The new photocatalyst enables efficient reduction of esters with blue light

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 a variety of 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 Journal of the American Chemical Society.

In a seemingly counterintuitive step, to break up—or chemically reduce in advance—esters, scientists actually add electrons to the compound. The addition of electrons forces the ester-containing groups to reduce to more basic components. Conventional ester reduction methods require excessive amounts of highly reactive and difficult to process metal reducing agents.

Now researchers are investigating the use of sustainable photocatalysts. Photocatalysts, or catalysts that are activated upon excitation by light, are known to promote the electron transfer process between the catalyst and organic compounds without the use of highly reactive metal reducing agents.

Conventional photocatalysts, which include expensive and non-renewable noble metals, reduce a limited amount of organic compounds and typically add only one electron to the compounds. The process, called single electron transfer (SET), must occur multiple times until the required number of electrons is added to achieve the target reduction of esters.

“Over the past decade, photocatalytic reactions have gained significant attention as desirable methods suitable for the UN’s Sustainable Development Goals (SDGs) in organic synthesis,” said co-correspondent 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 reducing agents. However, photocatalytic reactions via a multielectron transfer process have been less developed, so the photocatalytic reduction of esters to form alcohols, which requires four electrons, has remained undeveloped.” The photocatalytic reduction of esters to form alcohols is a huge challenge because it requires an unprecedented stepwise quadruple SET process,” said Okumura.

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

“Combining the N-BAP catalyst with oxalate as a trace-free reducing agent enables rapid sequential four-electron reduction of esters to form carbinol anions followed by protonation to form alcohols,” Okumura said.

“This work could prepare a new transformation of esters and is expected to contribute to a sustainable society as a green organic synthesis suitable for the SDGs.”