA-15 (Immobilization of ligand (imidazolidone) to chlorinated-SBA-15)
The solution of ligand (imidazolidone) (2 mmol) in dimethylformamide (10 mL) was added at 60% suspension of sodium hydride. The resulting mixture was stirred for 60 minutes. Then chloro-modified SBA-15 (4.5 g, 26.3 mmol) was added to the reaction mixture. The solution was warmed up gradually to 80 ° C and stirred for 18 hrs. The reaction was quenched with 1 N HCl (20 mL, 20 mmol) and the precipitated solid was collected by centrifugation, and then a pure product was obtained by washing with EtOH to produce modified SBA-15 (immobilization of ligand (imidazolidone) to chlorinated-SBA-15).
Preparation of Immobilization of Pd on the modified SBA-15 (catalyst)
To a suspension of modified SBA-15 (1 g) in dry ethanol (20 mL), 0.1 g of PdCl2 was added and the solution was refluxed for 3 hrs. The yellow light solid was separated by centrifuging and was dried afterwards. Unreacted palladium from the surface was removed using Soxhlet extracted with acetone.
Synthesis of indenopyrazolones
A mixture of phenylhydrazine (1.0 mmol), benzaldehydes (1.0 mmol), ninhydrin (1.0 mmol), and 6 mg functionalized SBA-15 (catalyst) in acetonitrile (10 mL) was stirred for the appropriate times. The reaction was monitored by TLC (n-hexane/ethyl acetate 7:3). After the completion of the reaction, the catalyst was recycled by a simple filtration. The catalyst was washed with a small amount of hot ethanol, dried in an oven at 80 °C for 6 hours, and then reused for the next run as indicated above for the model reaction. The solvent was evaporated using a rotary evaporator and the residue was washed with cold diethyl ether to get pure product. The characterization data of the compounds are presented below.
cis-3a,8b-Dihydro-3a,8b-dihydroxy-1,3-diphenylindeno[1,2-c]pyrazol-4(1H)-one (4a): Yellow solid; Mp. 221-223 °C, IR (KBr): νmax = 3435, 3053, 1736, 1458 cm-1. 1H NMR (400 MHz, CDCl3): δ (ppm) = 6.12 (s, OH); 6.17 (s, OH); 7.15 (d, J = 8.0 Hz, 1H, ArH); 7.18-7.30 (m, 3H, ArH); 7.40 (t, J = 7.6 Hz, 1H, ArH); 7.48 (t, J = 7.6 Hz, 1H, ArH); 7.52–7.66 (m, 5H, ArH); 8.22 (d, J = 7.6 Hz, 2H, ArH); 8.48 (d, J = 7.6 Hz, 1H, ArH). 13C NMR (100 MHz, CDCl3): δ (ppm) = 89.2 (C); 96.9 (C); 118.2 (2 CH); 121.4 (CH); 122.6 (CH); 123.2 (CH); 124.0 (CH); 126.2 (2 CH); 129.5 (2 CH); 130.7 (CH); 130.8 (C); 132.8 (2 CH); 133.5 (CH); 137.2 (C); 139.8 (C); 142.3 (C); 147.8 (C); 196.7 (C=O). Analysis for C22H16N2O3: calcd. C, 74.15; H, 4.53; N, 7.86%; Found C, 74.12; H, 4.56; N, 7.90%.
cis-3a,8b-Dihydro-3a,8b-dihydroxy-3-(4-methoxyphenyl)-1-phenylindeno[1,2-c]pyrazol-4(1H)-one (4b): Yellow solid; Mp. 211-214 °C, IR (KBr): νmax = 3422, 3282, 1702, 1592 cm-1. 1H NMR (400 MHz, CDCl3): δ (ppm) = 3.65 (s, OCH3); 6.12 (s, OH); 6.23 (s, OH); 6.88 (d, J = 8.2 Hz, 2H, ArH); 7.05 (t, J = 7.2 Hz, 1H, ArH); 7.18 (t, J = 7.4 Hz, 2H, ArH); 7.27 (t, J = 7.2 Hz, 1H, ArH); 7.37 (t, J = 7.4 Hz, 1H, ArH); 7.50 (d, J = 7.8 Hz, 1H, ArH); 7.55 (d, J = 7.4 Hz, 1H, ArH); 7.60 (d, J = 7.8 Hz, 2H, ArH); 8.02 (d, J = 8.4 Hz, 2H, ArH). 13C NMR (100 MHz, CDCl3): δ (ppm) = 55.4 (OCH3); 90.1 (C); 96.5 (C); 113.7 (2 CH); 117.8 (2 CH); 122.3 (CH); 123.7 (CH); 124.3 (C); 125.8 (CH); 128.6 (2 CH); 129.3 (2 CH); 130.4 (CH); 135.3 (C); 136.7 (CH); 142.8 (C); 143.3 (C); 147.7 (C); 160.4 (C); 197.5 (C=O). Analysis for C23H18N2O4: calcd. C, 71.49; H, 4.70; N, 7.25%; Found C, 71.36; H, 4.60; N, 7.17%.
