3.1 Characterization of nickel oxide nanoparticles
To investigate the impacts of microwave on the nickel oxide nanoparticlesʼ synthesis, the nickel oxide nanoparticles were prepared by applying microwave various power levels (450, 650 and 750 W). Figs. (1a, 1b and 1c) respectively illustrate the NiO nanoparticles scanning electron microscope images. The SEM image represented a spherical structure of the NiO nanoparticles, revealing that they were distributed widely under the effect of agglomeration, probably due to the presence of sodium dodecyl sulfate. The below fig. (1a) SEM image of NiO nanoparticles demonstrates an area of narrow distribution of 50-60 nm particle size, suggesting that 450 w is providing the suitable radiation power condition.
As can be seen from the obtained TEM images, the NiO nanoparticles are spherical in shape. The TEM image Fig. (2) shows that NiO mostly spherical nanoparticle sizes lie somewhere between 40-50 nm, while heavy agglomeration can be spotted in some areas. In this study the TEM and SEM imaging systems measurements confirmed the average particle size obtained from Debye-Scherer formula through XRD patterns.
Infrared spectrometer (IR) measuring NiO nanoparticles exhibited some absorption bonds at 1409 cm-1and 11590 cm-1 relative to NiO spinal structure. Also 600-1200 cm-1(Ni-O), 2850 and 2918 (CH2-SDS) and 1082, 1362 (S=O-SDS) adsorption bonds were consistent with the SDS-based synthesized NiO nanoparticles.
The following main elements were obtained from the energy dispersive X-ray (EDX) spectroscopy results: Ni (34%), O (28%), C (24%) and Na (18%). The NiO nanonoparticlesʼ XRD spectrum, synthesized under microwave optimal irradiations condition has been illustrated in Fig. (3). According to the Xpert high score software data, the synthesized sample XRD pattern with its JCPD card No. 47-1049 acknowledges the fruitful synthesis of NiO nanoparticles at 0.058 S-1 scanning rate across 10° to 80° in spinal phase spectrum. The below XRD pattern also contains –in addition to the NO diffusers–the Ni (OH)2 and SDS diffractions. A significant absorption peak of about 260 nm can be seen in the UV-VIS spectra of NiO nanoparticles.
3.2. Synthesis of pyrano [2, 3-d] pyrimidine derivatives by using NiO nanoparticles
To extend the present study to experimental research, so that high performance MCRs are developed for the provision of annulated bioactive molecules of pyrimidine [29, 31], here we claim that in aqueous media, through one-pot three-component domino Knoevenagel- Michael addition reactions, the NiO nanoparticles led to catalysis of fast, efficient, and simple synthesis of pyrano[2,3-d]pyrimidine derivatives (Scheme 1).
Initially, as a model reaction, the three-component reaction of 3-nitrobenzaldehyde 1a, barbituric acid 2a, and malononitrile 3 was employed to examine different solvents including: MeOH, EtOH, H2O, CHCl3 and CH2Cl2 (Table 1) in the presence of NiO nanoparticles (0.04 g). The appropriate chosen solvent for utilization in the reaction was H2O. Consequently, as Table 1 shows, the three-component model reaction was conducted while various amounts of catalysts were available. In order to come upon the optimal amount of catalyst for accessing pyrano [2, 3-d] pyrimidine 4a, the above investigation was conducted. According to the results, H2O with 0.04 g of NiO nanoparticles as the catalyst was the optimal reaction media for the formation of the products 4a.
Once the reaction conditions were optimized, the suitability and effectiveness of the same procedure were surveyed by conducting the reaction through various readily accessible and simple substrates under the optimum conditions. Thus, under the optimum conditions, different aromatic aldehydes containing electron‐donating and electron‐withdrawing groups, namely, NO2, N, N-dimethyl and OCH3 in the para and ortho positions of the benzene ring were came into reaction with malononitrile and barbituric or thiobarbituric acid. As entries 1–12 in Table 4 show, within short reaction times, the concerned products were achieved in moderate-good isolated yields. Also, as entries 8-12 in Table 2 suggest, under the optimum conditions, thiobarbiturate acid was employed, giving high yields of the desired products.
The proposed mechanism of the one-pot reaction between benzaldehyde derivatives, malononitrile and barbituric acid was described in Scheme 2. The acidic surface probably coordinates with the nitrogen and oxygen of the carbonyl carbon on which a partial positive charge appears. Based on this proposed mechanism, NiO nanoparticles are capable of activating aldehyde carbonyl group whilst accelerating barbituric acid enolization. The higher reactivity of the iminium group is utilized to facilitate Knoevenagel condensation between benzaldehyde 1 and malononitrile 2, which produce intermediate 6 that is attacked by the enolized barbituric acid 5 for generation of product 4 after proton transfer and tautomerization of intermediate 7 (Scheme 2).
In summary, by employing the multi-component reaction of malononitrile, barbituric acid, and aromatic aldehydes, in the presence of nanoparticles of NiO as a catalyst, an efficient and rapid synthesis route was developed for pyrano [2,3-d] pyrimidine derivatives in this study resulting in good yields. One can cite several advantages for the same synthesis: it is simple to prepare NiO nanoparticle as the catalyst, the easy accessibility to the starting materials, high reaction times, the clean reaction profile, the easy workup and green media.