Household microwave

This article describes conditions for using a household microwave oven to dry precipitates for the determination of Gh as AgGl, the determination of S04 as BaS04, and the determination of Ga + as GaG204 H2O. 

In the case of a diketone (e.g., 3-tosyloxypentane-2,4-dione), the formation of 5-acetyl-4-methyl-2-aryl-l,3-thiazole derivatives can be realized in very good yields (86-89%) (Scheme 7). All these experiments where performed in a Sears Kenmore unmodified household microwave oven (990 W) equipped with a turntable. The average bulk temperature was estimated by inserting a thermometer in the alumina bath housing the reaction vessel. 

Thiophenes of type 31 (X-Y = CH) were generated via Lawesson s reagent-mediated cyclization of 1,4-dicarbonyl compounds 30 under microwave irradiation in the absence of solvent [37]. The reaction was carried by mixing the two solid reagents in a glass tube inserted inside a household microwave apparatus and irradiating until the evolution of H2S ceased. An interesting application of this method is the preparation of liquid crystals and other ferro- and antiferroelectric material such as compound 33 (Scheme 10). 

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.

Several articles in the area of microwave-assisted parallel synthesis have described irradiation of 96-well filter-bottom polypropylene plates in conventional household microwave ovens for high-throughput synthesis. While some authors have not reported any difficulties in relation to the use of such equipment (see Scheme 4.24) [77], others have experienced problems in connection with the thermal instability of the polypropylene material itself [89], and with respect to the creation of temperature gradients between individual wells upon microwave heating [89, 90]. Figure 4.5 shows the temperature gradients after irradiation of a conventional 96-well plate for 1 min in a domestic microwave oven. For the particular chemistry involved (Scheme 7.45), the 20 °C difference between the inner and outer wells was, however, not critical. 

Gram quantities of aryl-2-(N,N-diethylamino) ethyl ethers, compounds of biological interest, have been prepared in a household microwave oven, with potassium hydroxide and glyme as the transfer agent, according to Eq. (11) [22]. 

Under solid-liquid PTC conditions 5,5-diethylbarbituric acid was N,N-dialkylated in a good yield in the presence of a lipophilic ammonium salts and potassium carbonate when reaction mixtures were irradiated in a household microwave oven (Eq. 26). 

In an utmost simplistic approach, an unmodified household microwave oven has been used in this study with excellent results and the generation of higher temperatures is simply avoided by intermittent heating [35]. 

Gordon used a household microwave oven for the transfer hydrogenation of benz-aldehyde with (carbonyl)-chlorohydridotris-(triphenylphosphine)ruthenium(II) as catalyst and formic acid as hydrogen donor (Eq. 11.43) [61]. An improvement in the average catalytic activity from 280 to 6700 turnovers h-1 was achieved when the traditional reflux conditions were replaced by microwave heating. 

A rapid preseparation technique was developed for the extraction of SAL from various chicken tissues using the irradiation of the sample in EtOH-2-PrOH for 9 s in a common household microwave oven. The extract was analyzed without further cleanup and detected via postcolumn reaction with DMABA at 86°C. Recoveries ranged between 87% and 100% (105). 

In a typical experiment, benzaldehyde (106 mg, 1 mmol) was added to the finely powdered paraformaldehyde (60 mg, 2 mmol). To this mixture, powdered barium hydroxide octahydrate (631 mg, 2 mmol) was added in a glass test tube and the reaction mixture was placed in an alumina bath (neutral alumina 125 g, mesh 150, Aldrich bath 5.7 cm diameter) inside a household microwave oven and irradiated for the specified time at its full power of 900 W intermittently or heated in an oil bath at 100-110 °C. On completion of the reaction, as indicated by TLC (hexane-EtOAc, 4 1, v/v), the reaction mixture was neutralized with dilute HC1 and the product extracted into ethyl acetate. The combined organic extracts were dried over anhydrous sodium sulfate and the solvent removed under reduced pressure. The pure benzyl alcohol (99 mg, 91%), however, is obtained by extracting the reaction mixture with ethyl acetate prior to neutralization and subsequent removal of the solvent under reduced pressure. 

The oxidation of benzyl alcohol la to benzaldehyde 2a is representative of the general procedure employed. Benzyl alcohol la (0.108 g, 1 mmol) and IBD (0.355 g, 1.1 mmol) doped on neutral alumina (1 g) are mixed thoroughly on a vortex mixer. The reaction mixture is placed in an alumina bath inside an unmodified household microwave oven and irradiated for a period of 1 min. On completion of the reaction, followed by TLC examination (hexane-AcOEt, 9 1, v/v), the product is extracted into dichloromethane and is neutralized with aqueous sodium bicarbonate solution. The dichloromethane layer is separated, dried over magnesium sulfate, filtered, and the crude product thus obtained is purified by column chromatography to afford pure benzaldehyde 2a in 94% yield. Alternatively, the crude products are charged on a silica gel column that provides io-dobenzene on elution with hexane followed by pure carbonyl compounds in solvent system (hexane-ethyl acetate, 9 1, v/v). 

A mixture of phenyl acetaldehyde 1 (0.6 g, 5 mmol) and morpholine 2a (0.48 g, 5.5 mmol) was placed in a small beaker and irradiated in an unmodified household microwave oven at its full power (900 W) for 2 min. Salicylaldehyde 4a (0.61 g, 5 mmol) and ammonium acetate (0.02 g, 0.25 mmol) were then added to the same reaction vessel, and the reaction mixture was further irradiated in the microwave oven at its 50% power for 5 min using a pulse technique. Upon completion of the reaction, followed by TLC, the reaction mixture was passed through a bed of basic alumina using hexane-ether (9 1, v/v) as an eluent to afford pure 2-aminomorpholinoisoflav-3-enes 6a (yield 80%, mp 103-105 °C). 

