Light and Electricity Join Forces to Forge Carbon-Carbon Bonds from Everyday Chemicals

Chemists have long dreamed of snapping together the carbon backbones of molecules the way children snap together building blocks — taking two simple, abundant starting materials and joining them directly to make something more valuable. A team at Harbin Institute of Technology in Shenzhen, China, has now brought that vision considerably closer to reality. In a study published in Nature Synthesis, Ping-Fu Zhong, Chao Yang, Lin Guo, Wujiong Xia and colleagues describe a photoelectrochemical strategy that welds two carbon atoms together directly from carboxylic acids, alcohols and aldehydes — the workhorse functional groups that chemists find ready-made in countless molecules — without any of the laborious pre-activation steps that such transformations traditionally demand.

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