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

The ubiquity of benzene rings in pharmaceuticals is undeniable; however, their planar geometry often imparts limitations such as metabolic instability and poor aqueous solubility, factors that can compromise drug efficacy and safety. Consequently, there has been an intense focus on developing non-planar, saturated bioisosteres that mimic the spatial arrangements of benzene substituents while addressing these shortcomings. Among these, bicyclo[2.1.1]hexane cores have garnered particular attention as three-dimensional surrogates that retain the geometric disposition of substituents akin to benzene derivatives yet confer improved solubility and metabolic resistance. Despite their promising attributes, synthetic access to diverse and functionalized BCH derivatives has been constrained by the lack of efficient, generalizable methods.

Historically, methods to assemble bicyclo[2.1.1]hexane skeletons have largely relied on cycloaddition approaches, including [2π+2π] cycloadditions involving 1,5-dienes and [2σ+2π] cycloadditions of bicyclo[1.1.0]butane precursors with unsaturated bonds. These strategies, while powerful, face challenges such as limited substrate scope, regio- and stereochemical control, and the need for prefunctionalized starting materials. Notably absent in the synthetic landscape until now has been a transition metal-catalyzed approach enabling direct intramolecular cyclization of β-alkynylcyclobutanone substrates—structures characterized by inherent ring strain and challenging reactivity profiles.

The key ingenuity of Liu’s team lies in their successful orchestration of a nickel-catalyzed “hydrometalation-5-exo-trig cyclization” cascade that surmounts the high strain and reactivity challenges intrinsic to the bicyclo[2.1.1]hexane formation. Utilizing (TMSO)₂MeSiH as a hydride source, the nickel catalyst generates an active Ni-H species in situ, which performs carbonyl-directed regioselective cis-hydroxynickelation of the alkyne moiety tethered to the cyclobutanone. This critical regioselectivity is guided by strong coordination between the nickel center and the carbonyl oxygen, an interaction elucidated and supported by complementary Density Functional Theory (DFT) computational studies.

Following initial hydrometalation, the resultant alkenylnickel intermediate undergoes an intramolecular nucleophilic addition into the cyclobutanone carbonyl, accompanied by ring closure to yield the strained bicyclo[2.1.1]hexane core. Subsequent protonolysis releases the bicyclic alcohol product, which serves as a versatile scaffold amenable to skeletal rearrangement under acidic or basic conditions. This rearrangement affords access to 1,2,4-trisubstituted bicyclo[2.1.1]hexanone derivatives, compounds poised for further functional elaboration and diversification.

This strategy overcomes the formidable challenges associated with such strained intermediates, particularly the propensity for β-carbon elimination or ring-opening side reactions often observed in metallacyclobutane species. Additionally, competing reduction of alkynes or carbonyl groups, common in hydride-rich reaction environments, was deftly circumvented due to precise reaction condition optimization and the directing influence of the carbonyl group. The efficiency, mildness, and regioselectivity of this nickel-catalyzed transformation position it as a paradigm-shifting approach in the synthesis of BCH-containing molecules.

The resultant bicyclo[2.1.1]hexanol and related ketone derivatives offer a high degree of structural similarity to 1,2,4-trisubstituted benzene rings, enabling their potential deployment as benzene bioisosteres in drug design. By replacing planar aromatic motifs with these saturated, three-dimensional cores, medicinal chemists can confer enhanced water solubility, improved metabolic stability, and reduced off-target reactivity to pharmaceutical candidates. Such physicochemical benefits are critical in overcoming long-standing obstacles in drug development pipelines.

Beyond their pharmaceutical potential, the synthetic accessibility of these BCH derivatives enables the exploration of novel chemical space hitherto less accessible, facilitating the preparation of complex molecular architectures for broader applications. The modularity of the synthetic procedure allows incorporation of diverse substituents, enriching the scope and enabling tailored property tuning for specific applications.

The study’s comprehensive use of DFT calculations not only corroborated the proposed reaction mechanism but also shed light on the pivotal role of carbonyl coordination in steering regioselectivity during hydrometalation. This mechanistic insight paves the way for rational design of related catalytic processes, spotlighting the integration of computational and experimental approaches as a powerful strategy in reaction development.

Published in the flagship journal of the Chinese Chemical Society, CCS Chemistry, this open-access research heralds a new frontier in organometallic catalysis and synthetic methodology. Such advances underscore the growing prominence of Chinese research institutions in contributing cutting-edge chemical science to the global community.

The work was supported by major funding bodies including the National Natural Science Foundation of China and the National Key Research and Development Program of China, further attesting to the strategic importance of this research in national scientific agendas. Professor Wen-Bo Liu led the project alongside doctoral and master’s students who contributed equally as co-first authors, symbolizing the holistic integration of mentorship and innovation.

As the method gains traction, further applications are anticipated in late-stage functionalization of drug-like molecules, enabling the rapid incorporation of BCH motifs into complex frameworks. This could foster the development of next-generation therapeutics with optimized pharmacokinetic and dynamic profiles.

In sum, the nickel-catalyzed intramolecular hydrometalative cyclization of β-alkynylcyclobutanones stands as a seminal accomplishment, deftly converting strained cyclic substrates into structurally versatile BCH derivatives under mild conditions with excellent regioselectivity. This breakthrough not only enriches synthetic toolbox but also advances the frontier of medicinal chemistry by providing robust pathways for three-dimensional benzene bioisosteres, a crucial step toward designing more effective and safer pharmaceuticals.

Subject of Research: Not applicable

Article Title: Ni-Catalyzed Regioselective Hydrometalative Cyclization of Alkynyl Cyclobutanones to Bicyclo[2.1.1]hexanes

News Publication Date: 30-Sep-2025

Web References: https://www.chinesechemsoc.org/journal/ccschem

References: DOI: 10.31635/ccschem.025.202506260

Image Credits: CCS Chemistry

Keywords

Catalysis