RESULTS AND DISCUSSION
Scheme 2 shows preparation of the immobilization of Pd on modified SBA-15 (catalyst).
The structure of ligand (imidazolidone) was confirmed by 1H NMR and FT-IR spectrum presented in Figs. 1 and 5, respectively.
The effect of modification on the structural framework of SBA-15 is monitored by a small-angle X-ray diffraction method (Fig. 2). A significant degree of long-range ordering of the structure has been determined by an intense diffraction peak (1 0 0). Two secondary high order peaks with lower intensities corresponding to (1 1 0) and (2 0 0) approve 2D-hexagonal planes of the mesoporous . On modification with ligand and metal, the intensities of the peaks are decreased which can be due to silylation inside the mesopores of SBA-15. The presence of similar peaks for Pd@modified SBA-15 indicates that the structural ordering of SBA-15 is not diminished during the silylation procedure. The 2D-hexagonal structure of SBA-15 is also preserved regardless of the ligand and metal loading.
The porous structure of SBA-15 and Pd@modified SBA-15 are analyzed by N2 sorption isotherms and pore size distributions (Fig. 3). Type IV adsorption-desorption isotherm with an H1 hysteresis loop is observed for both of them which can be characterized as mesoporous solids . The isotherm of Pd@modified SBA-15 indicates a lower N2 uptake, related to a diminution in pore volume and the specific surface area (Table 1). The height of the capillary condensation step slightly reduces due to pore blocking effect by changing in pore size distribution. Indeed, less uniformity of the mesopore size distribution is visible in functionalized SBA-15.
The FESEM of SBA-15 and Pd@modified SBA-15 manifest structural integrity and morphology (Fig. 4 a,b). The integrity of the porous structure of SBA-15 is preserved after loading Palladium and ligand. 2D Hexagonal network of the Pd@modified SBA-15 is determined by the TEM image that shows parallel channels (Fig. 4 c).
From the FT-IR spectra (Fig. 5), the SBA-15 spectrum displays a broad absorption band at 3439 cm-1 which is associated with the presence of silanol groups. A low-intensity band at 1632 cm-1 is attributed to the deformation modes of O-H bonds from adsorbed water. The Si-O-Si bond stretching vibrations with the bending vibrations are revealed at 1077 cm-1, 801 cm-1, and 455 cm-1. The stretching vibrations of the Si-OH and Si-O-Si bonds appear as a weak band at 960 cm-1. The peaks related to silica network of SBA-15 are observed with the same intensity in all processes of functionalization. In the spectrum of chlorinated-SBA-15, peaks at 3114 cm−1 and 1590 cm-1 are associated with the vibration of C–H bond in the propyl group. A peak at 709 cm-1 can be attributed to the C-Cl bonds. The results of the 1H NMR spectrum of ligand are approved by FT-IR spectrum. The FT-IR spectrum of ligand exhibits three sharp peaks at 1730, 1665 and 1610 cm−1 attributed to the C=O and C=C bonds, respectively. Moreover, the absorption band at 3450 cm−1 is attributed to the NH stretching vibrations. By loading Pd, the intensity of the absorption band at 3450 cm-1 has diminished and slightly moved towards the lower frequency region. The absorption band at 1520 cm-1 has appeared due to ring >C=N stretching vibration. Based on the results, the complexion of Pd ions with modified SBA-15 has been approved.
The peaks of carbon, nitrogen, oxygen, silicon, and palladium became visible in the EDX spectra which confirmed the uniform distribution of ligand and palladium over SBA-15 (Fig. 6). The weight percentages of elements calculated by EDX are: C (21.97%), N (5.75%), O (46.90%), Si (20.80%), Pd (4.58%).
