Tuning Endergonic Triplet Photosensitization Enables Bicyclo[1.1.0]butane–Alkene Cycloadditions

A new copper-based photocatalytic strategy could make it significantly easier to build complex, three-dimensional molecules that mimic the shape of benzene while avoiding some of the drawbacks associated with conventional photochemical methods. In a study published in Nature Chemistry, researchers report that specially designed heteroleptic copper(I) complexes can drive energy-transfer reactions between bicyclo[1.1.0]butanes and alkenes, producing bicyclo[2.1.1]hexanes with valuable substitution patterns. The approach expands the range of chemical partners that can participate in these light-driven cycloadditions, including electron-deficient alkenes, enynes, dienes and even simple aliphatic alkenes. The work suggests that copper, an abundant and comparatively inexpensive metal, may offer a more controllable alternative to highly reactive noble-metal and organic photocatalysts in demanding energy-transfer chemistry.

The motivation comes from a growing effort to replace flat aromatic rings with saturated, three-dimensional structures. Benzene is one of the most common motifs in medicines, agrochemicals and advanced materials, but its planar geometry can sometimes limit how a drug interacts with a biological target. Saturated benzene bioisosteres, by contrast, contain more three-dimensional carbon frameworks and can alter molecular shape, flexibility, polarity and metabolic behavior. Bicyclo[2.1.1]hexanes are especially attractive because their compact, rigid architecture can reproduce some of the spatial relationships found in substituted benzene rings without retaining the same aromatic electronic structure. Constructing these frameworks efficiently, however, requires the controlled formation of several carbon–carbon bonds under conditions that do not destroy sensitive functional groups.

Energy-transfer photocatalysis provides a way to accomplish this using visible light. Instead of relying on the photocatalyst to transfer an electron to or from a substrate, the catalyst absorbs light and transfers its electronic excitation energy to a reactant. That process can promote the reactant into a triplet excited state, a higher-energy configuration in which it can undergo reactions that are difficult or impossible from its ground state. For bicyclo[1.1.0]butanes, excitation can weaken the highly strained central bond and generate a reactive state capable of engaging an alkene in a formal [2+2] cycloaddition. The resulting ring system contains the compact bicyclo[2.1.1]hexane scaffold. Yet this same reactivity can become a liability if the excited intermediates are produced too quickly or at excessive concentrations.

Previous approaches often used powerful noble-metal complexes or strongly absorbing organic photocatalysts. These systems can generate reactive diradical intermediates rapidly after light absorption. A diradical contains two unpaired electrons, making it capable of forming new bonds but also prone to unwanted reactions. In the presence of alkenes, uncontrolled diradical chemistry can initiate chain processes and polymerization, consuming the starting material and generating complex mixtures. Such side reactions become particularly problematic when the alkene is electron-poor or when the reaction mixture contains multiple potentially reactive unsaturated groups. The result is a narrow substrate scope and limited tolerance for the functional groups needed in medicinal and materials chemistry.

The researchers addressed this problem by tuning the properties of copper(I) photosensitizers rather than simply increasing their photochemical power. Their catalysts combine a BINAP ligand, a well-known phosphorus-containing bidentate ligand, with a bidentate (pyrazolyl)pyridine ligand. The two ligands create a heteroleptic coordination environment around copper(I), meaning that the metal is bound by two different ligand systems. This arrangement influences the complex’s absorption characteristics, excited-state energy, molecular geometry and lifetime. In particular, the copper complexes can remain in their electronically excited states long enough to transfer energy to substrates in a controlled fashion, even when the energy-transfer event is endergonic.

An endergonic photosensitization step requires an input of energy rather than releasing it spontaneously. At first glance, that may seem disadvantageous for a photochemical reaction, but it can provide a critical form of control. Instead of immediately activating every available substrate molecule, the copper catalyst uses visible-light energy to access a higher-energy reactive state in a more moderated process. This slows the generation of diradical intermediates and reduces the likelihood that they will encounter one another or launch uncontrolled alkene polymerization. The extended excited-state lifetimes of the copper complexes are central to this behavior: the catalysts have more time to interact productively with the intended substrates, allowing the reaction pathway to compete successfully with destructive side reactions.

The study demonstrates that the strategy is not limited to idealized styrene derivatives. The copper photocatalysts can promote cycloadditions involving electron-deficient alkenes, a class of substrates that is often difficult to accommodate in radical and energy-transfer reactions. They also work with enynes and dienes, which contain multiple unsaturated bonds and can therefore participate in several competing photochemical processes. The reported scope extends further to aliphatic alkenes, which lack the conjugated aromatic systems that frequently help stabilize or guide photogenerated intermediates. This breadth indicates that the reaction is governed less by a single favorable electronic pairing and more by the carefully calibrated energy-transfer properties of the copper complex.

The researchers also show that the position of an aryl substituent in the final bicyclo[2.1.1]hexane can be controlled by choosing which reaction partner is activated. In one route, styrenes are used as the substrate that receives the relevant photochemical activation, leading to products with an aryl group at the 3-position of the bicyclic framework. In another, the bicyclo[1.1.0]butane is activated, producing compounds bearing the aryl substituent at the 2-position. This distinction offers synthetic flexibility because the same general combination of strained rings and alkenes can be used to access different molecular architectures. Such positional control is important when a saturated benzene replacement must reproduce a precise three-dimensional arrangement in a pharmaceutical candidate.

The broader significance of the work lies in its demonstration that photocatalytic performance can be improved through restraint rather than raw reactivity. Highly reactive catalysts are not always the most useful ones, particularly when a reaction proceeds through short-lived intermediates that can follow many competing pathways. By extending the excited-state lifetime of a copper complex and using an endergonic energy-transfer event to moderate substrate activation, the researchers create a photochemical environment in which productive bond formation is favored over runaway chemistry. Copper is also attractive from a practical perspective because it is more abundant and less costly than many precious metals used in photocatalysis. Although further studies will be needed to establish the method’s scalability, durability and compatibility with industrial processing, the findings point toward a new design principle for visible-light synthesis: carefully tuned energy transfer may unlock complex molecular transformations while keeping reactive intermediates under control.

Subject of Research: Copper(I)-photocatalyzed energy-transfer cycloadditions between bicyclo[1.1.0]butanes and alkenes to synthesize substituted bicyclo[2.1.1]hexanes.

Article Title: Modulating endergonic triplet photosensitization for cycloadditions between bicyclo[1.1.0]butanes and alkenes

Article References: Tang, W.Y., Guo, J., Nie, S. et al. “Modulating endergonic triplet photosensitization for cycloadditions between bicyclo[1.1.0]butanes and alkenes.” Nature Chemistry (2026). https://doi.org/10.1038/s41557-026-02233-3

Image Credits: AI Generated

DOI: https://doi.org/10.1038/s41557-026-02233-3

Keywords: copper photocatalysis, energy-transfer photocatalysis, triplet photosensitization, bicyclo[1.1.0]butanes, bicyclo[2.1.1]hexanes, cycloaddition, visible-light chemistry, BINAP, synthetic chemistry, benzene bioisosteres