Current Microwave Chemistry - Volume 7, Issue 3, 2020

Microwave chemistry is an emerging area of science mainly focusing on various applications of microwave energy into chemical processes. Microwave irradiation has enormous potential to provide controlled energy directly to the molecules of interest. On the other hand, homogeneous gold catalysis has emerged in the last two decades or so as one of the most promising fields in organic and organometallic chemistry. Its efficacy has been established many times for the construction of new C – X (X = O, N, S, etc.) and C – C bonds under mild reaction conditions. Although a significant number of reports have appeared in the literature regarding the homogeneous gold-catalyzed organic transformations under microwave conditions, this is the first review article which is going to appear in the literature. This mini-review is designed to give an interesting insight into various homogeneous goldcatalyzed organic reactions under microwave irradiation for the synthesis of a library of electronically and structurally diverse and biologically important organic molecules.

Background: Benzoxazole, containing a 1,3-oxazole system fused with a benzene ring, has a profound effect on medicinal chemistry research owing to its important pharmacological activities. On the other hand, the benzoxazole derivative has exhibited important properties in material science. Especially in recent years, microwave-assisted synthesis is a technique that can be used to increase diversity and quick research in modern chemistry. The utilization of microwave irradiation is beneficial for the synthesis of benzoxazole in recent years. In this focused review, we provide a metaanalysis of studies on benzoxazole in different reaction conditions, catalysts, and starting materials by microwave technique so far, which is different from conventional heating. Methods: Synthesis of different kind of benzoxazole derivatives have been carried out by microwave irradiation. The most used method to obtain benzoxazoles is the condensation of 2-aminophenol or its derivatives with aldehydes, carboxylic acids, nitriles, isocyanates, and aliphatic amines. Results: Benzoxazole system and its derivatives have exhibited a broad range of pharmacological properties. Thus, many scientists have remarked on the importance of the synthesis of different benzoxazole derivatives. Conventional heating is a relatively inefficient and slow method to convey energy in orientation to the reaction medium. However, the microwave-assisted heating technique is a more effective interior heating by straight coupling of microwave energy with the molecules. Conclusion: In this review, different studies were presented on the recent details accessible in the microwave- assisted techniques on the synthesis of the benzoxazole ring. It presents all examples of such compounds that have been reported from 1996 to the present. Benzoxazoles showed an extensive class of chemical substances not only in pharmaceutical chemistry but also in dyestuff, polymer industries, agrochemical, and optical brighteners. Thus the development of fast and efficient achievement of benzoxazoles with a diversity of substituents in high yield is getting more noteworthy. As shown in this review, microwave-assisted synthesis of benzoxazoles is a very effective and useful technique.

Microwave heating technology is considered one of the most likely to replace traditional heating methods due to its efficient, quick, and green heating transmission that meets the requirements of sustainable development. Microwave heating can strengthen chemical reactions and change the morphology of minerals, and it can save energy and achieve rapid and efficient heating, clean production, and emission reduction. Therefore, this paper summarizes the research status of microwave heating in the recovery of valuable metals (Cu, Au, V),) from metallurgical waste ore and rare earth elements from rare earth minerals in recent years, expounds the principle of microwave heating, and summarizes the previous experimental phenomena. Finally, the development potential, opportunities, and difficulties of microwave technology in future industrial applications are discussed.

Background: Schiff bases are excellent compounds. They were synthesized by condensation of active carbonyl and amines. They were widely used as a substrate in the preparation of industrial compounds as well as pharmaceutical purposes. They exhibit a wide range of biological activities. In this study, based on the importance of Schiff bases, a sustainable synthetic method was developed employing reflux and microwave irradiation. Objective: The aim is to develop a new synthetic method for imidazole Schiff base derivatives synthesis employing reflux, microwave irradiation, and ethanol as a green solvent. Methods: Synthesis of imidazole Schiff base derivatives was carried out under reflux and microwave irradiation conditions. Antibacterial activity of imidazole derivatives and standard drugs was examined against two Gram-positive bacteria (Staphylococcus aureus, Bacillus subtilis) and two Gramnegative bacteria (Proteus mirabilis, Escherichia coli). Results: Schiff bases were synthesized in the presence of microwave irradiation and ethanol in high yields 90-98% for 2-4 min. The antibacterial effects of Schiff bases were evaluated against both strains of Gram-positive and Gram-negative. Conclusion: In this paper, a novel series of imidazole Schiff base derivatives were synthesized using reflux, microwave irradiation, and ethanol. Antibacterial effects were investigated. The excellent advantages of microwave irradiation in the synthesis of imidazole derivatives include reduction of reaction time from an hour to a minute, high product yield. In this study, the measurement of antibacterial activity was also important. Imidazole derivatives with Cl, OH, and CH3 groups showed antibacterial effects.

Background: Researchers have pursued the new synthesis method. As a newly developed method, microwave (MW), ultrasound (US) and ultraviolet light (UV) assisted synthesis has drawn increasing interests. Under the synergistic effect, many materials with new structure, morphology and properties may be found. As an important rare-earth phosphate, DyPO4 was selected and the effect of MW, US and UV on the preparation was investigated. Methods: The DyPO4·1.5H2O nanostructures were prepared by MW, US, UV and their combination. Results: Hexagonal DyPO4·1.5H2O microcrystals obtained under MW irradiation were broomstick bundles. Needle-shaped products were formed in the presence of MW and US. Interestingly, the broom-sheaf-like structures can self-assemble into flower-shaped structures upon the irradiation of MW and UV. Whereas, MW/UV/US synergetic heating results in mixed morphologies of flower-like and needle-shaped structures. Conclusion: The growth of DyPO4 nanostructures can be tuned by selecting the combination of heating method of MW, US and UV.

