Microwave flash heating

Fig. 5.3 Heating profile for a typical Biginelli condensation in AcOH/EtOH (3 1) under sealed-vessel microwave irradiation conditions microwave flash heating (300 W, 0-40 s), temperature control using the feedback from IR thermography (constant 120 °C, 40-600 s), and active cooling (600-660 s).

Significant rate accelerations and higher loadings are observed when the micro-wave-assisted and conventional thermal procedures are compared. Reactions times are reduced from 12-48 h with conventional heating at 80 °C to 5-15 min with microwave flash heating in NMP at temperatures up to 200 °C. Finally, kinetic comparison studies have shown that the observed rate enhancements can be attributed to the rapid direct heating of the solvent (NMP) rather than to a specific nonthermal microwave effect [17]. 

Transition Metal Catalysis and Microwave Flash Heating in Organic Chemistry... 

The three component condensations of aldehydes, amines and elemental sulfur using 1-methyl-2-pyrrolidone as solvent employing microwave flash heating 100-180 °C for 2-20 min give the corresponding primary, secondary and tertiary thioamides (Scheme 6).23... 

Olofsson, K., Hallberg, A. and Larhed, M., Transition metal catalysis and microwave flash heating in organic chemistry, In Loupy, A. (Ed.), Microwaves in Organic Synthesis, Wiley Weinheim, 2002, pp. 379 403. 

The classical Hantzch dihydropyridine synthesis has been reported under microwave flash heating conditions45,46. More unusually, Hantzch products have also been obtained from Biginelli reaction under microwave flash heating conditions (Scheme 3.27)47. 

Scheme 7.7 Acid attachment employing microwave flash heating using (a) Merrifield resin and (b) commercially available chlorinated Wang resin as the polymer support.

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