Clean diesel power via microwave susceptible oxidation catalysts.

The problem of soot emissions from diesel engines is introduced and the possible solution of combining doped perovskites and microwave (mw) irradiation to "clean up" diesel soot filters is outlined. Eighteen doped perovskite catalysts are synthesized and tested for propane and CO oxidation, which are taken as model components for soot. The activity, selectivity, and SO2 tolerance are compared under conventional heating and mw irradiation. By combining mw irradiation and doped perovskites, one can create "hot spots" on the catalyst, resulting in efficient and selective heating of the active site, as well as less poisoning. Sr-doped and Ce-doped manganese perovskites show the highest activity. These catalysts are also the most selective, and have a high mw susceptibility. Optimal SO2 tolerance is displayed by Cr perovskites, from which the La0.8Ca0.2CrO3 combination uniquely converts propane before CO, and therefore can be used to remove >C2 hydrocarbons from a mix with CO. Possible mechanistic scenarios are presented and discussed.     

Porphyrins in Diels-Alder reactions. Improvements on the synthesis of barrelene-fused chlorins using microwave irradiation

The microwave irradiation technique was applied to the Diels-Alder reaction of tetrakis(pentafluorophenyl)porphyrin with pentacene and naphthacene. Both reactions proceed within minutes to afford the corresponding monoadducts in 83% and 23% yield, respectively. When compared with the yields obtained under classical heating (22% and no reaction, respectively), this represents an impressive improvement of these reactions. Bisadducts (bacteriochlorins and isobacteriochlorins) are also obtained in the reaction with pentacene; these compounds are not formed under classical heating.     

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.     

相转移催化与微波辐射合成三(2-氰乙基)胺

将微波辐射与相转移催化技术相结合,在加压密闭容器中,以氨水和丙烯腈合成了配位中间体三(2-氰乙基)胺,产率为61.2%,合成时间比传统方法缩短了8倍;在微波辐射下,产率为67.2%,合成时间缩短约90倍.对产物进行了元素分析,IR,¹H-NMR,ES-MS,及TG-DTA表征.     

一种新的催化方法：微波辐射-酶耦合成催化有机合成

微波辐射和酶催化是现代有机合成化学中两种强有力的催化手段。目前一种新 的研究动向是将两者结合起来用于催化有机合成反应,这种新型催化方法可以被称 作微波辐射-酶耦合催化（MIECC）。由MIECC方法催化的有机合成反应可以被分为 湿法和干法两大类型。介绍了这一新型耦合催化方法的研究进展以及一些典型 MIECC反应。对MIECC反应中可能存在的耦合催化机理,即微波在生物催化体系中的 “非热效应“进行了初步理论探讨,同时展望了这种新的耦合催化方法的前景。     

Microwave-assisted reactions in heterocyclic compounds with applications in medicinal and supramolecular chemistry

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 under conventional heating and modifications in selectivity (chemo-, regio- and stereoselectivity) have even been reported. Recent advances in microwave-assisted combinatorial chemistry include high-speed solid-phase and polymer-supported organic synthesis, rapid parallel synthesis of compound libraries, and library generation by automated sequential microwave irradiation. In addition, new instrumentation for high-throughput microwave-assisted synthesis continues to be developed at a steady pace. The impressive speed combined with the unmatched control over reaction parameters justifies the growing interest in this application of microwave heating. In this review we highlight our recent advances in this area, with a particular emphasis on cycloaddition reactions of heterocyclic compounds both with and without supports, applications in supramolecular chemistry and the reproducibility and scalability of organic reactions involving the use of microwave irradiation techniques.     

有机微波化学研究进展

本文综述了近几年来微波技术在有机合成方面的研究和应用进展。初步探讨了微波催化有机反应的作用机理, 并展望了有机微波化学的发展前景。     

Microwave-Assisted Solid Phase Organic Synthesis.Application to Indole Library Construction

Microwave-assisted organic synthesis (MAOS) has attained increasing popularity due to recent advancement in the instrumentation of microwave technology. Now, MAOS can be performed under controlled temperature and pressure to yield reproducible results. For combinatorial chemistry,the dramatically increased reaction rate under microwave irradiation at high temperature provides an ideal solution to those sluggish reactions, in particular the combinatorial reactions carried out on solid supports. In this presentation, we describe our results on microwave-assisted solid-phase organic synthesis (MASPOS) applied to the construction of indole libraries such as 5. Compounds 4 were synthesized on the Rink amide resins using IRORI MicroKanTM reactors encoded with a radio-frequency (Rf) tag. The resin-bound terminal alkynes 2, prepared via the amide bond, were cross-coupled with the nitroaryl triflate under the conditions adopted from the solution reactions developed by us1,2. The nitro group of 3 was then reduced and sulfonylated to give 4. Ring closure reactions within 4 with Cu(OAc)2 were examined initially in refluxing DCE for 24 h, but no indole product was detected after cleavage from the resin. Therefore, the same reactions were carried out under microwave irradiation at 200 ℃ for 10 min on a Personal Chemistry Emrys Creator, the desired indoles 5 were obtained in 60-95% overall yields calculated from 1 and in ＞90% purities in most cases3. It is necessary to mention that the IRORI microreactors cannot tolerate the high temperature and the resin-bound 4 must be transferred to the reaction vials for the microwave-assisted ring closure reactions. A traceless synthesis of an indole library via MASPOS will be discussed as well.4     

