Use of Microwaves

Essentials ( Inorganic Materials Synthesis, First Edition. C.N.R. Rao and Kanishka Biswas. 2015 Jdin WQey Sons, Inc. Published 2015 by John Wiley Sons, Inc. 

Industrially important sihcides have been synthesized by microwave reactions. A notable example is MoSi [13]. Metal phosphates have extensively been studied for applications as phosphors, catalysts and as cathode materials in lithium ion batteries. Several lanthanide orthophosphates, LnPO (Ln=La, Ce, Nd, Sm, Eu, Gd and Tb), are obtained by microwave heating of an aqueous solution of Ln(III) nitrate and NH H PO [14], Highly crystalline ohvine LiFePO nanorods are directly obtained within 5 min by microwave heating of lithium hydroxide, iron acetate and phosphoric acid in tetraethyleneglycol [15]. 

FIGURE 9.1 Schematic representation of microwave heating procedure (From Ref. 16, Chem. Mater. 24 (2012) 2558. 2012 American Chemical Society). See insert for color representation of the figure. ) 

---

Microwave Cooking Pads. A simple and effective method of reducing fat in meat products involves the use of microwavable heating pads. 

Microwaves have successfully been used for rewarming of blood for medical appHcations (157). Another successful appHcation, not commetciali2ed as of this writing, is the use of microwave heating for rapid tissue fixation (158,159). This procedure appears to reduce the time for tissue sample analysis... 

Catalysis can sometimes be improved through the use of microwaves, particularly pulsed microwaves (172). An important component of this process is beheved to be an appropriate metallized combination catalyst—susceptor (173). Microwave catalysis is an active area of research (174). 

Other chemical apphcations being studied include the use of microwaves in the petroleum (qv) industry (175), chemical synthesis (176,177), preparation of semiconductor materials (178), and the processing of polymers (179). 

Dryings of Castings and Other Products. The use of microwaves in the curing and drying of foundry cores is weU estabhshed (185). The best example is the use of microwaves for drying water-based core washes at 2450 MHz with up to 150 kW. These apphcations have not, however, found apphcation in manufacturing. Many similar drying apphcations have been examined (186,187). 

The longest wavelengths of the electromagnetic spectmm are sensitive probes of molecular rotation and hyperfine stmcture. An important appHcation is radio astronomy (23—26), which uses both radio and microwaves for chemical analysis on galactic and extragalactic scales. Herein the terrestrial uses of microwave spectroscopy are emphasized (27—29). 

The use of microwave irradiation as an energy source for chemical reactions and processes has been extensively investigated during recent years and has found wide application in various fields of chemistry and technology. The following presentation focuses on the scope and potential of microwaves in chemical analysis. 

Bu3Sn)20 BzCl. The use of microwaves accelerates this reaction. Bu2Sn(OMe)2 is reported to work better than Bu2SnO in the monoprotection of diols. The monoprotection of diols at the more hindered position can be accomplished through the stannylene if the reaction is quenched with PhMe2SiCl (45-77% yield).Microwave heating has been found to be effective for this transformation in some cases. ... 

The Bohlmann-Rahtz reaction has also been improved by the use of microwave... 

Alternative techniques, such as flash vacuum pyrolysis vide infra-, 30 —> 33), have been applied to the Gould-Jacobs reaction. Use of microwave has in some cases provided an... 

Over the last several years research groups have also explored the use of microwaves to increase the reaction rate and efficiency of the Biginelli reaction. In one example, polyphosphate ester (PPE) was used as the promoter under microwave conditions to deliver a variety of DHPMs 38 in yields ranging from 65-95% yield with reaction times typically below 2 minutes. ... 

The use of microwave irradiation for this reaction, compared to conventional thermal heating, was investigated. Chloroform used as solvent under the conventional heating did only allow a temperature of 60 °C and a direct comparison between the two methods is therefore somewhat unfair imder these circumstances. Nevertheless, the microwave-assisted method is attractive and proved useful for both primary and secondary amines resulting in highly substituted pyrazolo ring-fused pyridones 40 in 68-86% yields within only 10 min. 

The Suzuki reaction has been successfully used to introduce new C - C bonds into 2-pyridones [75,83,84]. The use of microwave irradiation in transition-metal-catalyzed transformations is reported to decrease reaction times [52]. Still, there is, to our knowledge, only one example where a microwave-assisted Suzuki reaction has been performed on a quinolin-2(lH)-one or any other 2-pyridone containing heterocycle. Glasnov et al. described a Suzuki reaction of 4-chloro-quinolin-2(lff)-one with phenylboronic acid in presence of a palladium-catalyst under microwave irradiation (Scheme 13) [53]. After screening different conditions to improve the conversion and isolated yield of the desired aryl substituted quinolin-2( lff)-one 47, they found that a combination of palladium acetate and triphenylphosphine as catalyst (0.5 mol %), a 3 1 mixture of 1,2-dimethoxyethane (DME) and water as solvent, triethyl-amine as base, and irradiation for 30 min at 150 °C gave the best result. Crucial for the reaction was the temperature and the amount of water in the... 

