A frustrated chemist might be tempted to whack the heck out of her compound when a synthesis just won’t go to plan. For mechanochemical reactions, this kind of a smack can actually be helpful. Some reactions can use mechanical energy as an input, and in some cases, this can make organic synthesis greener by eliminating the use of less desirable solvents.
Hajime Ito, Koji Kubota, and co-workers from Hokkaido University have now put mechanochemistry to work in a multifaceted synthesis that includes redox chemistry. With the help of a ball miller, they were able to perform both arylation and borylation reactions with yields as high as 80% (Science 2019, DOI: 10.1126/science.aay8224).The key was the incorporation of a piezoelectric material, which can generate an electric charge in response to applied mechanical stress.
Mechanochemistry is nothing new, but mechanoredox chemistry is a new twist on it, says Kubota. Compared to similar photoredox reactions without a mechanical input, the researchers were able to get higher yields in a shorter time. By taking advantage of the piezoelectric effect, the researchers were able to synthesize 18 functionalized arylfuran and arylthiophene compounds, and 10 functionalized phenyl boropinacolato compounds in 27 to 80% yields. In addition, Ito and co-workers were able to recover the catalyst after up to 5 milling reactions. This method worked so well that they were even able to activate the reaction on the benchtop with a hammer, to give 43% yield of their desired product.
They used a variety of piezoelectric compounds and milling frequencies, and found that BaTiO3 gave the highest yields in the arylation reaction. They also compared running the arylation reaction in the ball miller with no BaTiO3, and with BaTiO3 in solvent under ultrasound, and got less than 1% yield for each, which shows that the piezoelectric action was key. Ito and co-workers proposed that the reaction occurs through oxidative quenching of the BaTiO3 particles. Upon mechanical impact, the particles become highly polarized, and produce radicals that go on to react with the present organic molecules. The researchers recorded scanning electron microscope images of the BaTiO3 particles before and after milling, and showed that the particles were both deformed and smaller after the reaction, suggesting that they were getting sufficiently bashed up to transform mechanical energy into electrical energy inside the ball miller.
The group envisions the mechanoredox reaction as a greener replacement for a photochemical reaction, as no degassed solvents or inert atmospheres are needed. The ball miller needs electricity to run, but the required amount is similar to that needed to turn a stir bar, says James Mack, a mechano-organic chemist at the University of Cincinnati, who’s looked into the energy input difference between traditional solvent chemistry and mechanochemistry with a ball miller (Chem. Sci. 2017, DOI: 10.1039/c7sc00538e). The energetic pull is lower for mechanochemistry than heating an oil bath for certain reactions, he says. There are always tradeoffs involved, he says. But the mechanochemistry still takes far less energy than the processes required for dealing with solvents, such as vacuum removal.
Ito and co-workers say that this method can be applied to other redox reactions as well. “There are a lot more possibilities than just the transfer of conventional reactions to the ball-milling system,” Kubota says.