Microwave-induced electrostatic etching: generation of highly reactive magnesium for application in Grignard reagent formation

A detailed study regarding the influence of microwave irradiation on the formation of a series of Grignard reagents in terms of rates and selectivities has revealed that these heterogeneous reactions may display a beneficial microwave effect. The interaction between microwaves and magnesium turnings generates violent electrostatic discharges. These discharges on magnesium lead to melting of the magnesium surface, thus generating highly active magnesium particles. As compared to conventional operation the microwave-induced discharges on the magnesium surface lead to considerably shorter initiation times for the insertion of magnesium in selected substrates (i.e. halothiophenes, halopyridines, octyl halides, and halobenzenes). Thermographic imaging and surface characterization by scanning electron microscopy showed that neither selective heating nor a specific microwave effect was causing the reduction in initiation times. This novel and straightforward initiation method eliminates the use of toxic and environmentally adverse initiators. Thus, this initiation method limits the formation of by-products. We clearly demonstrated that microwave irradiation enables fast Grignard reagent formation. Therefore, microwave technology is promising for process intensification of Grignard based coupling reactions.