Synthesis of tetrazolo[1,5-a]pyrimidine-6-carbonitriles using HMTA-BAIL@MIL-101(Cr) as a superior heterogeneous catalyst

A one-pot three component reaction of benzaldehydes, 1H-tetrazole-5-amine, and 3-cyanoacetyl indole in the presence of a new hexamethylenetetramine-based ionic liquid/MIL-101(Cr) metal–organic framework as a recyclable catalyst was explored. This novel catalyst, which was fully characterized by XRD, FE-SEM, EDX, FT-IR, TGA, BET, and TEM exhibited outstanding catalytic activity for the preparation of a range of pharmaceutically important tetrazolo[1,5-a]pyrimidine-6-carbonitriles with good to excellent yields in short reaction time.

Fused heterocycles are a unique family of conjugated structures widely identified as core units in drug discovery 1,2 . In such important building blocks, the geometrically rigid bicyclic and polycyclic systems with 3D spatial alignment of substituents lead to outstanding biological performances as a result of improved binding capability to multiple receptors with high affinity [3][4][5] . Triazolopyrimidines are one of the most privileged fused heterocycles extensively known for their importance in synthetic chemistry and pharmaceutical science. The existence of a pyrimidine unit with a triazole ring in a single structure makes these skeletons as powerful synthetic intermediates, which have been shown to possess a wide range of biological functions such as antifungal 6 , antitumor 7 , antimicrobial 8 , antimalarial 9 , and antibacterial activities 10 . Thus, the synthesis of new triazolopyrimidines analogues continuously attracts noticeable attention in medicinal chemistry.
Indoles and their derivatives are important heterocyclic motifs with various bioactivities in the field of drug design 11 . The construction of a triazolopyrimidine unit with an incorporated indole nucleus could offer unique skeletons, named as tetrazolo [1,5-a]pyrimidine-6-carbonitriles, with outstanding biological activities such as antiproliferative and antitumor effects 12,13 . The exploitation of these privileged structures with the ability of binding to multiple receptors could allow medicinal chemists to quickly uncover many bioactive scaffolds across a wide range of therapeutics. Despite this significance, there are only quite a few synthetic methods for the preparation of these target candidates 12,13 , in which they suffer from several drawbacks such as high temperature, long reaction time, hazardous organic solvent, and the use of stoichiometric amount of toxic triethylamine as catalyst. Considering the growing demand for green and sustainable protocols to deliver complex organic compounds, the development of environmentally benign methodologies for the synthesis of tetrazolo[1,5-a]pyrimidine-6-carbonitriles is highly desirable.
Metal organic frameworks (MOFs) are currently utilized as versatile heterogeneous catalysts in synthetic methodologies due to their attractive structural features including high porosities, tunable pore sizes, large surface areas, and reasonable chemical and thermal stabilities 14,15 . Recently, incorporation of ionic liquids (ILs) into MOFs pores has attracted significant attention due to the opportunity of integrating the benefits of both ILs and MOFs in a wide range of catalytic purposes [16][17][18][19] . ILs are molten salts in liquid form at below 100 °C, which  20 . Water is considered to be one of the most significant impurities in ionic liquids. The moisture content is an important quality criterion of ionic liquids. Although some ionic liquids, are essentially insoluble in water, they can absorb a considerable amount of water resulting in changes in the physical and chemical properties like conductivity, thermal stability, and viscosity compared to those of the dry ionic liquid [21][22][23][24] . However, several drawbacks including low diffusion coefficients, recycling, packaging, and product purification limit their applications in chemical reactions. Accordingly, it has been shown that the incorporation of ILs into the highly porous materials such as MOFs could overcome this disadvantages 25 .
MOF-catalyzed multicomponent reactions (MCRs) have recently received significant interest for the preparation of complex heterocyclic skeletons with a high level of atom efficiency. While most of the methodologies employed in multicomponent reactions require toxic organic solvents to provide the desired products, MOFs have exhibited to possess better catalytic performance under solvent-free conditions, highlighting the significance of these porous materials as green alternatives in catalytic transformations 26 . In continuing our interest in the development of multicomponent reactions [27][28][29][30][31] , herein, we report a facile one-pot, three-component protocol for the synthesis of a series of tetrazolo[1,5-a]pyrimidine-6-carbonitrile derivatives in the presence of a Cr-based IL/MOF composite as catalyst under solvent-free conditions.

