Conventional Heating Conditions Research Articles

Organic Base-Promoted C-N- and C-O-Coupled Domino Cyclization Strategy: Syntheses of Oxazine-6-ones and 4-Pyrimidinols.

Oxazine-6-one and 4-pyrimidinol are two important frameworks in pharmaceutical production. Herein, we disclosed a simple, efficient, inexpensive organic base-promoted and additive-stimulated protocol for the syntheses of variably functionalized oxazine-6-ones and 4-pyrimidinols employing acetonitrile solvent under conventional heating conditions using an oil bath through C-N and C-O coupled domino steps. This simple practicable productive protocol utilizes easily producible cheap precursors, namely, benzimidates or benzamidines, with differently substituted dicyano-olefins, and it comprises step economy, robustness, and moisture insensitive conditions affording high yield that avoids the use of transition-metal catalysts, multistep with multicomponent strategy, and harsh reaction conditions involving hazardous chemicals. This method is scalable into gram-scale production with good yield.

A Review of the Synthesis of Oxazoline Derivatives.

Oxazolines are important heterocyclic systems due to their biological activities, such as antibacterial, antimalarial, anticancer, antiviral, anti-inflammatory, antifungal, antipyretic, and antileishmanial. They have been widely applied as chiral auxiliaries, polymers, catalysts, protecting groups, building blocks, and ligands in asymmetric synthesis. Due to their importance, many synthetic routes to prepare oxazoline moieties have been investigated and developed by researchers around the world. In this review, we summarized several synthetic methodologies published in the literature. The main substrates are nitriles, carboxylic acids, and acid derivatives, which react with a variety of reactants under conventional heating, microwave irradiation or ultrasound irradiation conditions. Syntheses via intramolecular cyclisation from amides have also been reported. Many publications have highlighted procedures based on solvent-free conditions using eco-friendly, reusable, and easy-availability catalysts.

BF3@K10: An Efficient Heterogeneous Montmorillonite Catalyst for the Halogenation of N-Heterocycles

AbstractHalogenated N-heterocycles are an essential structural building block in medicinal chemistry. Herein, we describe an economical and efficient protocol for the regioselective halogenation of several N-heterocycles (pyrimidines, a pyrazole, 2-aminopyridine, theophylline, and an imidazo[1,2-a]pyridine) with BF3-doped montmorillonite (BF3@K10). The new catalyst was characterized by FTIR and 11B NMR spectroscopy, XRD, SEM, and EDS. The developed strategy provides easy and fast access to iodo-, bromo-, and chloro-N-heterocycles under mild conditions. This method was used to synthesize nine new halogenated pyrimidine derivatives. The reaction is simple and general, affording good to excellent yields of products under conventional heating or microwave conditions in the presence of BF3@K10 as an ecofriendly, inexpensive, and efficient catalyst. This protocol is clearly superior to the conventional route because it offers short reaction times, high yields, and easy workup.

A comparative investigation: evaluating the epoxidation of styrene with binuclear molybdenum(VI) complexes under conventional heating and microwave-assisted conditions

Two bis-[cis-MoO2]2+ core bearing complexes, designated as [{MoVIO2}2(L1)(H2O)2] (1) and [{MoVIO2}2(L2)(H2O)2] (2), were created by reacting MoO2(acac)2 with dihydrazone ligands, namely H4L1(i) and H4L2(ii) (generated from a 2:1 ratio of hydrazide of terephthalate and salicylaldehyde or o-vanillin) in a 2:1 ratio in a methanolic solution. These complexes were extensively characterized using several spectroscopic techniques. The shape of the complex [{MoVIO2}2(L1)(H2O)2] (1) was confirmed by SC-XRD, revealing an octahedral geometry. Moreover, the crystallographic analysis of complex (1) elucidates the substitution of a single facet of the water molecule with DMF, accompanied by the inclusion of an additional DMF molecule within the crystal lattice. These synthesized compounds were evaluated as potential catalysts for the epoxidation of styrene in the existence of oxidant (H2O2). The oxidation reactions were conducted using both conventional and microwave-assisted methods, and the obtained data were compared in terms of activity and selectivity.

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.

