Microwave Irradiation Conditions Research Articles

Effect of the Ferrocene Nucleus on the Microwave-Accelerated Esterification Reaction

Microwave irradiation has been utilized in the syntheses of a wide variety of organic compounds due to its shorter reaction times and improved selectivities. Since ferrocene is an organometallic compound consisting of an iron center sandwiched between two cyclopentadienyl rings, the iron moiety was expected to efficiently absorb microwave irradiation, thereby resulting in the ferrocene molecules exhibiting an antenna effect. Thus, we herein confirmed this antenna effect by employing an esterification reaction as a model reaction, using both ferrocene and benzene derivatives under microwave irradiation and conventional heating conditions for comparison purposes. It was also found that the reaction could be carried out at lower temperature using ferrocene derivatives.

Synthesis of silver nanoparticles with polyols under reflux and microwave irradiation conditions

Four polyols were used as stabilisers to synthesise silver nanospheres under reflux and microwave irradiation conditions. The nanoparticles were covered with a protective up to 10 nm silicon dioxide shell. The shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) technique was used to analyse the self-assembled monolayer of 4-mercaptobenzoic acid (4-MBA) by applying the produced silver core-silica shell nanoparticles. The article provides a detailed description of the synthesis, analysis and application of the nanoparticles based on UV/Vis, HR-TEM and SHINERS data.

A convenient approach for the synthesis of 4,4'-((substituted phenyl)methylene)bis(3-methyl-1H-pyrazol-5-ol) and their antioxidant and antimicrobial activities

A facile one-pot, three component, convenient approach for the synthesis of 4,4'-((substituted phenyl)methylene)bis(3-methyl-1H-pyrazol-5-ol) derivatives (3a–l) has been accomplished from the reaction of two moles of 5-methyl-2,4-dihydro-3H-pyrazol-3-one (1), with one mole various aromatic aldehydes (2a–l) in the presence ZnO nanoparticles as an efficient heterogeneous catalyst under microwave irradiation (MWI) and solvent-free conditions in good to excellent yields. The major advantages of the present method are, short reaction times, magnetically recyclable catalyst and high yields. The newly synthesized compounds were synthesized and characterized by melting point, IR, NMR, mass spectra. Furthermore the compounds were evaluated for their antioxidant and antimicrobial activities which showed good activities when compared to standards.

Chemo-/Regio-Selective Synthesis of Novel Functionalized Spiro[pyrrolidine-2,3'-oxindoles] under Microwave Irradiation and Their Anticancer Activity.

A novel series of nitrostyrene-based spirooxindoles were synthesized via the reaction of substituted isatins 1a-b, a number of α-amino acids 2a-e and (E)-2-aryl-1-nitroethenes 3a-e in a chemo/regio-selective manner using [3+2] cycloaddition (Huisgen) reaction under microwave irradiation conditions. The structure elucidation of all the synthesized spirooxindoles were done using 1H and 13C NMR and HRMS spectral analysis. The single crystal X-ray crystallographic study of compound 4l was used to assign the stereochemical arrangements of the groups around the pyrrolidine ring in spiro[pyrrolidine-2,3'-oxindoles] skeleton. The in vitro anticancer activity of spiro[pyrrolidine-2,3'-oxindoles] analogs 4a-w against human lung (A549) and liver (HepG2) cancer cell lines along with immortalized normal lung (BEAS-2B) and liver (LO2) cell lines shows promising results. Out of the 23 synthesized spiro[pyrrolidine-2,3'-oxindoles], while five compounds (4c, 4f, 4m, 4q, 4t) (IC50 = 34.99-47.92 µM; SI = 0.96-2.43) displayed significant in vitro anticancer activity against human lung (A549) cancer cell lines, six compounds (4c, 4f, 4k, 4m, 4q, 4t) (IC50 = 41.56-86.53 µM; SI = 0.49-0.99) displayed promising in vitro anticancer activity against human liver (HepG2) cancer cell lines. In the case of lung (A549) cancer cell lines, these compounds were recognized to be more efficient and selective than standard reference artemisinin (IC50 = 100 µM) and chloroquine (IC50 = 100 µM; SI: 0.03). However, none of them were found to be active as compared to artesunic acid [IC50 = 9.85 µM; SI = 0.76 against lung (A549) cancer cell line and IC50 = 4.09 µM; SI = 2.01 against liver (HepG2) cancer cell line].

