Current Microwave Chemistry - Volume 9, Issue 1, 2022

Aim: The research work aims to develop a sustainable microwave-assisted scheme for the synthesis of 4-(benzylidene amino)-6-phenylpyrimidine-5-carbonitrile congeners. Background: 4-(benzylidene amino)-6-phenylpyrimidine-5-carbonitrile scaffolds are novel molecules having various pharmacological activities, such as neurodegenerative, antimicrobial, and anticancer. Schiff base congeners are considered efficient pharmacophores for research. These activities are due to the presence of azomethine (CH=N) group in the Schiff base compounds. Objective: The aim of the study was to synthesise different novel Schiff base scaffolds of pyrimidine nuclei by green chemistry in good yield. Methods: The 4-(benzylidene amino)-6-phenylpyrimidine-5-carbonitrile scaffolds were prepared by two-step reactions. Both steps were microwave-assisted. The first step was to synthesize 4-amino-6- phenylpyrimidine-5-carbonitrile as an intermediate compound. This compound was synthesized by using benzaldehyde, malononitrile and formamidine hydrochloride. The reaction conditions are as follows: temperature- 101°C, pressure- 300W, time- 50 min. The final Schiff base congeners were obtained by reacting them with various aromatic aldehydes in the second step. The yield, reaction condition, and time consumption were all very acceptable for the green synthetic methods rather than the conventional schemes. Results: Microwave-assisted method was more efficient. The reactions were less time-consuming, and the overall yield of the all-synthesized compounds was in the range of 72-81%. The synthesized congeners were characterized by different spectroscopic methods. The main functional group, azomethine, was considered by the IR peak at 1611 cm^-1 wavelength. Conclusion: This microwave-assisted synthetic method thus emerges as more eco-friendly due to a much-reduced usage of organic solvents, leading to less harmful residues. Using this scheme, we synthesized different Schiff base congeners with satisfactory chemical yields.

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, CN, 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.

Background: Choline chloride-thiourea/sulfuric acid is a powerful and efficient green catalyst used for one-pot synthesis of quinazoline-4 (3H)-one derivatives via a reaction between various amines, acetic anhydride, and anthranilic acid under microwave irradiation and solventfree conditions (4a-q). Microwave irradiation, which is a faster, more cost-effective, less energyintensive, and more efficient method than conventional heating, has been used to synthesize some quinazolinone derivatives. Introduction: For the past ten years, one of the major subjects in synthetic organic chemistry has been green synthesis, which has used efficient and environmentally friendly methods to synthesize biological compounds. The use of catalysts has significant advantages, including ease of preparation and separation, chemical and thermal stability, and environmental friendliness due to features such as reusability, low cost, and efficient and easy workup techniques. Therefore, the mechanism is performed by a non-toxic organic catalyst that uses the least amount of energy and chemical reactants in accordance with the principles of green chemistry and the least waste. Methods: One-pot and sequential addition methods have been used to synthesize quinazolinone derivatives. In the sequential addition method, the reaction was started by adding acetic anhydride and anthranilic acid to the reaction vessel under microwave irradiation and continued by adding choline chloride thiourea/sulfuric acid as efficient, recyclable green catalysts and the desired amine. In vitro, the well diffusion method against different pathogenic strains was used to evaluate the antimicrobial activity of quinazoline-4 (3H)-one derivatives. Pathogenic strains used were Candida albicans ATCC 10231 (yeast), Aspergillus niger ATCC 16404 (fungus), Escherichia coli ATCC 8739, Pseudomonas aeruginosa ATCC 9027 (bacteria) and ATCC 6538, and Staphylococcus aureus S. epidermidis ATCC 12228. Pyrimidine-containing compounds, in which the 3- hydroxyl, 2,5-dimethoxy, 4-bromo, 4128;methoxy, and 4128;chloro groups are attached to the phenyl ring of pyrimidine, exhibit antimicrobial properties. Results: In a short reaction time, a variety of biologically active quinazolinone derivatives were synthesized with high efficiency. According to the results, it was found that with aliphatic amines, the reaction time was shorter, and the reaction efficiency was higher. Products synthesized from aromatic amines had more antibacterial properties. Conclusion: In this work, a variety of 2-methyl-quinazoline-4 (3H)-one derivatives (4a–q) were synthesized as potent antibacterial agents under microwave irradiation and solvent-free conditions in the presence of ChCl-thiourea/H2SO4 as an efficient, eco-friendly, and recyclable catalyst.

