A efficient, regioselective and solvent free synthesis of 5,9-dihydropyrimido[4,5-e][1,2,4]triazolo[1,5-a]pyrimidine-6,8(4H,7H)-diones derivatives in presence nanocatalyst sulfuric acid grafted with silica-3-aminotriazol

In this research, SO₃H-functionalized Silica grafted 3-amino triazol has been prepared and identified through FT-IR, XRD, EDX, Fe-SEM, and TGA spectra. 3-chloropropyltriethoxysilane was first reacted with silica nanoparticles and then treated with 3-amino-1H-1,2,4-triazoles and grafted finally with chlorosulfonic acid to prepare SiO₂@(CH2)3-AT/SO₃H (SNP_S-AT/SO₃H). Afterward, the catalytic efficiency has been investigated as a homogeneous and recyclable catalyst for the solvent-free synthesis of 5,9-dihydropyrimido[4,5-e][1,2,4]triazolo[1,5-a]pyrimidine-6,8(4H,7H)-diones by the one-pot multi-component reaction of aromatic aldehydes with 3-amino-1H-1,2,4-triazoles and 1,3-dimethyl barbituric acid under thermal (100 °C) conditions.

Introduction

In recent times, nanoparticles have been investigated intensively due to the excellent attributes that produce them fitting for different applications including wetness sensors, pigments, drugstores, chemistry, and catalysts [1].

The creation of new and effective nanomaterials as catalyst supports is a challenge in new-fashioned research in catalysis that provides a good catalyst nanoparticle. Therefore, the use of them as heterogeneous supports has received increasing attention. Because they offer great surface areas on which both catalysis and interaction between the particles and their support material can occur [2].

Amidst various nanostructured substances, silica has maintained a pre-eminent position both in scientific studies and industrial design because of its facility of preparation and wide range of applications [3].

As the size distribution of silica nanoparticles is the fundamental criterion correlated to the quality of the terminal product, control of these agents is very essential. Recent research explicates that the sol-gel method has been confirmed to be a hopeful method for creating spherical silica nanoparticles, using tetraethylorthosilicate (TEOS) as a precursor due to its easiness and Inexpensive. This technique includes hydrolysis and polycondensation of the precursor in the presence of a base respectively. Which leads to the production of silanol groups that act as active reaction sites. This results in the formation of a cross-sectional three-dimensional grid structure and eventually siloxane (Si-O-Si) bridges, as explained in Scheme 1 [4].

Scheme 1: Sol-gel synthesis of silica NPs.

Much research has been done on this method and it has been proven that the concentration of precursor, catalyst, co-solvent, water, and reaction temperature are among the parameters affecting the size and distribution of nanoparticles [4-6].

During the past several years, much notice has been paid to the manufacturing and use of acid catalysis in organic transformations. constantly, Homogeneous acid catalysts such as Chlorosulfonic acid, Hydrofluoric acid, Sulfuric acid, and Phosphoric acid catalytic and chemical processes received more attention from chemists [7-9]. However, the separation of these acid catalysts from the product and the need to neutralize acidic waste will limit their use. In this context, the use of nanoparticles as heterogeneous catalysts, reusable and strong solid acids in organic reactions, are often considered to follow the principles of green chemistry, which is catalyzed operations expend the least of energy and reagents and minimize waste [10,11].

The expansion and promotion of eco-friendly reactions is the most significant assignment in the simultaneous Scientists and chemical manufacturers. Therefore, due to the deficits of solvent used such as toxicity, high price, and difficulty of separation, performing chemical reactions in the condition absence of solvent has been highly regarded by researchers in industry and academia [12].

In chemistry, a multicomponent reaction (MCR), sometimes referred to as a “Multi-component assembly method” (MCAP), is a chemical reaction where more than two reactants combine in a sequential operation to give extremely selective products that retain the majority of the atoms of the starting material [13,14].

