Current Microwave Chemistry - Volume 9, Issue 2, 2022

Using combined microwave-assisted flow chemistry approaches is one of the most active areas of microwave chemistry and green synthesis. Microwave-assisted organic synthesis (MAOS) has contributed significantly to developing green synthetic methods, while flow chemistry applications are quite popular in industrial chemistry. The combination of the two has farreaching advantages. In early studies, the flow chemistry concept was applied in domestic microwave ovens already indicating strong potential for future applications. The relatively small diameter of the flow reactors can address the limited penetration depth of microwaves, which is a major impediment in large-scale batch reactors. With the commercial availability of dedicated microwave synthesizers with tunable frequencies and better temperature control, the possibilities to apply flow synthesis grew even broader. The developments focus on several issues; the two major ones are the design and application of reactors and catalysts. Common reactor types include microwave- absorbing, such as silicon carbide, and microwave-transparent materials, such as borosilicate glass, quartz, or Teflon, with the catalyst or solvent adjusted accordingly. Several heterogeneous catalysts are considered strong microwave absorbers that can heat the reaction from inside the reactor. Such materials include clays, zeolites, or supported metal catalysts. Here, the major advances in design and applications and the benefits gained will be illustrated by synthesizing fine chemicals, from organic compounds to nanoparticles and new materials.

Manganese is a vital metal resource, and increased consumption of manganese is leading to the shortage of high-grade manganese ore resources. However, a large number of low-grade manganese ore resources ((Mn<30%) accounts for about 60% of the total manganese resources) have not been effectively utilized because of the lack of efficient industrial utilization methods. Researching new technologies for reducing low-grade pyrolusite is an urgent problem to be solved. Microwave is an effective and environmentally friendly heat source widely used in mining, metallurgy, and chemistry. Different substances have different dielectric constants. The difference in dielectric constant affects the absorption rate of substances, resulting in different heating rates for different substances when heated by microwaves. Microwave is widely used in the metal smelting process because of its unique heating method. So far, few works have been done to verify that microwave heating can effectively promote the reduction of pyrolusite. This article summarizes some current methods of reducing low-grade pyrolusite and compares them with the method of reducing pyrolusite by microwave heating. In addition, this article introduces the principle of microwave- enhanced reduction of pyrolusite and discusses the opportunities and challenges faced by microwave heating technology in its subsequent development. The aim is to analyze and study the promoting effect of microwave heating technology on the reduction of pyrolusite, further improve the utilization of low-grade pyrolusite, and provide new methods and approaches for the comprehensive utilization of mineral resources and provide assistance in industrial production.

Background: A variety of methods have been reported for the synthesis of pyrano[2,3- d]pyrimidines in the literature with some limitations, and generally used expensive catalysts, harmful solvent and prolonged reaction time. This paper describes an efficient and rapid multicomponent synthesis of pyrano[2,3-d]pyrimidine through condensation of aromatic aldehyde, malononitrile and barbituric acid catalysed by agro-waste solvent catalyst under microwave irradiation. The present method provides several added advantages such as being environmentally friendly, simple work-up, inexpensive, and shorter reaction time affording excellent yields. The synthesized compounds were confirmed by various spectroscopic analyses such as FT-IR,¹H- &¹³C-NMR and mass spectrometry. Objective: Develop an eco-friendly method for the synthesis of pyrano[2,3-d]pyrimidine derivatives. Methods: We have selected Water Extract of Lemon Fruit Shell ash extract solvent as a greener homogenous organo catalysts, and reaction is accelerated by microwave irradiation for the inexpensive synthesis of pyrano[2,3-d]pyrimidine derivatives. Results: The pyrano[2,3-d]pyrimidine derivatives are prepared using an agro-waste-based catalyst, which avoids the use of the external base, additives and solvent in multi-component reactions. Further, the rate of the reaction is accelerated by custom-made microwave irradiation. The use of microwave irradiation showed many advantages over conventional methods such as reaction required less time, more yield and fewer by-products. Further, the custom-made microwave oven has the advantage of no spillage of any organic reagent or solvent to the microwave oven walls, because the reaction vessel is connected to a reflux condenser and direct exposure is avoided. Conclusion: In conclusion, we have developed a simple, efficient, agro-waste-based catalytic approach for the synthesis pyrano[2,3-d]pyrimidine derivatives employing WELFSA as an efficient agro-waste-based catalyst under microwave conditions. The method is found to added advantages of less hazardous, eco-friendly, metal-free, chemical-free, short reaction time, simple workup and isolated product in good to excellent yields.

Background: A variety of natural products reveal the presence of the 1H-1,2,3-triazole moiety in their chemical structures. In general, these molecules also play a significant role in the agrochemical, medicinal and pharmaceutical industries. Microwave-assisted reactions have attracted great interest for researchers to synthesize 1H-1,2,3-triazole compounds in shorter times with increased yields. Objective: The objective of this study is to optimize the purity and yield of the product, shorter the reaction time, and make the reaction more eco-friendly with the help of microwave-assisted organic synthesis. Methods: The present work elucidates a very simple but efficient and rapid, highly productive synthesis of various substituted 1H-1,2,3-triazole series, using the Suzuki-Miyaura cross-coupling reaction, employing microwave irradiation in water with tetrabutylammonium bromide (TBAB). Utilizing (S)- (-) ethyl lactate as the starting material, the synthesis of the substituted 1H-1,2,3- triazole aryl bromide (1) was achieved. Results: This compound (1) was subjected to the Suzuki-Miyaura cross-coupling reaction under microwave irradiation, using a variety of aryl boronic acids in an aqueous medium, to attain high yields of the target products, namely 3a-w. Overall, this is an environmentally benign, very efficient technique under microwave irradiations as a green and eco-friendly source. Only those methodologies that involve microwave-assisted reactions during synthesis in a related manner have been reviewed. Conclusion: Microwave-assisted Suzuki-Miyaura cross-coupling reactions in the water of substituted 1H-1,2,3-triazole series can be employed to quickly explore and increase molecular diversity in synthetic chemistry. In this respect, microwave-mediated methods help researchers to make helpful studies.

