The reaction relies on alkyl sulfonylhydrazides as the alkyl donors, a class of reagents that the team found can release alkyl radicals under remarkably gentle conditions. Conventional directed C–H alkylation protocols typically depend on alkyl halides or other electrophiles that demand elevated temperatures, strong bases, or aggressive activators to enter the catalytic cycle. By swapping in sulfonylhydrazide-derived donors, the researchers sidestepped that energetic barrier entirely. The hydrazide framework fragments to generate the carbon-centred radical directly, which is then captured within the nickel catalytic cycle, allowing the entire sequence to proceed below 50 degrees Celsius while remaining redox-neutral and operationally simple.

Directing groups sit at the heart of the strategy. The substrates carry an amide-type directing group that coordinates to nickel and positions the metal catalyst adjacent to the target C–H bond, enabling selective activation of the C(sp2)–H bond in arenes and heteroarenes. This chelation-assisted approach enforces site selectivity, so the alkylation occurs predictably at the position dictated by the directing group rather than at whichever reactive site happens to be most accessible. The team demonstrated the method on more than seventy examples, spanning simple benzamides through to heavily decorated heterocycles of the kind that populate the internal libraries of pharmaceutical companies.

The heterocycle compatibility is perhaps the feature that will resonate most strongly with practitioners of medicinal chemistry. Nitrogen-containing rings such as pyridines, pyrimidines, and related azines are ubiquitous in approved drugs, yet they are notoriously problematic substrates for metal-catalysed C–H functionalization because the ring nitrogen poisons many catalysts or redirects reactivity in unwanted ways. The Scripps team showed that the sulfonylhydrazide–nickel combination tolerates a wide range of these heteroaromatic systems, opening a practical route to alkylated heterocycles that previously required multi-step sequences or gave poor yields under existing protocols.

Beyond simple primary alkyl groups, the method accepts complex secondary alkyl donors, including fragments derived from elaborated building blocks, which dramatically expands its utility. Late-stage functionalization experiments demonstrated that the reaction can be performed on molecules already bristling with functional groups, appending an alkyl unit to a sophisticated scaffold without disturbing esters, ethers, halides, or other sensitive motifs. The researchers also applied the chemistry in the context of natural product synthesis, underscoring that the transformation is not merely a curiosity of model substrates but a genuinely useful tool for constructing molecules of real structural and biological complexity. The reaction was shown to be scalable, a further indication of its practical character.

Underpinning the synthetic scope is a mechanistic picture that the team assembled through a combination of experimental probes and computational analysis. Density functional theory calculations, alongside mechanistic experiments, point to an asynchronous, amine-assisted C–H activation pathway. Rather than proceeding through a single, synchronous transition state in which the C–H bond breaks in concert with metal–carbon bond formation, the activation appears to unfold in a stepwise, nonsynchronous fashion, with an amine component of the catalyst system assisting the deprotonation or proton-shuttling events that accompany metalation. This asynchronous character lowers the energetic cost of C–H cleavage, helping to explain why the reaction succeeds at such low temperatures where classical concerted metalation–deprotonation pathways would stall.

The choice of nickel as the catalyst metal is itself significant. Nickel has earned a reputation as the spirited workhorse of modern cross-coupling, prized for its abundance relative to palladium and its unusual willingness to engage radical intermediates. In this system, the sulfonylhydrazide-derived alkyl radicals are intercepted within the nickel manifold, and the mechanistic studies suggest that radical capture and C–H activation are choreographed within a single catalytic framework. The chemoselectivity observed across the substrate screen—where the reaction finds the directed C–H bond even in the presence of multiple potentially reactive sites—highlights how the interplay between the directing group, the nickel complex, and the gently generated radical donor produces a reaction that is both fast and discerning.

The broader context of this work is the long-running effort in the pharmaceutical industry to escape flatland. Decades of analyses of approved drugs and clinical candidates have shown that molecules richer in three-dimensional character, with more saturated carbon frameworks, tend to enjoy better clinical success rates, improved solubility, and more favourable promiscuity profiles. Yet most robust cross-coupling chemistry remains oriented toward joining flat fragments: aryl to aryl, aryl to vinyl. Methods that reliably weld sp2 ring systems to sp3 alkyl fragments remain comparatively scarce, and those that exist often require photoredox catalysts, electrochemical apparatus, elevated temperatures, or electrophilic alkyl halides that are themselves unstable or difficult to prepare. A thermal, redox-neutral, nickel-catalysed protocol that works below 50 degrees Celsius represents a meaningful addition to that limited toolbox.