The synthesis of 5,9-dihydropyrimido [5,4- e][1,2,4]triazolo[1,5-a]pyrimidine-6,8(4H,7H)-diones derivatives is an attractive example of these MCRs. 1,2,4-Triazolopyrimidines are unique in the family of organic compounds because of their hopeful biological and pharmacological activities [15,16]. Research on these compounds has proven the medicinal properties of these compounds, such as anticancer, antiviral, antitumor, anti-inflammatory, antimicrobial, antifungal, antihistaminic, analgesic [17], anticonvulsive, and sedative [15] properties activities. Moreover, these compounds are used as cannabinoid receptor inverse agonists, anti-Alzheimer’s, anti-parasitic, and inhibitors of the bromodomain [18]. Accordingly, we have afforded a professional strategy for the synthesis of 1,2,4-Triazolopyrimidines derivatives utilizing accessible primitive materials, eco-friendly procedures, and a simple workup that has a great purpose.

In view of the increasing public concern over environmental degradation and future resources, and in continuation of our previous reports in the field of heterogeneous catalysts [19-23]. In line with this research, silica nanoparticles were synthesized via sol-gel precipitation method, then 3-chloro propyltriethoxysilane was reacted with silica nanoparticles and treated with 3-amino-1H-1,2,4-triazoles and finally immobilization with chlorosulfonic acid as a novel heterogeneous catalyst for one-pot synthesis of 5,9-dihydropyrimido[e-4,5][1,2,4]triazolo[a-1,5]pyrimidine-6,8(4H,7H)-diones via three-component couplings of 3-amino-1H-1,2,4-triazoles (3-AT) 1, 1,3-dimethyl barbituric acid 2, and aldehydes 3 under solvent-free conditions (Scheme 2).

Scheme 2: One‐pot synthesis of 5,9-dihydropyrimido[5,4-e][1,2,4]triazolo[1,5-a]pyrimidine-6,8(4H,7H)-diones derivates in the presence Nano catalyest SNP_S-AT/SO₃H.

Experimental materials and equipment

All reagents and chemicals used including tetraethyl orthosilicate (TEOS), 3-chloropropyl tri methoxy silane (CPTMS), 3-amino-1H-1,2,4-triazoles(3-AT), Chlorosulfuric acid, potassium carbonate 1,3-dimethyl barbituric acid, and aldehydes, toluene, dichloromethane (DCM) and ethanol in this research were purchased from Fluka or Merck. The reaction process and the purity determination of the compounds were monitored by thin-layer chromatography (TLC) on commercial plates of silica gel SILG/UV254 (EtOAc: n-hexane, 1:3). In order to determine the identity and purity of the compounds, the melting point was determined by a measuring device of the German company Kruss. Fourier transform infrared (FTIR) spectra were recorded with potassium bromide pills and the FT-IR-680 plus spectrometer (Jasco, Japan) in the frequency range of 400-4000 cm^-1. Another analysis used in determining the structure of the compounds synthesized in this study is Nuclear Magnetic Resonance. ¹H NMR spectra in DMSO were registered on a BRUKER 400 MHz UltraShield spectrometer and ¹³C NMR spectra were recorded at 100 MHz using TMS as an internal standard (Bu Ali Research Institute, Mashhad, Iran). To determine the size and shape of the crystal, X-ray powder diffraction (XRD) patterns with Cu Kα radiation (λ= 0.15418 nm) model X’Pert Pro made in Germany, have been used. Energy dispersive spectroscopy (EDS) was achieved utilizing a TESCAN Vega model device. The morphology of the particles was examined via the Field Emission Scanning Electron Microscope (FE-SEM) under the acceleration voltage of 26 kV.

Synthesis of SiO₂ nanoparticle

The SiO₂ nanoparticles applied in this research, of the sol-gel procedure, due to their ability to form pure and homogenous results in gentle conditions, have been synthesized [24]. TEOS (6.2ml) was dispersed in ethanol (100 ml) and sonicated for 15 minutes to mix completely. Subsequently, ammonium hydroxide (6.5ml) as a catalyst was added to the mixture, under fast magnetic stirring for 6 h at room temperature. Then filtered by Centrifuges (4000 rpm, 30 min), to remove the solvent. finally washed (3x10) with an ethanol solution and water, and eventually, hydrophilic silica nanoparticles were obtained and dried at 60 °C for 12 h in a vacuum [25,26].