The TGA analysis demonstrates that Pd@modified SBA-15 has higher weight loss than that of neat SBA-15 (Fig. 7). The weight loss was measured at about 28% up to 300 °C in two steps: the removal of physically absorbed water in the first step and, in the second step, the thermal decomposition of the organic functional group which starts above 300 °C. Consequently, the high decomposition temperature (above 300 °C) is evidence of the high thermal stability of complex Pd/ligand.
The atomic concentration of catalyst and electronic state of the palladium has been characterized by X-Ray photoelectron spectroscopy (Fig. 8). N 1s, C 1s, and Pd 3d peaks are found on the side of Si 2s, Si 2p, and O 1s peaks that approve the loading of ligand and metal on the SBA-15 surface. The N 1s spectrum of Pd@modified SBA-15 shows a peak at 401 eV due to σ*(N-H) resonance. Also, a shoulder is observed at 400 eV which may be caused by π*(NHC=O). Amide π* resonances occur at very similar energies as the σ*(N–H) resonance. The intense and broad C 1s spectrum has exhibited a peak at 285 eV for C–Si, C–H*, σ*(C–C), and σ*(C–N) resonances . The presence of the SiO2 structure is confirmed by binding energies at 103 and 150 eV for Si 2s and Si 2p, respectively . The binding energy of O 1s is 533.52 eV which could be assigned to Si-O, C-O, and C=O. Two distinct peaks at 337.1 and 339 eV have been observed in Pd 3d spectrum of Pd@modified SBA-15 which is associated with 3d 5/2 and 3d 3/2, respectively. Atomic percentage concentrations of the Pd@modified SBA-15 calculated by XPS: C (77.58), N (6.87), O (9.41), Si (5.42), Pd(0.15).
NH3-TPD analysis has been used to calculate the surface acidity of the catalyst (Fig. 9). The TPD curve of initial SBA-15 exhibits no peaks that determined SBA-15 material has no acidity. The immobilization of Pd/ligand complex leads to improving the acidity of the catalyst as opposed to the initial SBA-15. The TPD curve of Pd@modified SBA-15 represents a sharp peak at 300 °C and a peak around 450 °C correlated with weak and medium acid sites, respectively. Also, there is a shoulder peak at ca. 550 °C that is attributed to strong acid sites. Indeed, there are some weak, medium, and strong acid sites in the Pd@modified SBA-15 that make it a very effective catalyst.
Using the reaction of phenylhydrazine, benzaldehyde, and indan-1,2,3-trione as a model procedure, we conducted it in the presence of Et3N, p-TSA nano-NiO, Ligand, SBA-15, modified SBA-15, and Pd@modified SBA-15. The results are summarized in Table 2. We found that the reaction gave very useful results in the presence of Pd@modified SBA-15 (6 mg for a 1 mmole scale reaction) in acetonitrile at room temperature. Furthermore, we conducted the reaction of phenylhydrazine and indan-1,2,3-trione with other aromatic aldehydes and consistently found satisfactory results (Table 3). Yields were slightly higher for aldehydes substituted with electron-withdrawing groups.
The reusability of our nanocatalyst was examined for the model reaction, and it was found that product yields lessened only to a very small extent on each reuse (run 1, 92%; run 2, 92%; run 3, 91%; run 4, 91%; run 5, 90%; run 6, 90%).
Scheme 3 shows a plausible mechanism for this process in the presence of Pd@modified SBA-15. At the start, the activated aldehyde by Pd@modified SBA-15 is condensed with the phenylhydrazine to give intermediate I, which attacks indan-1,2,3-trione to afford the zwitterionic intermediate II. Its tautomer III undergoes an intramolecular nucleophilic addition reaction, which affords the product by an H-atom-transfer reaction. A highly regiospecific synthesis and crystal structure of indenopyrazolone was reported by Yavari and coworkers . The cis configuration of the hydroxy groups was confirmed by NMR (both –OH groups are involved in intramolecular H-bond and X-Ray crystal) [29-31].
In conclusion, we have reported an efficient procedure for the synthesis of indenopyrazolones through a three-component reaction involving phenylhydrazine, aromatic aldehydes and indan-1,2,3-trione. Our procedure uses Pd@modified SBA-15 at room temperature in acetonitrile. The catalyst has been characterized by XRD, FE-SEM, TEM, EDX, XPS, FT-IR, N2 adsorption analysis, TPD, and TGA-DTA. Very satisfactory yields, the excellent reusability of the catalyst, low catalyst loading, and easy separation of products are the merits of this method.
CONFLICTS OF INTEREST
The authors announce that there are no conflicts of interest.