The mechanism of photothermal catalytic reforming of biomass tar: An exploratory study based on density functional theory

Tar removal is a hot topic in the biomass gasification field. Photothermal catalytic reforming (PCR) is an emerging approach that shows high efficiency, but the mechanism and tar cracking reaction pathway under PCR are still not clear. In this paper, as a widely recognized catalyst that has both thermal and photo-catalysis effects, Ni/TiO2 was investigated and the model was calculated by density functional theory (DFT). Toluene was selected as the model tar compound, and the cracking performances were analyzed in both PCR and conventional heating conditions. It was found that H2O and toluene can be stably adsorbed on the catalyst surface but the adsorption sites are different, which provides support for the comparisons of toluene cracking pathways. The energy barriers of CC bond breaking reactions were calculated and the toluene cracking pathway showed significant changes under PCR, due to the formation of oxygen vacancy and hydroxyl radical. Toluene mainly occurs in the dissociation reaction of methyl group under conventional conditions, but toluene tends to open aromatic ring to form chain hydrocarbons under PCR conditions. It indicates that PCR shows greater activation on the aromatic ring which is crucial for tar cracking. This study confirms the change of the toluene cracking pathway under photothermal conditions by the DFT calculation method and also provides guidance for the optimization of the tar removal process and photothermal catalyst preparation.

Continuous-Flow Microwave Heating Inactivation Kinetics of α-Amylase from Bacillus subtilis and a Comparison with Conventional Heating Conditions

The inactivation kinetics of an α-amylase enzymatic time-temperature integrator (TTI) from Bacillus subtilis (BAA) under continuous-flow microwave (MW) and conventional heating conditions were evaluated and compared in this study. The TTI dispersed in a buffer solution (pH 5.0 to 6.9) at 20 °C initially, and it was continuously circulated through two helical coils connected in a series for heating. The two coils were positioned in two domestic microwave ovens (2450 MHz and 1000 W nominal capacity each) and connected by a short tube. The sample flow rates were adjusted to result in a specific exit temperature in the range of 65 to 80 °C. A short fully insulated helical coil at the exit of the second oven was used as a holding tube. Test samples were drawn either at the exit of the second MW oven or immediately after the holding tube. The decimal reduction times obtained under conventional batch heating conditions decreased from 66 to 24 s as the temperature changed from 70 to 75 °C at pH 5.0 while they decreased from 8 to 5 s under MW in the same temperature range, but at pH 6.0, they increased both under conventional and microwave heating conditions (138 to 120 s and 89 to 61 s, respectively). The D-values under conventional thermal holding were four–eight times higher than under a continuous-flow MW heating condition. By varying the pH, the D-values could be modified to suit the validation of appropriate processing conditions.

Improved Process for the Synthesis of 3-(3-Trifluoromethylphenyl)propanal for More Sustainable Production of Cinacalcet HCl.

Cinacalcet (I), sold as hydrochloride salt, is a calcimimetic drug which has been approved for the treatment of secondary hyperparathyroidism in patients with chronic renal disease and for the treatment of hypercalcemia in patients with parathyroid carcinoma. Here, an improved method for the synthesis of 3-(3-trifluoromethylphenyl)propanal (II), a key intermediate for the preparation of I, is described. The protocol required a Mizoroki-Heck cross-coupling reaction between 1-bromo-3-(trifluoromethyl)benzene and acroleine diethyl acetal, catalyzed by Pd(OAc)2 in the presence of nBu4NOAc (tetrabutylammonium acetate), followed by the hydrogenation reaction of the crude mixture of products in a cascade process. Palladium species, at the end of the reaction, were efficiently recovered as Pd/Al2O3. The procedure was developed under conventional heating conditions as well as under microwave-assisted conditions. The obtained mixture of 1-(3,3-diethoxypropyl)-3-(trifluoromethyl)benzene (III), impure for ethyl 3-(3-trifluoromethylphenyl) propanoate (IV), was finally treated, under mild conditions, with potassium diisobutyl-tert-butoxyaluminum hydride (PDBBA) to obtain after hydrolysis 3-(3-trifluoromethylphenyl)propanal (II), in an excellent overall yield and very high purity. Microwave conditions permitted a reduction in reaction times without affecting selectivity and yield. The final API was obtained through reductive amination of (II) with (R)-(+)-1-(1-naphthyl)ethylamine (V) using a catalyst prepared by us with a very low content of precious metal.