A mixture of benzaldehyde la (106 mg, 1 mmol) and 2-aminopyridine 3a (94 mg, 1 mmol) was irradiated in an unmodified household microwave oven for 1 min (at full power of 900 W) in the presence of montmorillonite K-10 clay (50 mg). After addition of benzyl isocyanide 2a (117 mg, 1 mmol), the reaction mixture was further irradiated successively (2 min) at 50% power level for a duration of 1 min followed by a cooling period of 1 min. The resulting product was dissolved in dichlorometliane (2x5 mL) and the clay was filtered off. The solvent was removed under reduced pressure and the crude product was purified either by crystallization or by passing it through a small bed of silica gel using EtOAc-hexane (1 4, v/v) as eluent to afford 4a. 

A household microwave oven operating at 2450 MHz was used at its full power, 650 W. A neat mixture of benzopyran derivative 1 or 3 (1 mmol) and hydrazine (1.2-3 mmol) in a 10-mL glass beaker was thoroughly mixed for about 5 min, then it was placed in an alumina bath inside the household microwave oven and irradiated. Maximum temperature reached in the alumina after 10 min was about 150 °C. After cooling, methanol (ca. 4 mL) was added to the mixture and the separated solid was filtered off and washed with a small amount of methanol to give the products 2 and 4. 

In a small beaker oxime (10 mmol) and freshly prepared clayfen reagent (6.6 mmol of iron(III) nitrate) were mixed together to make an intimate mixture. The beaker was placed in a household microwave oven for the specified time. The residue was washed with CH2C12 (10 mL) and filtered. The filtrate was evaporated to dryness to afford the corresponding carbonyl compound. 

It should be stressed that the majority of literature reports on the acceleration of chemical reactions by microwave irradiation (even more than 1000 fold) come from the initial period of the application of microwaves in organic synthesis (i.e. from late 1980 s and early 1980 s). At this time there were no dedicated microwave scientific reactors available on the market, and most of theses reactions were conducted in household microwave ovens. Recently, applying modern microwave reactors, scientists have verified a number of these reports, and it turns out that the claimed acceleration of the majority chemical reactions were attributed to difficulties with proper temperature measuments rather than to non-thermal micowave effects. Sometimes it was found that these effects were results of faster delivering of energy to the reaction systems [35,38]. 

In the literature, first microwave-assisted experiments on organic synthesis employed multimode household microwave ovens [12,13]. More recently, the use of microwave reactors for chemical syntheses has become more advanced, and, at the moment, some chemical journals specializing in organic chemistry intend to refuse manuscripts in which experiments were carried out in a domestic microwave oven (even including ovens with... 

Even though in some cases the original works given in the references have been conducted in household microwave ovens, all the experiments presented in the book have been carried out in the dedicated microwave reactors (see Section 3.4), which can result in differences in reaction temperatures and times while compared to the original references. 

The reduction was carried out applying a household microwave oven. In contrast to the Zn-AcOH reduction of Parthenin, which was performed under conventional heating conditions, the microwave-assisted protocol was found to be useful in preventing the deoxygenation of the compound and the reduction of the exocyclic methylene group. 

A rapid parallel solvent-free synthesis of a representative 28-membered library of phthalimides was achieved utilizing a household microwave oven under highly optimized conditions [35]. Thus, the highest irradiation area inside the household microwave oven was determined to ensure better reproducibility of results (Scheme 12). 

In a closely related publication, carboxylic acids were employed instead of acid chlorides in a microwave-assisted direct synthesis of 2-substituted benzoxazoles [79]. The reactions with 2-aminophenol were performed in a household microwave oven and worked well with aromatic, heteroaromatic, aj/i-unsalurated and arylalkyl carboxylic acids (35-82% yields). Phthalic acid formed only mono-benzoxazoles, while the use of succinic acid led to a mixture of mono- and bis-benzoxazoles. Phthalic and succinic anhydrides could... 

Microwave irradiation considerably improved the reaction speed and yields of 2,4-disubstituted quinolines in a MCR of aldehydes, anilines and alkynes [111]. The cyclocondensation was catalyzed by montmorillonite clay doped with copper bromide and was completed within 3-5 minutes (pulsed irradiation technique—1 min with 20 s off interval), when performed in a household microwave oven. Oil-bath heating at 80 °C for 3-6 hours was necessary to achieve comparable yields of quinolines (71-90%) (Scheme 42). 

The Friedlander annulation is one of the most straightforward approaches towards poly-substituted quinolines. Thus, a 22-membered library of quinolines was synthesized in a TsOH-catalyzed cyclocondensation-dehydration of 2-aminoaryl ketones and 2-aminoarylaldehydes with ketones in a household microwave oven (with power control) under solvent-free conditions [112]. It was observed that the Friedlander reaction occurred readily also in an oil-bath (at 100 °C). To compare the conventional and dielectric heating conditions precisely, a purpose-built monomode microwave system with temperature control was employed instead of the household oven. The experiments at 100 °C under otherwise identical conditions demonstrated that the dielectric heating protocol was only slightly faster. Products were isolated by a simple precipitation-neutralization sequence (in the case of solid products) or neutralization-extraction for oily or low melting point products (Scheme 43). 

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