Background: A series of β-diketone hydrazones have been synthesized via condensation of isoniazid with series of β-diketone. The structures of the Schiff bases are established by elemental and spectroscopic techniques. The prepared compounds were screened for antibacterial and antioxidant potential by DPPH free-radical scavenging activity and Ferric reducing antioxidant power (FRAP) assays. Methods: β-diketone hydrazine derivatives were synthesized by simple condensation between various β-diketones and isoniazid. The titled compounds were synthesized following both conventional and microwave irradiation methods. The in vitro antibacterial activity of synthesized derivatives was evaluated against gram-positive (B. subtilis, S. aureus and S. pyogenes) and gram-negative (E. coli, and K. pneumonia) bacterial strains and expressed in terms of zone of inhibition and also screened for antioxidant activity. Results: The yield of products was appreciably increased in shorter reaction times with the aid of microwave-assisted synthesis. Therefore, it follows the green chemistry approach by making the above reactions eco-friendly. The synthesized compounds were characterized using FT-IR, 1H NMR, 13C NMR, and elemental analysis techniques. The spectroscopic techniques showed the formation of β-diketone hydrazone compounds. Theoretical data show good agreement with the experimental results. Some of the compounds displayed significant antibacterial and antioxidant potentials when compared to the standard drug. Conclusion: In the present research work, we report the synthesis of some novel β-diketone hydrazone derivatives. A high yield of compounds was noted under microwave-assisted reaction in shorter reaction times. The results revealed that the synthesized Schiff bases showed good radical scavenging activity.

Aims: To synthesise biologically active thiazolidin-4-one by microwave irradiation method and evaluate against different species of bacteria, fungi and Plasmodium falciparum. Background: Microwave irradiation method is serviceable for rapid and sustainable synthesis. In this present study, Thiazolidin-4-one bearing pyrimidine derivatives have been synthesized by microwave irradiation method. Objective: Thiazolidin-4-one is a valuable motif because of its broad-spectrum biological evaluation. It is famous for many types of biological profiles, mainly antimicrobial, anti-tuberculosis, anti- convulsant, antihypertensive, hypoglycemic agent and antimalarial. This biological response leads our attention towards the change of Thiazolidin-4-one skeleton to enhance potential. Present study aims to carry out a rapid synthesis of Thiazolidin-4-one derivative of pyrimidine by microwave- assisted heating. Methods: 4-(4-substituted phenyl)-6-(substituted aryl) pyrimidin-2-amine was the key intermediate required for the synthesis of 3-(4-(Substituted phenyl)-6-(substituted aryl) pyrimidin-2-yl)-2-(4-hydroxy phenyl) thiozolidin-4-one (5A-J), which was prepared by using microwave irradiation. The structures of all newly synthesized motifs were characterized by spectral analysis (IR, 1H NMR, 13C NMR spectroscopy) and screened for antibacterial activity against Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus and Streptococcus pyogenes; antifungal activity against Candida albicans, Aspergillus niger, Aspergillus Clavatus; anti-tuberculosis activity against M. tuberculosis H37RV and antimalarial activity against Plasmodium falciparum. Results: Higher yield with less time-consuming method is the main advantage of Thiazolidin- 4-one bearing pyrimidine motifs synthesis. The excellent biological response of compounds 5B, 5C, 5D, 5G, 5H, 5I, and 5J was observed. Conclusion: As compared to conventional method, less time is required for the preparation of Thiazolidin- 4-one analogues by using advantageous microwave irradiation method. Thiazolidin-4-one derivatives showed improved biological activity.

Objective: A simple and highly efficient microwave-promoted procedure for the synthesis of imidazo[1,2-a]pyridine derivatives from the reaction of aromatic ketones, N-bromosuccinimide, and 2-aminopyridines in lemon juice was designed. The main advantages of this protocol, such as clean reaction profile, mild reaction condition, high yield, and minimum reaction time, were compared to other previously developed methods. Methods: A mixture of aromatic ketones (1a-m) (0.005 m), N-bromosuccinimide (NBS) (0.005 m), and lemon juice (10 ml) was irradiated by microwave at 400-watt power at 85°C, and the formation of α- bromoketones was monitored by Thin Layer Chromatography (TLC). After completion of α-bromination, 2-aminopyridines (0.005 m) was added to the reaction mixture and it was further irradiated at the same reaction condition. After completion of the reaction, the reaction mass was poured in ice-cold water, the solid product obtained was filtered, washed with cold water, and recrystallized from aqueous ethanol. Results: In the present investigation, we have developed an environmentally benign, easy, and highly efficient one-pot procedure for the synthesis of 2-phenylimidazo [1, 2-a] pyridines from the reaction of aromatic ketones, N-bromosuccinimide, and 2-aminopyridines in lemon juice as a natural acid catalyst and solvent under microwave irradiation. This new protocol offers very attractive features such as minimum reaction time, clean reaction profile, mild reaction condition, and green aspects such as avoid poisonous catalyst, hazardous solvents, ease of the work-up procedure, and higher yield. Conclusion: In the present investigation, we have developed an environmentally benign, easy, and highly efficient one-pot procedure for the synthesis of 2-phenylimidazo [1, 2-a] pyridines from the reaction of aromatic ketones, N-bromosuccinimide, and 2-aminopyridines in lemon juice as a natural acid catalyst and solvent under microwave irradiation. This new protocol offers very attractive features such as minimum reaction time, clean reaction profile, mild reaction condition, and green aspects such as no need for a poisonous catalyst and hazardous solvents, ease of work-up procedure, and higher yield.