Asymmetric Organocatalysis under Microwave Irradiation-Thermal Effect

Asymmetric Organocatalysis under Microwave Irradiation-Thermal Effect [Pg.593]

Microwave (MW) irradiation is the most popular non-conventional method of accelerating organic reactions, used also as an alternative to conventional heating. [Pg.593]

The microwave irradiation was generated using a reactor setup that allowed accurate measurements of internal reaction temperature using fiber-optic probes. Experiments were carried out under at least two different power values, and also with simultaneous external cooling when a higher power of microwave radiation was applied. The same method of temperature measurement was also used in experiments with conventional heating. [Pg.595]

In conclusion, Kappe s group demonstrated the absence of any differences between conventional and microwave heating in proline-catalyzed Mannich and aldol reactions as well as no evidence for specific or non-thermal microwave effects. In all cases, in contrast to the previous literature reports, the results obtained with microwave irradiation could be reproduced by conventional heating at the same reaction temperature and time in an oil bath. The differences observed in previous publications could be a result of incorrect temperature measurements [36]. After Kappe s [35] publication several articles appeared in the literature concerning the application of microwaves in asymmetric organocatalysis, mostly in aldol and Michael type reactions operating via enamine as well as iminium catalysis. [Pg.595]

Erase s group [38] also developed a microwave-assisted a-sulfamidation of 2-arylaldehydes and ketones with chloramine-T catalyzed by L-proline (2mol%). Under optimized conditions (200 W, 60 °C, 30 min, with air cooling) products were obtained in high yields (71-91%) but unfortunately no information about enantioselectivity was reported. [Pg.596]

82% yield and 97% ee. The temperature tolerance for this reaction, as well as Westermann s [32] studies, encouraged Rodriguez and Bolm to explore the effect of microwaves. After 4h irradiation at 48 °C (with air cooling) the yield of the [Pg.594]

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