Thermal Effects in the Organocatalytic Asymmetric Mannich Reaction
作者:Belén Rodríguez、Carsten Bolm
DOI:10.1021/jo060064d
日期:2006.3.31
The proline-catalyzed direct asymmetricMannichreaction between cyclohexanone, formaldehyde, and various anilines is thermally accelerated. With only 0.5 mol % of catalyst, Mannich products with up to 98% ee have been obtained after a short period of time in reactions performed under microwave irradiation. In situ reduction of the resulting ketones affords N-aryl amino alcohols in up to 86% yield
can be performed underflowconditions by using inductivelyheatedflow reactors. These include transfer hydrogenations, heterocyclic condensations, pericyclic reactions, organometallic reactions, multicomponent reactions, reductive cyclizations, homogeneous and heterogeneous transition‐metal catalysis. Silica‐coated iron oxide nanoparticles are stable under many chemical conditions and the silica shell
Microwave-Assisted Asymmetric Organocatalysis. A Probe for Nonthermal Microwave Effects and the Concept of Simultaneous Cooling
作者:Masood Hosseini、Nikola Stiasni、Vera Barbieri、C. Oliver Kappe
DOI:10.1021/jo0624187
日期:2007.2.1
[GRAPHICS]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.
Direct Catalytic Enantioselective α-Aminomethylation of Ketones
作者:Ismail Ibrahem、Jesús Casas、Armando Córdova
DOI:10.1002/anie.200460678
日期:2004.12.3
Direct organocatalytic enantioselective α-aminomethylation of ketones
The scope and limitations of the direct organocatalytic asymmetric α-aminomethylation of ketones are disclosed. The proline-catalyzed classical Mannich reactions between unmodified ketones, aqueous formaldehyde and aromatic amines furnished the desired Mannich bases in high yield with up to >99% ee. Moreover, methyl alkyl ketones were regioselectively α-aminomethylated at the methylene carbon affording