Specific Microwave Effect Research Articles

Microwave-Specific Effects in Various TiO2 Specimens. Dielectric Properties and Degradation of 4-Chlorophenol

Microwave specific effects on various potential TiO2 photocatalyst specimens were examined for the photodecomposition of 4-chlorophenol (4-CP) with regard to physical and chemical properties and by the characterization of dielectric properties such as the dielectric loss factor (ε′′), the dielectric constant (ε′), and the dielectric loss tangent (tan δ = ε′′/ ε′). Dielectric properties were measured in both dispersed TiO2 particles in a 4-CP aqueous solution and in TiO2 pellets prepared from the specimens’ powders. The irradiation efficiency of the microwave for the TiO2 particles in aqueous media is discussed on the basis of the changes in the dielectric loss factor and penetration depth from the increase in the temperature of water. Changes in crystalline shape and the estimated band gap energies were analyzed before and after irradiation by the microwaves. Of particular significance, the band gap energies were estimated by an in situ observation method under UV and microwave irradiation conditions.

Probing “microwave effects” using Raman spectroscopy

The use of in situ Raman spectroscopy is reported as a tool for probing the effects of microwave irradiation on molecules. Our results show no evidence for localized superheating, an often-cited specific microwave effect. While the microwave energy may interact with the polar molecules more so than with non-polar ones, the conversion of electromagnetic energy into kinetic energy is slower than conversion of kinetic energy into thermal energy. As a result, more polar molecules are not at a temperature greater than that of the bulk.

Microwave Effects on Chemical Functionalization of Hydrogen-Terminated Porous Silicon Nanostructures

This article reports on the chemical functionalization of hydrogen-terminated porous silicon (PSi−H) nanostructures with alkenes, aldehydes, and alkyl halides under microwave irradiation. The technique allows for the introduction of different functional groups on the surface. The use of microwave irradiation has a net effect on the hydrosilylation reaction rate. However, no specific microwave effect was observed when the freshly prepared PSi−H surface was modified with an alkene bearing an aldehyde function. The reaction takes place at both the carbon−carbon and the carbon−oxygen double bonds. The polar aldehyde functional group did not show a dominant effect in directing the reaction selectively at the carbonyl group.

A kinetic study on the activating power of lithium ions on the porphyrin metallation by lanthanides and on a microwave specific effect

A kinetic study of the lithium ion activating effect on porphyrin metallation has been investigated using monohydroxy-tritolyl porphyrin and the lanthanides erbium and gadolinium in dimethylacetamide. The powerful, concentration-dependent, catalytic effect of lithium, observed in both cases with classical heating was considerably enhanced under microwave irradiation at the same temperature thus underscoring a specific microwave effect.

Microwave Activation of Enzymatic Catalysis

Microwave irradiation can be used to regulate biocatalysis. Herein, the utilization of hyperthermophilic enzymes in a microwave reactor is reported. While these enzymes are inactive at low temperatures, they can be activated with microwave irradiation. To the best of our knowledge, this is the first illustration of a specific microwave effect in enzymatic catalysis.

Microwave Synthesis of CdSe and CdTe Nanocrystals in Nonabsorbing Alkanes

Controlling nanomaterial growth via the "specific microwave effect" can be achieved by selective heating of the chalcogenide precursor. The high polarizability of the precursor allows instantaneous activation and subsequent nucleation leading to the synthesis of CdSe and CdTe in nonmicrowave absorbing alkane solvents. Regardless of the desired size, narrow dispersity nanocrystals can be isolated in less than 3 min with high quantum efficiencies and elliptical morphologies. The reaction does not require a high temperature injection step, and the alkane solvent can be easily removed. In addition, batch-to-batch variance in size is 4.2 +/- 0.14 nm for 10 repeat experimental runs. The use of a stopped-flow reactor allows near continuous automation of the process leading to potential industrial benefits.

