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6.3 Microwave Assisted Organic Synthesis
Description
Chapter 6
Microwave Assisted Organic Synthesis (MAOS) uses microwave energy to heat and thus drive
chemical reactions. Microwave irradiation efficiently heats the materials by a ‘microwave
dielectric heating’ effect. This phenomenon is dependent on the ability of a specific material
(solvent or reagent) to absorb microwave energy and convert it to heat. The irradiation produces
efficient internal heating by direct coupling of microwave energy with the molecules. The
energy of microwave photon is too low to break chemical bonds and therefore microwaves
cannot induce chemical reactions.
Microwave heating enables a wide range of reactions to be performed quickly and easily.
Among them are allylic alkylations, ring closing metathesis, cycloadditions, C-H bond
activation, numerous rearrangements and reactions directed towards combinatorial chemistry
using polymer-supported substrates. This technique is able to contain reactions involving
superheated solvents. This allows the use of solvents from which it is easy to isolate products.
Achieved environmental benefits
• higher energy efficiency (related to dramatic enhancement of reaction rates, higher yields,
and less by-products)
• possible avoidance of transition metal catalyst and hence avoidance of toxic waste
• excellent control of reaction parameters.
Cross-media effects
Wahrscheinlich keine.
Operational data
“It appears that the combination of rapid dielectric heating by microwaves with sealed vessel
technology (autoclaves) will most likely be the method of choice for performing MAOS in the
future” [107, Kappe, 2004].
The choice of solvent is not governed by the boiling point but rather by the dielectric properties
of the reaction medium which can be easily tuned by, for example, addition of highly polar
materials such as ionic liquids.
Applicability
Applicable to a number of reaction types. Simple (simplicity of the programming and ease of
use) and effective technology.
At the moment, several litres can be processed at a time. Continuous flow reactors are nowadays
available for both single- and multimode cavities that allow the preparation of kilograms of
materials. However, at present, there are no documented published examples of the use of
microwave technology for organic synthesis on a production-scale level above 1000 kg per
batch. Several manufacturers of microwave systems are working to transfer the technology from
a laboratory scale into process plants.
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