KCC-1-NH2-DPA: an efficient heterogeneous recyclable nanocomposite for the catalytic synthesis of tetrahydrodipyrazolopyridines as a well-known organic scaffold in various bioactive derivatives

Abstract In this study, a novel approach has been used for the efficient synthesis of tetrahydrodipyrazolopyridine derivatives (5a–m) via a four-component one-pot condensation reaction of aromatic aldehydes, hydrazinehydrate, ethyl acetoacetate, and ammonium acetate in the presence of KCC-1-npr-NH2-DPA as an advanced nano-catalyst in ethanol under reflux conditions at 30 min. For this purpose, mesoporous fibrous nano-silica functionalized by dipenicillamine as a novel nanocatalyst (KCC-1-npr-NH2-DPA) was synthesized using a hydrothermal protocol. KCC-1-npr-NH2-DPA nano-catalyst is easily recyclable eight times without the considerable loss of catalytic activity. Other remarkable features include the short reaction time, simple work-up procedure and providing excellent yields (89–98%) of the products under mild reaction conditions. Furthermore, the effects of solvent, concentration of catalyst, time and temperature for the synthesis of tetrahydrodipyrazolopyridine (5a) were studied. Graphical Abstract


Introduction
Pyrazolopyridines represent a well-known organic scaffold in various bioactive derivatives and have various pharmacological properties. These compounds exhibit significant biological properties, including anti-bacterial [1,2], anti-microbial [3,4], anti-fungal [5], anti-tumor [6], anti-virus [7], anti-Leishmania [8], HIF 1-a-prolyl hydroxylase inhibitors [9], B-Raf V600E3 inhibitors [10], protein kinase inhibitors [11], PDE4B inhibitors [12], dopaminergic properties [13] and cancer cell lines growth inhabitation activities [14]. The main problems for the synthesis of pyrazolopyridine compounds are long reaction times, utilization of non-reusable and toxic catalyst and use of particular conditions. Therefore, looking for simple and efficient methods for the synthesis of pyrazolopyridines is essential. For these reasons, multicomponent reactions (MCRs) are specially well suited for variety-oriented synthesis [15]. MCRs provide suitable conditions with high demand in advanced organic synthesis. It is especially accurate in case of heterocycle compounds [16] as those reactions comfort formation of various bonds in one-pot operation [17]. Development of several synthetic methodologies to achieve various diversities of pyrazolopyridines derivatives, exhibits the growing interests into these compounds [18]. MCRs have also be widely utilized for synthesis of functional polymers and polymeric composites with great potential for biomedical and environmental applications [19][20][21][22]. Thus, the synthesis of pyrazolopyridines by MCRs with a suitable catalyst could enhance their efficiency from an ecological points of view.
Several kinds of catalysts have been used to advance the reactions using MCRs, such as acetic acid [23], carbonaceous material (C-SO 3 H) [24], p-TSA [25] and L-proline [26]. In recent years, the use of nanocatalysts has increased quickly and caused in the advancement of several active and capable nanocatalyst for various procedures [27][28][29][30]. These materials have several advantages over formal catalysts, such as excellent activity and high stability. In addition, metal nanoparticles with a superior support provides a large area for the discovery of novel and highly active nanocatalyst for significant reactions, which also promises the advantage of recycling. Recently, surface functionalized mesoporous systems have appeared as one of the most significant research areas in the concerning of advanced functional materials [28,31,32]. Particularly, researchers reported synthesis of fibrous nano-silica (KCC-1), as a new nano-silica with high surface area (typically >700 m 2 g -1 ), broad pore size distribution, large pore sizes [33], ease of surface modification, low density, suitable stability, and low toxicity with good biocompatibility [9,11,12]. Also, this dendritic fibrous nanosilica showed special activities in vast fields such as heterogeneous catalysis [34], gas capture, solar energy harvesting, energy storage [35], medical diagnosis, targeting of drugs [7,[36][37][38], DNA adsorption [39], drug delivery applications [18], bio sensing [40] and CO 2 mitigation [41].
In this study, we reported the use of KCC-1-npr-NH 2 -DPA (d-pencil amine) nanocatalyst as an efficient material for the synthesis of tetrahydrodipyrazolopyridine (5a-l) compounds by MCR of aromatic aldehydes, hydrazinehydrate, ethyl acetoacetate and ammonium acetate under reflux conditions in ethanol as a solvent (Scheme 1). We found that KCC-1-npr-NH 2 -DPA nanocatalyst produce our desired compounds in high yields (89-98%) with excellent recovery and simple work-up procedure. In addition, KCC-1-npr-NH 2 -DPA has a good recycling properties and this advantage is important from economic point of view.