Microwave-Assisted Reactions of α-Diazosulfoxides to Form α-Oxosulfines

Corrected by: Microwave-Assisted Reactions of α-Diazosulfoxides to Form α-OxosulfinesSynlett 2008; 2008(05): e2-e2DOI: 10.1055/s-2008-1066989

Microwave-enhanced radical reactions at ambient temperature : Part 3: Highly selective radical synthesis of 3-cyclohexyl-1-phenyl-1-butanone in a microwave double cylindrical cooled reactor

The microwave-enhanced synthesis of 3-cyclohexyl-1-phenyl-1-butanone (CPB) from crotonophenone and iodocyclohexane in the presence of t-BuOOH and triethylborane was examined at ambient temperature and under reflux conditions in bulk solution using no less than four different synthetic protocols. Chemical yields of CPB (93 and 95%, respectively) obtained after 60 and 120 min under microwave dielectric heating coupled to a cooling system (MW/Cool protocol) for a reaction temperature of 20–24 °C exceeded the yields from the reaction occurring at room temperature (24 °C; no microwaves; 40% and 72%) for the same reaction times and temperature. The competitive radical reaction to produce CPB and a small amount of by-products generated from the thermally-induced synthesis by the non-selective radical reaction was restricted solely to the process occurring at ambient temperature (RT protocol), under microwave dielectric heating to reflux (MW protocol) and heating with an oil-bath also to reflux (Oil-bath protocol). By contrast, no by-products were generated or detected for the reaction taking place by the MW/Cool method. Comparison of product yields under conditions of identical temperature conditions must involve some specific microwave effect in the synthesis of the title compound.

Diastereoselectivity in the Staudinger reaction: a useful probe for investigation of nonthermal microwave effects

The effect of microwave irradiation on the selectivity, especially stereoselectivity, is one of the most important issues in microwave-assisted organic reactions. The diastereoselectivity in Staudinger reactions involving the representative ketenes and the corresponding matched imines has been used as a probe to investigate carefully the existence of the specific nonthermal microwave effects. The results indicate that the microwave irradiation-controlled stereoselectivity in the Staudinger reaction is in fact the contribution of temperature. No specific nonthermal microwave effect was found in the Staudinger reaction.

Microwave-Assisted Asymmetric Organocatalysis. A Probe for Nonthermal Microwave Effects and the Concept of Simultaneous Cooling

A series of five known asymmetric organocatalytic reactions was re-evaluated at elevated temperatures applying both microwave dielectric heating and conventional thermal heating in order to probe the existence of specific or nonthermal microwave effects. All transformations were conducted in a dedicated reactor setup that allowed accurate internal reaction temperature measurements using fiber-optic probes. In addition, the concept of simultaneous external cooling while irradiating with microwave power was also applied in all of the studied cases. This method allows a higher level of microwave power to be administered to the reaction mixture and, therefore, enhances any potential microwave effects while continuously removing heat. For all of the five studied (S)-proline-catalyzed asymmetric Mannich- and aldol-type reactions, the observed rate enhancements were a consequence of the increased temperatures attained by microwave dielectric heating and were not related to the presence of the microwave field. In all cases, in contrast to previous literature reports, the results obtained either with microwave irradiation or with microwave irradiation with simultaneous cooling could be reproduced by conventional heating at the same reaction temperature and time in an oil bath. No evidence for specific or nonthermal microwave effects was obtained.

An efficient one-step regiospecific synthesis of novel isoxazolines and isoxazoles of N-substituted saccharin derivatives through solvent-free microwave-assisted [3+2] cycloaddition

Novel isoxazolines and isoxazoles of N-substituted saccharin derivatives are synthesized in good yields by 1,3-dipolar cycloaddition of N-allyl or propargyl N-substituted saccharin with arylnitrile oxide under solvent-free microwave irradiation. In this process, the yields were significantly improved over conventional heating, without alteration of the selectivity. The regioselectivity as well as the nonthermal specific microwave effect are discussed.

Improvement and simplification of synthesis of 3-aryloxy-1,2-epoxypropanes using solvent-free conditions and microwave irradiations. Relation with medium effects and reaction mechanism

Some 3-aryloxy-1,2-epoxypropanes, interesting as potential synthons in β-adrenergic receptor antagonists preparation, were obtained in excellent yields (65–96% within 2–17 min) by microwave activation (monomode system) using solid–liquid solvent-free phase transfer catalysis (PTC). The best results for the O-alkylation of some phenols with epichlorohydrin were obtained using TBAB and NaOH/K 2CO 3 (1:4 mol/mol) as phase transfer catalyst and more acceptable basic system, respectively. These new procedure is compared with classical methods. Significant specific microwave effect (non-purely thermal) was evidenced in all cases. They were discussed in terms of reaction medium and mechanism, taking into account the variations in polarity of the systems.

