Microwaving a Grignard

02 September 2011 - Microreview

What happens when you microwave a piece of metal? Investigations as reported on youtube (here, here or here) are inconclusive. A fork does nothing, a spoon will spark somewhat and microwaving a ball of aluminum foil involves a lot of violent arcing.
What happens when you microwave a Grignard reaction?. Microwave chemistry holds the promise of faster reactions with higher yields due to efficient energy transfer but the presence of magnesium scrap makes any outcome uncertain. As Grignards are notoriously difficult to get started as a result of Mg surface passivation an investigation makes sense.

An early adaptor is Grunenthal GmbH from Aachen Germany which holds a patent on Grignard microwaving since 2004 (Link). The first to microwave a Grignard in academia were Nilsson et al. (DOI) and Suna et al. (DOI), both a year later in 2005 and both reporting success. More success stories followed: Scammells (2007) (DOI) and Hulshof (2010) (DOI).

From the Hulshof research the following observations emerged: much arcing between the Mg turnings resulted in microcavities and formation of spherical Mg particles from Mg melting, all of this resulting in high Mg surface to volume ratio and hence reduced reaction initiation time. In the reaction of bromothiophene and carbon dioxide initiation time was reduced 20-fold. The Grignard of 2-chloropyridine formed twice as fast but its instability and polymerisation prevented CO2 quenching. Bromobenzene in the same reaction was not impressed and no differences were reported.

In a very recent contribution 2011 Kappe et al. reinvestigated the microwave assisted Grignard formation of 2-chloropyridine (DOI). This time using a methanol quench they too reported initiation rate acceleration.They also discovered what happens when you turn up the microwave power to the limit: the reaction will stop. This is not an instance of the fabled and controversial microwave effect. It is simply the THF solvent that starts to decompose and spoil the Mg surface.
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