Tuning the morphologies of ZnO for enhanced photocatalytic activity

Facile one-pot method to prepare three-dimensional hierarchical nanostructures of ZnO with good control over their morphologies and enhanced photocatalytic activity was developed by proper selection of template, base and microwave irradiation conditions in pure water. Synthesized ZnO products were characterized by XRD, SEM, BET, and TGA. The effect of morphology on the decolorization of rhodamine B solution under UV light irradiation was investigated. The ZnO nanorods originating from the center (Tetrapods) exhibits enhanced photocatalytic activity in the degradation of Rhodamine B under UV light irradiation. This high degradation efficiency (98.86 %) can be attributed to the large percentage of the exposed [1 0 1] facet, (facet dependent) even though surface area is relatively lower. Photocatalytic study reveals that the decomposition of Rhodamine B follows a first order kinetics regardless of the morphology of ZnO particles. The rate constant for nano tetrapods of ZnO particles is the highest (0.036 min−1), while the rate constant for nano coral reefs is lowest (0.003 min−1). Moreover, the photocatalyst exhibits excellent durability, allowing it to be utilized for at least five consecutive cycles without losing its efficiency. Based on the results obtained from two separate synthesis and characterization experiments performed under identical reaction conditions, we can conclude that our material and synthesis protocol exhibit a high degree of reproducibility. This study signifies the dependence of enhanced photocatalytic activity on morphology but not the surface area of material.

Linear Tetraquinanes by Microwave Assisted [3+2] Cycloaddition and Ring‐Rearrangement Metathesis

AbstractHerein, an improved synthetic strategy to linear tetraquinanes is reported via a [3+2] cycloaddition and ring‐rearrangement metathesis as key steps. The linear tetraquinanes reported earlier were synthesized in a nine step sequence with an overall yield of 9 % while the same tetraquinanes are now prepared in four steps with a 3‐fold increase in overall yield of 29 %. Failure of the conventional [3+2] cycloaddition using allyltriisopropylsilane was overcome by using microwave irradiation (MWI) conditions which is performed for the first time for this purpose. The strategy reported here does not involve any protection and deprotection steps and can be helpful for the synthesis of polyquinane based natural products and synthesis of medicinally important targets.

Synthesis by microwave irradiation of new pyrazole-imidazoline-pyrimidine analogs: Physicochemical and photophysical properties and their biological activity against Trypanosoma cruzi

Pyrazole derivatives are pharmacologically important agents that possess a wide spectrum of biological activities. Thus, the optimization of their synthetic approaches is of great interest for medicinal and pharmaceutical chemistry. In this work, eleven new 7-aryl-2,3-dihydro-7H-imidazo[1,2-c]pyrazolo[4,3-e]pyrimidines 1(a-k) were synthesized employing microwave irradiation conditions (150 W, 15–45 min). All compounds were completely characterized through Fourier Transform Infrared Spectroscopy (FT-IR), Nuclear Magnetic Resonance (NMR), and High-resolution Mass Spectrometry (HRMS) analyses. The in silico prediction of physicochemical properties showed that all analogs do not violate Lipinki's rule with prediction of good oral bioavailability. Analogs showed low activity against T. cruzi. Correlated experimental methods have been applied to investigate the photophysical properties of 1(a-k), describing their absorption properties in function of substituents. The molecular set exhibits a cyan emission, enabling it to investigate the basic fluorescence spectroscopy in solution.

Efficient synthesis, crystallography study, antibacterial/antifungal activities, DFT/ADMET studies and molecular docking of novel α-aminophosphonates

An efficient method for the synthesis of a new series of α-aminophosphonates has been developed in a one-pot Kabachnik-Fields reaction of 4-methylaminophenol with various aldehydes and triethylphosphite under microwave irradiation and neat conditions using ZnO nanoparticles as a reusable and heterogeneous catalyst, with 82–93% yield at 250 Hz within 2-5 min. A single crystal of the studied compound 3e was selected for X-ray diffraction analysis, it crystallizes in the monoclinic crystal system with P 21/n space group. All the compounds were evaluated for their antimicrobial against a panel of Gram-negative pathogenic bacteria such as Escherichia coli, Klebsiella pneumoniae, Acinetobacter baumannii, Serratia marcescens, Morganella morganii, Pseudomonas aeruginosa and Gram-positive :Staphylococcus aureus and against fungi : Candida albicans, Candida krusei, Candida kefyr, Candida lusitaniae, and Candida tropicalis. Further in silico target hunting reveals the antibacterial activity of the designed compounds by inhibiting Dihydropteroate synthase and all the designed compounds have shown significant drug-like characteristics.

