There’s now a safer way to make cyclopropanes—three-carbon rings—thanks to a new reaction that generates carbenes from alkylthianthrenium salts. Carbenes react with double bonds to create cyclopropanes, but previous methods of generating carbenes rely on diazo or dihalo compounds, which can be explosive and toxic. The new transformation promises to help chemists who make drugs and agrochemicals, where cyclopropanes are popular motifs (Nature 2026, DOI: 10.1038/s41586-026-11108-z).
A team led by Tobias Ritter at the Max Planck Institute for Kohlenforschung developed the new reaction. Ritter’s research team has been working with thianthrene compounds for several years and has shown that they can transform aromatic C–H bonds, tack tritium onto complex molecules, and append alkyl groups to double bonds.
“Because there’s no hazard, we could just mix all the solids and then shake it in a ball mill.”
“Thianthrene can do amazing things,” Ritter says. By studying the molecule’s physical organic properties, Ritter and his team realized that alkylthianthrenium salts could transfer carbenes to iron catalysts, which could then react with double bonds to create cyclopropanes.
The approach can generate unusual carbenes, including cyclopropyl and alkylnyl carbenes, which are currently inaccessible from dihalo or diazo reagents. What’s more, the thianthrene-generated carbenes can also participate in other types of reactions, such as σ-bond insertions and sigmatropic rearrangements.
David A. Nagib, who studies organosulfur compounds at the Ohio State University and was not involved with the work, says using thianthrenium salts to make carbenes is clever. “I always love seeing new strategies to access carbenes, especially ones that avoid the safety hazards of diazo reagents and thus provide scalable access to new classes of carbenes,” he says in an email.
The reaction also works on a large scale in a solvent-free reaction, which could make it useful for process chemists. “Our iron catalyst is not very soluble, which is great on small scale. But when we wanted to scale it up, it was very hard to stir,” Ritter says. “Because there’s no hazard, we could just mix all the solids and then shake it in a ball mill.” Using this approach, his team was able to do cyclopropanations on a decagram scale.
Alkylthianthrenium salts are currently not commercially available, so chemists have to synthesize them. But Ritter says his laboratory will soon publish a paper on an easier way to make the reagents.