Background: Ultrasonic research can be used to recognise molecular connections and the structural behaviour of molecules. The combination of ultrasonic velocity, density, and viscosity data provides a plethora of information regarding ion interactions, dipoles, hydrogen bonding, multi-polar, and dispersive forces. Objective: This study aimed to determine the molecular interaction between solute dextran of different concentrations and distilled water as a solvent. The “ultrasonic speed (U), density (ρ), and viscosity (η)” at 323 K have been determined at four different frequencies, i.e., “1MHz, 5MHz, 9MHz, and 12 MHz”. The derived acoustical parameters, such as “acoustic impedance (Z), adiabatic compressibility (β), intermolecular free length (Lf), relaxation time (Τ), Gibb’s free energy (ΔG),” have been determined from the experimental data. The significance is given to the subjective data with respect to the nature and quality of the interactions of the particles between solute and solvent in the liquids solutions. Methods: The density was measured by a specific gravity bottle, viscosity by Ostwald’s viscometer, and ultrasonic velocity through an ultrasonic interferometer of the solution. Moreover, the thermo-acoustic parameters were calculated using the measured parameters. Results: Ultrasonic wave propagation affects the physical properties of the medium and hence furnishes information on the physics of liquid and solution. The measured parameters like ultrasonic velocity, density, and viscosity have been used to understand the solute-solute and solutesolvent interactions in the solution containing dextran with water. Conclusion: The effect of frequency on thermo-acoustic parameters has been studied. The nature of the forces between molecules, such as hydrogen bonds, charge transfer complexes, and breaking of hydrogen bonds and complexes, has been interpreted from the above studies.

Background: The study effect of microwave radiation on the catalytic properties of transition metal ferrites synthesized by ceramic and sol-gel methods in the oxidation reaction of carbon monoxide into dioxide. Objective: This work aims to study the effect of microwave radiation on the production of cobalt, copper, nickel, and manganese ferrites by sol-gel combustion technology using various organic reagents and to study their catalytic activity in the oxidative conversion of carbon monoxide into dioxide. Methods: Microwave treatment was carried out on a setup based on an EM-G5593V microwave oven (Panasonic) with a magnetron power varying from 300 to 800 W with an operating frequency of 2450 MHz. X-ray phase analysis of the products was carried out on a Bruker D 2Phazer automatic diffractometer. The measurement of the specific surface area of the samples was determined by low-temperature nitrogen adsorption by the multipoint BET method on a SORBI-MS instrument (ZAO META, Russia). IR spectra were recorded using FT-IR LUMOS Bruker spectrometers. The micrographs of samples were analyzed on the Sigma VP (Carl Zeiss Jena) equipment. Results: It was determined that the most active catalysts for the oxidation of carbon monoxide to dioxide are ferrites obtained by the sol-gel combustion method using microwave radiation to "ignite" the gel. Conclusion: The use of microwave radiation in the preparation of ferrites by the sol-gel method with combustion can initiate a self-propagating exothermic combustion reaction in the entire volume of the sample in a very short time, measured in seconds. The catalytic activity in the oxidative conversion of carbon monoxide obtained by this method of ferrites is comparable to the activity of ferrites obtained by the sol-gel method with traditional combustion.

Background: The 1,2,3-triazole, pyrazole, and coumarin-based derivatives have received much attention due to their wide coverage of biological properties. The present work describes the microwave synthesis of novel triazolyl pyrazolyl pyrazoline substituted coumarins. The structures of all the newly synthesized compounds are characterized by spectral analysis and screened for their in vitro antimicrobial activity by the Broth dilution method. Methods: Using the synthetic method, the targets were prepared by reacting various 3-{3-[3-(5- methyl-1-aryl-1H-1,2,3-triazol-4-yl)-1-phenyl-1H-pyrazol-4-yl]acryloyl} coumarins (coumarin chalcones) (3a-d) with hydrazine hydrate or aryl hydrazine (5a-c) in the presence of acetic/propionic acid under microwave irradiation. Results: The structures of all the synthesized compounds were established by IR, ¹H-NMR, ¹³CAPT, and selected mass spectral data. The target compounds were also screened for their in vitro antimicrobial efficiency against a representative panel of pathogenic strains, specifically Gram-positive bacteria (Staphylococcus aureus, Bacillus subtilis), Gram-negative bacteria (Escherichia coli, Salmonella typhi), and Fungi (Candida albicans, Aspergillusniger). Conclusion: In conclusion, the target compounds were obtained by Microwave Irradiation (MWI) technique in good yield with a short reaction time. Among all the synthesized compounds, 4c, 4h, 6a, 6h, and 6l were found to have significant activity against bacterial and fungal strains.