Microwave-aided pyrolysis and fragmentation of tar-rich coal and rocks for in situ coal conversion and clean utilization

British Petroleum predicts that coal will account for the largest share in energy resources for power generation until 2040. The U.S. Energy Information Administration forecasts that coal will remain a key component of the country's national electricity portfolio at least until 2040. However, due to serious issues including high carbon emissions, environmental pollution, surface subsidence, resource wastage, and mining accidents associated with traditional coal mining and utilization patterns, international communities have called for the abandonment of coal use and adoption of renewable resources. To overcome the disadvantages, we proposed a novel approach for achieving in situ conversion and power generation using subsurface coal. An earthworm-shaped intelligent unmanned mining machine (IUMM) was designed to use high-power microwave energy to pulverize and pyrolyze coal into particles that can be used for power generation via chemical conversion. In this study, we used IUMM-based technology for in situ conversion and utilization of tar-rich coal and recovery of oil and gas from the coal. We quantified the pyrolysis efficiency of the tar-rich coal and the surrounding sandstone as well as the gas–liquid overflow rate of the coal under various high-power microwave irradiation conditions. The critical microwave energy threshold for discriminating tar-rich coal from the surrounding sandstone was determined, which will help to realise automatic pyrolysis of tar-rich coal by IUMM and minimise the damage caused by high-energy microwave radiation to the surrounding rock. This work aids in the production of cleaner energy, while ensuring energy security.

Analysis of microwave irradiation on the lethal conditions for insect pest in rice and its influence and regulation on the quality of rice

Abstract In view of the problem that conventional insecticidal methods cannot inhibit the hatching of insect eggs in rice, microwave heating was used to kill the insect eggs hidden in the rice grains. The lethal conditions of microwave irradiation on insect pests in rice and the effects of microwave irradiation on the quality of rice were studied, and the process conditions that can both ensure the quality of rice and kill insects were put forward. The microwave energy density parameter was used to evaluate the lethal conditions of microwave irradiation for insect pests in rice. The results showed that the lethality ratio of insect pests was 100% when the microwave energy density was higher than 0.0325 kW h/kg. Both microwave power density and microwave power affected the quality of rice. The results concluded that the cracking ratio, chalkiness and amylose content were increased with the increase of microwave power at the same microwave power density, while the free fatty acid content was decreased. Through the study of variable microwave power process, it was found that the cracking ratio and chalkiness can be controlled below 1.5% when the heating rate of rice at the later stage of microwave irradiation was less than 0.2 °C/s. The use of two-stage variable microwave power process can eventually ensure the disinfestation and the quality of rice simultaneously.

Aqueous microwave assisted novel synthesis of isothiocyanates by amine catalyzed thionation of isocyanides with Lawesson's reagent

A green and sustainable chemistry approach for organic synthesis is described here, which involves microwave exposure of reactants in the presence of water. A general, practical, and highly efficient protocol for the preparation of a broad range of functionalized isothiocyanates has been developed from their corresponding isocyanides using Lawesson's reagent and catalytic amount of amine bases, particularly triethylamine under microwave irradiation conditions. As opposed to established approaches that employ toxic or volatile electrophilic liquids (thiophosgene, its derivatives, or CS2). In addition, these compounds were synthesized also by conventional heating procedures for comparison. Comparison between conventional and microwave-assisted synthesis was carried out by comparing total reaction time and percentage yield. The results suggest that microwave-assisted syntheses lead to higher yields within very short reaction times. From isocyanides to isothiocyanates using Lawesson’s reagent in the presence of triethylamine under conventional and microwave heating conditions.

Access to distal meta-C-H functionalization of arylmethanesulfonic acid derivatives.

