Portal de Periódicos da CAPES

Microwave-Assisted Asymmetric Organocatalysis. A Probe for Nonthermal Microwave Effects and the Concept of Simultaneous Cooling

2007; American Chemical Society; Volume: 72; Issue: 4 Linguagem: Inglês

10.1021/jo0624187

1520-6904

Masood Hosseini, Nikola Stiasni, Vera Barbieri, C. Oliver Kappe,

Asymmetric Synthesis and Catalysis

A series of five known asymmetric organocatalytic reactions was re-evaluated at elevated temperatures applying both microwave dielectric heating and conventional thermal heating in order to probe the existence of specific or nonthermal microwave effects. All transformations were conducted in a dedicated reactor setup that allowed accurate internal reaction temperature measurements using fiber-optic probes. In addition, the concept of simultaneous external cooling while irradiating with microwave power was also applied in all of the studied cases. This method allows a higher level of microwave power to be administered to the reaction mixture and, therefore, enhances any potential microwave effects while continuously removing heat. For all of the five studied (S)-proline-catalyzed asymmetric Mannich- and aldol-type reactions, the observed rate enhancements were a consequence of the increased temperatures attained by microwave dielectric heating and were not related to the presence of the microwave field. In all cases, in contrast to previous literature reports, the results obtained either with microwave irradiation or with microwave irradiation with simultaneous cooling could be reproduced by conventional heating at the same reaction temperature and time in an oil bath. No evidence for specific or nonthermal microwave effects was obtained.