For decades, one of chemistry’s most useful molecular linkages has also been one of its most frustrating. Dialkyl ethers—molecules in which an oxygen atom connects two carbon-based groups—appear throughout pharmaceuticals, natural products and materials, yet building them becomes increasingly difficult when the carbon atoms surrounding oxygen are crowded. A new study published in Nature reports a radical-based strategy that could open a much wider route to these challenging structures. The work, led by J. J. Großkopf, J. Z. Wang and J. W. Gu, introduces a catalytic method for joining carboxylic acid-derived fragments with alcohols under relatively mild conditions. Most importantly, the approach is designed to bypass the steric and mechanistic limitations that have long constrained conventional ether synthesis.
Ethers are often assembled through nucleophilic substitution, a familiar reaction family in which an oxygen nucleophile attacks a carbon atom bearing a leaving group. In a classic S_N2 reaction, that attack must occur from the back side of the carbon–leaving-group bond. The geometry of this transition state makes the process highly sensitive to steric congestion: as the carbon center becomes more substituted, the incoming nucleophile has less room to approach. Tertiary carbon centers are particularly problematic because they generally resist S_N2 displacement. An alternative, the S_N1 pathway, involves ionization to produce a carbocation before the nucleophile arrives. Although this can accommodate more substituted carbon atoms, carbocations are unstable and reactive intermediates. They can undergo elimination, rearrangement or other side reactions, and reactions involving chiral centers may lose valuable stereochemical information.
The new strategy takes a fundamentally different route by using carbon-centered radicals rather than conventional ionic intermediates. Radicals contain an unpaired electron and follow reaction pathways governed by distinct electronic and geometric requirements. In the reported process, radicals derived from carboxylic acids are generated from redox-active esters, a class of activated compounds widely used in modern photochemical and radical chemistry. These intermediates can be converted into carbon radicals under mild conditions, allowing the carbon fragment to participate in bond formation without requiring a highly crowded nucleophilic substitution transition state. The researchers describe the key step as heteroatom homolytic substitution, or het-S_H2, a relatively underused mechanism in which a carbon-centered radical forms a bond to a heteroatom while displacing another radical-derived component.
The term “homolytic substitution” reflects the central difference between this chemistry and polar substitution. In a conventional S_N2 reaction, two electrons from a nucleophile are used to form the new bond as the leaving group departs with an electron pair. In a homolytic process, bond-making and bond-breaking events are organized around single-electron pathways. The radical attacks the oxygen-containing reaction partner and enables the formation of a new carbon–oxygen bond through a transition state that is less constrained by steric crowding. This does not mean that steric effects disappear entirely, but it offers a new way to reduce the severe penalty that makes highly hindered ethers difficult to access using traditional methods. The result is a mechanistic platform tailored to a longstanding synthetic bottleneck.
To make the transformation practical, the researchers combine a titanium-based catalytic system with visible-light photoredox catalysis. Photoredox chemistry uses light-absorbing catalysts to control the movement of single electrons, often enabling reactive intermediates to be generated at room temperature or under otherwise gentle conditions. In this system, visible light helps establish the redox environment needed to activate the carboxylic acid-derived redox-active ester and generate the carbon-centered radical. Titanium then participates in the catalytic sequence that brings the radical pathway together with the alcohol component. The division of labor between light-driven electron transfer and titanium-mediated reactivity allows the overall transformation to proceed through a controlled catalytic cycle rather than through uncontrolled radical generation.
According to the study, the method works across several difficult substitution patterns that are especially important for medicinal chemistry. These include 3°–2° ethers, in which a tertiary carbon fragment is connected to a secondary one; 3°–1° structures, joining tertiary and primary fragments; 2°–2° ethers; and 2°–1° combinations. Such patterns can be challenging because increasing substitution near the oxygen atom simultaneously hinders nucleophilic attack and raises the likelihood of elimination or rearrangement under ionic conditions. By accessing these architectures through radical coupling, the method expands the range of carbon frameworks that can be connected through oxygen. This breadth is significant because the value of a synthetic method is determined not only by whether it produces one target molecule, but also by how reliably it handles the structural diversity found in real drug candidates.
One especially notable application involves bicyclo[1.1.1]propane, or BCP, ether bioisosteres. BCP groups are compact, three-dimensional motifs increasingly used in medicinal chemistry to replace flat aromatic rings or other structural elements. Their unusual geometry can influence molecular shape, lipophilicity, metabolic stability and the way a compound interacts with a biological target. Connecting a BCP-containing fragment to an ether linkage, however, can create a sterically demanding environment that is not easily reached by standard substitution chemistry. The reported radical platform provides a route to these crowded BCP ether structures, potentially allowing medicinal chemists to explore molecular shapes that would otherwise be difficult to synthesize. In drug discovery, that ability can be crucial: a single change in three-dimensional architecture may alter potency, selectivity or pharmacokinetic behavior.
The researchers also demonstrate the value of the chemistry for late-stage diversification, a strategy in which a complex molecule is modified near the end of a synthesis rather than being rebuilt from the beginning. Late-stage methods are highly prized because they can generate families of analogues quickly from a common pharmaceutical scaffold. Instead of designing and optimizing a separate multistep route for every ether analogue, chemists may be able to install different carboxylic acid-derived fragments onto an alcohol-containing molecule under the catalytic conditions. This can accelerate structure–activity relationship studies, in which medicinal chemists systematically vary molecular substituents to determine how structure affects biological performance. The ability to alter congested regions of a drug-like molecule is particularly valuable because those regions are often the least accessible to conventional reaction design.
Beyond the specific products described in the study, the work highlights a broader change in how chemists think about heteroatom bond formation. Carbon–oxygen bonds are among the most common linkages in bioactive molecules, yet the most accessible reactions for making them do not always provide the substitution patterns that modern drug discovery demands. The het-S_H2 concept offers a complementary logic: rather than forcing a hindered carbon center to behave as a conventional electrophile, it turns that fragment into a radical and uses single-electron chemistry to reach the desired ether. The combination of titanium catalysis, visible light and redox-active esters therefore represents more than an additional ether-forming reaction. It is a blueprint for using radical intermediates to construct bonds to heteroatoms in situations where polar chemistry struggles. If its scope and practicality continue to hold across broader molecular families, the platform could become a powerful tool for exploring previously inaccessible regions of pharmaceutical chemical space.
Subject of Research: Radical-based synthesis of sterically congested dialkyl ethers through heteroatom homolytic substitution.
Article Title: Dialkyl ether synthesis through heteroatom homolytic substitution
Article References: Großkopf, J.J., Wang, J.Z., Gu, J.W. et al. “Dialkyl ether synthesis through heteroatom homolytic substitution.” Nature (2026). https://doi.org/10.1038/s41586-026-11043-z
Image Credits: AI Generated
DOI: 10.1038/s41586-026-11043-z
Keywords: dialkyl ethers, heteroatom homolytic substitution, radical chemistry, photoredox catalysis, titanium catalysis, redox-active esters, medicinal chemistry, BCP ether bioisosteres, late-stage diversification, drug discovery