Herein, we report the meta-C-H olefination of arylmethanesulfonates aided by a potentially versatile aliphatic nitrile-directing group under microwave irradiation conditions with fair to very good yields and good to outstanding regioselectivities. Significantly, the protocol showed broad substrate scope including olefin-derived drugs and cyclic olefins. Remarkably, a dual meta-C-H bond was amenable to generating the bis-olefination products.

Synthetic approaches of medicinally important Schiff bases: An updated Review

The Schiff base defined by an imine or azomethine (-CH= N-) group, is mostly synthesized by the condensation reaction of carbonyl compounds (Aldehyde or Ketone) with compounds consisting of amine moiety. Schiff bases are among the most chiefly used organic compounds, revealing a wide range of applications, such as electroluminescent effects, fluorescence properties, nonlinear optical and chemosensory properties. The typical Schiff bases are crystalline solids that are basic, although at least some of them combine with strong acids to generate insoluble salt. Schiff bases are widely used in the pharmaceutical, electronic, cosmetic and polymer industries. Schiff bases use various alpha-amino acids and aldehydes in acidic or basic conditions. Schiff bases form a new class of drugs that can strengthen the immune system and also be used in the treatment of various ailments. The C=N imine bond's electrophilic carbon and nucleophilic nitrogen offer great binding chances with many nucleophiles and electrophiles, which can be used to suppress specific diseases, enzymes or DNA replication. These Schiff bases are synthesized from various aldehydes or ketones and amines under stirring conditions, catalyst-free, reflux conditions, conventional methods, microwave irradiation and ultrasonic conditions. Thus, Schiff bases and their derivatives can be synthesized using various techniques and may be further used for enormous biological applications with potent effects.

Biological applications of Schiff bases: An overview

Schiff bases are the compounds containing the azomethine group (-HC=N-). They are formed by the condensation of ketones or aldehydes with a primary amine. Formation of Schiff base generally takes place under acid/base catalysis or with heat. The common Schiff bases are crystalline solids, which are basic but at least some form insoluble salts with strong acids. Schiff bases are used as intermediates for the synthesis of amino acids or as ligands for the preparation of metal complexes having a series of different structures. The electrophilic carbon and nucleophilic nitrogen in the C=N imine bond provide excellent binding opportunities with different nucleophiles and electrophiles thereby inhibiting targeted diseases, enzymes or DNA replication. These Schiff bases are synthesized from various aldehydes and amines under stirring conditions, catalyst-free, reflux conditions, microwave irradiation and ultrasonic conditions. These compounds serve a wide range of applications in pharmaceutical industries as well as chemical industries. There are a series of biological activities exhibited by these compounds which include antimicrobial, anti-inflammatory, antiviral and antioxidant activities. Apart from exhibiting medicinal properties, Schiff base compounds play an important role as ligands in coordination chemistry. Thus, Schiff bases and their derivatives may be further used for enormous biological applications with potent effects.

Synthesis, Cytotoxic Activity, Crystal Structure, DFT, Molecular Docking Study of β-Enaminonitrile Incorporating 1H-Benzo[f]Chromene Moiety

In this work, we used microwave irradiation conditions to synthesize β-enaminonitrile (4), which was affirmed using single crystal X-ray diffraction and the different spectral data. Two tumor cell lines, MCF-7 and MCF-7/ADR, as well as two normal cell lines, HFL-1 and WI-38, were used to assess the anticancer activity of compound 4. The studied molecule exhibited potent efficacy against the MCF-7 and MCF-7/ADR cell lines compared with the reference drugs. Furthermore, target compound 4 had feeble activity against HFL-1 and WI-38. The chemical reactivity was discussed using DFT and QTAIM analysis to study the intrinsic electronic properties of compound 4. A molecular docking study was also conducted to examine their binding affinity to the EGFR. Compound 4 revealed a stable binding mode at the enzyme active pocket more than the reference inhibitor. The docking analysis was performed for molecule (4).