Microwaves in organic and medicinal chemistry

Preface. Personal Foreword. 1. Introduction: Microwave Synthesis in Perspective. 1.1 Microwave Synthesis and Medicinal Chemistry. 1.2 Microwave: Assisted Organic Synthesis (MAOS) - A Brief History. 1.3 Scope and Organization of the Book. 2. Microwave Theory. 2.1 Microwave Radiation. 2.2 Microwave Dielectric Heating. 2.3 Dielectric Properties. 2.4 Microwave Versus Conventional Thermal Heating. 2.5 Microwave Effects. 2.5.1 Thermal Effects (Kinetics). 2.5.2 Specific Microwave Effects. 2.5.3 Non-Thermal (Athermal) Microwave Effects. 3. Equipment Review. 3.1 Introduction. 3.2 Domestic Microwave Ovens. 3.3 Dedicated Microwave Reactors for Organic Synthesis. 3.4 Multimode Instruments. 3.4.1 Milestone s.r.1. 3.4.2 CEM Corporation. 3.4.3 Biotage AB. 3.4.4 Anton Paar GmbH. 3.5 Single-Model Instruments. 3.5.1 Biotage AB. 3.5.2 CEM Corporation. 3.6 Discussion. 4. Microwave Processing Techniques. 4.1 Solvent-Free Reactions. 4.2 Phase-Transfer Catalysis. 4.3 Reactions Using Solvents. 4.3.1 Open-versus Closed-Vessel Conditions. 4.3.2 Pre-Pressurized Reaction Vessels. 4.3.3 Non-Classical Solvents. 4.4 Parallel Processing. 4.5 Scale-Up in Batch and Continuous-Flow. 5. Starting with Microwave Chemistry. 5.1 Why Use Microwave Reactors? 5.2 Translating Conventionally Heated Methods. 5.2.1 Open and Closed Vessels? 5.2.2 Choice of Solvent. 5.2.3 Temperature and Time. 5.2.4 Microwave Instrument Software. 5.3 Reaction Optimization and Library Generation - A Case Study. 5.3.1 Choice of Solvent. 5.3.2 Catalyst Selection. 5.3.3 Time and Temperature. 5.3.4 Reinvestigation by a Design of Experiments Approach. 5.3.5 Optimization for Troublesome Building Block Combinations. 5.3.6 Automated Sequential Library Production. 5.4 Limitations and Safety Aspects. 6. Literature Survey Part A: General Organic Synthesis. 6.1 Transition Metal-Catalyzed Carbon-Carbon Bond Formations. 6.1.1 Heck Reactions. 6.1.2 Suzuki Reactions. 6.1.3 Sonogashira Reactions. 6.1.4 Stille Reactions. 6.1.5 Negishi, Kumada, and Related Reactions. 6.1.6 Carbonylation Reactions. 6.1.7 Asymmetric Allylic Alkyations. 6.1.8 Miscellaneous Carbon-Carbon Bond-Forming Reactions. 6.2 Transition Metal-Catalyzed Carbon-Heteroatom Bond Formations. 6.2.1 Buchwald-Hartwig Reactions. 6.2.2 Ullmann Condensation Reactions. 6.2.3 Miscellaneous Carbon-Heteroatom Bond-Forming Reactions. 6.3 Other Transition Metal-Mediated Processes. 6.3.1 Ring Closing Metathesis. 6.3.2 Pauson-Khand Reactions. 6.3.3 Carbon-Hydrogen Bond Activation. 6.3.4 Miscellaneous Reactions. 6.4 Rearrangement Reactions. 6.4.1 Claisen Rearrangements. 