Connection CPTMS upon silica nanoparticles (SiO₂ @ (CH2)3Cl)

Powder-white silica nanoparticle (1 g) was added to 60ml of dry toluene, and the reaction mixture was sonicated for 15 minutes to dissolve better. Subsequently, CPTMS (2 ml), dissolved in toluene, drop-drop into a rounded balloon was added and stirred under an argon atmosphere for 24 h. When the reaction mixture reaches room temperature, then filtered and washed with distilled water and ethanol. The resulting powder was dried in a vacuum at 80 C for 6 hours [7,19].

Grafted 3-AT with silica nanoparticles modified with (SiO₂@(CH2)3-AT)

A mixture of the powder obtained from the previous step (SiO₂ @ (CH2)3Cl) (1 g) and anhydrous toluene(30-mL) in a round-bottomed flask, for achieving a stable suspension, was dispersed by ultrasonic for 15 min. Then suspension of 3-AT (5 mmol) as a base, and K₂CO₃ (5 mmol), in dry toluene, was added to the reaction mixture under conditions (reflux, Ar atmosphere, time 48 h). Ultimately, the following off-white powder was filtered, then it was washed several times with (EtOH/ H₂O) to remove surplus 3-AT, and SiO₂@(CH2)3-AT dried at 100 °C in a Vacuum for 24 h.

Procedure for preparation of the SiO₂@(CH2)3-AT/SO₃H (SNP_S-AT/SO₃H).

In the flask containing SiO₂@(CH2)3-AT (1 g), dried DCM (10 mL), and Et3N (0.2 mL) were added, and it was stirred to obtain a uniform solution. Subsequent 10 min, a solution of chlorosulfonic acid (1 mL) in dry DCM (3 mL) was slowly added drop-drop manner to the solution of SiO₂@(CH2)3-AT at temperature 0 ˚C. Next addition was completed, and the flask contents were stirred at room temperature for 3-5 h. In the end, the residue, beige powder, is filtered and washed with DCM and ethanol before being dehydrated in an oven at 60 ◦C.

Catalyst titration for determining the acidity of SNP_S-AT/SO₃H

Acid-base titrations were employed to measure the acidity of the amount of acid stabilized on the surface of the SiO₂@(CH2)3-AT, 100 mg of the catalyst was stirred in 20 ml double distilled water and titrated versus 0.1 N NaOH solution in the presence (2 drops) of phenolphthalein reagent. According to this investigation, the pH of the solution is 3.5. Also, the results resumed catalyst approval, with no substantial decrease in the amount of acid loading of the catalyst.

Assessment of the catalytic activity of SNP_S-AT/SO₃H by the synthesis of 1,2,4-Triazolo Pyrimidines

In brief, for the synthesis of 5,9-dihydropyrimido[5,4- e][1,2,4]triazolo[1,5-_a]pyrimidine-6,8(4H,7H)-diones derivates: A mixture SNP_S-AT/SO₃H (0.005 g), 3-AT (1 mmol), 1,3-dimethyl barbituric acid (1 mmol), and aryl aldehyde (1 mmol) were stirred at 100 °C for 10 - 15 min in under solvent-free conditions. The rate of product formation was determined by TLC (EtOAc: n-hexane, 1:3). Subsequent completion of the reaction, the Balloon contents were cooled to room temperature, then warmed ethanol (5 mL) was added to the mixture, and the catalyst was collected by utilizing of Centrifuges instrument, and the powder was recrystallized from EtOH to yield the pure product.

