Modified commercial microwave ovens

Reactions involving a solvent under reflux require the use of modified commercial microwave ovens. In these modified systems the oven is perforated on the top to accommodate a reflux condenser and a 10 cm pipe is used to avoid microwave leakage the turnable dish is replaced by a magnetic stirrer or by monomode reactors especially designed for chemical synthesis [15]. 

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. 

More recently, these conditions have been modified through use of microwave heating. Sejias and co-workers reported the use of a commercial microwave oven to affect the cyclization albeit with lesser control over the reaction temperature. ... 

In a modified Graebe-Ullman reaction, pyridylbenzotriazole 223 was converted to a-carboline 224 in an efficient manner but in moderate yield (Scheme 38) <20060L415>. Microwave irradiation was the energy source for both a-carboline synthesis and the preparation of 223. The advantages of this procedure are that the starting materials are commercially available and lower reaction times are used resulting in fewer undesirable side products. The style of microwave oven, amount of pyrophosphoric acid, power level, and time were all optimized. 

The microwave sintering was performed in a kitchen oven, commercially available, that has been modified by the addition of a temperature controller and an insulation box and silicon carbide susceptors. The modifications include a thermocouple (Pt-PtRh) and a power controller (with both manual and automatic options) for temperature control. The insulation box consists of a small chamber fabricated from low-density insulation board. Low density and very low dielectric loss is required for... 

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