Comparison of Microstructures of Magnetite Reduced by H2 and CO under Microwave Field

The reduction of magnetite in H2 and CO atmospheres was compared using a microwave-heating technique. The reduction of magnetite in a mixed H2 + CO atmosphere was compared with respect to the effects of a microwave field and a conventional field. Microstructural changes were observed using an electron microscope. The results show that the metallization rate and reduction degree of the H2-reduced magnetite are much higher than those of the magnetite reduced by CO at 900–1100 ℃. The Fe phase generated by H2 reduction forms a cavity structure, and the Fe phase generated by CO reduction forms a dense block. Under conventional heating conditions, the higher the H2 content in a pure CO atmosphere, the better the reduction effect. Under the effect of a microwave field, the atmosphere with the highest reduction rate was 50% H2 + 50% CO. Compared with conventional heating, the bubble holes formed by reduced iron in microwave field are larger under the same conditions.

Diversity-Oriented Synthesis of Benzo[f][1,4]oxazepine-, 2H-Chromene-, and 1,2-Dihydroquinoline-Fused Polycyclic Nitrogen Heterocycles under Microwave-Assisted Conditions.

An efficient, diversity-oriented synthesis of oxazepino[5,4-b]quinazolin-9-ones, 6H-chromeno[4,3-b]quinolines, and dibenzo[b,h][1,6]naphthyridines was established involving a substrate-based approach under microwave-assisted and conventional heating conditions in high yields (up to 88%). The CuBr2-catalyzed, chemoselective cascade annulation of O-propargylated 2-hydroxybenzaldehydes and 2-aminobenzamides delivered oxazepino[5,4-b]quinazolin-9-ones involving a 6-exo-trig cyclization-air oxidation-1,3-proton shift-7-exo-dig cyclization sequence. This one-pot process showed excellent atom economy (-H2O) and constructed two new heterocyclic rings (six- and seven-membered) and three new C-N bonds in a single synthetic operation. On the other side of diversification, the reaction between O/N-propargylated 2-hydroxy/aminobenzaldehydes and 2-aminobenzyl alcohols delivered 6H-chromeno[4,3-b]quinolines and dibenzo[b,h][1,6]naphthyridines involving sequential imine formation-[4 + 2] hetero-Diels-Alder reaction-aromatization steps. The influence of microwave assistance was superior to conventional heating, where the reactions were clean, rapid, and completed in 15 min, and the conventional heating required a longer reaction time at a relatively elevated temperature.

A comprehensive study on the pyrolysis behavior of pine sawdust catalyzed by different metal ions under conventional and microwave heating conditions

The Russia-Ukraine conflict and the COVID-19 pandemic have made fossil energy more urgent, and the catalytic pyrolysis of biomass is conducive to energy transformation to achieve global sustainable development. In this paper, the influence mechanisms of different metal ions on biomass pyrolysis under conventional heating and microwave heating conditions were studied. Through thermogravimetric analysis, it was found that the existence of metal ions could change the pyrolysis behaviors of biomass, leading to different degrees of changes in the main pyrolysis temperature and range. Compared with conventional heating conditions, metal ion-loaded biomass samples exhibited higher heating rates under microwave heating conditions due to the possible hotspot phenomenon, resulting in increased gas yields and decreased bio-oil yields. Among them, the trivalent iron ion exhibited excellent catalytic properties for gas generation, with a high gas yield of 57.9% and a bio-oil yield of 12.1%. The components in bio-oil were greatly simplified by microwave irradiation, the number of the bio-oil compounds from the pyrolysis of Fe-loading pine sawdust was reduced to 77, and the GC-MS area of light compounds with carbon number less than 10 was increased to 84.4%. Phenol and furan in bio-oil are also catalytically converted into aromatic hydrocarbons, which are ideal chemical raw materials.