Spectral data

9-(3-hydroxyphenyl)-5,7-dimethyl-5,9-dihydropyrimido[4,5-e][1,2,4]triazolo[1,5-a]pyrimidine-6,8(4H,7H)-dione (4k). Light brown solid, M.p.: 210-213 °C, FT-IR (KBr, cm^-1): 3380 (OH), 3262 (NH), 3012 (CH aromatic), 2958, 2914 (CH aliphatic), 1703, 1688 (C=O), 1599 (C=N), 1532, 1460 (C=C), 1388, 1267(C-N), 875 (Bending vibrations aromatic ring). ¹H NMR (400 MHz, DMSO-d₆) δ: 9.61 (s, 1H, NH), 9.47 (s, 1H, OH), 8.27 (s, 1H, Ar-H), 6.96-7.22 (m, 4H, Ar-H), 5.15 (s, 1H, CH), 2.92 (s, 3H, CH₃), 2.80 (s, 3H, CH₃). ¹³C NMR (100 MHz, DMSO-d₆) δ: (162.7, 157.1) (C=O amin), 156.2 (C-OH), 151.9 (C-ring) (149.4, 148.6) (C- triazole ring), 138.1, 130.0, 120.0, 114.1, 111.9, 91.6 (C-Aromatice), 59.8 (C-benzyl), 29.1, 28.9 (CH₃).

Results and discussion

The synthesis process of SNP_S-AT/SO₃H nanocatalyst is illustrated in Scheme 3. In the First Stage, SiO₂ nanoparticles were fabricated through the sol-gel procedure. In the next step, the surface of silica nanoparticles was functionalized via (3-chloropropyl) trimethoxysilan molecule. Then, chloride the group was replaced by 3-AT, and finally SiO₂@(CH₂)₃/AT nanostructure was sulfonated by chlorosulfonic acid to synthesize the desired nanocatalyst SNP_S-AT/SO₃H. The prepared nanocatalyst was characterized by FT-IR spectroscopy, EDX, XRD, FE-SEM, and TGA.

Scheme 3: Schematic for the synthetic steps of SNP_S-AT/SO₃H.

Figure 1 displays the FT-IR spectra of SiO₂, SiO₂@(CH2)3Cl, SiO₂@(CH₂)₃/AT, and SNP_S-AT/SO₃H. In the FTIR spectrum of SiO₂, a strong and evident peak at 1080 cm^-1 can be related to the Si–O–Si symmetric stretching vibrations, also the absorption peaks at 800 and 466 cm^-1 are due to the symmetric stretching vibration and bending vibration of Si-O, respectively. In addition, the absorption peak at 950 and 3415 cm^-1 is attributed to the bending vibration of Si-OH and the vibrations of the hydroxyl group, respectively (Figure 1a) [27]. The absorption bands at 2800 - 2956 cm^-1 correspond to the stretching vibration of a C–H band of CPTMS [28]. Furthermore, the absorption peak at 649cm^-1 indicates the presence of the C-Cl bond [29] (Figure 1b). New peaks at 1493, 1560, and 1644 cm^-1 may be assigned to C-N and C=N stretching, respectively [30,31]. Also, the intensity of the C-Cl peak decreased due to the presence of AT (Figure 1c). The successful sulfonation on the surface of the SiO₂@(CH₂)₃/AT can be confirmed based on the vibrations at 1000 - 1300 cm^-1 [32]. The stretching vibration of SOH bonds at 1070 cm^-1 and the vibrational modes of N-SO₂ bond at 1088 overlapped with a broad peak at 1080 -1262 those of strong Si-O-Si [33-36]. That attributed interaction of the sulfuric acid groups on the surface SiO₂@(CH₂)₃/AT (Figure 1d). Accordingly, a comparison of all IR spectra confirms the successful preparation of the desired nano-catalyst SNP_S-AT/SO₃H

Figure 1: Comparative FT–IR spectra of SiO₂, SiO₂@(CH2)3Cl, SiO₂@(CH₂)₃/AT, SNP_S-AT/SO₃H.

In addition, SNP_S-AT/SO₃H was characterized using Energy Dispersive X-ray Spectroscopy (EDS) analysis. This technique confirmed that the as-prepared catalyst was composed of Si, C, O, N, and S elements Figure 2.

Figure 2: EDS spectrum of the SNP_S-AT/SO₃H.

In order to verify the structure of the obtained nanocatalyst (SNP_S-AT/SO₃H), XRD analysis was performed and the resultant pattern of the prepared sample is displayed in Figure 3. As can be seen, a broad peak centered at 2θ = 24. shows which is consistent with an amorphous nature of SiO₂ structure as reported in the literature [19]. The XRD pattern showed the nanoparticles SiO₂@(CH₂)₃/AT and in comparison with SiO₂, the existence of nitrogen in the structure confirmed the formation of SiO₂@(CH₂)₃/AT. The other present groups in the catalyst did not show any changes in the nanocatalyst structure.

