Carbon-carbon bond formation is the backbone of organic synthesis, and the reactions that accomplish it define what molecules are practically accessible. Bonds between two sp3-hybridised carbons, the tetrahedral carbons that give molecules their three-dimensional shape, are particularly valuable. Saturated, three-dimensional frameworks are strongly associated with success in drug development, a trend famously described as escaping flatland, because flat, aromatic-rich molecules often suffer from poor solubility and off-target effects. Yet while chemists have become adept at stitching together flat sp2 carbons through classic cross-coupling chemistry, the alkyl-alkyl variant remains far more difficult, because the reactive intermediates involved are short-lived and prone to unproductive side reactions.

One powerful way to make sp3-sp3 bonds is radical chemistry. Radicals, species bearing an unpaired electron, are notoriously reactive, which makes them both useful and dangerous in a synthesis flask. When two radicals meet, they can combine to form a new carbon-carbon bond, a process known as radical-radical cross-coupling. The catch is controlling the encounter. Most successful strategies to date rely on pairing a persistent radical, one that survives long enough to accumulate in solution, with a transient radical, one that lives for only a fleeting moment. The persistent radical effect then steers the chemistry toward selective cross-coupling rather than a chaotic mixture of products. Directly coupling two transient radicals, both generated from separate precursors, has remained a formidable challenge.

There is one venerable exception: Kolbe electrolysis, first described by Hermann Kolbe in 1849. In this electrochemical process, carboxylic acids are oxidised at an anode, losing an electron and releasing carbon dioxide to generate carbon-centred radicals that then dimerise. Kolbe electrolysis is one of the few methods capable of directly coupling two transient radicals, but it works best when both radicals come from the same or very similar carboxylic acids. Getting radicals from two chemically distinct precursors, say a carboxylic acid on one side and a boronic acid on the other, to meet and combine selectively has largely been out of reach, because the two precursors oxidise at different potentials and their radicals tend to react with themselves rather than with each other.

The Northwestern team’s solution rests on two intertwined ideas. The first is redox-matched alternating-polarity electrolysis. Instead of applying a constant current in one direction, the researchers alternate the polarity of the electrodes, so that each half-cycle generates one type of radical under conditions matched to its own oxidation chemistry. This waveform control allows carboxylic acids to be oxidised to alkyl radicals during one polarity phase, while organoboron reagents are activated during the complementary phase, giving both radical pools a chance to coexist and combine. Alternating current electrolysis has been gaining momentum in organic electrosynthesis precisely because it lets chemists run incompatible oxidative and reductive events in the same vessel, and the team’s earlier mechanistic work on alternating-polarity electrolysis for carbon-centred radical generation laid the groundwork for the present advance.

The second idea is controlled activation of redox-active species through fluoride. Organoboron reagents are not normally easy to oxidise directly, but the researchers used controlled fluoride activation to tune the redox behaviour of the boronic acids, making them competent radical precursors under the electrolysis conditions. Cyclic voltammetry studies, an electrochemical technique that maps out at which potentials molecules gain or lose electrons, helped the team identify the redox-matched window in which both precursors could be activated without one simply overwhelming the other. By carefully choreographing electrode polarity and chemical activation, the method achieves what conventional constant-current electrolysis cannot: a productive meeting of two transient radicals born from entirely different functional groups.

The scope of the transformation is what makes the result more than an electrochemical curiosity. The researchers demonstrated the cross-coupling of alkyl carboxylic acids with alkyl boronic acids to form sp3-sp3 carbon-carbon bonds, exactly the connectivity that is prized in drug-like scaffolds but difficult to access. They also showed that the platform extends to homocoupling reactions, in which two identical fragments are joined, and to a net carboxylic acid-alkene cross-coupling. In the latter variant, an alkene is first converted in situ into an alkyl boronic acid through hydroboration, the addition of a boron-hydrogen bond across the double bond, and the resulting boronic acid then enters the electrolytic coupling. This effectively lets chemists use cheap, widely available alkenes as radical precursors without isolating the intermediate.

Perhaps most striking is the way the new chemistry dovetails with established catalytic reactions. The team showcased tandem processes in which the electrochemical radical coupling is combined with Suzuki coupling, the palladium-catalysed workhorse reaction that joins boronic acids with aryl halides, and with Buchwald-Hartwig amination, which forms carbon-nitrogen bonds. This means a single molecular sequence can build a carbon-carbon bond electrochemically and then diversify the product through trusted catalytic steps, multiplying the value of each starting material. For medicinal chemists, who routinely need to prepare families of closely related analogues to optimise a drug candidate, such downstream diversification from a common carboxylic acid-boronic acid coupling product is exactly the kind of flexibility that accelerates discovery.