Microwave Inhibition of the Hydrogenation of CO2 for Methane Formation

The complex equilibria associated with the hydrogenation of CO2 to form valuable products are of considerable interest for the remediation of CO2. Due to the potential usefulness of these reactions, alternative methods of driving them more favorably using alternative energy sources, such as microwave radiation, are also of interest. We report here a study of the methanation reaction that yields CH4, which was studied under microwave and conventional convective heating conditions. An essential part of the science that arises from such studies is characterizing the microwave-specific effects on the reaction. From the determined equilibrium constants, we found that microwave radiation strongly inhibited the formation of CH4. The effective thermodynamic parameters obtained from a van’t Hoff plot reflect this inhibition. The enthalpy under microwave conditions is −38.63 kJ/mol, compared to −180.02 kJ/mol under conventional heating methods. Similarly, the free energy across the temperature range is less negative, suggesting a decrease in spontaneity under microwave conditions. We also observed a significant difference in the entropy, with entropy of −52 J/mol under microwave conditions and −206 J/mol under conventional conditions. This is consistent with prior observations that microwaves do not accelerate exothermic reactions. As discussed, this difference may arise from the microwave-driven dissociation of the reactant before CH4 bonds are made. Analysis of this system is of interest in understanding how the microwave inhibits the formation of the products of exothermic reactions. These results are of significance for understanding the broad area of microwave driven heterogeneous catalysis.

An efficient microwave assisted copper catalyzed C-3 amination of 3-bromopyrazolo[1,5-a]pyrimidine

Treatment of 3-bromopyrazolo[1,5-a]pyrimidine with a variety of 1° or 2° alkylamines with 20 mol% CuI, 40 mol% l-proline, and Et3N (2 equiv) in DMSO under microwave heating at 130 °C (4 h) gave the corresponding C-3 aminated products in good to excellent isolated yields (54–90 %; ave. yield = 77 %). The reaction worked well for simple 1° alkylamines (e.g. butylamine, benzylamine, cyclohexylamine, and isopropylamine), and 1° amines with CH2- or CH2CH2-linked heterocycles (e.g. furan, thiophene, pyridine, and/or indole) were also well tolerated. 2° Alkylamines were also good substrates for this reaction (e.g. piperidine, pyrrolidine, and morpholine). The reaction worked well under conventional heating conditions on both the mg or gram-scales, with representative gram-scale reactions giving products in 72–76 % yield under conventional heating. Coupling to a pegylated azide (11-azido-3,6,9-trioxaundecan-1-amine, 65 % yield microwave) demonstrated compatibility of this coupling methodology for potential click-type conjugation of the pyrazolo[1,5-a]pyrimidine to biologically relevant target ligands.

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.

Non-thermal effect of microwave on organic sulfur removal from coal by microwave with peroxyacetic acid

This work investigated the non-thermal microwave effect during organic sulfur removal from coal by microwave irradiation combined with oxidation assistants. XANES was used to analyze the change of sulfur forms in the desulfurization process of high organic sulfur coal under microwave heating and conventional heating conditions. The effects of microwave fields on the molecular properties, bond strength and reaction kinetic of organic sulfur-containing groups were studied by quantum chemical calculation. Results showed that the desulfurization rate with microwave heating improved by 26% at the same temperature compared with conventional heating. The XANES analysis indicated that microwave radiation promoted the transformation of low-valence sulfur into high-valence sulfur, which resulted in the sulfur in the treated coal mainly existing as heterocyclic sulfur and oxidized sulfur. For the theoretical calculation, it is observed that the nucleophilic reaction activity of sulfur-containing groups could be improved and the strength of CS bands could be reduced under the extra electric fields due to the non-thermal microwave effect. Besides, the reaction activation energy was reduced and the reaction rate was increased under the same extra electric fields, promoting the formation of oxidized sulfur in coal. Therefore, there was a competitive relationship between sulfur-containing bond dissociation and group oxidation under the action of microwave non-thermal effect, which provided a reference for coal desulfurization research.

Preparation and Characterization of NiCoFe2O4 Nanoparticles as an Effective Catalyst for the Synthesis of Trisubstituted Imidazole Derivatives Under Solvent-free Conditions.

