Modified domestic oven

For a more representative investigation, the authors synthesized a fragment of the acyl carrier protein (6S 74ACP) using preformed active esters in N,N-dimethyl-formamide [22]. Each coupling step required only 4 min of irradiation in a modified domestic oven with an average coupling yield of >99% (Scheme 7.2). 

The reaction of C60 with o-quinonemethide, prepared from o-hydroxybenzyl alcohol (110) (Scheme 9.32), was performed in a modified domestic oven at 800 W and gave 111 in 27% yield after only 4 min [72], Although Eguchi et al. [83] reported the same reaction with a slightly better yield (31%) by thermolysis in a sealed vessel, the microwave approach to this adduct has the advantage of simplicity, and avoids the risks associated with high-pressure conditions. 

We are all familiar with use of microwave radiation in cooking food where it increases the speed of reaction. Recently this method has been used to synthesize solid state materials such as mixed oxides. The first solid state reaction experiments were performed in modified domestic ovens, and these are still used, but more specialised (and expensive ) ovens have also been developed to give more control over the conditions. We shall briefly consider how microwaves heat solids and liquids because this gives us insight into which reactions will be good candidates for this method. 

More recently Peng and Song reported the rapid synthesis of a library of hydrazides in a MW-US combined reactor (Scheme 9.20)139. Unlike the aforementioned system that employed decalin as an energy transfer medium for the ultrasound irradiation, in their modified domestic oven, the horn was immersed directly into the reaction mixture. 

Titanium oxide/polystyrene core-shell nanospheres have been prepared using a combined sol-gel/microwave-assisted emulsion polymerization procedure using a modified domestic oven. The sol-gel process gave stable Ti02 colloids that were used as seeds for the subsequent polymerization step accomplished in 1.5 h at 70 °C. 

Fig. 3.1 Modified domestic household microwave oven. Inlets for temperature measurement by IR pyrometer (left side) and for attaching reflux condensers (top) are visible. A magnetic stirrer is situated below the instrument.

The reactions were carried out rapidly in a modified domestic microwave oven with mountings in the side wall for sufficient inert gas supply. Furthermore, a dedicated custom-made solid-phase reaction vessel was employed under atmospheric pressure conditions. 

To avoid explosion hazards [70], a modified domestic microwave oven was used. Higher yields were obtained in shorter periods of time on use of microwave irradiation (39% yield after 20 min irradiation) in comparison with thermal conditions. Longer irradiation times led to a decrease in the yield of 75, because of increased bis adduct formation. 

There are a number of published methods for the safe modification of domestic microwave ovens. The improved safety of this type of system is particularly attractive, the advantage of modifying an oven in this way is that the reaction vessel is neither sealed nor directly open to the microwave oven. Thus, reactions can be carried out using a flask attached to a reflux condenser. There are two advantages of using this type of system (i) a wide variety of solvents can be used because the fire hazard is significantly reduced compared to an open vessel system, (ii) reactions can be carried out under inert conditions. 

Modified microwave ovens. The accuracy and safety factor in microwave assisted organic synthesis can be increased by causing a slight variation in domestic microwave oven. The modified microwave oven differs from domestic microwave oven in having a hole on top of cavity. This allows the introduction of a tube (acting as an air cooler) surmounted by a water cooler to maintain reaction s solvent reflux or under inert atmosphere, or allowing the chemist to follow multistep procedures of chemical synthesis. 

Palchik [182] obtained nanosized amorphous iron oxide (Fe203) by the pyrolysis of iron pentacarbonyl, Fe(GO)5, in a modified domestic microwave oven in refluxing chlorobenzene as solvent under air. The reaction time was 20 min. Separate particles of iron oxide, 2-3 nm in diameter, were obtained together with aggregated spheres with a diameter of 25-40 nm. Differential scanning calorimetry measurements showed an amorphous/crystalline phase transition at about 250 °C. 

This method is not only limited to halide-based ionic liquids. Synthesis of ionic liquids containing a tetrafluoroborate anion and imidazolium cation has been performed successfully [16] by use of a modified domestic microwave oven with pulsed irradiation (5 x 30 s). Approximately 90% yields of the desired ionic liquids were usually obtained compared with 36% after the same reaction time using conventional heating. Microwave irradiation has been used in the synthesis of l-ethyl-3-methylimidazolium benzoate and dialkyl imidazolium tetrachloroaluminate ionic liquids [17, 18]. These reactions were again performed in a domestic microwave oven using pulsed irradiation. 