Figure 3: XRD pattern of the SNP_S-AT/SO₃H.

In another assay, the size and morphology of the SNP_S-AT/SO₃H were recognized using FE-SEM. Based on these results, the size of the as-prepared catalyst particles size is about 20 nm, and the nanocatalyst (SNP_S-AT/SO₃H) particles are in the nanometer scale and have a nearly spherical shape (Figure 4).

Figure 4: FE-SEM images of the SNP_S-AT/SO₃H.

Thermogravimetric analysis (TG) of the SiO₂-AT/SO₃H shows in (Figure 5) the thermal stability of the as-prepared sample. The results showed a weight loss (about 4%) at temperatures below 100 degrees °C corresponding to the elimination of H₂O and other organic solvents; however, the second weight loss observed in the temperature range of 170 to 280 °C (about 4%) attributed to the organic components located on the surface of the NPs. Also, The weight loss related to sulfonic acid groups (about 2%) was observed at 300 °C - 380 °C. The largest weight loss (about 8%) can be assigned to the thermal decomposition of the silica NPs at 480 °C - 550 °C. The thermal decomposition of the differential Thermogravimetric analysis (DTG) shows that the decomposition of the organic structure is mainly at a temperature of 550 ° C, and the catalyst is quite stable up to a temperature of 200 ° C. Therefore, the stability of the synthesized nanocatalyst was confirmed [19,32].

Figure 5: TG analysis of SiO₂-AT/SO₃H.

The catalytic activity of the SNP_S-AT/SO₃H was investigated for the synthesize 5,9-dihydropyrimido[5,4- e][1,2,4]triazolo[1,5-a]pyrimidine-6,8(4H,7H)-diones. In order to optimize the reaction conditions, we selected 3-AT, 1,3-dimethyl barbituric acid, and 4-choloro benzaldehyde as the modal reaction. We used several protic, aprotic solvents and solvent-free conditions in the presence of various amounts of the SNP_S-AT/SO₃H catalyst in the modal reaction. Results Table 1 shows that solvent-free conditions obtain better yields than aprotic and protic solvents. Also, the influence of temperature was investigated for this reaction and it was found that the reaction would yield further at 100 °C temperature. Finally, the thermal-assisted model reaction is efficiently carried out by adding catalytic amounts of NPs-AT/SO₃H (0.005 g) in a solvent-free condition.

Table 1: Optimization of the reaction.
Entry	solvent	Catalyst (g)	Temperature (°C)	Time (min)	Yielda (%)
1	EtOH	0.007	80 °C	35	80
2	MeOH	0.007	65 °C	35	75
3	CHCl3	0.007	60 °C	70	60
4	CH2Cl2	0.007	50 °C	65	60
5	H2O	0.007	100 °C	40	85
6	Solvent-free	0.007	110 °C	13	96
7	Solvent-free	0.007	100 °C	20	96
8	Solvent-free	-	100 °C	80	trace
9	Solvent-free	0.003	100 °C	50	60
10	Solvent-free	0.005	100 °C	20	96
a Isolated yield.

In order to develop the scope of this reaction, a wide range of both electron-rich and electron-poor aryl aldehydes were used. It was located that yield electron-rich more than electron-poor aryl aldehyde reacted well in this process. All the corresponding products were obtained with excellent yields. The products were isolated and confirmed by a comparison of their physical data and spectroscopic techniques with those reported in the literature. The results are summarized in Table 2 [15,18,24].

Table 2: SNPS-AT/SO3H catalyzed synthesis of compound 4 5,9-dihydropyrimido[5,4-e][1,2,4]triazolo[1,5-a]pyrimidine-6,8(4H,7H)-diones 4.
Entry	Product	Time (min)	Yield (%)a	M.p.( C) [Ref]
4a		45	90	230-232 [231-232][15]
4b		60	93	216-217 [218-219][15]
4c		20	94	206-208 [206-208][18]
4d		40	85	229-231 [228-229][15]
4e		30	90	286-288 [285-286][17]
4f		25	94	263-265 [263-265][17]
4g		20	96	250-252 [251-252][17]
4h		35	95	248-249 [248-249][18]
4i		30	95	274-276 [274-275][17]
4j		60	80	304-307 [302-304][15]
4k		15	94	210-213b
4l		35	84	194-196 [15]
4m		25	93	238-240 [15]
4n		30	90	249-251[15]
4o		35	82	237-241 [15]
4p		90	30	260-263 [15]
a Isolated yield b Novel compounds.

