Inorganic nanoparticles possess a high surface-to-volume ratio and, owing to their exposed surface features, they have increasingly been recognized as an important tool in efficient, highly selective catalysis [1-4]. Recently, metal and metal oxide nanocrystals, which preserve essential features of heterogeneous catalysts, have been examined for their organocatalytic, electrical conductivity, and photocatalytic properties [5-9].Nanocatalysis offers considerable potential for developing processes that align with 'green chemistry' principles by seeking to decrease energy requirements and optimizing the use of the available resources. Nanoparticles increase the exposed surface area of a catalyst's active component, thereby enhancing the contact between reactants and catalyst, while mimicking homogeneous catalysts.Recently, we showed the importance of Cu₂O rhombic dodecahedra nanocrystals for the regioselective synthesis of 1, 2, 3-triazoles and 3, 5-disubstituted isoxazoles in green media [10, 11]. In 2014, we demonstrated the first 1, 3-dipolar cycloaddition to 1, 2, 3-triazoles in water using Au nanocrystals [12].

Highly substituted tetrazoles are important therapeutic scaffolds that are present in natural products and pharmaceutical molecules [13-16]. Moreover, they are regarded as a bioisosteric substituent of carboxylic acid during the synthesis of bioactive compounds [17]. Because of their excellent metabolic stability, hydrogen bonding properties, and lipophilicity, these scaffold moieties can facilitate interactions between ligands and receptors to accelerate transport across cell membranes [18-20]. Figure 1 demonstrates the presence of tetrazole moieties in natural products (A); an antihypertensive drug, Losartan (B); and Tomelukast (C), where it is used for its anti-leukotriene and anti-asthmatic properties [21, 22]. Furthermore, the tetrazole compound dimethyl thiazolyl diphenyl tetrazolium bromide (MTT) is used in cell-proliferation assays [23].

Many synthetic methods have been employed for the synthesis of tetrazoles, but the most common is the reaction of NaN₃ with organic nitriles. In 2001, for the first time, Sharpless and coworkers [24, 25] synthesized 5-substituted 1H-tetrazoles by reacting NaN₃ with organic nitriles using ZnCl₂ as the catalyst. Since then, a plethora of homogeneous and heterogeneous catalysts have been used in the synthesis of tetrazole derivatives, including AlCl₃ [26], TBAF [27], ZnO [28], Zn/Al hydrotalcite [29], Yb(OTf)₃ [30], Zn hydroxyapatite [31], Zn(OTf)₂ [32], Cu₂(OTf)₂ [33], Cu₂O [34], ZnBr₂ under microwave irradiation [35], FeCl₃–SiO₂ [36], Fe(OAc)₂ [37], CdCl₂ [38], In(OTf)₃ [39], I₂-or silica-supported NaHSO₄ [40], nano-CuFe₂O₄ [41], CoY zeolite [42], Zn–Cu alloy [43], Cu(OAc)₂ [44], CuSO₄–5H₂O [45], Et₃N.HCl [46], mesoporous ZnS [47], chitosan-derived magnetic ionic liquid [48], AgNO₃ [49], AgNPs [50], WAlPO-5 microspheres [51], CAN-HY zeolite [52], B(C₆F₅)₃ [53], 4-(N, N-dimethylamino)pyridinium acetate [54], cuttlebone [55], CAES [56], diphenyl phosphorazidate [57], Fe₃O₄@chitin [58], activated fuller earth [59], and an ONO pincer-type Pd(Ⅱ) complex [60]. In other work, Volter and coworkers [61] recently demonstrated a one-pot conversion of aromatic aldehydes to tetrazole derivatives.

Most of these synthetic methodologies suffer one or more drawbacks. As well as being expensive, some methods require high catalyst loading, long reaction times, and harsh reaction conditions that use the explosive hydrazoic acid. Meanwhile, other routes give low product yields. To overcome these drawbacks, it remains essential to develop an atom-efficient, economic, green, one-pot synthetic methodology that is applicable to a wide range of substrates. In comparison to conventional heating, the application of microwave irradiation accelerates the chemical processes and transformation and results in higher product yields, lower energy usage, different reaction selectivities, and a milder reaction profile. Recent work has focused on organic reactions using CuO nanoparticles because of their nontoxic, nonhazardous, and recyclable nature [62, 63]. To develop this area further, we therefore considered combining microwave irradiation and CuO nanoparticles to create a green synthetic methodology with an efficient reusable catalyst to synthesize 5-substituted 1H-tetrazoles. Specifically, as part of our wider program oriented towards the synthesis of bioactive heterocycles [64-67], we envisaged a concise route to 5-substituted 1H-tetrazole derivatives that involves the microwave-assisted (3+2) dipolar cycloaddition of readily available nitriles with NaN₃ using recyclable CuO nanoparticles.