Materials and methods
All chemical materials and solvents were purchased from Merck, Sigma Aldrich and Fluka in high purity and used without further purification. Melting points were measured in open capillaries using an Electrothermal MEL-TEMP apparatus (model 9200) and are uncorrected. X-ray diffraction (XRD) patterns of KCC-1-based materials were recorded on a Siemens D 5000 X-Ray diffractometer (TX, USA) with a Cu K a anode (k ¼ 1.54 A ) operating at 40 kV and 100 mA. Scanning electron microscopy (SEM) images and Energy-dispersive X-ray spectroscopy (EDX) were recorded with FEG-SEM MIRA3 TESCAN, Czech Republic) at 1000 kV. Transmission electron microscopy (TEM) analysis was conducted on a Carl Zeiss LEO 906 electron microscope operated at 100 kV (Oberkochen, Germany). Brunauer-Emmett-Teller (BET) was recorded on a Micromeritics NOVA 2000 apparatus at 77 K using nitrogen as the adsorption gas (FL, USA). The particle size distribution and zeta potential values were determined using Malvern particle size analyzer (Malvern, UK). The purity determination of the products and reaction monitoring were accomplished by TLC on silica gel poly gram SILG/UV 254 plates.

Preparation of KCC-1
KCC-1 was synthesized according to the procedure described by Bayal et al. [42]. Briefly, 1 g cetyl trimethylammonium bromide (CTAB) was added to 10 mL deionized water and after addition of 0.6 g urea, the mixture was stirred for about 3 h at room temperature. Then, 2 gr of tetraethyl orthosilicate (TEOS), 30 mL cyclohexane and 1.5 mL hexanol was added to the flask and sonicated for 30 min. To continue, the mixture was refluxed at 80 C for 24 h. Afterwards, the mixture was cooled to the room temperature and centrifuged to collect the KCC-1 as white sediment. The collected KCC-1 was washed several times with deionized water and ethanol and was dried at 60 C for 24 h. Finally, KCC-1 was calcinated at 550 C for 6 h to remove the CTAB as templating agent. Scheme 1. Synthesis of tetrahydrodipyrazolopyridines (5a-l) in the presence of KCC-1-npr-NH 2 -DPA.

Preparation of KCC-1-npr-NH 2
In order to functionalize the KCC-1 surface with NH 2 moieties, 0.02 g KCC-1 was dispersed in the 1.2 mL dried toluene and sonicated for 30 min. Then 50 lL 3-aminopropyl triethoxysilane (APTES) was added to the mixture and refluxed for 20 h at 80 C. The mixture was separated and washed with toluene several times and dried at 80 C for 24 h.

Preparation of KCC-1-npr-NH 2 -DPA
To a magnetically stirred, 10 mg of dipencilamine (DPA), 5 mg of 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide and 3 mg of N-hydroxysuccinimide were added to the 50 mL of N,N-dimethyl sulfoxide (DMSO) and stirred for 2 h at room temperature and named as flask 1. In another flask, to a mixture of DMSO: toluene (1: 7) (35 mL), 1 g of KCC-1 and 10 lL of APTES were added and stirred for 2 h at room temperature. Subsequently, 'flask 1' was added to the 'flask 2' and stirred for 24 h at room temperature. Finally, the KCC-1-npr-NH 2 -DPA was collected by centrifugation and washed several times with anhydrous toluene and was dried at 50 C and stored in a refrigerator as a white powder (Scheme 2).

General procedures for preparation of tetrahydrodipyrazolopyridine (5a-l)
A mixture of ethyl acetoacetate (2 mmol) (1) and hydrazine hydrate (2.0 mmol) (2) and KCC-1-NH 2 -DPA (0.1 g) in EtOH (4 mL) was magnetically stirred for 10 min at 25 C followed by addition of aromatic aldehyde (1.0 mmol) (3) and ammonium acetate (4.0 mmol) (4). The reaction mixture was heated under reflux conditions for 30 min and then cooled to 25 C. Then the mixture monitored by TLC and after completion of the reaction, the nanocatalyst (KCC-1-NH 2 -DPA) were separated by filtration. The formed precipitate was washed with warm EtOH and recrystallized in EtOH to afford the pure product. The catalyst was washed with acetone/water and used for next runs without a considerable loss of efficiency (Scheme 1).

