Researchers from the University of Osaka develop a rhodium-catalyzed reaction that inserts a single carbon atom into carbon–carbon single bonds, joining two planar molecules into rigid 3D spirocyclic frameworks

Osaka, Japan – Flat is fine for a sheet of paper, but many of the most useful molecules need depth. Now, researchers from Japan have found a way to give them that depth using nothing more than a single carbon atom.

In a study to be published in Nature, researchers from the University of Osaka have developed a new reaction that uses one carbon atom as a connecting point to join two planar molecules into a 3D structure.

The resulting molecules are known as spirocycles, in which two rings share just one carbon atom. Because the two halves sit at right angles to each other, like two playing cards slotted together into a cross, spirocyclic frameworks are rigid and stable. This has made them valuable in solar cells and other electronic materials. However, building them is challenging because the central carbon atom must form four separate bonds.

“A lone carbon atom would be the ideal ‘glue’ for this job, because it can form four bonds at a single point,” says lead author of the study Hayato Fujimoto. “But free carbon atoms are extremely unstable, and previous substitutes could only join molecules through their double bonds. We wanted to find a way to work with carbon–carbon single bonds instead.”

To do this, the team combined a rhodium catalyst with a stable reagent called PINC, which acts as a ‘packaged’ carbon atom. The catalyst cleaves carbon–carbon single bonds in flat molecules called biphenylenes, and PINC delivers a single carbon atom into the gap. The result is four new carbon–carbon bonds around one central carbon, creating a rigid 3D spirocyclic framework.

The researchers confirmed the origin of the central carbon by labeling PINC with a traceable form of carbon, which ended up exclusively at the heart of the new molecule. The reaction also worked on a Gram scale using a standard benchtop setup.

“What excites us most is the flexibility,” explains Mamoru Tobisu, senior author. “By controlling the temperature, we can join either two identical molecules or two different ones, giving us access to a diverse range of 3D structures that are otherwise difficult to make.”

The team also showed that the method tolerates a variety of chemical groups and can even pair a biphenylene with a carbon–carbon double bond, further expanding the types of structures it can produce.

By turning simple, flat starting materials into complex 3D architectures in a single step, this approach is expected to facilitate the design and synthesis of new functional molecules, with potential applications ranging from electronic materials to solar cells.
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The article, “Catalytic transfer of carbon atoms enables spirofusion of C–C σ-bonds,” is to be published in Nature at DOI: https://doi.org/10.1038/s41586-026-11242-8

About The University of Osaka
The University of Osaka was founded in 1931 as one of the seven imperial universities of Japan and is now one of Japan's leading comprehensive universities with a broad disciplinary spectrum. This strength is coupled with a singular drive for innovation that extends throughout the scientific process, from fundamental research to the creation of applied technology with positive economic impacts. Its commitment to innovation has been recognized in Japan and around the world. Now, The University of Osaka is leveraging its role as a Designated National University Corporation selected by the Ministry of Education, Culture, Sports, Science and Technology to contribute to innovation for human welfare, sustainable development of society, and social transformation.
Website: https://resou.osaka-u.ac.jp/en