Silicon carbide (SiC) is a strongly microwave absorbing chemically inert ceramic material that can be utilized at extremely high temperatures due to its high melting point and very low thermal expansion coefficient. Microwave irradiation induces a flow of electrons in the semiconducting ceramic that heats the material very efficiently through resistance heating mechanisms. The use of SiC carbide reaction vessels in combination with a single-mode microwave reactor provides an almost complete shielding of the contents inside from the electromagnetic field. Therefore, such experiments do not involve electromagnetic field effects on the chemistry, since the semiconducting ceramic vial effectively prevents microwave irradiation from penetrating the reaction mixture. The involvement of electromagnetic field effects (specific/nonthermal microwave effects) on 21 selected chemical transformations was evaluated by comparing the results obtained in microwave-transparent Pyrex vials with experiments performed in SiC vials at the same reaction temperature. For most of the 21 reactions, the outcome in terms of conversion/purity/product yields using the two different vial types was virtually identical, indicating that the electromagnetic field had no direct influence on the reaction pathway. Due to the high chemical resistance of SiC, reactions involving corrosive reagents can be performed without degradation of the vessel material. Examples include high-temperature fluorine-chlorine exchange reactions using triethylamine trihydrofluoride, and the hydrolysis of nitriles with aqueous potassium hydroxide. The unique combination of high microwave absorptivity, thermal conductivity, and effusivity on the one hand, and excellent temperature, pressure and corrosion resistance on the other hand, makes this material ideal for the fabrication of reaction vessels for use in microwave reactors.
The development of multistep continuous flow reactions for the synthesis of important intermediates for the pharmaceutical industry is still a significant challenge. In the present contribution the biaryl-hydrazine unit of Atazanavir, an important HIV protease inhibitor, was prepared in a three-step continuous flow sequence in 74% overall yield. The synthesis involved Pd-catalyzed Suzuki–Miyaura cross-coupling, followed by hydrazone formation and a subsequent hydrogenation step, and additionally incorporates a liquid–liquid extraction step.
Applying continuous flow processing in a high-temperature/high-pressure
regime, n-alkyl chlorides can be prepared in high
yields and selectivity by direct uncatalyzed chlorodehydroxylation
of the corresponding n-alcohols with 30% aqueous
hydrochloric acid. Optimum conditions for the preparation of n-butyl and n-hexyl chloride involve the
use of a glass microreactor chip, a reaction temperature of 160–180
°C (20 bar backpressure) and a residence time of 15 min.
Es ist die Feldstärke, nicht die Temperatur! Das Wechseln der elektrischen Feldstärke in einem Mikrowellenexperiment kann das Ergebnis der Reaktion völlig verändern. Betrachtet wird die Bildung des Grignard‐Reagens aus Mg‐Metall und Arylhalogeniden: Während eine niedrige Feldstärke den Initiationsschritt beschleunigt, wird bei hoher Feldstärke bei gleicher Temperatur die Mg‐Insertion unterdrückt, und es kommt zu Lösungsmittelzersetzung und Passivierung des Mg‐Metalls (siehe Schema).
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