Characterization of KCC-1, KCC-1-NH2 and KCC-1-NH2-Cys
The synthesis of KCC-1-npr-NH 2 -DPA nanocatalyst involved several stages (Scheme 2). To investigate the morphology of KCC-1-npr-NH 2 -DPA, FE-SEM images were recorded (Figure 1(a)). Moreover, the structure and size of the KCC-1-npr-NH 2 -DPA NPs were evaluated utilizing (TEM) (Figure 1(b)). The uniform fibers of the KCC-1 with high surface area have several Si-OH groups that could grows from the inner to outside as shown in the TEM images. Also, the TEM images revealed the porous, fibrous and spherical form of the nanomaterials where the fibrous system is as a result of using the CTAB for the NPs design, with the fibrous-sphere revealing the formation of KCC-1-based NPs. The size of the KCC-1-npr-NH 2 -DPA is about 25 nm. As shown in Figure S1, the KCC-1-npr-NH 2 -DPA have a constant particle size range of 20-35 nm. EDX results indicates the atomic structure of the produced compound and that the KCC-1 is composed only with Si and O. Though, the carbon is arising from the SEM grid and CTAB as a pattern agent. Moreover, after functionalization with APTES, the weight percent of N, O, S and C are increased which proofs the efficacious surface modification of KCC-1 with APTES and DPA ( Figure S2).
The powder XRD patterns of KCC-1-npr-NH 2 -DPA are shown in Figure 2. The XRD pattern of KCC-1-NH 2 -DPA was performed from 3.0 (2h) to 70.0 (2h) to investigate the crystallinity of the produced nanomaterial in order to obtain additional information about their molecular structures. As can be observed, the result indicated that crystallinity was increased from KCC-1 to KCC-1-npr-NH 2 -DPA, because of two significant peaks. The wide peak between 20 and 30 is related to the amorphous silica. Thus, the XRD templates of the KCC-1-npr-NH 2 -DPA is similar to the fibrous mesoporous silica with DPA.
The N 2 adsorption-desorption isotherms of KCC-1-npr-NH 2 -DPA NPs are shown in Figure S3. The BET and BJH analyses of the KCC-1-npr-NH 2 -DPA were used to determine the porous structure of the nanoparticles ( Figure S3c). The specific surface area and porosity of the materials were determined using the adsorption isotherm and calculated by BET. Also, BJH technique was used to evaluate the pore volume of the KCC-1, KCC-1-npr-NH 2 and KCC-1-npr-NH 2 -DPA. The BET surface area of KCC-1, KCC-1-npr-NH 2 and KCC-1-npr-NH 2 -DPA was obtained as 617, 367 and 78 m 2 g À1 . Also, the average pore size is 6.7 nm. The pore volumes, pore size, and surface area of KCC-1, KCC-1-npr-NH 2 and KCC-1-npr-NH 2 -DPA are clearly proven by the reported results.
FTIR was employed to confirm the proper functionalization of the KCC-1 fibrous structure with -NH 2 and DPA moieties. As shown in Figure S4, the typical peaks of the silica based nanomaterials could be seen in the range of 1049-1075 cm À1 representing the Si-O-Si asymmetric stretching. Also, a Si-OH peak could be observed at 799 cm À1 which showed the asymmetric bending and stretching vibration. In addition, the peak at around 1377 cm À1 is assigned to the amide bonds between the carboxyl of DPA and amine group of the KCC-1-npr-NH 2 .