6.4.2 Domino/Tandem Claisen Rearrangements. 6.4.3 Squaric Acid-Vinylketene Rearrangements. 6.4.4 Vinylcyclobutane-Cyclohexene Rearrangements. 6.4.5 Miscellaneous Rearrangements. 6.5 Diels-Alder Cycloaddition Reactions. 6.6 Oxidations. 6.7 Catalytic Transfer Hydrogenations. 6.8 Mitsunobu Reactions. 6.9 Glycosylation Reactions and Related Carbohydrate-Based Transformations. 6.10 Multicomponent Reactions. 6.11 Alkylation Reactions. 6.12 Nucleophilic Aromatic Substitutions. 6.13 Ring-Opening Reactions. 6.13.1 Cyclopropane Ring-Opening. 6.13.2 Aziridine Ring-Openings. 6.13.3 Epoxide Ring-Opening. 6.14 Addition and Elimination Reactions. 6.14.1 Michael Additions. 6.14.2 Addition to Alkenes. 6.14.3 Addition to Alkenes. 6.14.4 Addition to Nitriles. 6.15 Substitution Reactions. 6.16 Enamine and Imine Formations. 6.17 Reductive Aminations. 6.18 Ester and Amide Formation. 6.19 Decarboxylation Reactions. 6.20 Free Radical Reactions. 6.21 Protection/Deprotection Chemistry. 6.22 Preparation of Isotopically Labeled Compounds. 6.23 Miscellaneous Transformations. 6.24 Heterocycle Synthesis. 6.24.1 Three-Membered Heterocycles with One Heteroaton. 6.24.2 Four-Membered Heterocycles with One Heteroatom. 6.24.3 Five-Membered Heterocycles with One Heteroatom. 6.24.4 Five-Membered Heterocycles with Two Heteroatom. 6.24.5 Five-Membered Heterocycles with Three Heteroatom. 6.24.6 Five-Membered Heterocycles with Four Heteroatom. 6.24.7 Six-Membered Heterocycles with One Heteroatom. 6.24.8 Six-Membered Heterocycles with Two Heteroatom. 6.24.9 Six-Membered Heterocycles with Three Heteroatom. 6.24.10 Larger Heterocyclic and Polycyclic Ring Systems. 7. Literature Survey Part B: Combinatorial Chemistry and High-Throughput Organic Synthesis. 7.1 Solid-Phase Organic Synthesis. 7.1.1 Combinatorial Chemistry and Solid-Phase Organic Synthesis. 7.1.2 Microwave Chemistry and Solid-Phase Organic Synthesis. 7.1.3 Peptide Synthesis and Related Examples. 7.1.4 Resin Functionalization. 7.1.5 Transition Metal Catalysis. 7.1.6 Substitution Reactions. 7.1.7 Multicomponent Chemistry. 7.1.8 Microwave-Assisted Condensation Reactions. 7.1.9 Rearrangements. 7.1.10 Cleavage Reactions. 7.1.11 Miscellaneous. 7.2 Soluble Polymer-Supported Synthesis. 7.3 Fluorous Phase Organic Synthesis. 7.4 Grafted Ionic Liquid-Phase-Supported Synthesis. 7.5 Polymer-Supported Reagents. 7.6 Polymer-Supported Catalysts. 7.6.1 Catalysts on Polymeric Supports. 7.6.2 Silica-Grafted Catalysts. 7.6.3 Catalysts Immobilized on Glass. 7.6.4 Catalysts Immobilized on Carbon. 7.6.5 Miscellaneous. 7.7 Polymer-Supported Scavengers. 8. Outlook and Conclusions. Index.

