At Scripps Research, chemist Phil Baran and his team have solved one of chemistry's most stubborn puzzles: how to snap together highly reactive molecular fragments while preserving the precise 3D shapes that make medicines work. Their breakthrough, published in Science, uses a surprisingly elegant solution—a nickel catalyst that acts like a protective cage, holding volatile radicals just long enough for them to bond without scrambling their orientation.
This matters because nearly every pharmaceutical drug on the market depends on molecular handedness, the property chemists call chirality. Imagine a left hand trying to fit into a right glove—it simply won't work. A drug molecule must have the correct 3D "handedness" to lock into its biological target and produce the intended effect. The mirror-image version often fails entirely, or worse, binds in an unintended way and triggers unwanted side effects. Yet creating and preserving this 3D information while linking carbon atoms has remained notoriously difficult, especially when working with free radicals—the most reactive molecules in chemistry that normally lose their orientation almost instantly.
The research team's method joins a sulfonyl hydrazide (which already carries the desired 3D information) with an alkyl halide through a "caged radical rebound" mechanism. One radical is briefly trapped on the nickel catalyst in a protected environment, allowing it to snap back and form a new bond before it can escape and lose its handedness. The result is remarkable: the reaction maintains 80–96 percent enantiospecificity, meaning the product usually keeps its starting handedness, while delivering practical yields between 40 and 90 percent.
What makes this approach especially powerful for drug discovery is its simplicity and versatility. The reaction is redox-neutral, requiring no extra chemicals to drive it forward. It needs no specialized additives or shape-directing helper molecules, and it runs under standard lab conditions. More importantly, it tolerates the chemical features that drug chemists rely on to build and fine-tune medicines—free amines, olefins, heterocycles, aryl bromides—without triggering unwanted side reactions.
The team tested the reaction on dozens of starting materials, focusing on piperidine and pyrrolidine scaffolds, the chemical structures found throughout pharmaceuticals. One particularly striking example: a medicinally relevant piperidine building block that previously required seven synthesis steps, including a laborious separation of its left- and right-handed forms, was prepared in a single coupling step at 60 percent yield with 95 percent stereoretention. The researchers also synthesized stenusine, a natural product that certain beetles excrete from their feet to glide across water, using fewer steps than previous techniques.
"Organic chemistry is fundamentally about forming carbon-carbon bonds, and doing so with control over 3D structure is one of the most important obstacles to overcome," Baran says. "Our approach lets us connect the most reactive pieces and still get precise results."
The work extends Baran's earlier advances in radical-based cross-coupling, methods that are already reshaping how pharmaceutical companies design molecules. With the ability to couple two preassembled, complex fragments directly—and to scale the reaction up to gram quantities—this breakthrough opens new pathways for faster, more efficient drug discovery.