Feb 19 2019
Photoelectrochemical (PEC) cells are extensively researched for the transformation of solar energy into chemical fuels. They use photoanodes and photocathodes to “split” water into oxygen and hydrogen, respectively. PEC cells can function under mild conditions with light, which also perfect renders them for other catalyzing reactions that convert organic molecules into high added-value chemicals, similar to those used to make drugs.
Image credit: EPFL
However, PEC cells have seldom been used in organic synthesis till now, except in a few recent conceptual attempts that have tested just a handful of basic substrates. Generally, PEC cells are still largely not investigated for broad-scope synthetic methodologies of functional organic molecules.
They could, however, prove most useful in one of the most appealing synthetic approaches for pharmaceuticals and agrochemicals, called “direct amination”. It involves incorporating an amine group to an organic molecule without having to pre-activate the molecule using a supplementary processing step.
Direct amination generally necessitates high temperatures, and also needs what is called a “directing group”—a chemical unit that fixes the reaction site but has no other use, and which frequently has to be taken off before using the new compound in applications.
Currently, the labs of Xile Hu and Michael Grätzel at EPFL’s Institute of chemical sciences and engineering (ISIC) have formulated a new technique for aminating arenes—hydrocarbons with a ring in their structure—without needing a directing group.
“Our method is operationally simple and can be used to synthesize a broad range of nitrogen-containing heterocycles relevant to drug discovery,” writes Lei Zhang, the lead author of the research. To establish the fact, the scientists used their technique to produce numerous pharmaceutical molecules, including derivatives of the muscle relaxant metaxalone and the antimicrobial benzethonium chloride.
Based on a PEC cell, the technique catalyzes the reaction with light and the affordable, Earth-abundant semiconductor hematite.
Pioneering studies in Michael Grätzel’s lab have yielded robust hematite samples that are efficient for water splitting, but hematite has never been used to catalyze organic synthesis.
Xile Hu, Institute of Chemical Sciences and Engineering (ISIC), EPFL.
In the present study, hematite was discovered to function well for direct amination under visible light, while its high stability denotes a long lifespan as a working catalyst. Plus, since it harvests light, the photoelectrocatalysis used here guzzles less energy than direct electrocatalysis.
This is an important demonstration of principle for using PEC cells for the production of high added-value chemicals and pharmaceuticals. The work merges two traditionally separated fields, namely photoelectrochemistry and organic synthesis. There are plenty of untapped opportunities for this approach, and we are excited to further explore these opportunities.
Xile Hu, Institute of Chemical Sciences and Engineering (ISIC), EPFL.