Microwave-Enhanced Reaction Rates for Nanoparticle Synthesis

Microwave reactor methodologies are unique in their ability to be scaled-up without suffering thermal gradient effects, providing a potentially industrially important improvement in nanocrystal synthetic methodology over convective methods. Synthesis of high-quality, near monodispersity nanoscale InGaP, InP, and CdSe have been prepared via direct microwave heating of the molecular precursors rather than convective heating of the solvent. Microwave dielectric heating not only enhances the rate of formation, it also enhances the material quality and size distributions. The reaction rates are influenced by the microwave field and by additives. The final quality of the microwave-generated materials depends on the reactant choice, the applied power, the reaction time, and temperature. CdSe nanocrystals prepared in the presence of a strong microwave absorber exhibit sharp excitonic features and a QY of 68% for microwave-grown materials. InGaP and InP are rapidly formed at 280 degrees C in minutes, yielding clean reactions and monodisperse size distributions that require no size-selective precipitation and result in the highest out of batch quantum efficiency reported to date of 15% prior to chemical etching. The use of microwave (MW) methodology is readily scalable to larger reaction volumes, allows faster reaction times, removes the need for high-temperature injection, and suggests a specific microwave effect may be present in these reactions.

Synthesis and properties of new poly(ether–ester)s containing aliphatic diol based on isosorbide. Effects of the microwave-assisted polycondensation

Microwave irradiation was applied to synthesize to the bulk synthesis of novel poly(ether–ester)s based on diol-ether of isosorbide ( 1) and adipoyl chloride ( 2) or terephthaloyl chloride ( 3). Thus, the poly(ether–ester)s ( 4 and 5) consist partially of isosorbide. In order to check the influence of microwaves and possible specific non-thermal microwave effects, the reactions were comparatively performed inside a thermostated oil bath under similar conditions. The reaction conditions were varied to optimize both yields and molecular weights of poly(ether–ester)s. The reaction proceeded roughly five times faster under microwave irradiation, the polycondensation being almost completed (yields upto approximately 95%) within 5 min to afford a series of novel poly(ether–ester)s based with relatively high average molecular weights ( M w upto approximately 8000). The resulting poly(ether–ester)s were characterized by NMR ( 1H and 13C), FT-IR spectrometry, SEC measurements and MALDI-TOF mass spectrometry. Thermal properties of the poly(ether–ester)s ( 4 and 5) were investigated by means of differential scanning calorimetry (DSC).

Microwave Accelerated Synthesis of N‐Phenylmaleimide in a Single Step and Polymerization in Bulk

AbstractSummary: This paper reports the microwave mediated direct synthesis of N‐phenylmaleimide (3) from maleic anhydride (1) and aniline (2) by using microwave irradiation. Good yields and very short reaction times were the main aspects of the method. The reaction conditions and kinetics of this process were investigated. Microwave irradiation in a boiling solvent showed the existence of a significant specific microwave effect. The microwave homopolymerization of 3 in bulk using 2,2′‐azoisobutyronitrile (AIBN) as a free radical initiator was also carried out.Kinetic curves for the synthesis of N‐phenylmaleimide in a MW and in an oil bath at 144 °C (boiling xylene).magnified imageKinetic curves for the synthesis of N‐phenylmaleimide in a MW and in an oil bath at 144 °C (boiling xylene).

Microwaves in Organic Synthesis. Thermal and Non‐Thermal Microwave Effects

AbstractFor Abstract see ChemInform Abstract in Full Text.

Enviro-economic, facile, one-pot synthesis of novel spiro[pyrazolo [4,3-c] [1,5] benzothiazepines] using microwave irradiation

An efficient method for the exclusive one-pot synthesis of novel pyrazolo (4,3-c)(1,5)- benzothiazepines (7a-e) possessing a spiro-3H-indoline nucleus is described. The investigation of the reaction between substituted o-aminothiophenols (5a-e) with 3-pyrazolidinyl-2H-indol-2- one (3) to form the title products has been found interesting in view of the fact that different reaction sites are available in the key intermediate 3 which may lead to a mixture of products. Conventionally, 3 did not undergo reactions with 5a-e even under drastic conditions of prolonged reflux using strong acidic or basic catalysts in high boiling organic solvents for many days. However, the exclusive formation of the title products in a satisfactory yield (71 %) was achieved under microwave irradiation coupled with various inorganic supports indicative of a very strong specific microwave effect.

Microwaves in organic synthesis. Thermal and non-thermal microwave effects

Microwave irradiation has been successfully applied in organic chemistry. Spectacular accelerations, higher yields under milder reaction conditions and higher product purities have all been reported. Indeed, a number of authors have described success in reactions that do not occur by conventional heating and even modifications of selectivity (chemo-, regio- and stereoselectivity). The effect of microwave irradiation in organic synthesis is a combination of thermal effects, arising from the heating rate, superheating or "hot spots" and the selective absorption of radiation by polar substances. Such phenomena are not usually accessible by classical heating and the existence of non-thermal effects of highly polarizing radiation--the "specific microwave effect"--is still a controversial topic. An overview of the thermal effects and the current state of non-thermal microwave effects is presented in this critical review along with a view on how these phenomena can be effectively used in organic synthesis.

Synthesis of phosphonium salts under microwave activation — Leaving group and phosphine substituents effects

The specific nonpurely thermal effects of microwaves were evidenced according to neutral or charged leaving groups during nucleophilic substitution of benzylic electrophiles with triphenylphosphine and tributylphosphine. Microwave (MW) irradiation considerably enhanced the reactions with charged alkylating agents, especially under solvent-free conditions. Results are interpreted considering the magnitude of MW effects according to the position of the transition state along the reaction coordinates.Key words: microwave irradiation, specific effects, phosphonium salts, leaving group effects.