Unless otherwise indicated all common reagents and solvents were used as-obtained from commercial suppliers without further purification. CuO nanopowder ( < 50 nm particle size TEM) was purchased from Sigma-Aldrich. ¹H NMR (400 MHz) and ¹³C NMR (100 MHz) were recorded on a Bruker DRX400 spectrometer. Chemical shifts were recorded in ppm relative to the internal solvent peak. The coupling constant, J, is given in Hz. Multiplicities of peaks are given as: s (singlet), d (doublet), t (triplet), and m (multiplet). Mass spectra were recorded on a Perkin Elmer Calrus600 GC-MS spectrometer. IR spectra were recorded by a Bomen DA8 3FTS spectrometer. Powder X-Ray diffraction (PXRD) was conducted with Cu-K_α radiation (λ = 0.154178 nm) on an X-ray diffractometer. Transmission electron microscopy (TEM) was carried out with an accelerating voltage of 200 kV. Microwave-assisted reactions were performed in a Catalyst Scientific Microwave oven system (CATA R; Catalyst System, Pune, India) operating at 2450 MHz and equipped with a glass vial extension by a condenser. An external system was used to monitor and control the reaction temperature.

In a round-bottomed flask, we added a mixture of benzonitrile 1 (0.052 g, 0.50 mmol, 1.0 equivalent) and NaN₃ (0.048 g, 0.75 mmol, 1.5 equivalent) to 5 mL of DMF containing 5 mol% of CuO nanoparticlesunder a N₂ atmosphere. The reaction mixture was irradiated by microwaves at 420 W for 15 min at 75 ℃. The progress of the reaction was monitored by TLC. After the reaction had finished, the mixture was filtered to remove the catalyst. The filtrate was acidified with 5 N HCl (20 mL) to neutralize the product, and extracted with ethyl acetate (2×10 mL). The combined organic layer was dried over anhydrous MgSO₄ before the combined filtrate was subjected to evaporation to obtain the crude compound, which was purified over a silica gel column (60–120 mesh) using 50% ethyl acetate in hexane as the eluent to obtain the corresponding 5-phenyl-1H-tetrazole (2a) as the product.

5-Phenyl-1H-tetrazole (2a). Pale-white solid. ¹H NMR (400 MHz, DMSO-d₆) δ 8.04–8.03 (m, 2H), 7.59 (m, 3H), 4.06 (brs, NH); ¹³C NMR (100 MHz, DMSO-d₆) δ 131.2, 129.4, 126.9, 124.2; IR (KBr, cm^-1): 3431, 2980, 2835, 2684, 2600, 2337, 1739, 1606; MS (EI) m/z: 146 (M+); HRMS (EI, m/z) calculated for C₇H₆N₄: 146.0592; found 146.0583 (M+).

5-(p-Tolyl)-1H-tetrazole (2b). White solid. ¹H NMR (400 MHz, DMSO-d₆) δ 7.92 (d, J = 8.0 Hz, 2H), 7.40 (d, J = 8.0 Hz, 2H), 2.38 (s, 3H); ¹³C NMR (100 MHz, DMSO-d₆) δ 155.2, 141.1, 129.9, 126.9, 121.4, 21.0; IR (KBr, cm^-1): 3431, 2916, 2845, 2690, 2447, 1878, 1732; MS (EI) m/z: 160 (M+).

5-(4-Methoxyphenyl)-1H-tetrazole (2c). Pale-white solid. ¹H NMR (400 MHz, DMSO-d₆) δ 7.99 (d, J = 8.0 Hz, 2H), 7.16 (d, J = 8.0 Hz, 2H), 3.85 (s, 3H); ¹³C NMR (100 MHz, DMSO-d₆) δ 161.4, 154.8, 128.6, 116.3, 114.8, 55.4; IR (KBr, cm^-1): 3075, 2931, 2742, 1610, 1502; MS (EI) m/z: 176 (M+); HRMS (EI, m/z) calculated for C₈H₈N₄O: 176.0698; found 176.0666 (M+).