In this study, NiCoFe2O4 nanoparticles were prepared and used as a reactive catalyst for the synthesis of trisubstituted imidazoles. The multicomponent reactions of aromatic aldehydes, benzil and ammonium acetate were carried out under solvent-free and conventional heating conditions. In this reaction, some imidazole derivatives with high purity, short reaction times and high yields were obtained. The prepared nanocatalyst was characterized by FT-IR, XRD, EDX, VSM and SEM techniques. Moreover, the organic products were identified by melting point, FT-IR and 1H NMR analyzes.

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.

Regio- and Stereoselective Synthesis of Dispiropyrrolidineoxindole Derivatives by Using Classical and Microwave Conditions, and Molecular Docking Studies of Them as Potential Cyclin Dependent Kinases (CDK5) Inhibitors

A Solvent-free microwave-assisted green approach has been presented the synthesis of dispiropyrrolidineoxindole compounds with regio- and stereoselectivity and the excellent yields in short reaction times. A comparative study of the synthesis of dispiropyrrolidineoxindole derivatives by conventional heating and microwave condition has been described by the cycloaddition of azomethine ylides generated from sarcosine and isatin with the dipolarophile aurones. Molecular docking studies were carried out to explore the synthesized bispirooxindoles as inhibitors of CDK5. One of these compounds exhibited excellent binding affinity for cyclin dependent kinases (CDK5). Also, the structures were established by spectroscopic techniques as well as single crystal X-ray analysis.

Thermal catalytic device for toluene decomposition via direct heating

A thermal catalytic device that reduces the energy required for the catalytic oxidation of volatile organic compounds (VOCs) was developed. The catalytic oxidation of VOCs is typically performed under indirect heating conditions resulting in high energy consumption. The proposed device drastically decreases the energy consumed and increases the amount of toluene decomposed during catalytic oxidation because the catalysts are heated directly via a carbon nanotube (CNT) element. The proposed device consists of a glass-fiber textile coated in CNTs, α-MnO<sub>2</sub> nanostructures, and Pt nanostructures. The effect of different α-MnO<sub>2</sub> nanostructures (granular and urchin-like) on device performance was investigated. Moreover, the effect of each device component on the toluene decomposition efficiency and energy consumption of the device was explored by determining the toluene concentration of gaseous toluene after the catalytic oxidation process and the associated energy consumption. The device featuring urchin-like α-MnO<sub>2</sub> nanostructures coated in a thin layer of Pt achieves higher toluene decomposition efficiency than the device featuring granular α-MnO<sub>2</sub> nanostructures coated in Pt nanoparticles. Moreover, the device featuring urchin-like α-MnO<sub>2</sub> nanostructures coated in a thin Pt layer achieves higher toluene decomposition efficiency and lower energy consumption under direct heating conditions than under conventional indirect heating conditions.

Optimization of Holographic Imprinting Process for Metallic Glass using Thermoplastic Forming

The thermoplastic forming (TPF) process of metallic glass (MG) is a unique and powerful method that cannot be performed using conventional crystalline alloys. Because the mechanical and thermal properties of MGs are more favorable with smaller sample sizes, TPF is particularly useful for microscale and nanoscale part molding and micro-patterning. One of the promising commercial MG applications that can take full advantage of these characteristics is hologram patterning. Holograms can be used to identify unique brands, using characteristics with patterns that are difficult to replicate. Their excellent aesthetic qualities can also greatly contribute to increased product value. In this study, we developed and performed a TPF process for actual holographic imprinting with Mg-based MGs, and further investigated the TPF processing window, covering a wide range of temperature and process time conditions through thermal analysis, with ultra-fast heating rates ranging from 100 to 25000 K/s using Flash-DSC. The results of this study serve as a practical guide for identifying the full range of TPF processing windows including conventional and ultrafast heating conditions for micro-scale and nanoscale molding of various MGs. Moreover, a methodology is proposed to identify the general TPF processing window (<i>η</i><108Pa· s) and the ideal TPF processing window (<i>η</i><104Pa· s) by estimating the viscosity (<i>η</i>) of the supercooled liquid. Accordingly, this study is expected to be utilized to optimize the TPF process of MGs and promote the commercialization of related industries.