The sulfonylhydrazide donor chemistry builds on recent demonstrations that these reagents can serve as a general redox-neutral platform for radical cross-coupling, but the present study extends that logic into the domain of directed C–H activation, where the substrate itself dictates where the new bond forms. The combination is powerful: the directing group provides the site selectivity, the nickel catalyst provides the bond-forming machinery, and the hydrazide reagent provides the alkyl fragment under the mildest possible activation conditions. Because the donors are straightforward to prepare from the corresponding hydrazines and carbonyl or sulfonyl precursors, practitioners can access a diverse panel of alkyl partners without exotic reagent synthesis.

For the synthetic community, the practical implications are immediate. A chemist seeking to methylate, ethylate, or append a branched alkyl group to a pyridine or benzamide scaffold can now contemplate a single-step operation conducted on a warm hotplate rather than a high-thermal or photochemical setup. The demonstrated scalability means medicinal chemistry campaigns can generate gram quantities of alkylated analogues for structure–activity studies, while the late-stage compatibility means proven lead compounds can be diversified without rebuilding them from scratch. As mechanistic understanding of amine-assisted, asynchronous C–H activation deepens, the design principles uncovered here are likely to inform the next generation of mild, selective, and sustainable C–H functionalization methods, bringing the long-promised efficiency of direct C–H chemistry closer to routine practice in laboratories focused on discovering the medicines of tomorrow.

Subject of Research: Mild nickel-catalysed directed C(sp2)–H alkylation of (hetero)arenes using alkyl sulfonylhydrazide radical donors

Article Title: Chemoselective Ni-catalysed directed C(sp2)–H alkylation at low temperature using alkyl sulfonylhydrazides

Article References: Wang, S., Cagan, D. A., Cao, Y., Vokits, B. P., Palkowitz, M. D., Kawamata, Y., Baran, P. S., & Engle, K. M. (2026). Chemoselective Ni-catalysed directed C(sp2)–H alkylation at low temperature using alkyl sulfonylhydrazides. Nature Synthesis. https://doi.org/10.1038/s44160-026-01158-6

Image Credits: AI Generated

DOI: 10.1038/s44160-026-01158-6

Keywords: nickel catalysis, C–H activation, C–H alkylation, sulfonylhydrazides, radical cross-coupling, heterocycles, late-stage functionalization, medicinal chemistry, organometallic chemistry, directing groups, Nature Synthesis, asynchronous mechanism

A team of chemists at Seoul National University led by Seung Youn Hong has now demonstrated a conceptually different route to the same goal, one that does not require bending a sulfur atom at all. Writing in Nature Catalysis, Woojin Lee, Jason Kim and colleagues describe a spirocyclic organosulfur scaffold in which four sulfur atoms are anchored to a single carbon centre, a geometric arrangement that forces the sulfur p orbitals into direct spatial overlap with one another. The result is a highest-occupied molecular orbital that is no longer localized on a single atom but spread across four sulfur centres simultaneously, and whose energy is substantially elevated compared with ordinary sulfides. In effect, the molecule manufactures its own reactivity not by distorting a single atom but by making several atoms share a common electronic frontier.

The underlying phenomenon, known as through-space orbital interaction, has been familiar to physical organic chemists for decades under the name spiroconjugation, and it has been exploited in optoelectronic materials, where spiro-linked frameworks tune the electronic properties of organic semiconductors. What the Seoul team has done is to transport this concept from materials science into catalysis. In their spirocyclic design, the rigid carbon framework holds the four sulfur atoms in a fixed three-dimensional relationship, ensuring that their lone-pair orbitals approach each other closely enough to mix. Computational analyses and sulfur K-edge X-ray absorption near-edge structure measurements, performed in collaboration with Jongwoo Lim’s group, confirmed that the highest-occupied molecular orbital is a genuine four-centred orbital with markedly raised energy, a signature that prototypical acyclic sulfides do not display.

Elevated orbital energy translates directly into chemical power. The highest-occupied molecular orbital governs a molecule’s nucleophilicity, its willingness to donate electron density into an incoming electrophile. A four-atom orbital spread across sulfur centres means that the sulfide form of the catalyst is an unusually potent nucleophile, far more electron-rich than a conventional thioether of comparable structure. The researchers showed that this enhanced nucleophilicity allows the spirocyclic sulfide to attack N-halosuccinimides, common halogen-transfer reagents, with pronounced ease, forming a sulfonium intermediate that carries an activated halogen ready for delivery to an arene substrate.