5-(3-Nitrophenyl)-1H-tetrazole (2d). White solid. ¹H NMR (400 MHz, CDCl₃) δ 8.55 (s, 1H), 8.06 (d, J = 7.6 Hz, 1H), 8.37–8.35 (m, 1H), 8.17 (s, 1H), 7.74 (d, J = 7.6 Hz, 1H); ¹³C NMR (100 MHz, DMSO-d₆) δ 162.3, 148.3, 133.1, 131.2, 126.1, 125.6, 121.5; IR (KBr, cm^-1): 3094, 2970, 1643, 1525; MS (EI) m/z: 191 (M+); HRMS (EI, m/z) calculated for C₇H₅N₅O₂: 191.0443; found 191.0554 (M+).

5-(3-Chlorophenyl)-1H-tetrazole (2e). Yellow solid. ¹H NMR (400 MHz, DMSO-d₆) δ 7.84 (s, 1H), 7.78 (d, J = 6.8 Hz, 1H), 7.42 (d, J = 6.8 Hz, 2H); ¹³C NMR (100 MHz, DMSO-d₆) δ 162.3, 133.9, 131.4, 130.9, 126.5, 126.4, 125.6; IR (KBr, cm^-1): 3431, 3331, 2972, 2827, 2465, 1654, 1556; MS (EI) m/z: 180 (M+); HRMS (EI, m/z) calculated for C₇H₅ClN₄: 180.0203; found 180.0200 (M+).

5-(m-Tolyl)-1H-tetrazole (2f). White solid. ¹H NMR (400 MHz, DMSO-d₆) δ 7.84 (d, J = 7.6 Hz, 1H), 7.78 (d, J = 6.8 Hz, 1H), 7.72 (m, 1H), 7.44–7.39 (m, 2H); ¹³C NMR (100 MHz, DMSO-d₆) δ 138.8, 131.9, 129.3, 127.4, 124.1, 123.9, 20.9; IR (KBr, cm^-1): 3061, 2918, 2696, 2605, 1562; MS (EI) m/z: 160 (M+).

5-(4-Bromophenyl)-1H-tetrazole (2g). Yellow solid. ¹H NMR (400 MHz, DMSO-d₆) δ 7.99 (d, J = 8.4 Hz, 2H), 7.83 (d, J = 8.4 Hz, 2H); ¹³C NMR (100 MHz, DMSO-d₆) δ 155.0, 132.5, 128.9, 124.7, 123.6; IR (KBr, cm^-1): 2995, 2899, 2723, 2617, 1734, 1600; MS (EI) m/z: 223 (M+); HRMS (EI, m/z) calculated for C₇H₅BrN₄: m/z 223.9698; found 223.9790 (M+).

5-(4-Chlorophenyl)-1H-tetrazole (2h). Yellow solid. ¹H NMR (400 MHz, DMSO-d₆) δ 8.04 (d, J = 8.4 Hz, 2H), 7.66 (d, J = 8.4 Hz, 2H); ¹³C NMR (100 MHz, DMSO-d₆) δ 154.9, 135.9, 129.5, 128.7, 123.2; IR (KBr, cm^-1): 2997, 2762, 2632, 1691, 1606; MS (EI) m/z: 180 (M+); HRMS (EI, m/z) calculated for C₇H₅ClN₄: 180.0203; found 180.0273 (M+).

3-(1H-tetrazol-5-yl)aniline (2i). Brown solid. ¹H NMR (400 MHz, DMSO-d₆) δ 7.18 (t, J = 8.4 Hz, 1H) δ 6.86 (d, J = 6.8 Hz, 3H), 5.59 (S, 2H); ¹³C NMR (100 MHz, DMSO-d₆) δ 149.9, 130.5, 119.9, 119.2, 118.9, 116.2, 111.9; IR (KBr, cm^-1): 3433, 3352, 3223, 2225, 1625; MS (EI) m/z: 161 (M+).