Although reports of ionic liquid synthesis in domestic microwave ovens are promising, synthesis in these systems has several disadvantages. For chloroalumi-nates there were problems with the reproducibility of the synthesis. In domestic microwave ovens, control of the reaction conditions is difficult and overheating of the reaction mixtures can easily occur. A modified procedure was published, again using a domestic oven, but placing the reaction vessel in a beaker of water [19]. 

Khadilkar and Madyar have developed a large scale continuous synthesis of Hantzsch l,4-dihydropyridine-3,5-dicarboxylates in aqueous hydrotope solution, using a modified domestic microwave oven [81]. The authors used novel reusable aqueous hydrotope solution as a safe alternative to inflammable organic solutions, in a microwave cavity, for synthesis of commercially important calcium blockers such as nifedipine, nitrendipine, and a variety of other 1,4-dihydropyridines (DHP) (Scheme 10.38). Nitrendipine (R = 3-N02, R = Me) has been obtained in 94% yield (50 g) after 24 min by microwave irradiation of the reaction mixture (final temperature 86 °C) at a flow rate of 100 mL min. The reaction mixture was circulated through the microwave cavity in four cycles of 6 min each a 2-min gap between each cycle was imposed to avoid excessive heating. 

Domestic ovens can be inexpensively and safely modified, however this almost eliminates these disadvantages and enables independent temperature measurement and reasonable temperature control. For temperature measurement an IR thermometer or, better, a fiber-optic thermometer [75-77] has been recommended. Such a batch microwave reactor made by modification of a domestic microwave oven is depicted in Fig. 13.1 and has been described elsewhere (Refs. [51, 75-77, 141-144] and references cited therein). 

The copolymerization of methacrylic acid, 2-(dimethylamino)ethyl ester, and thiourea under microwave irradiation was studied by Lu et al. (Scheme 14.8) [19]. It was shown that the copolymers can be used to coordinate Cu(II) to afford coordinated copolymers which can, in turn, be used as heterogeneous catalysts in the polymerization of methyl methacrylate. The reactions were performed in a modified domestic microwave oven with a continuous power regulation. 

Homogenous atom-transfer radical polymerization of methyl methacrylate under microwave irradiation conditions has also been studied by Zhu et al. [23, 24]. In a typical run, a small amount of CuCl, N,N,N, N",N"-pentamethyl-diethylenetriamine, with ethyl 2-bromobutyrate as activator-initiator system, were placed in a 10-mL glass tube with 1 mL DMF and 5 mL methyl methacrylate. The tube was sealed and placed in a two-necked reaction flask filled with hexane, so that temperature was controlled by the boiling point of the solvent during reflux in a modified domestic microwave oven. Linear first-order rate plots, linear in-... 

A more detailed approach to the synthesis of a series of functionalized Merrifield resins has been reported [25]. In a modified domestic microwave oven, using a reflux condenser, reaction rates were dramatically enhanced compared with conventional methods - as high conversions were achieved within 25 min (Scheme 16.2). These microwave-mediated pathways are convenient methods for rapid and efficient solid-phase synthesis, using PS-Merrifield resin as either a support or a scavenger (see Section 16.6). 

Because the reaction occurred in solution and, in these early days, in an effort to avoid explosion hazards [18], a modified domestic microwave oven was used. By use of microwave irradiation higher yields of the desired products were obtained more quickly (39% yield after irradiation for 20 min) than under thermal condi-... 

A modified domestic microwave oven with a refluxing system (Kenwood Microwave, 2.45 GHz, 900 W) is used to carry out the reactions. The experimental setup is shown in Figure 4.2. In all the experiments the microwave is operated at a specific cycling mode on 21 seconds, off 9 seconds, with the total power always at 900 W. The cycling mode was chosen in order to avoid the bumping of the solvent. The reactions were carried out in a closed hood with air-exhaust ventilation. 

In the series of papers, synthesis of aliphatic polyamides and polyimides under microwave irradiation was described by Imai et al. [67-71]. The reactions were carried out in a modified domestic microwave oven with a small hole on the top of the oven so that nitrogen was introduced to a 30 ml wide-mouth vial adapted as a reaction vessel. In the case of polyamides synthesis, they were prepared of both CO-amino acids and nylon salt type monomers while polyimides were obtained from the salt form of monomers composed of aliphatic diamines and pyromellitic acid or its diethyl ester in the presence of a small amount of a polar organic medium (Fig. 11). 

Fig. 6.1 a Domestic multimode oven, b A modified MW oven for MW photochemistry experiments. (A) Magnetron, (B) reaction mixture with the EDL and a magnetic stir bar, (C) aluminium plate, (D) magnetic stir, (IT) infrared pyrometer, (F) circulating water in a glass tube and (G) dummy load inside the oven cavity, c Photochemistry a MW oven (the EDL floats on the liquid... 

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