¹H NMR and ¹³C NMR spectra proved the suggested products. 9-aryl-5,9-dihydropyri-mido[4,5-d][1,2,4]triazolo[1,5-a]pyrimidine-6,8(4H,7H)-diones 4 was formed of the reaction aromatic aldehydes 1, dimethyl barbituric acid 2 and 3-amino-1H-1,2,4-triazoles 3 in the presence of nanocatalyst SNP_S-AT/SO₃H and in the absence of solvent. Generally, both regioisomers 4 and 5 are possible reactions, but in this reaction, the alternative product 5 wasn’t formed. 1H NMR spectroscopic analysis confirms this claim.

The suggested mechanism for the formation of 5,9-dihydropyrimido[5,4-e][1,2,4]triazolo[1,5-a]pyrimidine-6,8(4H,7H)-diones are shown in Scheme 4. Initially, 1,3-diethyl barbituric 2 and aryl aldehyde 3 are activated by SNP_S-AT/SO₃H. Then arylidene-1,3-diethyl barbituric acid 6 is achieved from the Knoevenagel condensation reaction between 1,3-diethyl barbituric acid with aryl aldehyde. A Michael-type addition between Compound 6 and 3-amino-1H-1,2,4-triazole 1 gives the intermediate 7, and subsequent heterocyclization and dehydration give the corresponding products 4. Scheme 4. A plausible mechanism for the formation of 5,9-dihydropyrimido[5,4-e][1,2,4]triazolo[1,5-a]pyrimidine-6,8(4H,7H)-diones 4 catalyzed by SNP_S-AT/SO₃H.

Scheme 4: A plausible mechanism for the formation of 5,9-dihydropyrimido[5,4-e][1,2,4]triazolo[1,5-a]pyrimidine-6,8(4H,7H)-diones 4 catalyzed by SNP_S-AT/SO₃H.

SNP_S-AT/SO₃H (0.005 g) was added to a mixture of 3-AT (1 mmol), 1,3-dimethyl barbituric acid (1 mmol), and 4-chlorobenzaldehyde (1 mmol) in optimal conditions. After completing the reaction, ethyl acetate (5 ml) was added to the reaction mixture SNP_S-AT/SO₃H catalyst was removed from the product obtained using a filtering. The catalyst was washed accurately with EtOH: H₂O (3×10) and dried at 100 °C in a vacuum oven overnight. Using this method, the catalyst recovered is reused up to 6 times with preservation of performance, and only activity decreases insignificantly after six catalytic cycles (Figure 6). The FT-IR spectra of the catalyst after six cycles (a) have shown the same spectral fingerprint of the newly prepared catalyst (b) designating recycled catalyst chemical stability (Figure 7).

Figure 6: Recoverability and reusability of the catalyst in the synthesized 5,9-dihydropyrimido[5,4- e][1,2,4]triazolo[1,5-a]pyrimidine-6,8(4H,7H)-diones 4.

Figure 7: FT-IR spectra for catalyst chemical stability SNP_S-AT/SO₃H.

Conclusion

In conclusion, a new nanocatalyst [SNP_S-AT/SO₃H] was successfully synthesized and characterized for the formation of 5,9-dihydropyrimido[5,4-e][1,2,4]triazolo[1,5-a]pyrimidine-6,8(4H,7H)-diones. The efficiency of this catalysis under solvent-free conditions has made this method green, and environmentally friendly. Also, The catalyst showed an excellent yield in a short time. Another advantage of this catalyst is easy separation from the reaction mixture and reuse without any considerable loss in catalytic activity.

Supporting

The authors gratefully acknowledge partial support of this work by Yasouj University, Iran.

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