A Proline-Based 'Redox–Release' Platform for Programmed Access to Diverse Sulfur(VI) and Fluoroalkyl Radicals

Chemists have long relied on a toolbox of reactive fragments called radicals to build complex molecules. These fragments are like single-handed versions of stable atoms or groups of atoms; because they are missing a partner electron, they are eager to grab onto other molecules to form new bonds. This eagerness allows scientists to construct shapes and connections that are impossible to make using standard, two-electron chemistry. However, a persistent problem has limited how freely scientists can use these tools. Traditionally, the specific type of radical a researcher wants to create is locked to a specific way of waking it up. If a scientist needs one kind of radical, they must use a special precursor molecule and a specific set of conditions, such as a particular light source or a specific metal catalyst. If they need a different kind of radical, they often have to switch to a completely different precursor and a different set of rules. This dependency means that building a library of diverse reactive pieces requires a vast and complicated collection of starting materials and methods, making it difficult to design a single, unified system that can produce many different types of radicals on demand.

A team of researchers at the University of Wisconsin-Madison, working with colleagues from the University of Pavia and Eli Lilly, has developed a new strategy that breaks this lock. They created a modular platform that allows them to generate a wide variety of sulfur-based and fluorine-containing radicals from a single, common starting structure. The key to their success is a molecule built on a proline backbone, which is a simple, naturally occurring amino acid. By attaching different chemical groups to this backbone, the team created a family of precursors that all behave the same way when triggered. Whether the goal is to release a sulfur-based fragment or a fluorine-based one, the activation process remains identical. The researchers demonstrated that they could switch between releasing these different radicals simply by changing the attached group, while keeping the method of release constant. This approach, which they call "redox-release," effectively decouples the identity of the radical from the method used to generate it, offering a streamlined path to diverse chemical building blocks.

The core of this system lies in how the proline-based precursors are designed. The researchers attached the reactive group they wanted to release—such as a sulfonyl fluoride or a trifluoromethyl group—to the nitrogen atom of the proline ring. The part of the molecule that actually gets activated by light or electricity is located on the carboxylic acid end of the proline, far away from the group being released. This physical separation ensures that the electronic properties of the group being released do not interfere with the activation step. When the researchers shine light on the mixture in the presence of a catalyst, the proline backbone undergoes a specific chemical change that creates a temporary, unstable intermediate. This intermediate then snaps apart in a predictable way, ejecting the desired radical and leaving behind a harmless byproduct. The team tested this with a library of precursors bearing different sulfur and fluorine groups and found that they all activated at nearly the same energy levels, regardless of the specific group attached. This uniformity confirmed that the proline scaffold, not the attached group, was controlling the process.

To prove the system worked, the team applied it to a series of reactions where these newly generated radicals were added to simple carbon chains known as alkenes. In one set of experiments, they used a reductive activation method, which involves adding an electron to the precursor, to generate radicals and attach them to various alkene substrates. They successfully added fluorosulfonyl groups and aryl sulfonyl groups to a wide range of molecules, including those containing sensitive functional groups like esters, nitriles, and halides. In another set of experiments, they used an oxidative activation method, which involves removing an electron, to generate radicals from acid-based precursors. This pathway allowed them to release trifluoromethyl and difluoromethyl groups, which are crucial components in many modern pharmaceuticals and materials. The ability to switch between these two activation modes using the same underlying scaffold meant they could access a broad spectrum of reactive intermediates without needing to redesign their entire setup for each new target.

The practical value of this platform became clear when the researchers applied it to complex, real-world molecules. They took existing drugs and natural products, such as the anti-inflammatory drug indomethacin and the natural product safrole, and successfully attached these new sulfur and fluorine groups to them. This process, known as late-stage functionalization, is highly prized in drug discovery because it allows scientists to quickly modify a finished molecule to test new properties without having to rebuild the entire structure from scratch. The team showed that their method was gentle enough to work on these delicate molecules without damaging other parts of the structure. Furthermore, the products they created were not just endpoints but versatile tools for further chemistry. The fluorosulfonyl groups they installed could be easily converted into other useful chemical handles, such as sulfonamides or sulfones, which are common in medicinal chemistry. In one striking example, they used this method to shorten the synthesis of a specific drug inhibitor from nine steps down to four, significantly improving the efficiency of the process.

To understand exactly how this fragmentation occurred, the team combined experimental observations with computer simulations. They tracked the reaction using chemical traps that caught the intermediate radicals, confirming that the process followed the path they had predicted. The computer models showed that once the proline backbone was activated, the breaking of the bond to release the radical happened very quickly and with a low energy barrier. This low barrier explained why the process worked so smoothly for such a diverse range of attached groups. The simulations also revealed that for certain fluorine-containing groups, the release involved a two-step fragmentation where a sulfur dioxide molecule was first ejected, followed by the release of the final fluorine radical. This detailed mechanistic insight confirmed that the system was robust and that the fragmentation pathway was the same regardless of the specific radical being generated.

The implications of this work extend beyond the specific reactions demonstrated. By establishing a system where the method of activation is independent of the radical's identity, the researchers have provided a new framework for designing chemical reactions. This approach suggests that in the future, chemists could design a single, universal platform capable of releasing almost any desired radical simply by swapping out the attached group, rather than having to invent a new activation strategy for each new target. The study highlights that the limitations previously thought to be inherent to radical chemistry were actually due to the design of the precursors, not the nature of the radicals themselves. With this new "redox-release" platform, the generation of reactive intermediates becomes a programmable process, opening the door to more efficient synthesis of complex molecules and accelerating the discovery of new materials and medicines.