Experimental Investigation of Microwave-Specific Effect on Nazarov Cyclization

Abstract A copper-catalyzed Nazarov cyclization using an aryl vinyl ketone derivative containing a 1,3-dicarbonyl moiety was carried out under microwave irradiation conditions. The Nazarov cyclization was dramatically accelerated and the kinetic rate of the microwave irradiation was 5.8 times faster than that by conventional heating conditions. A theoretical interpretation of the microwave-specific effect as physical phenomena of the molecules based on chemical kinetics and dynamics was attempted. The microwave-specific effect in terms of the pre-exponential factor in the Arrhenius equation is discussed, thus a hypothesis of the conformational equilibrium activation by microwaves is theoretically supported. To further investigate the microwave-specific effect, a catalytic asymmetric Nazarov cyclization was finally performed. As the enantioselectivity is defined as the function of the reaction temperature, observation of the same enantioselectivity implies the same reaction temperature. Interestingly, when the asymmetric Nazarov cyclization was conducted using a chiral copper catalyst, the Nazarov reaction was subsequently enhanced by the microwave irradiation without any loss of the enantioselectivity. It is suggested that the drastic enhancement with the retention of the enantioselectivity was caused not by a thermal effect, but by a microwave-specific effect on the enantioselective reaction.

Mild Acidic Charcoal Catalyzed Synthesis of 3,4-dihydropyrimidin-2(1H)-one/-thione Derivatives

A green and efficient method for the multicomponent synthesis of 3,4-dihydropyrimidin-2(1H)-ones and -thiones using acetic acid supported on activated charcoal as a mild acid catalyst in ethanol under both conventional as well as microwave irradiation conditions has been developed. The catalyst system found more efficient under microwave irradiation conditions than conventional conditions with shorter reaction times and excellent yields.

A novel, swift, and effective green synthesis of morpholino-pyridine analogues in microwave irradiation conditions

We report a facile, efficient, and environmentally benign approach for preparing morpholino-linked 1,4-dihydropyridine derivatives. The four-component fusion of morpholin-4-amine, diethyl acetylene dicarboxylate, malononitrile, and chosen substituted benzaldehydes occurred under microwave irradiation under mild conditions in the presence of potassium carbonate as a catalyst. This protocol offers remarkable benefits such as excellent yields (92–98 %), easy handling, simple workup, economy, cleaner reaction profile, green conditions, shorter reaction time (≤10 min), and avoiding column chromatography for purification.

Selective hydrogenation of furfural using TiO2-Fe2O3/C from Ti-Fe-MOFs as sacrificial template: Microwave vs Continuous flow experiments

A TiO2-Fe2O3/C catalyst was synthesized utilizing Metal-Organic-Frameworks (MOFs) as a sacrificial template prepared by the solvothermal method and pyrolyzed in an inert nitrogen (N2) atmosphere. This material was characterized using various techniques, including X-ray diffraction, N2 porosimetry, and X-ray photoelectron spectroscopy; these techniques confirmed its transformation into TiO2-Fe2O3/C. TiO2-Fe2O3/C was employed in the hydrogenation of furfural using isopropanol and formic acid as hydrogen donors and solvents to produce furfuryl alcohol. This reaction has been studied under microwave irradiation and continuous flow conditions. Results obtained (70% conversion and 100% selectivity in a continuous flow reactor) represent a significant advance in the design of selective biomass-derived conversion strategies, taking into account the frequently uncontrolled reactivities of such molecules that result in several reaction byproducts. This provides the opportunity to continue discovering selective processes with promising applications for the advancement of green chemistry.

Microwave-assisted synthesis of biodegradable and antibacterial thiophene, furan and thiazole derivatives

The rise of bacterial resistance to common clinical antibiotics is calling for alternative techniques to synthesize antibacterial drugs with high biodegradability. For instance, microwave-assisted synthesis is a powerful technique that accelerates organic reactions. Here, we report the synthesis of 19 thiophene derivatives, five furan derivatives, and two thiazole derivatives, using Suzuki–Miyaura cross-coupling by microwave irradiation. Compound biodegradability was measured by the closed bottle test. Results show that microwave irradiation conditions have reduced the reaction time and increased the yield, and this method uses less organic solvent compared to conventional heating. Several compounds show broad antibacterial activity. One compound showed significant antibacterial activity against Streptococcus pyogenes with a minimal inhibitory concentration of 0.097 µg/mL and against Salmonella enterica with a minimal inhibitory concentration of 0.78 µg/mL. Two furan derivatives with the best activity against pathogenic bacteria were found biodegradable and non-toxic for environmental bacteria present in wastewater treatment plants.