Aim: The research aims to develop a sustainable microwave-assisted scheme for Synthesizing 5-(benzylidene amino)-1-phenyl-1H-pyrazole-4-carbonitrile congeners. Background: 5-(benzylideneamino)-1-phenyl-1H-pyrazole-4-carbonitrile scaffolds are novel molecules having various pharmacological activities such as neurodegenerative, anti-microbial, anticancer. Schiff base congeners are considered as efficient pharmacophores for research. These activities are due to the presence of azomethine (CH=N) group in the Schiff base compounds. Objectives: To synthesise different novel Schiff base compounds of pyrazole nuclei by green chemistry with a decent yield. Methods: The 5-(benzylideneamino)-1-phenyl-1H-pyrazole-4-carbonitrile scaffolds were prepared by two-step reactions. Both steps were microwave-assisted. The first step was to synthesize 5- amino-1-phenyl-1H-pyrazole-4-carbonitrile as an intermediate compound. This compound was synthesized by using phenyl hydrazine and 2-(ethoxymethylene)malononitrile. The temperature, pressure, and time required for this reaction were 102°C, 300W, and 45 minutes respectively. In the second step, the final Schiff base congeners were attained by reacting this compound with several aromatic aldehydes. The yield, reaction condition, and time consumption were all acceptable for the green synthetic methods rather than the conventional schemes. Results: The microwave-assisted method was more efficient. The reactions were less timeconsuming, and the overall yield of the all-synthesized compounds was 75-82%. Different spectroscopic methods characterized the synthesized congeners. The IR peak is considered the main functional group (azomethine) at 1611 cm-1 wavelength. Conclusions: This microwave-assisted synthetic scheme thus appears more environmentally due to a significant reduction in organic solvents, resulting in fewer hazardous residues. Using this scheme, we prepared different Schiff base congeners with satisfactory chemical yields.

Background: The organic and peptide synthesis, various nanotechnology, and biochemistry processes are being carried out using microwave irradiation. The use of microwaves for synthesis has increased in the past two decades. The microwave offers several advantages such as ease of handling, lesser reaction times, quality of the product, and eco-friendly, which is green. The conventional method of synthesis, on the other hand, requires a longer time, is difficult to handle and maintenance of temperature is also difficult. The use of microwave-assisted reactions over conventional methods is advantageous in medicinal chemistry research as they will be less timeconsuming and crucial in drug discovery and development. On the other side, they might not work in bulk synthesis due to their limited capacity for loading the reaction mixture. Objective: The present work aims to compare reaction time, temperature and percentage of yield of the microwave-assisted synthesis method against the conventional method. Methods: A novel, simple, and green method was developed for the synthesis of tri-substituted imidazoles by microwave irradiation. Both derivatives from conventional and microwave-assisted synthesis were characterized by IR spectroscopy, Mass spectrometry, and 1H-NMR spectroscopy. The same derivatives were also synthesized by the conventional method for comparison. Results: A comparison of both methods was made by comparing the reaction time and the percentage yield. It was found that microwave-assisted reactions produced greater yield in the minimal time, though at different reaction temperatures. Conclusion: It can be concluded from the present comparison study that the use of the microwave for synthesis provides numerous advantages; thus, newer molecules are developed quickly anthat are developed quickly. To further proceed in this direction and to produce evidences, synthesis of more derivatives may be required. The only disadvantage is that it cannot be used for bulk synthesis of the compounds.

Background: In previous work, we successfully prepared CuO/Al2O3 catalysts and evaluated their catalytic activity, kinetics and degradation mechanism for Fenton-like oxidation of p-nitrophenol (PNP) under microwave irradiation. However, we did not study the effect of important preparation parameters on the activities of catalysts. Objectives: (1) The effect of preparation conditions: CuSO4 concentration of the impregnating solution, Al2O3 to CuSO4 solution ratio, type and concentration of precipitant and calcination temperature on the physico-chemical properties and catalytic activity were studied. (2) The catalytic performance of the Fenton-like oxidation reaction of PNP under microwave irradiation was evaluated and correlated with the characterization results. (3) The stability and catalytic mechanism of the catalysts were investigated. Methods: The CuO/Al2O3 catalyst was prepared by the impregnation deposition method. The 20 g pretreated Al2O3 particles were immersed in 0.6 mol/L Cu (NO3)2 solution and 0.4 mol/L NaOH solution for 24 h before and after. After cleaning and drying, the samples were calcined in an air muffle furnace for 4 h at a certain temperature to obtain CuO/Al2O3 catalyst. Then the catalyst was characterized and catalyzed. Results: XRD, BET and FESEM results have demonstrated that the catalyst claimed at 300 and 350°C showed a smaller size, a higher specific surface area and a better distribution of the CuO species than their counterparts prepared at higher calcination temperatures. The CuO/Al2O3 catalyst claimed at 300 and 350°C also showed higher removal efficiencies for PNP than other catalysts prepared at higher calcination temperatures. Conclusion: It was found that the catalysts prepared at 350°C as calcination temperature showed higher surface area, smaller CuO particle size, and uniform CuO particle size distribution, and consequently showed better catalytic activities with better stability and reusability. Moreover, the XPS results of the catalysts showed a decrease in the Isat/Ip ratio after microwave enhanced Fenton- like reaction, confirming that CuO species has been reduced to Cu2O to some extent.