Selective introduction of a fluorine atom into carbohydrates and a nucleoside by ring-opening fluorination reaction of epoxides

Ring-opening fluorination reactions of epoxides using tetrabutylammonium bifluoride (TBABF)-KHF₂, or Et₃N-3HF under microwave irradiation were applied for the introduction of a fluorine atom into the carbohydrate molecules. When TBABF-KHF₂ was used as the fluorination reagent, a fluorine atom was introduced regioselectively and various functional groups can tolerate the conditions. When Et₃N-3HF was used under microwave irradiation, the reaction time could be remarkably shortened compared with the conventional oil-bath heating.     

Application of hydrogen peroxide encapsulated in silica xerogels to oxidation reactions

Hydrogen peroxide was encapsulated into a silica xerogel matrix by the sol-gel technique. The composite was tested as an oxidizing agent both under conventional and microwave conditions in a few model reactions: Noyori's method of octanal and 2-octanol oxidation and cycloctene epoxidation in a 1,1,1-trifluoroethanol/Na2WO4 system. The results were compared with yields obtained for reactions with 30% H2O2 and urea-hydrogen peroxide (UHP) as oxidizing agents. It was found that the composite has activity similar to 30% H2O2 and has a several advantages over UHP such as the fact that silica and H2O are the only products of the composite decomposition or no contamination by urea or its derivatives occurs; the xerogel is easier to heated by microwave irradiation than UHP and could be used as both an oxidizing agent and as solid support for microwave assisted solvent-free oxidations.     

微波有机合成反应的新进展

综述了近来微波辐射技术在有机合成应用中的新进展。着重介绍了微波有机合 成反应技术及其在重要有机合成反应中的应用。     

Green chemical syntheses and applications within organophosphorus chemistry

Application of the microwave (MW) technique offers many advantages in organophosphorus syntheses. Reluctant reactions may take place on MW irradiation. In most cases, MWs make the reactions more efficient in respect of rate, selectivity and yield. The benefits are shown via representative examples. MW irradiation may replace a catalyst, or simplify catalytic systems. The synthesis of catalysts incorporating heterocyclic P-ligands is also discussed. Where it was relevant, structural chemical details were also provided.     

Application of microwave method to the solid phase synthesis of pseudopeptides containing ester bond

A new procedure was developed for reducing the reaction time and improving the yield of esterification reaction in solid phase synthesis of pseudopeptides containing an ester bond by utilizing microwave irradiation. We selected a pseudodipeptide (Fmoc-LysΨ[COO]Leu-NH₂) and optimized the microwave-assisted esterification reaction in solid phase synthesis using Fmoc chemistry. For this, microwave-assisted esterification reactions with different reaction time, temperature, and solvents were performed using 1,3-diisopropylcarbodiimide (DIC) as the coupling reagent. We synthesized several pseudodipeptides containing an ester bond by using the optimized microwave irradiation method. The purity and yield of the pseudodipeptides synthesized in this way were better than those obtained without microwave irradiation. Furthermore, we applied this methodology for synthesizing pseudopeptides (6- and 12-mer) corresponding to the α helical peptide. The microwave-assisted esterification reaction afforded the target pseudopeptides with high yield (∼80%) and purity within 12 min, whereas the reaction without microwave irradiation afforded the target compound with poor yield (∼45%) and low purity.     

Microwave-assisted rapid decomposition of persulfate

Microwave irradiation has been a promising alternative to conduct several chemical reactions. In this work the microwave effects in potassium persulfate decomposition rate, under controlled conditions of temperature and microwave power, were evaluated. Higher decomposition rate constants were obtained in microwave irradiated reactions in comparison with conventional heated ones. To study the effect of high power microwave irradiation, a pulsed irradiation strategy was developed, in which the samples were repeatedly heated within short intervals of time at high power levels (500 or 1400 W). A great decomposition percentage was achieved in shorter irradiation times, showing the kinetic advantages of microwave-assisted reactions. However, it was found no differences in the reaction yields, even when high power levels were involved, proving that microwave enhancements may arise only from the ability to quickly provide a large amount of energy to the reaction medium.     