What makes the system a true catalyst rather than a stoichiometric reagent is the reversibility of this chemistry. Once the sulfonium species transfers its halogen to an aromatic ring, the catalyst is restored to its sulfide state and can re-enter the cycle. Mechanistic studies, including kinetic analyses designed to establish reaction orders, revealed a striking form of multi-atom cooperativity: the same four-centred orbital architecture that makes the sulfide state hyper-nucleophilic also endows the corresponding sulfonium state with an unusual capacity for halogen transfer. Both halves of the catalytic cycle, reduction and oxidation in a formal sense, are amplified by the shared orbital, which is precisely the kind of dual behaviour that geometrically constrained phosphorus catalysts achieve through distortion of a single centre.

The flagship application demonstrated by the team is electrophilic aromatic halogenation, one of the oldest and most industrially important reactions in organic chemistry. Aryl chlorides and bromides are ubiquitous in pharmaceuticals and agrochemicals; the chlorine atom alone appears in a large fraction of blockbuster drugs, where it modulates potency, metabolic stability and membrane permeability. Yet installing halogens onto complex, electron-poor or sterically congested arenes remains difficult, and existing organocatalytic halogenation methods each carry limitations in substrate scope or reactivity. The spirocyclic organosulfur catalyst proved capable of halogenating arenes that are otherwise hard to functionalize, exhibiting a level of activity that clearly distinguishes it from prototypical organosulfur compounds such as simple sulfides and even from previously reported specialized halogenation catalysts.

Of particular interest to the pharmaceutical community is the catalyst’s performance in late-stage functionalization. The researchers subjected drug-like molecules, scaffolds decorated with multiple sensitive functional groups, to the halogenation conditions and found that the spirocyclic catalyst could install halogen atoms selectively onto aromatic positions without disturbing the rest of the molecule. This kind of chemoselective, late-stage diversification is a prized capability in medicinal chemistry, where a single advanced intermediate may serve as the branching point for dozens of analogues in a structure-activity relationship study. A metal-free, organocatalytic platform that tolerates complex molecular architecture expands the synthetic toolkit available for such campaigns and avoids the toxicity and environmental concerns associated with many metal-based halogenation catalysts.

The study also carries conceptual weight for the broader field of main-group chemistry. Over the past fifteen years, chemists such as those working on geometrically constrained phosphorus(III) compounds and low-valent bismuth platforms have shown that main-group elements can mimic transition metals in catalysis, performing oxidative addition, redox cycling and bond activation once thought to be the exclusive province of the d-block. That programme, however, has been built almost entirely on the geometric-distortion principle, which is inherently element-specific. By showing that through-space orbital interactions can achieve an analogous electronic effect in a divalent chalcogen, the Seoul team has added a second, orthogonal design axis. The strategy is in principle transferable to other divalent elements and to other spirocyclic architectures, suggesting a generalizable route to multicentred main-group catalysts that does not depend on forcing unusual bond angles.

The experimental evidence underpinning the claims is unusually thorough for a conceptual advance. Crystallographic data for three representative spirocyclic compounds were deposited with the Cambridge Crystallographic Data Centre, establishing the solid-state structures and the spatial relationships between the sulfur atoms. Sulfur K-edge XANES spectroscopy, supported by simulations, probed the electronic structure of the sulfur centres in both the sulfide and sulfonium states, while quantum-chemical calculations mapped the composition and energy of the four-centred highest-occupied molecular orbital. Energy decomposition analyses traced the halogen-transfer step to the cooperative involvement of multiple sulfur atoms, providing a mechanistic rationale for why the multicentred catalyst outperforms its single-centred counterparts.

For a reaction as old as aromatic halogenation, the arrival of a genuinely new catalytic principle is noteworthy in itself. But the deeper significance of the work may lie in the demonstration that molecular orbitals can be engineered through architecture rather than distortion, that the spatial arrangement of identical atoms around a shared framework is itself a designable variable in catalyst development. If the spirocyclic strategy generalizes as the authors suggest, chemists may soon be building multicentred main-group catalysts for a wide range of transformations, from redox chemistry to bond activation, by simply deciding which atoms to bring within orbital reach of one another. The periodic table, it appears, still has unexploited modes of cooperation waiting to be switched on.

Subject of Research: Through-space orbital interactions in a spirocyclic tetrathiide scaffold that generate a four-centred high-energy HOMO for multicentred organosulfur catalysis

Article Title: Harnessing through-space orbital interactions for multicentred organosulfur catalysis

Article References: Harnessing through-space orbital interactions for multicentred organosulfur catalysis. (n.d.). https://doi.org/10.1038/s41929-026-01602-y

Image Credits: AI Generated

DOI: 10.1038/s41929-026-01602-y

Keywords: organosulfur catalysis, through-space orbital interactions, spiroconjugation, frontier molecular orbitals, main-group chemistry, electrophilic aromatic halogenation, multi-atom cooperativity, late-stage functionalization, sulfonium intermediates, organocatalysis, divalent chalcogens, Nature Catalysis