Microwave oven effect

AI2O3, H2O, hexanes, 81-98% yield. These conditions are selective for the primary derivative. TBDPS and TMS ethers are also cleaved. The use of alumina in a microwave oven is also effective (68-93% yield). ... 

Independently, Caddick et al. reported microwave-assisted amination of aryl chlorides using a palladium-N-heterocyclic carbene complex as the catalyst (Scheme 99) [lOlj. Initial experiments in a domestic microwave oven (reflux conditions) revealed that the solvent is crucial for the reaction. The Pd source also proved very important, since Pd(OAc)2 at high power in DMF gave extensive catalyst decomposition and using it at medium and low power gave no reaction at all. Pd(dba)2/imidazohum salt (1 mol% catalyst loading) in DME with the addition of some DMF was found to be suitable. Oil bath experiments indicated that only thermal effects are governing the amination reactions. 

It is clearly connected with the effect of stirring and the presence of a nucleation regulator [15]. It is also related to the microwave power. It has been shown that the effect is eliminated when the experiments are performed on well-stirred mixtures [16, 17] using low microwave power. It could be essentially a consequence of the absence of stirring, i. e. in closed vessels inside a domestic microwave oven. 

Rapid N-alkylation of saccharin (Eq. 24) by a series of halides was performed on silica gel via its sodium salt in a microwave oven. TEBA was shown to have a useful co-catalytic effect (Tab. 5.11). 

The cycloaddition of ketone 54 could be effected in a sealed glass tube in a modified microwave oven to afford the tricyclic system stereoselectively. This major adduct arose via the preferred transition state, in which the nonbonded interactions were minimized, because of the alignment of the dienophile beneath the triene unit furthest from the MOM substituent. This pattern of n-facial selectivity implies that, with the natural C2 stereoselectivity, the preferred geometry should provide the relative stereochemistry required for taxol itself. 

The conversion of aryl iodides into aryl phosphonates, a useful precursor to aryl phosphonic acids, was performed in a Teflon autoclave by Villemin [51]. A domestic microwave oven was used for these experiments and the reaction times for classic heating were effectively reduced from 10 h to 4-22 min. The reactivity of iodides was good whereas the use of bromides resulted in lower yields and reactions with tri-flates were very slow (Eq. 11.35). It is notable that the reactions were brought to completion with short reaction times in a non-polar solvent. 

We have mentioned microwave ovens a few times already. Although we might employ such an oven regularly, no one has actually seen microwave radiation. The photons are wholly invisible. The phenomenon of sight can be simplified to the photo-effected transformation of a pigment related to retinal within the retina at the back of the eye. Retinal is derived from vitamin A. 

There are several ways to define efficiency. One general definition says that energy efficiency is the ability to produce a desired effect with minimum energy expenditure. For example, suppose that you want to bake a potato. You can use a microwave oven or a conventional oven. 

Solvent effects. As any modem cook knows, a microwave oven is a superb means of cooking any foodstuff containing water, when irradiated with microwaves of sufficient frequency, will absorb energy and so the food gets hot and therefore cooks . Unfortunately, microwaves are at the very heart of EPR spectroelec-trochemistry experiments, so care is therefore required to remove all traces of water. 

The diodes used are normaily high voltage types used in microwave ovens or other HV circuits such as in TV s. These can be linked in series if the rating is not high enough to provide the necessary blocking effects. 

A rather grisly example of biological effects of microwave radiation is provided by the story, possibly apocryphal, of the woman who put her cat in a microwave oven to dry after it had come in out of the rain. The cat exploded And one hears tales of crows and seagulls being shot out of the sky with intense microwave beams. 

A number of other methods have been reported for the acceleration of Diels-Alder reactions, including the use of a microwave oven,877 water as a solvent (a hydrophobic effect),878 5 M LiC104 in Et20 as solvent,879 absorption of the reactants on chromatographic absorbents,880 and the use of an ultracentrifuge881 (one of several ways to achieve reaction at high pressures).882... 

Table2 The effect of microwave oven pressure on surface area and pore volume of the synthesis samples ...

Clayfen (0.125 g) was thoroughly mixed with neat benzoin lc (0.106 g, 0.5 mmol) in the solid state using a vortex mixer and the material was placed in an alumina bath inside the microwave oven and irradiated. Upon completion of the reaction, monitored on TLC (hexane-AcOEt, 10 1, v/v), the product 2c was extracted into methylene chloride. That the effect is not purely thermal is supported by the fact that this reaction could be completed in 18 h in an oil bath at a comparable temperature of 65 °C. 

A mixture of 3-chloroaniline le (2.10 mL, 20.0 mmol) and diethyl phenylmalo-nate 2e (8.62 mL, 40.0 mmol) is stirred and irradiated in a microwave oven (type Milestone Ethos 900) for 15 min at 500 W power under a gentle stream of nitrogen (in order to effectively remove the formed ethanol during the reaction). The mixture is allowed to attain room temperature and diluted with diethyl ether. The precipitate is collected by filtration and washed with diethyl ether to furnish pure 7-chloro-4-hydroxy-3-plienylquiiiolin-2(l//)-one 3e (4.36 g, 81%) as white crystals. 

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