How a Simple Non-Covalent Trick Solves a 50-Year Problem in Asymmetric Radical Chemistry

Lausanne, Switzerland, Nishant Shrivastava: Imagine trying to build a complex molecule where every atom has to be placed in a specific 3D arrangement, but the key intermediate lives for only a few nanoseconds. That has been the nightmare of synthetic chemists for decades. Now, a team at EPFL has found a way to tame these fleeting radicals without the usual painstaking catalyst design. Their secret? A handshake, not a bond.

The challenge has always been about control. Radicals are highly reactive, short-lived species that zip through reactions faster than you can say 'stereocenter.' Traditional methods to make them behave in a chiral fashion required building elaborate covalent catalysts from scratch. Each new catalyst meant months of synthesis, and if it didn't work, you started over. The EPFL team asked a different question: what if the catalyst could assemble itself on the fly?

Their answer, published in Nature on June 1, 2026, is a system that uses two commercially available components: a chiral phosphoric acid and a simple molecule called 2-mercaptopyridine. When mixed, they form a non-covalent complex. No chemical bonds, just supramolecular interactions. This complex then acts as a chiral relay station, guiding hydrogen atoms with precision during a radical reaction. The result is a method that can create tertiary stereocenters, the kind of structural features found in many blockbuster drugs, with high selectivity and under mild conditions.

What makes this approach so compelling is its modularity. Want to try a different chiral environment? Swap out the phosphoric acid. Need to adjust the reactivity? Change the mercaptopyridine. The system is like a Lego kit for asymmetric catalysis. This flexibility dramatically accelerates the screening process, allowing researchers to test dozens of catalyst combinations in the time it used to take to make one.

The team demonstrated the power of their method by tackling a classic problem: the deracemization of 2-aryl pyrrolidines. These ring structures are everywhere in pharmaceuticals, from neurological drugs to cancer treatments. Using visible light and their self-assembled catalyst, they could take a racemic mixture and enrich it to nearly perfect optical purity. The reaction worked across a broad range of substrates, tolerating functional groups that would typically cause trouble.

Mechanistically, the process is elegant. A photoredox cycle generates a thiyl radical from the mercaptopyridine. This radical selectively plucks a hydrogen atom from the substrate, creating a carbon-centered radical. Then, the same chiral complex delivers a hydrogen atom back, but with a specific handedness. It is a molecular relay race, and the baton is chirality itself. The non-covalent assembly ensures that the chiral information is passed with high fidelity, avoiding the racemization that plagues many radical reactions.

This work represents a conceptual shift. For decades, the field of asymmetric catalysis has been dominated by covalent catalysts, which are powerful but rigid. Non-covalent systems offer dynamic adaptability. They can form, break, and reform, which is particularly useful when dealing with reactive intermediates. The EPFL team has shown that for radical chemistry, this flexibility is not just a convenience; it is a strategic advantage.

The implications go beyond pharmaceuticals. Agrochemicals, materials science, and natural product synthesis all rely on the precise construction of chiral centers. The ability to do this under ambient conditions with visible light aligns with the principles of green chemistry. It reduces the need for harsh reagents and high temperatures, making the process more sustainable and easier to scale.

What is particularly exciting is the potential for generalization. The researchers anticipate that this platform can be adapted for other transformations, such as enantioselective halogenation, oxygenation, or fluorination. The modular nature of the catalyst system means that each new reaction type may only require a simple component swap. The detailed supplementary data, including NMR spectra and mechanistic probes, provides a solid foundation for other labs to build upon.

In a field where progress often comes in small increments, this discovery feels like a leap. It is a reminder that sometimes the most elegant solutions come from rethinking the fundamentals. Instead of fighting the transient nature of radicals, the EPFL team learned to work with it, using a dynamic, self-assembling catalyst that mirrors the adaptability of biological systems. For synthetic chemists, this is not just a new tool; it is a new way of thinking about molecular control.