The use of microwave irradiation for decarboxylation reactions is well appreciated [107-110]. Still, only one example of a decarboxylation performed on 2-pyridone starting materials has been reported (Fig. 10) [111]. Notably, this decarboxylation reaction is a selective and reagent-free method performed in N-methyl-2-pyrrohdin one (NMP) and microwave irradiation at 220 °C for 10 min. The products 65 were isolated in excellent yields (92-99%) by a simple aqueous work-up (Fig. 10). 

The one-pot, three-component synthesis of a 20-membered dihydrotri-azine hbrary was also dramatically accelerated through the use of microwave irradiation [79]. Heating a subset of substituted anilines, cyanoguanidine and acetone in the presence of concentrated hydrochloric acid for 35 min at 90 °C in a single-mode microwave reactor gave the corresponding 2,2-dimethyl-1,2-dihydro-s-triazine hydrochloride 51 in comparable yield to conventional conductive heating methods but in a much shorter reaction time and increased purity (Scheme 21). 

The use of microwave-assisted multicomponent cycloaddition reactions allows unique heterocyclic scaffolds to be assembled rapidly from readily accessible starting materials. The three-component reaction of M-alkyl-l,4-DHP... 

Since 1986, when the very first reports on the use of microwave heating to chemical transformations appeared [147,148], microwave-assisted synthesis has been shown to accelerate most solution-phase chemical reactions [24-27,32,35]. The first application of microwave irradiation for the acceleration of reaction rate of a substrate attached to a solid support (SPPS) was performed in 1992 [36]. Despite the promising results, microwave-assisted soHd-phase synthesis was not pursued following its initial appearance, most probably as a result of the lack of suitable instriunentation. Reproducing reaction conditions was nearly impossible because of the differences between domestic microwave ovens and the difficulties associated with temperature measurement. The technique became a Sleeping Beauty interest awoke almost a decade later with the publication of several microwave-assisted SPOS protocols [37,38,73,139,144]. There has been an extensive... 

Researchers at the Bristol-Myers Squibb Pharmaceutical Research Institute in Wallingford investigated the use of microwave irradiation for the rapid... 

The use of microwaves to heat organic reactions has attracted considerable interest in the last 15 years. This technique allows to reduce the time of chemical transformations and, consequently, the formation of by-products is reduced, often with improved yields and purity of the products. Practically every kind of transformation has been tested imder microwave irradiation, in many instances giving better results than conventional heating [ 1 ]. 

The use of microwaves for the preparation of aromatic five-membered heterocycles has been intensely investigated with excellent results in terms of yields and purities of the products prepared. The Paal-Knorr reaction, namely the cyclocondensation of a 1,4-dicarbonyl compound to give furans, pyrroles and thiophenes has been successfully carried out with the aid of microwaves. 

Different approaches towards pyrazoles have been described using microwave-accelerated cyclizations. A classical approach to pyrazoles is the cyclization of a j6-diketone with hydrazines. A series of 5-trichloromethyl-pyrazoles 58 and pyrazolium chlorides 59 were synthesized by reaction of a 4-methoxy-trihalo-3-alken-one 57 with differently substituted hydrazines (Scheme 19). The use of microwave and conventional heating for making pyrazoles gave... 

The availability of functionalized 2(lH)-pyrazinone in combination with the use of microwave accelerated solid-phase chemistry constitutes a sohd foundation for generating large libraries of compoimds suitable for medicinal chemistry. The authors have also shown that the scaffold can be further functionalized using the principles of cUck-chemistry , thereby paving the way towards highly substituted 2-pyridone structures [45-47]. 

This technique has also been applied in the synthesis of carbostyril analogues 15 and as in the previous example also this reaction is favored by an electron-rich group in the aniline-ring and an electron-poor group attached to the electrophihc specie (b. Scheme 6) [55]. The use of microwave irradiation can reduce the reaction times from 18-58 h to 80 min and the products are generally isolated in high yields and purities. 

Another possibility to achieve more efficient CM is the use of microwave irradiation [149]. Its use sometimes leads to drastically shortened reaction times and higher yields compared to thermal heating while the i /Z-ratio remains unaffected. 

The use of microwaves for heating chemical reactors has increased dramatically over the past decade [428]. 

The principles behind MAP liquid-phase and gas-phase extractions are fundamentally similar and rely on the use of microwaves to selectively apply energy to a matrix rather than to the environment surrounding it. MAP gas-phase extractions (MAP-HS) give better sensitivity than the conventional static headspace extraction method. MAP-HS may also be applied in dynamic applications. This allows the application of a prolonged, low-power irradiation, or of a multi-pulse irradiation of the sample, thus providing a means to extract all of the volatile analytes from the matrix [477]. 

The use of microwave digestion-wet oxidation overcomes sample preparation problems for many polymer-based materials. However, this will result in a reduction in sensitivity compared with an ashing procedure, because of dilution. Use of an aqueous phase is not... 

---