Results
In this study, HMTA-BAIL ionic liquid was incorporated into the MIL-101(Cr) MOF pores to construct a novel IL/MOF hybrid composite, denoted as HMTA-BAIL@ MIL-101(Cr).
The structure of the resultant new nanoporous material was then investigated by several techniques such as powder X-ray diffraction patterns (PXRD), field emission scanning electron microscopy (FE-SEM), energydispersive X-ray (EDX), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), The N 2 adsorption-desorption isotherm (BET) and transmission electron microscopy (TEM). The obtained results from spectroscopic techniques verified that HMTA-BAIL was successfully incorporated into MIL-101(Cr). This is due to physical interactions between cation moieties of the HMTA-BAIL with the linker parts of MOF and also anion species of the IL with the unsaturated Cr metal cluster of MOF which lead to physical adsorption of ILs in the surfaces and the pores of the MOF 32 . Field emission scanning electron microscopy. Figure 2 represents the FE-SEM images of MIL-101(Cr) and HMTA-BAIL@MIL-101(Cr). The images showed that MIL-101(Cr) as a support maintained its morphology during the synthesis of HMTA-BAIL@MIL-101(Cr), with a characteristic octahedral shape form. These results indicated that the ionic liquid incorporation had not significantly affected the morphology of the MIL-101(Cr) framework, which is in good agreement with the PXRD observations.
EDX analysis was carried out to determine the elemental compositions of the MIL-101(Cr) and HMTA-BAIL@MIL-101(Cr) (Fig. 3). The obtained spectra confirmed the existence of C, O, Cr, F and N as the only elementary components of the samples (Fig. 3a,b).  Transmission electron microscopy. In another study, to investigate the particle size and morphology of the hexamethylenetetramine-based ionic liquid/MIL-101(Cr) metal-organic framework, TEM images of the HMTA-BAIL@MIL-101(Cr) are shown in Fig. 7. As indicated, the obtained results were clearly showed that the encapsulation of hexamethylenetetramine-based Ionic liquid into MIL-101(Cr) MOF leads to formation of irregular shapes with an increasing in the particle size.

Discussion
To investigate the catalytic activity of the synthesized novel HMTA-BAIL@MIL-101(Cr), the three-component reaction of 4-bromobenzaldehyde, 1H-tetrazole-5-amine and 3-cyanoacetyl indole was selected as a model reaction. The effects of different solvents as well as solventless conditions on the reaction progress were examined. It was observed that the solvent-free conditions displayed the best yield (75%) compared to the organic solvents (Table 1, entry 6). The effect of the amount of catalyst was then investigated, in which 0.01 g showed to be the optimum quantity (Table 1, entry 6). The reaction progress was subsequently studied at different temperatures as one of the most important factors in the control experiments. When the temperature reduced from 100 to 70 °C, a significant decrease of the reaction yield was observed from 96 to 72%. High temperature (120 °C) also did not provide a better result in the model reaction (    ) and Brønsted acidity of the ionic liquid in the highly ordered crystalline structure of the composite could be the main reason for its outstanding catalytic activity.
In continue, to evaluate the influence of various loading of HMTA-BAIL into MIL-101(Cr), the preparation of HMTA-BAIL/MIL-101(Cr) was carried out using different amounts of HMTA-BAIL (0.05, 1.0, 1.5 and 2.0 g of IL in the presence of 1.0 g of MIL-101(Cr)) and the obtained HMTA-BAIL/MIL-101(Cr) composites were used in model study. As shown in Table 2, the best result was obtained by using 1.0 g of the HMTA BAIL. An increase in the amount of HMTA-BAIL to more than 1.0 g showed no substantial improvement in yield, whereas it was reduced by decreasing the amount of HMTA-BAIL to 0.5 g and entry 2 was chosen to be the target catalyst ( Table 2).
The obtained initial findings inspired us to extend this methodology to other substrates. A series of benzaldehydes were employed in the multicomponent reaction under the optimized reaction conditions ( Table 3). The scope of benzaldehydes displayed good tolerance of electron-withdrawing and electron-donating substituents to give the corresponding tetrazolo[1,5-a]pyrimidine-6-carbonitriles 4 in high to excellent yields (88-98%) within short reaction time (15 min). (Table 4). After completion of the reaction, the catalyst was easily separated from the reaction mixture by a simple filtration and was then reused for the next run. A negligible in the catalytic activity of the HMTA-BAIL@ MIL-101(Cr) was observed after four cyclic experiments. The obtained results exhibited the merit of the new porous material as an effective and recyclable catalyst for the synthesis of the tetrazolo[1,5-a]pyrimidine-6-carbonitriles. The chemical structure of recovered HMTA-BAIL@MIL-101(Cr) was verified using XRD pattern and FT-IR spectrum. There is no significant difference between the XRD and FT-IR of the fresh and recovered nanocomposite (Fig. 8). Also, the acid sites (1.23 mmol/g) of the catalyst after 6 times reused had no dramatic changes based on the acid-base titration measurement, in comparison with acid sites of the fresh HMTA-BAIL@ MIL-101(Cr) nanocomposite (1.26 mmol/g). These facts prove that the efficiency, appearance and structure of HMTA-BAIL@MIL-101(Cr) catalyst remained intact in recycles and there was no considerable deformation or leaching after 6 runs.

