Microwave-Assisted Domino Cyclization Reactions

Background: Microwave-assisted domino cyclization reactions have attracted great interest for researchers to synthesize complex compounds in shorter times with increase yields. The domino reactions were used in various synthetic approaches and many drug deliveries in medicinal chemistry with microwave assisted approach. Methods: Microwave irradiation has been applied for the various domino reactions. The research related to microwave assisted domino cyclization was reviewed and the important methodologies are collected from 2011-2021. Results: Only those methodologies that involve microwave-assisted domino cyclization reactions during synthesis in a related manner have been reviewed. Along with some recent syntheses that are microwave-assisted regarding new heterocyclic moieties are summarized. Conclusion: Microwave-assisted domino cyclization reactions can be employed to quickly explore and increase molecular diversity in synthetic chemistry. We hope that this review will be helpful to find out complex molecule synthesis by microwave-assisted domino cyclization reactions. This review aimed to explain the applications of microwaves for the domino reactions from 2011-2021. In this respect, the microwave mediated methods help researchers to make helpful studies.

Synthesis of Schiff’s Bases of 1,2,4-Triazole Derivatives under Microwave Irradiation Technique and Evaluation of Their Anti-diabetic Activity

Background: Schiff bases play key role for the generation of a large number of biologically active compounds via cycloaddition, replacement and ring closure reactions. Objective: The objective of this study is to optimize the purity and yield of product, reaction time, eco-friendly reaction by the help of microwave assisted organic synthesis. Methods: New series of Schiff’s bases of triazole derivatives were achieved via multicomponent reactions. The starting material benzohydrazide 1 was obtained by esterification of bezoyl chloride with methanol in presence of concentrated sulphuric acid followed by the reaction with hydrazine hydrate. Benzohydrazide was allowed to react with carbon disulphide in ethanolic potassium hydroxide solution to yield potassium dithiocarbazinate 2 which undergoes cyclization by reacting with hydrazine hydrate to afford 4-[amino]-5-phenyl-4H-1,2,4-triazole-3-thiol (3). Further, various Schiff’s bases 4a-f were obtained by reacting 1,2,4-triazole-3-thiol with different substituted benzaldehydes under microwave irradiations as a green and eco-friendly energy source. Results: The structures of the newly synthesized compounds were elucidated in accordance with their spectral data and elemental analysis. Conclusion : The obtained compounds exhibited significant in-vivo anti-diabetic activity as compared to the standard drug Metformin. The anti-diabetic effect was investigated by using Alloxan-induced diabetic model.

Microwave Induced Green chemistry approach towards synthesis of heterocyclic compounds via C-N Bond Forming Reactions

: The synthesis of heterocyclic compounds via carbon-nitrogen (C-N) bond formation reaction is considered as an emerged and efficient protocol in the field of synthetic chemistry. The C–N bond-forming reactions can proceed through condensation, coupling, ring-opening, cyclization or ring closure process etc. The reactivity pattern of these reactions mainly depends upon the reaction conditions as well as the type of catalysts and reacting substances that are associated with the synthesis of heterocyclic compounds containing the C-N system including pyrazole, imidazole, pyridine, pyrimidine, thiazole, tetrazole, isoxazole, benzothiazine and benzimidazole etc. Further, the technique of microwave-induced synthesis becomes an alternative strategy for the sustainable production of structurally diverse organic compounds. This method provides a cleaner reaction, faster reaction rate, atom economy and energy-efficient. So, the utilization of microwave radiation in organic synthesis becomes resource-friendly and eco-friendly processes. It follows the green chemistry approach by using safer solvents, renewable starting materials and green catalysts. The unique feature of this method is to generate various types of bioactive or medicinal agents.

Microwave-assisted organo-catalyzed C-C and C-X (heteroatom) bond-forming reactions-An overview

: Organocatalysis defines small organic molecules exclusively containing carbon, hydrogen, oxygen, nitrogen, sulfur and phosphorous atom to speed-up the chemical reactions. Researcher demonstrated large area of applications in various organic transformations catalyzed by the organocatalysts, due to their less moisture sensitivity and air, easy abundance, less polluting, not interfere with the final product and inexpensive. This highlights high demand and direct benefits in the pharmaceutical intermediate and fine chemical manufacture compared to other conventional transition metal and enzyme catalysts. This review article intends to compile literature reported application of the microwave accelerated organocatalyzed carbon-carbon and carbon–heteroatom bond formation reactions reported in the literature.