A fast synthesis for Zintl phase compounds of Na3SbTe3, NaSbTe2 and K3SbTe3 by microwave irradiation

The microwave irradiation technique was used to prepare three Zintl phase compounds Na₃SbTe₃, NaSbTe₂ and K₃SbTe₃. The as-prepared products were analyzed and characterized by XRD, EDX and SEM techniques. Higher microwave oven power and shorter irradiation time are required for the synthesis of Na₃SbTe₃, whereas lower oven power and longer irradiation time are needed for NaSbTe₂. Moderate microwave irradiation conditions facilitate the formation of pure K₃SbTe₃. Pure phase of Na₃SbTe₃ are directly obtained by this technique for the first time. Compared with the traditional high-temperature solid-state synthesis, the microwave reaction required a considerable shortened reaction time for the preparation of the three Zintl compounds. The initial driving force for these reactions originates from the interaction of microwave electric field with alkali metals (Na and K) and Sb powders.     

Non-thermal effects of microwaves on protease-catalyzed esterification and transesterification

The non-thermal effects of microwave irradiation on enzyme-catalyzed reactions have been evaluated by keeping the reaction temperature constant during irradiation. Subtilisin-catalyzed transesterification and α-chymotrypsin-catalyzed esterification have been carried out in six solvents of differing polarities and at three different temperatures. In all cases, microwave irradiation was found to increase the initial reaction rates by 2.1-4.7 times at all hydration levels. It is also shown that microwave irradiation can be used in conjunction with other strategies (like pH tuning and salt activation) for enhancing initial reaction rates.     

Microwave Effects Due to Anionic or Cationic Initiators in Emulsion Polymerization Reactions

Summary: Emulsion polymerization reactions were performed under microwave irradiation and conventional heating using anionic or cationic initiators and surfactants. Microwave irradiation promoted higher reaction rates for both initiators and surfactants, in comparison with the conventional heating. The effect of high power microwave irradiation was studied using a method of cycles of heating and cooling, where rapid polymerization reactions were obtained. In the reactions with anionic initiator and surfactant, a decrease in the particle diameters was observed with microwave heating, and even smaller particles were obtained using high power microwave irradiation. Moreover, the decrease in the particle size was acompanied by an increase in the polymer molecular weight. On the other hand, these effects were not observed for reactions with cationic initiator and surfactant.     

Applications of pi-photon-induced transparency in two-frequency pulse electron paramagnetic resonance experiments

An approach to pulse electron paramagnetic resonance (EPR) experiments which are based on two different resonance fields is introduced. Instead of using two microwave (mw) sources or a magnetic field jump, bichromatic pulses consisting of a transverse microwave field with frequency omega(mw) and a longitudinal radio frequency field with frequency omega(rf) are employed. Such bichromatic pulses excite a number of multiple photon transitions at frequencies omega(mw)+komega(rf) (k in Z). The pi-photon-induced transparency phenomenon is used to select the required transitions. This approach is used in the stimulated soft electron spin echo envelope modulation and the four-pulse double electron-electron resonance experiments. The results obtained using the bichromatic pulse approach are in agreement with those obtained with the standard pulse EPR techniques. It is shown that applying bichromatic pulses is straightforward and advantageous in several respects.     

Chemical Recycling and Liquefaction of Rigid Polyurethane Foam Wastes through Microwave Assisted Glycolysis Process

Glycolysis of polyurethane rigid foams (PUFs) was performed utilizing microwave irradiation at atmospheric pressure. The effects of various metal hydroxide and acetate catalysts as well as different microwave powers were investigated. All reactions were monitored by FTIR spectroscopy. The recovered liquid product containing OH functional groups was applied as a portion of the polyol in formulation of a new polyurethane rigid foam product. The reactivity factors and densities were compared with the foam produced from totally virgin polyol.     

A polymer imidazole salt as phase-transfer catalyst in halex fluorination irradiated by microwave

A new imidazole polymer salt was synthesized in order to develop a high efficiency phase-transfer catalyst for multi-phase reactions. The polymer salt was prepared easily by co-polymerization of 1-1′-(1,4-butamethylene)bis(imidazole) and 1,2-dibromoethane, and has the properties of excellent chemical and thermal stability and high ionic conductivity. It was applied as phase-transfer catalyst in the fluorination of chloronitrobenzenes under the irradiation of microwave and gave excellent yields of corresponding fluoronitrobenzenes. In addition, the enhanced mechanism of microwave was studied and found “non-thermal” effect was a great factor.