The significance of the work lies in the feedstocks as much as in the chemistry. Carboxylic acids and organoborons are among the most plentiful and well-behaved functional groups in the synthetic repertoire, stable enough to store and handle, and available in enormous structural variety. Electrochemistry itself has undergone a renaissance in the past two decades, because electricity can serve as a clean, tunable oxidant or reductant, replacing stoichiometric chemical oxidants and offering precise control over reaction outcomes through potential and current. By merging waveform-controlled electrolysis with rational redox matching, the Northwestern group has pushed that renaissance into territory that classical methods, including more than a century and a half of Kolbe chemistry, could not reach.

There remain practical considerations before such methods become routine. Alternating-polarity electrolysis requires equipment and optimisation that many synthetic laboratories are still adopting, and the interplay of electrode material, supporting electrolyte, solvent and waveform parameters demands careful tuning for each new substrate combination. The mechanistic picture, probed through cyclic voltammetry and experimental studies of radical generation, will continue to be refined as the community explores the boundaries of the approach. Nevertheless, the demonstration that two transient radicals from distinct, abundant precursors can be generated in the same pot and persuaded to couple selectively marks a genuine expansion of what electroorganic synthesis can do. For a field whose ultimate goal is to make any desired molecule efficiently, sustainably and in three dimensions, the ability to weld carboxylic acids and organoborons together with nothing more than electrons is a milestone worth watching.

Subject of Research: Electrochemical radical-radical cross-coupling of alkyl carboxylic acids and organoboronic acids

Article Title: The radical-radical cross-coupling of alkyl carboxylic acids and organoborons

Article References: Zhong, J., Boudjelel, M., Evans, J. M., Vanswearingen, I., & Malapit, C. A. (2026). The radical-radical cross-coupling of alkyl carboxylic acids and organoborons. Nature Chemistry. https://doi.org/10.1038/s41557-026-02237-z

Image Credits: AI Generated

DOI: 10.1038/s41557-026-02237-z

Keywords: electrochemistry, radical cross-coupling, carboxylic acids, organoborons, Kolbe electrolysis, C(sp3)-C(sp3) bonds, organic synthesis, alternating polarity electrolysis, Nature Chemistry, synthetic methodology, persistent radical effect, medicinal chemistry

The bond at the heart of the work is the C(sp3)–C(sp3) linkage, the connection between two tetrahedral, saturated carbon atoms. These bonds form the skeletal framework of the vast majority of drugs, agrochemicals and natural products, yet they remain among the hardest bonds for synthetic chemists to forge selectively. Conventional cross-coupling methods, which earned the Nobel Prize in Chemistry in 2010, excel at joining flat, sp2-hybridized carbons but struggle with the stubborn, unreactive sp3 carbons that dominate three-dimensional molecular architecture. Radical approaches — in which two carbon-centred radicals recombine — offer an appealing alternative, but they come with a fundamental catch.

That catch is diffusion control. When two free radicals meet in solution, they combine at essentially the rate at which random molecular motion brings them together. The chemistry offers no inherent preference for coupling two different radicals over two identical ones, so reactions tend to produce statistical mixtures: the desired cross-coupled product alongside wasteful homodimers of each partner. Compounding the problem, the transition-metal catalysts typically used to impose order on this chaos are exquisitely sensitive to the redox environment of the reaction. Radical generation is inherently a redox process, and the reactive intermediates it creates can push a delicate nickel or iron catalyst into inactive oxidation states, collapsing the very selectivity the catalyst is meant to provide.

The Harbin team’s solution is an elegant act of chemical bookkeeping: decoupling radical generation from catalytic redox maintenance. Instead of asking a single catalytic system to both create radicals and keep the cross-coupling catalyst in its productive state, the researchers split the labour. Light handles the first job through photoinduced ligand-to-metal charge transfer, a process in which a photon excites an electron from a ligand — here, a carboxylate or alkoxide bound to an iron centre — up to the metal, tearing the bond apart and releasing an alkyl radical under remarkably mild conditions. Electricity handles the second job, continuously regulating the redox balance of the catalytic cycle through the applied potential at the electrode, so that the metal catalyst neither accumulates in an over-reduced state nor burns out through over-oxidation.

Bringing these two energy inputs together with a dual-metal catalyst — iron for radical generation and nickel for selective bond formation — the system accomplishes an impressive range of transformations. Double decarboxylative couplings join two carboxylic acids by expelling carbon dioxide from both partners. Dehydroxymethylative couplings convert alcohols into alkyl radicals by cleaving a C–C bond adjacent to the hydroxyl group. Deformylative couplings do the same for aldehydes, stripping the formyl group to reveal the underlying alkyl fragment. In every case, the starting materials are native functional groups — the very handles already present in feedstock chemicals, metabolites and drug candidates — requiring no conversion into redox-active esters or other activated derivatives.