Results and discussion
Having proven the complete, proper and correct synthesis of the nanocatalyst the catalytically performance was evaluated for the synthesis of tetrahydrodipyrazolopyridines. In order to optimize the MCR conditions and obtain well catalytic activity, synthesis of tetrahydrodipyrazolopyridine was used as a model and investigated under different reaction parameters including the amount of the catalyst, time, temperature, and solvent type. Initially, the effect of solvent on the synthesis of tetrahydrodipyrazolopyridine compounds using the KCC-1-npr-NH 2 -DPA was studied. According to obtained results, the type of solvent has significant effect on the performance of the nanocatalyst. For example, cyclohexane, n-Hexane and CCl 4 , which are nonpolar solvents, gave tetrahydrodipyrazolopyridine at a lower yield than other solvents (   entries 16 and 17). In contrast, the utilization of water caused in an increased yield of 70%, while the yield was considerably increased up to 97% when ethanol was used as an organic solvent in the presence of KCC-1-npr-NH 2 -DPA NPs. In this research, it was found that conventional heating under reflux conditions in ethanol (as a solvent; Table 1) for 30 min in the presence of 0.0001 g of KCC-1-npr-NH 2 -DPA gave more efficient conditions for desired tetrahydrodipyrazolopyridines. We also examined the essential role of temperature in the synthesis of tetrahydrodipyrazolo pyridine in the presence of KCC-1-npr-NH 2 -DPA NPs as the catalyst. In this case, the tetrahydrodipyrazolo pyridines were obtained with excellent isolated yield at 76 C and results clearly indicated that reaction completion is related to reaction temperature. The optimum   temperature for this reaction was 76 C. The higher temperatures lead to alterations in the efficiency of the reaction (Table 1, entries 1 and 2). Next, the amount of catalyst necessary to complete the reaction efficiently was investigated. It was detected that the variation in the KCC-1-npr-NH 2 -DPA NPs amount had an efficient influence. The highest amount of KCC-1-npr-NH 2 -DPA was 0.1 mg, which obtained a desired product at 98% yields ( Figure 3). Also, excellent yields of tetrahydrodipyrazolo pyridine using this catalyst system for 30 min were obtained (Figure 4).
To further investigate the efficiency of the nanocatalyst, we compared the catalytic performance of our catalyst with different control experiment and the results are shown in Table 2. Originally, a standard reaction was accomplished using KCC-1, KCC-1-npr-NH 2 , DPA and KCC-1-npr-NH 2 -DPA; and the results confirmed that the desired product was not formed ( Table 2, entries 1 and 4) after 1 h of reaction time in different amounts. When KCC-1npr-NH 2 -DPA was used as the catalyst, a reaction was performed and completed (Table 2, entry 4).
Ultimately, the reaction conditions were optimized, and to carry out this approach, we specially evaluated this methodology utilizing hydrazine hydrate, ethyl acetoacetate, ammonium acetate and a variety of different substituted aromatic aldehydes in the presence of KCC-1-npr-NH 2 -DPA in ethanol under reflux conditions. As shown in Table 3, the type of substituents on the aromatic ring and electronic effects did not show extremely evident effects in terms of yields under the reaction conditions. Aromatic aldehydes containing electron-donating groups and electron-withdrawing were used and reacted well to afford the desired tetrahydrodipyrazolo pyridines in excellent yields with high purity.
It is undeniable that for a catalytic process, the recovery and reuse of catalyst materials is highly preferable. In this regard, the recyclability of the KCC-1-npr-NH 2 -DPA was investigated using the model reaction of hydrazine hydrate, ethyl acetoacetate, aromatic aldehydes and ammonium acetate under identical reaction conditions. After the completion of reaction, the recovered catalyst from the reaction mixture was washed with acetone/water and dried at room temperature and reused for subsequent reactions. It is obvious that the heterogeneous property of the KCC-1-npr-NH 2 -DPA facilitates the effective recovery of the nanocatalyst   from the reaction mixture during the work-up procedure so that the catalyst could be recycled and reused up to eight consecutive trials without remarkable loss of its catalytic activity ( Figure 5) and the recyclability test of catalyst was stopped after eight runs. Thus, these results indicated that the nanocatalyst was stable and could tolerate the MCR conditions. In order to show the special efficiency of the KCC-1-npr-NH 2 -DPA in comparison with different catalysts witch used for similar reactions, we summarize numerous results for the synthesis of 4-(3,5dimethyl-1,4,7,8-tetrahydrodipyrazolo [3,4-b:4',3'-e] pyridin-4-yl)phenol (5j) in Table 4. Our study has some advantages in compare with other mention studies including high yield of synthetic compound, reasonable time reaction and easy catalyst recovery. In this regard, some of other reported have shorter reaction time.

Conclusions
In summary, we prepared a new fibrous oregano-silica (KCC-1-npr-NH 2 -DPA) and used it as a nanocatalyst for the synthesis of tetrahydrodipyrazolo pyridine derivatives in ethanol as a solvent with excellent yield under reflux reaction conditions. This catalyst could be recovered and reused at least eight times with no significant decrease in its activity and selectivity. Our method has some advantages, containing short reaction times, mild reaction conditions, the reusability of the heterogeneous catalyst, high yields and convenient workup process.

Disclosure statement
No potential conflict of interest was reported by the authors.

Funding
This research was supported by Tabriz University and Tabriz University of Medical Sciences.

Notes on contributorss
Sajjad Azizi received his M.Sc. in organic chemistry from Tabriz University. He has published mainly on organic synthesized using hetrogenis nano-catalysts. Also, He is a researcher in the field of pharmaceutical science based on analytical methods. He is a researcher with over 145 publications and research interests in nano-catalyst and nanomaterial base electrochemistry.
Nasrin Shadjou received her Ph.D. in organic chemistry from K.N. Toosi University of Technology, Tehran, Iran. She currently is an associated professor of Nanochemistry at Department of Nanochemistry, Nano Technology Research Center, Urmia University. She is a researcher with over 156 publications and research interests in nanocatalyst and nanomaterial base electrochemistry. Her research interests include the preparation of metal silica mesoporous and graphene quantum dot materials in particular smart silica materials, and their applications in health science and environmental technology.