5-Benzyl-1H-tetrazole (2j). Pale-white solid.¹H NMR (400 MHz, DMSO-d₆) δ 7.36–7.32 (m, 2H) 7.28–7.26 (m, 3H), 4.29 (s, 2H); ¹³C NMR (100 MHz, DMSO-d₆) δ 136.4, 129.2, 129.1, 127.5, 29.3; IR (KBr, cm^-1): 3101, 2949, 2854, 1529; MS (EI) m/z: 160 (M+); HRMS (EI, m/z) calculated for C₈H₈N₄: 160.0749; found 160.0649 (M+).

5-(o-Tolyl)-1H-tetrazole (2k). White solid.¹H NMR (400 MHz, DMSO-d₆) δ 7.69-7.51 (m, 1H), 7.49–7.38 (m, 3H), 2.48 (s, 3H); ¹³C NMR (100 MHz, DMSO-d₆) δ 137.6, 131.8, 131.2, 129.9, 126.8, 124.3, 20.9; IR (KBr, cm^-1): 2966, 2713, 1737; MS (EI) m/z: 160 (M+).

5-(o-Bromophenyl)-1H-tetrazole (2l). Yellow solid. ¹H NMR (400 MHz, CDCl₃) δ 7.71-7.65 (m, 2H), 7.48–7.44 (m, 2H); ¹³C NMR (100 MHz, CdCl₃) δ 143.4, 134.35, 133.2, 127.7, 125.3, 125.0, 118.8; IR (KBr, cm^-1): 3088, 2916, 2223, 1732, 1538; MS (EI) m/z: 223 (M+).

4-(1H-tetrazol-5-yl)benzaldehyde (2m). White solid. ¹H NMR (400 MHz, DMSO-d₆) δ 10.09 (s, 1H), 8.26 (d, J = 8.0 Hz, 2H), 8.12 (d, J = 8.0 Hz, 2H), 3.38 (brs, NH); ¹³C NMR (100 MHz, DMSO-d₆) δ 192.7, 165.1, 137.5, 130.4, 127.6; IR (KBr, cm^-1): 3406, 2864, 1666, 1575; MS (EI) m/z: 174 (M+); HRMS (EI, m/z) calculated for C₈H₆N₄O: 174.0542; found 174.0513 (M+).

Methyl 4-(1H-tetrazol-5-yl)benzoate (2n). White solid. ¹H NMR (400 MHz, DMSO-d₆) δ 8.19–8.14 (m, 4H), 3.89 (s, 3H); ¹³C NMR (100 MHz, DMSO-d₆) δ 166.0, 155.9, 132.1, 130.6, 129.2, 127.7, 52.9; IR (KBr, cm^-1): 3097, 3010, 2954, 1710; MS (EI) m/z: 204 (M+); HRMS (EI, m/z) calculated for C₉H₈N₄O₂: 204.0647; found 204.0623 (M+).

To determine the catalytic suitability of CuO nanoparticles, we first carried out optimization studies using CuO with benzonitrile (1a) and NaN₃ as model substrates. The results from these experiments are summarized in Table 1. An initial attempt to use 2.5 mol% of CuO nanoparticles as the catalyst under solvent-free conditions at 100 ℃ for 6 h did not yield any 2a product (Table 1, entry 1). Similarly, no 2a was produced when the reaction was carried out using acetonitrile or THF as the solvent at refluxing temperatures (Table 1, entries 2 and 3). Subsequent analysis of the reaction mixtures indicated the presence of starting material only. We then focused on reactions with polar protic solvents such as EtOH at refluxing temperature for 10 h, and obtained a yield of 2a of 50% (Table 1, entry 4). Meanwhile, using H₂O-IPA as the solvent for a reaction at 100 ℃ did not yield any 2a (Table 1, entry 5). This suggested that careful choice of solvent is required to obtain high yields of tetrazole derivatives.

Changing the solvent polarity from polar protic to polar aprotic by using a solvent such as DMSO was shown to be ineffective at boosting the yield for this reaction with multiple spots (Table 1, entry 6). The use of dioxane and ethyl acetate as aprotic solvents also failed to produce 2a (Table 1, entries 7 and 8). However, when the reaction was carried out in DMF for 10 h at 130 ℃, an 80% yield of 2a was achieved (Table 1, entry 9). Increasing the catalyst loading to 5 mol% did not markedly increase the yield after 10 h (Table 1, entry 10). However, performing the same reaction (with 5 mol% catalyst loading) under 15 min of microwave irradiation with DMF as the solvent produced a 99% yield of 2a (Table 1, entry 11). The final result was derived from an experiment conducted without the CuO nanoparticles; this did not yield any 2a product (Table 1, entry 12).