The substrate scope reported in the study is strikingly broad. The method accommodates primary, secondary and even sterically hindered tertiary carbon centres, and it works across every pairing: primary-to-primary, primary-to-secondary, secondary-to-secondary, primary-to-tertiary and even tertiary-to-tertiary couplings. Quaternary carbon centres — carbons bonded to four other carbons, a structural motif that confers biological potency but that synthetic chemists find notoriously difficult to construct — fall within reach. The authors also demonstrated applications to the synthesis of pharmaceutically relevant intermediates, suggesting the chemistry could shorten routes to drug candidates by allowing medicinal chemists to couple molecular fragments at late stages rather than building complex scaffolds from scratch.

How does the system avoid the statistical chaos that plagues ordinary radical recombination? The mechanistic studies point to a process called SH2 radical sorting, or bimolecular homolytic substitution at a metal centre. Rather than letting two radicals find each other freely in solution, the nickel catalyst captures one radical to form a high-valent organonickel species, and the second radical then attacks that complex directly, displacing the coupled product in a single, ordered step. This pathway imposes selectivity that diffusion-controlled recombination cannot, channelling the reaction toward the cross-coupled product. Electron paramagnetic resonance spectroscopy and cyclic voltammetry provided experimental support for this picture, tracking the radical intermediates and the oxidation states of the nickel catalyst as the reaction proceeded.

One of the most intriguing mechanistic findings concerns a seemingly humble additive: a sacrificial alkyl bromide. Mechanistic analysis indicated that this reagent prevents over-reduction of nickel, sustaining a Ni(II)/Ni(III) catalytic cycle — the oxidation-state shuttle that mediates the selective radical cross-coupling. In an electrochemical setting, where the electrode continuously pumps electrons into the system, keeping the catalyst from sliding into an inactive low-valent state is a genuine challenge. The sacrificial bromide acts as a redox buffer, preferentially accepting electrons that would otherwise reduce the nickel catalyst beyond its productive cycle. It is a small molecular chaperone with an outsized role in keeping the entire catalytic machine running.

The significance of the work extends beyond any single reaction. It sits at the confluence of three rapidly advancing fields: photocatalysis, electrocatalysis and first-row transition-metal catalysis. By using earth-abundant iron and nickel rather than precious metals, and by drawing energy from light and electricity rather than stoichiometric chemical oxidants and reductants, the approach aligns with the growing demand for sustainable synthesis. The electrochemical component also offers something batch chemistry rarely can: a tunable dial. By adjusting the applied potential, chemists can influence which intermediates form and which oxidation states persist, effectively programming the reaction’s selectivity through waveform control — a concept that has already transformed related electrochemical transformations such as modern versions of the century-old Kolbe electrolysis.

For the broader chemistry community, the study offers a template for taming radical reactions that were previously considered too promiscuous to be useful. If two carbon radicals can be generated from the most ordinary functional groups and then sorted selectively into a single cross-coupled product under mild conditions, the retrosynthetic imagination of chemists expands considerably: complex molecules could be conceived as combinations of simple carboxylic acid, alcohol or aldehyde fragments, joined where needed by a photon, an electron and a pair of cheap metal catalysts. The Harbin team’s data, including raw NMR, cyclic voltammetry and EPR datasets deposited openly on Zenodo, will allow other groups to scrutinize and build upon the mechanism. What remains to be seen is how far the platform can stretch — whether it can be rendered enantioselective, extended to heteroatom-bearing fragments, and scaled from milligram demonstrations to the gram and kilogram quantities that industrial synthesis demands. For now, the message is clear: the once-chaotic world of radical coupling is being brought under disciplined control, one photon and one electron at a time.

Subject of Research: Photoelectrochemical C(sp3)–C(sp3) radical cross-coupling of native functional groups using iron–nickel dual catalysis

Article Title: Photoelectrochemical C(sp3)–C(sp3) cross-coupling using native functional groups

Article References: Zhong, P.-F., Yang, C., Guo, L., & Xia, W. (2026). Photoelectrochemical C(sp3)–C(sp3) cross-coupling using native functional groups. Nature Synthesis. https://doi.org/10.1038/s44160-026-01162-w

Image Credits: AI Generated

DOI: 10.1038/s44160-026-01162-w

Keywords: photoelectrochemistry, cross-coupling, C(sp3)–C(sp3) bonds, radical sorting, nickel catalysis, iron catalysis, ligand-to-metal charge transfer, decarboxylation, SH2 mechanism, electrocatalysis, photocatalysis, organic synthesis