Performing chemical reactions in a microwave has recently gained much attention as a method to safely and rapidly accelerate reactions. Unfortunately, the reactions must be performed in a closed space, separate from the user, because no one wants to be cooked from the inside out.
Andreas Kirschning and coworkers at Leibniz Universitat Hannover and the Henkel corporation (both in Germany) have instead used a magnetic field to accelerate chemical reactions. Magnetic nanoparticles in their small-scale setup generate heat in a safe magnetic field, and can improve total chemical reaction yields compared to conventional heating.
Experimental setup.
The scientists used a glass microreactor (9 mm internal diameter, 14 cm length) so that their reactions could be performed on a small scale under 5x atmospheric pressure. The microreactor was packed with iron oxide magnetic nanoparticles.
These nanoparticles generate heat when they are exposed to a constantly changing magnetic field, in this case, 25 kHz, a medium-level frequency. The nanoparticles are coated with silica to prevent nanoparticle degradation in air, and to enable easy chemical derivitization of the nanoparticle surfaces for enhanced nanoparticle reaction capabilities.
Chemical reactions.
The scientists performed eleven chemical reactions with their system, and in selected cases compared the results to that obtained with conventional heating.
One of these reactions was what is known as a Claisen rearrangement. This reaction was heated to 170ºC by the nanoparticles (in a medium-frequency magnetic field), and gave an 85% yield. Only a 62% yield was obtained by conventional heating.
A catalytic transformation, known as a Buchwald-Hartwig amination, heated to 90ºC, gave a 75% yield. A Wittig reaction gave a 98% yield, with a 30:1 preference for a configuration known as E over the Z configuration. Such configurations are important in that they dictate chemical properties needed by chemists and other scientists.
Additionally, the scientists appended palladium molecules onto the surfaces of their nanoparticles. This enabled carbon-carbon bond forming reactions known as Heck and Suzuki-Miyaura couplings. Reaction yields ranged from 63% to 84%.
The future.
Different temperatures were obtained in the scientists' setup because the amount of heat put out by the nanoparticles does not depend solely on the frequency of the magnetic field. Other considerations, such as the microreactor diameter, are also important.
This is analogous to using microwaves in chemical synthesis. Microwaves put out heat depending on factors beyond microwave frequency, such as the identity of the solvent.
The scientists intend to next investigate the use of microreactors that can withstand higher temperatures and pressures, in order to further accelerate their chemical reactions using magnetic fields. Their method will be of great use to chemists who need to safely and efficiently heat small-scale reactors to quickly synthesize useful molecules.
for more information:
Ceylan, S.; Friese, C.; Lammel, C.; Mazac, K.; Kirschning, A.
Inductive heating for organic synthesis by using functionalized
magnetic nanoparticles inside microreactors.
Angew. Chem. Int. Ed. 2008, 47, 8950-8953.