Scheme 1 illustrates the nano-CuO catalyzed synthesis of 5-substituted 1H-tetrazole derivatives that involve the (3+2) cycloaddition reaction using DMF as the solvent with microwave irradiation. Once the reaction had finished, an acidic treatment produced excellent yields of the substituted 1H-tetrazole derivatives.

Encouraged by the synthetic efficiency of the reaction protocol described above, we further investigated the scope of this reaction with a diverse range of nitriles to produce the results that are summarized in Table 2.

The time required to complete the reaction was typically 15–18 min. Following the simple work-up described in Section 2, the corresponding 5-substituted 1H-tetrazole derivatives were obtained with excellent yields. To verify whether the observed catalysis was truly heterogeneous or not, additional control experiments were performed. Here, CuO nanoparticles were stirred in DMF at 130 ℃ for 5 h. After cooling at room temperature, the solution was centrifuged and the precipitated nanocrystals were carefully collected. Freshly measured quantities of 1a and NaN₃ were added to the solution to investigate whether any of the cyclized product 2a was obtained. However, after 20 min of microwave irradiation at 75 ℃, no tetrazole product was observed. These results ruled out any possible contribution from homogeneous catalysis by leached Cu(Ⅱ) species in this (3+2) cycloaddition reaction, indicating that the observed catalytic activity was entirely heterogeneous in nature. Finally, the crude products were purified by column chromatography before undergoing spectroscopic characterization using ¹H NMR, ¹³C NMR, and mass spectroscopy (MS).

Aromatic nitriles containing either unsubstituted or electron-withdrawing groups reacted efficiently to obtain the corresponding 1H-tetrazole derivatives, producing excellent yields after 15 min. Meanwhile, substrates with electron-donating substituents produced slightly lower yields after reactions of 18 min owing to the electrophilic nature of the nitrile moiety attached to the aromatic rings. The benzylic nitriles also reacted comprehensively with NaN₃ to obtain high yields of corresponding 1H-tetrazoles, whereas aliphatic nitriles appeared unreactive under the present reaction conditions. To diversify the synthetic methodology, we attempted to react organic azides, but no corresponding tetrazoles were obtained.

The optimized synthesis was next tested at a larger scale: employing 2 g of 1a and 15 min of microwave irradiation obtained a 99% yield of 2a.

We then examined the recyclability of the CuO nanocrystals. Following the microwave irradiation during the reaction, the reaction mixture was treated with ethyl acetate and water. The CuO nanoparticles were collected by centrifuging the mixture and washed successively with water and acetone before being dried under a vacuum. Another cycle of the reaction was then carried out using the same nanocrystals, with this process repeated for five runs. As shown in Fig. 2, even in the fifth run the catalyst facilitated the reaction to the extent that a 90% yield of 2a was obtained alongside a 95% conversion and with > 95% catalyst being recovered.

The TEM images and PXRD peaks of the fresh CuO nanoparticles and those recovered after completing five cycles of the (3+2) cycloaddition reaction shown in Fig. 3 exhibit no noticeable morphological differences, supporting the conclusion that they may be used as recyclable catalysts.

We have developed an efficient, environmentally friendly, microwave-assisted approach for the synthesis of biologically useful 5-substituted 1H-tetrazole derivatives. Reactions required 15–18 min to produce yields that reached 99% using CuO nanoparticles. Moreover, the CuO nanoparticles were recycled five times without marked loss of catalytic activity. Work is ongoing in our laboratory to further develop CuO-nanoparticle-catalyzed synthesis of bioactive heterocycles for potential biological applications.

Acknowledgments

The authors thank the Chancellor and Vice Chancellor of VIT University for providing opportunity to carry out this study. Further the authors wish to thank the management of this university for providing seed money as research grant. Kaushik Chanda thanks CSIR-Govt of India for funding through Grant No. 01(2913)/17/EMR-Ⅱ. The authors are thankful to anonymous reviewers for providing constructive comments for the overall improvement of the manuscript.