Microwave-assisted Palladium-catalyzed C-H bond Functionalizations Towards the Synthesis of Bio-inspired Heterocycles

: C-C or C-heteroatom bond formation from direct C-H bond activation of several heteroarenes containing suitable directing groups has now emerged as an efficient and straightforward strategy for the design of complex heterocyclic molecules as well as their late-stage functionalization. The most common problem of several C-H bond activation reactions is high temperature, long reaction time and unwanted side reactions where recent examples of MW assisted C-H bond activation showed the requirements of low temperature and short completion time and thus proved its efficacy in terms of heating effect and conversion rate of conventional heating methods. The schemes discussed in the present review depict the reaction conditions along with a look into the mechanism involved to render a deep understanding of the catalytic role of palladium-catalysis. In some examples, the optimization procedure of the corresponding strategy has been illustrated through tables, i.e., choice of catalyst, solvent screening, loading of the catalyst and percentage yield with different substrates. Each of the described illustrations has been analyzed considering a wide variety of reactants, reaction conditions, and transition metals employed as the catalyst. This review definitely allows to introduce the synthetic chemists in understanding the challenges associated with the previous methods as well as their drawbacks and future opportunities in choosing substrates, catalyst and reaction conditions. This review would be alluring to a wider range of synthetic chemists in academia and industrial R&D sectors working with heterocyclic chemistry. In this short perspective, an outline of recent eloquent examples of a variety of palladium-catalyzed C-H bond activation involving bio-oriented heterocycles achieved in the past ten years is nicely presented and the pros and cons of each strategy are highlighted so that the researchers could get enough scope for further designing and modification of developed protocols.

Surface Modification of Polybenzimidazole (PBI) with Microwave Generated Vacuum Ultraviolet (VUV) Photo-oxidation

Background: Polybenzimidazole (PBI) is used in high temperature proton exchange membrane fuel cells (HT-PEMFCs) and redox flow batteries, where proton transfer occurs with the nitrogen-containing groups in PBI, and in aerospace applications exposed to oxygen and radiation. Objective: The objective is to investigate VUV photo-oxidation of PBI for the first time in order to incorporate polar functional groups on the surface to potentially enhance proton conductivity in HT-PEMFCs. Methods: A low-pressure microwave discharge of Ar generated 104.8 and 106.7 nm vacuum UV (VUV) radiation to treat PBI with VUV photo-oxidation. Analysis was done with X-ray Photoelectron Spectroscopy (XPS), Atomic Force Microscopy (AFM), water contact angle (WCA) and Thermal Gravimetric Analysis (TGA) to detect changes in chemistry, surface roughness, hydrophilicity, and adhesion, respectively. Results : XPS showed: an increase in the O concentration up to a saturation level of 15 ± 1 at %; a decrease of the C concentration by about the same amount; and little change in the N concentration. With increasing treatment time, there were significant decreases in the concentrations of C-C sp2, C-C sp3 and C=N groups, and increases in the concentration of C=O, O-C=O, O-(C=O)-O, C-N, and N-C=O containing moieties. The water contact angle decreased from 83° for pristine PBI down to 43°, making the surface more hydrophilic, primarily due to the oxidation, since AFM detected no significant changes in surface roughness. TGA analysis showed an improvement of water adhesion to the treated surface. Conclusion: Microwave generated VUV photo-oxidation is an effective technique for oxidizing the surface of PBI and increasing hydrophilicity.

Microwave-assisted C-C and C-heteroatom bond formations in aqueous medium

: In recent times, microwave assisted chemistry have gained enormous attraction in organic synthesis owing to its versatile advantages such as avoidance of harsh reaction condition, increase of yield, eliminates of by product, shorter reaction time and removal of wastages. Besides, water as a reaction medium further includes more benefit as it eliminates all the drawbacks of toxic solvent which may cause injuries for our mother earth. Furthermore C-C and C-heteroatom bond formation reactions are very significant as most of the drug as well as bioactive compounds contain heterocycles or C-hetero bond which is a key tool of chemistry. This article demonstrates the advancement on the topic of the microwave assisted C-C and C-heteroatom bond formation reactions in aqueous medium.