Recycling and reusing of the catalyst. The reusability of the HMTA-BAIL@MIL-101(Cr) was investigated for the synthesis of 7-(3-bromophenyl)-5-(1H-indol-3-yl)tetrazolo[1,5-a]pyrimidine-6-carbonitrile 4c
In order to demonstrate the efficacy of the presented methodology, the catalytic activity of HMTA-BAIL@ MIL-101(Cr) in the preparation of tetrazolo[1,5-a]pyrimidine-6-carbonitrile 4c was compared with the only previous report (Table 5). It can be seen that the presented method has several advantages over the reported methodology such as a green chemical approach with no need to use toxic solvents, high yield, and short reaction time.
A plausible mechanism for the synthesis of tetrazolo[1,5-a]pyrimidine-6-carbonitriles catalyzed by HMTA-BAIL@MIL-101(Cr) is shown in Scheme 1. It is suggested that HMTA-BAIL@MIL-101(Cr) serves as a dual Brønsted/Lewis acid catalyst (IL/Cr 3+ active sites), increasing the electrophilicity of the carbonyl groups of the aldehyde and the intermediates. Firstly, the activated carbonyl of the benzaldehyde undergoes a Knoevenagel condensation reaction with 3-cyanoacetyl indole to afford the intermediate A, followed by the condensation reaction with 1H-tetrazole-5-amine to produce the intermediate B. The intramolecular cyclization of the intermediate B with a subsequent auto-oxidation reaction finally gives the desired product 4.

Experimental
Materials and analysis. The high purities chemicals were bought from Sigma-Aldrich and Merck. The substances with the commercial reagent grades were utilized with no more purifying. The melting point was unmodified and defined in a capillary tube over a melting point microscope (Boetius). 1 H NMR and 13 C NMR spectra were attained on Bruker 250 MHz spectrometer with DMSO-d 6 as a solvent and utilizing TMS as an internal standard. Recording FT-IR spectra was performed on Magna-IR, spectrometer 550. Powder XRD (X-ray diffraction) was performed on a Philips diffractometer (X'pert Co.) with Cu Kα mono chromatized radiation (λ = 1.5406 Å). Transmission electron microscopy (TEM) was performed with a Jeol JEM-2100UHR, operated at 200 kV. The microscopic morphology of the products was observed through SEM (LEO, 1455VP). The energy dispersive analysis of X-ray was used to perform compositional analysis (EDX, Kevex, Delta Class 1). A Table 2. Optimization of the loading amounts of HMTA-BAIL for the synthesis of 4c. Reaction conditions: 4-bromobenzaldehyde (1 mmol), 1H-tetrazole-5-amine (1 mmol), and 3-cyanoacetyl indole (1 mmol). a Isolated yield.     www.nature.com/scientificreports/ After evaporation of the solvent, the crude product was recrystallized from EtOH to yield the pure tetrazolo[1,5a]pyrimidine-6-carbonitrile derivatives. All the known synthesized compounds were confirmed by carefully comparing their spectral data and physical properties with the reported literature. Spectroscopic data and copies of IR, 1 HNMR, and 13 CNMR spectra for the new compounds are provided in the Supporting Information.

Conclusions
A facile approach for the synthesis of a number of tetrazolo[1,5-a]pyrimidine-6-carbonitrile derivatives via an one pot three-component reaction of various benzaldehydes, 1H-tetrazole-5-amine, and 3-cyanoacetyl indole was reported. The reactions were conducted under solvent-free conditions at 100 °C catalyzed by a novel hexamethylenetetramine-based ionic liquid/MIL-101(Cr) metal-organic framework composite. The current methodology offers several advantages including high to excellent yields of tetrazolo[1,5-a]pyrimidine-6-carbonitriles in short reaction time, eco-friendly procedure, low catalyst loading, and reusability of the catalyst (Supplementary information S1).