Oxidation chemistry plays a fundamental role in both synthetic chemistry (the kind carried out in laboratories to create new compounds) and biological chemistry (the kind that occurs in living organisms). These processes have a well-defined reactivity, which means altering them to produce different reactions is difficult and usually requires complex steps. A clear example of this challenge is ozonolysis.

Ozonolysis is a classic reaction in which ozone (O₃) breaks the double bonds of alkenes (a type of molecule with C=C bonds). This reaction is well known and widely taught in organic chemistry courses. However, the ozonolysis of aromatic compounds remains an unsolved challenge. Why? Because aromatic compounds are more stable than alkenes, meaning ozone will preferentially react with alkenes before targeting aromatics. The result is an uncontrolled, non-selective reaction that destroys the molecule instead of modifying it in a useful way.

A new approach to achieving selectivity in oxidation reactions

To tackle this challenge, the Photoactivated Processes Unit, in collaboration with Aachen University and the University of Rouen Normandy, has developed the first method capable of selectively ozonising aromatic compounds—even in the presence of alkenes. This breakthrough opens a new direction in the field, as it challenges long-held beliefs in oxidative chemistry.

Their strategy uses photoexcited nitroarenes as an alternative to ozone. These compounds are non-toxic and remain stable under exposure to purple light. When activated by this light, the nitroarenes specifically target aromatic rings and break carbon-carbon (C–C) bonds within them. This makes it possible to “disassemble” stable aromatic molecules that were previously nearly impossible to modify, enabling access to products that were very difficult to synthesise using traditional techniques.

An innovative mechanism in organic chemistry

The study introduces a completely new way of thinking about organic chemistry: it shows that by changing the excited state of nitroarenes (from a state called n,π⁎ to another called π,π⁎), it is possible to control precisely which types of molecules they react with. In other words, the same class of compounds can behave in completely different ways, simply by changing how they are excited by light. This shift means that instead of reacting with alkenes as they normally would, the nitroarenes selectively react with aromatic compounds.

This level of precise control over reactivity has not been seen before with any other reagent. Its impact is significant, with applications in both academic research and industrial fine chemistry. For instance, this technique has already been used to modify more than ten complex medicines and natural products, including structurally intricate compounds such as nicergoline and ilaprazole.

The study proposes a new way of understanding how the excited states of molecules can be used to steer chemical reactions in a precise and selective manner—something that could completely transform how oxidation processes are designed in the future, with benefits across many areas of chemical science.

The work has been published in Science journal and can be accessed at the following link:

DOI: 10.1126/science.ads3955