Blog

The sweet smell of strawberries and other fruits is due to a chemical compound called an ester, which is also found in many fats and polyesters. The ubiquitous compound can be broken down to produce desired alcohols and other chemicals for use in various industries, including pharmaceuticals and cosmetics, but the process can be costly, both financially and environmentally.

Now, a team of researchers from the National Institute of Natural Sciences (NINS) in Japan has developed a new approach using light as an energy source. They published their findings on June 14 in Journal of the American Chemical Society.

In a seemingly counterintuitive move, to break down—or reduce, in the chemical stake—esters, scientists actually add electrons to the compound. The addition of electrons forces the groups making up the ester to be reduced to more basic components. Conventional ester reduction methods require an excess amount of highly reactive and difficult-to-handle metal reducing agents.

Researchers are now exploring the use of sustainable photocatalysts. Photocatalysts, or catalysts that are activated when excited by light, are known to promote an electron transfer process between the catalyst and organic compounds without using highly reactive metal reductants.

Conventional photocatalysts, involving expensive and non-renewable noble metals, reduce limited organic compounds and typically add only one electron to the compounds. Called single-electron transfer (SET), the process must continue repeatedly until the desired number of electrons is added to achieve the target reduction of esters.

“In the past decade, photocatalytic reactions have attracted considerable attention as desirable methods relevant to the United Nations Sustainable Development Goals (SDGs) in organic synthesis,” said co-author Shintaro Okumura, an assistant professor at the Institute for Molecular Sciences (IMS). ) of NINS.

“Photocatalysts promote redox reactions using visible light as an energy source in the absence of metal reductants. However, photocatalytic reactions through a multielectron transfer process are less developed, so the photocatalytic reduction of esters to form alcohols, which requires four electrons , remains undeveloped.. The photocatalytic reduction of esters to form alcohols is a huge challenge because it requires an unprecedented sequential quadruple process,” Okumura said.

To achieve this quadruple SET process, the researchers developed a new photocatalyst, which they named “N-BAP”. When irradiated with blue light, the photocatalyst initiates the reaction to produce a chemical group that reacts with water and a second carbon chemical group. With the addition of oxalate, a negatively charged molecule that occurs widely in nature, the reaction can add four electrons in rapid succession—leading to the desired alcohols.

“The combination of an N-BAP catalyst with oxalate as a traceless reductant allows rapid sequential four-electron reduction of esters to generate carbinol anions, with subsequent protonation to yield alcohols,” Okumura said.

“This work may pave the way for the novel transformation of esters and is expected to contribute to a sustainable society as a green organic synthesis suitable for the SDGs.”