RESEARCHERS at Brown University, US have created a nanocatalyst capable of simultaneously performing all four reactions to make benzoxazole, a chemical widely used in pharmaceuticals.
Benzoxazole is used to make antibacterials, antifungals and non-steroidal anti-inflammatory (NSAID) painkillers, but it is made using a series of four reactions with multiple catalysts. Post-doctoral chemistry researcher Chao Yu and graduate student Xuefeng Guo set out to find a way to make the compound in a more sustainable, environmentally friendly way.
“If you're running four different reactions separately, then you've got four different steps that require solvents and starting materials, and they each leave behind waste contaminated with byproducts from the reaction,” said Brown chemistry professor Christopher Seto, in whose laboratory the research was partly carried out. “But if you can do it all in one pot, you can use less solvent and reduce waste.”
The researchers made the catalyst by growing silver-palladium nanoparticles on nanorods made from oxygen-deficient tungsten oxide. Making benzoxazole begins with formic acid, nitrobenzene and an aldehyde, which react in a series of four reactions. All the reactions resulted in a near-quantitative yield, ie the maximum possible. The reaction series was carried out at 80?C and took eight hours. Conventionally, producing benzoxazole requires a temperature of 130?C and takes around two days.
As well as requiring only one pot, less time and less harsh conditions, the catalyst enables the use of more environmentally benign starting materials.
“The reaction chain requires a hydrogen source for its initial step. That source could be pure hydrogen gas, which is difficult to store and transport, or it could be extracted from a chemical compound. A compound called ammonia borane is often used for this purpose, but the new catalyst enables formic acid to be used instead, which is cheaper, greener and less toxic,” Yu said.
The researchers found that the catalyst was stable over at least five cycles. They also used it to produce quniazoline, derivatives of which are found in antimalarial and anti-cancer drugs. The researchers say their research represents “an emerging line” of green chemistry research.
Journal of the American Chemical Society doi.org/b6fh
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