New photocatalyst enables efficient ester reduction with blue light

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

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

In a seemingly counterintuitive move, to break down esters – or reduce them, in the chemical battle – scientists actually add electrons to the compound. The addition of electrons forces the groups that make up the ester to reduce to more basic components. The conventional ester reduction methods require an excessive amount of highly reactive and difficult to handle metal reducing agents.

Now researchers are investigating the use of sustainable photocatalysts. Photocatalysts or catalysts that activate when excited by light are known to promote an electron transfer process between the catalyst and organic compounds without using the highly reactive metal reducing agents.

Conventional photocatalysts, which use expensive and non-renewable precious metals, reduce the number of organic compounds and typically add only one electron to the compounds. This process is called single-electron transfer (SET) and must continue several times until the desired number of electrons are added to achieve the intended reduction of esters.

“Over the past decade, photocatalytic reactions have received considerable attention as desirable methods suitable for the United Nations Sustainable Development Goals (SDGs) in the field of organic synthesis,” said co-corresponding author Shintaro Okumura, assistant professor at the Institute for Molecular Science (IMS). ) from NINS.

“Photocatalysts promote redox reactions by using visible light as an energy source in the absence of metal reductants. However, photocatalytic reactions via a multi-electron transfer process are 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 extremely challenging because it requires an unprecedented sequential four-way SET process, Okumura said.

To achieve this quadruple SET process, the researchers developed a new photocatalyst they called “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-based chemical group. With the addition of oxalate, a negatively charged molecule commonly found in nature, the reaction can add four electrons in rapid succession, resulting in the desired alcohols.

“The combination of N-BAP catalyst with oxalate as a traceless reducing agent enables the rapid succession of four-electron reduction of esters to generate carbinoanions, with subsequent protonation to give alcohols,” Okumura said.

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