Kontrolliertes Erhitzen mit Mikrowellen in der modernen organischen Synthese

Abstract

Die offene Flamme konnte erst gezielt als Heizquelle für Reaktionsgefäße in der Synthesechemie verwendet werden, nachdem Robert Bunsen 1855 den nach ihm benannten Brenner erfunden hatte. Der Bunsen‐Brenner wurde später durch den Heizpilz, das Ölbad und die Heizplatte als Wärmequellen für chemische Reaktionen abgelöst. In den vergangenen Jahren ist das Erhitzen und Beschleunigen chemischer Reaktionen durch Mikrowellenenergie populär geworden. Diese Heiztechnik hat sich von einer Laborkuriosität zu einer etablierten Methode entwickelt, die an Universitäten und in der Industrie stark genutzt wird. Die effiziente Verringerung der Reaktionszeiten (von Tagen und Stunden auf Minuten und Sekunden) ist nur einer der vielen Vorteile des schnellen Erhitzens mit Mikrowellen (“microwave flash heating”). Dieser Aufsatz gibt einen Überblick über neuere Anwendungen des kontrollierten Mikrowellenerhitzens in der modernen organischen Synthese und diskutiert einige grundlegende Vorgänge.

Palladium(0) Nanoparticles on Glass‐Polymer Composite Materials as Recyclable Catalysts: A Comparison Study on their Use in Batch and Continuous Flow Processes

“…[36] A reaction medium with a high tan d at the standard operating frequency of a microwave reactor (2.45 GHz) is required for good absorption and, consequently, for efficient heating. As indicated above our reactor design should in principle allow for the selective heating of the strongly microwave absorbing [36] palladium nanoparticles by microwave energy, since both the chosen reactor material (PEEK) as well as the porous glass polymer composite Raschig rings should be microwave transparent. In order to verify this hypothesis we have conducted an extensive set of microwave irradiation experiments with all the materials used in the reactor construction.…”

“…[24b,35] In addition, it is unknown how the mode of heating (thermal heating or microwave irradiation) as well as the type of process (batch or continuous flow) chosen, effects nanoparticle-mediated catalysis. For investigating the influence of microwave irradiation [36] on nanoparticle structure and the catalytic process in general, the PASSflow reactor shown in Figure 1 c was fitted into a commercially available single mode microwave reactor de- Figure 1. PASSflow reactor system as employed in the present work: a) Precipitation polymerization inside a megaporous glass carrier; b) monolithic porous glass polymer composite Raschig rings inserted into a c) PASSflow reactor (rings are aligned on a perforated tube; gaskets between the rings prevent bypass); d) housing is made of PEEK polymer.…”

“…[38] One of the advantages of microwave dielectric heating over conventional heating in the work described here is the fact that strongly absorbing materials such as metal particles can be heated selectively in a reaction mixture. [36,39] It can therefore be imagined that the intrinsic temperature localized around those particles is significantly higher than that of the bulk solution. [36,39] In the development of our reactor concept ( Figure 1) we incorporated the opportunity to selectively heat ("activate") the palladium nanoparticles by microwave irradiation since both the chosen reactor material (PEEK) as well as the porous glass polymer composite Raschig rings can be considered to be microwave transparent (see below).…”

“…[36,39] It can therefore be imagined that the intrinsic temperature localized around those particles is significantly higher than that of the bulk solution. [36,39] In the development of our reactor concept ( Figure 1) we incorporated the opportunity to selectively heat ("activate") the palladium nanoparticles by microwave irradiation since both the chosen reactor material (PEEK) as well as the porous glass polymer composite Raschig rings can be considered to be microwave transparent (see below). [40] Our detailed studies in this field are disclosed in this account which focuses on the catalytic performance of Pd nanoparticles under different conditions and experimental setups.…”

H3PW12O40: An Efficient and Recyclable Heterogeneous Catalyst for the Selective Synthesis of 2-Aryl-5,6-dihydro-4H-1,3-oxazines and 2-Aryl-1,4,5,6-tetrahydropyrimidines

“…This is a consequence of the selective absorption of microwave energy by polar molecules or polar transition state intermediates formed in the course of the reaction. The notable features of the microwave approach are enhanced reaction rates, formation of purer products in high yields and easier manipulation [25,26]. The use of solid acid catalysts with microwave irradiation provides even more benign processes.…”