Thianthrenium salts enable safer, broader carbene transfer

Nature reports that alkylthianthrenium salts can act as effective sources of carbenes for making cyclopropanes and other products formed through carbene transfer. The approach addresses a long-standing trade-off in this area: many useful cyclopropanation reactions rely on hazardous starting materials or intermediates, while safer sulfonium salts have generally been poor carbene donors.

The work is significant because it allows variation in both components of the reaction. Chemists can alter the olefin—the carbon–carbon double-bond partner—and the carbene-derived partner, rather than being restricted to diversification on only one side. That wider choice could make the chemistry more useful for preparing different three-membered carbon frameworks.

Why cyclopropanation is difficult

Carbenes are highly reactive, divalent carbon intermediates. They can function as one-carbon building blocks, or synthons, in the construction of cyclopropanes: small rings containing three carbon atoms. The synthetic value of these rings has made cyclopropanation an important reaction, even though the materials used to generate the necessary reactive species can present serious safety concerns.

According to Nature’s report, both established and newer cyclopropanation methods have involved explosive starting materials or intermediates. The attraction of these compounds is their ability to deliver carbene reactivity, but that same reactivity complicates handling and scale-up.

Sulfonium salts offer a potentially safer alternative. However, they have not matched more conventional carbene precursors in cyclopropanation because they usually do not release carbenes efficiently enough. This has left a gap between the safety profile chemists would prefer and the reactivity needed for the transformation.

There has also been a limitation in the range of products accessible through reported cyclopropanation reactions. Although different structures can be introduced through either the olefin or the carbene partner, the two sources of diversity have not generally been available at the same time. Nature points to reactions restricted to activated olefins, including styrenes, as examples of that constraint.

A different role for thianthrenium salts

The reported chemistry uses alkylthianthrenium salts, a type of sulfonium salt containing thianthrene. Nature describes these compounds as conceptually distinct from other carbene precursors. In this setting, they serve as the basis for thianthrenium ylides, which can participate in carbene-transfer reactions.

The proposed explanation for their performance involves two properties of thianthrene: its considerable steric bulk and its low Lewis basicity. Together, these features are suggested to prevent the reaction from becoming trapped in energetically favourable but unproductive local minima. Such minima are present for other sulfonium salts on the reaction’s potential-energy surface, according to the report.

In practical terms, the proposed effect is that the thianthrenium system can follow a productive route to carbene transfer rather than settling into competing arrangements. This helps explain how a salt belonging to a class previously regarded as inefficient for this purpose can support cyclopropanation.

The result is not limited to one narrowly defined substrate combination. The reported method permits diversity in both the olefin and the carbene partner, extending the structural options available when cyclopropanes are assembled. The article presents that combination as a central advantage of the chemistry rather than as a minor adjustment to existing methods.

Safety and broader reactivity

Reactivity is only part of the appeal. Nature reports that thianthrenium salts have a safety profile suitable for scale-up, including in the solid state using ball-milling. That point is important because the solid-state operation of many other cyclopropanation reactions would be dangerous when carried out in this way.

The ability to combine strong carbene-transfer reactivity with safer handling could make the method relevant beyond small-scale laboratory experiments. The reported safety characteristics do not remove the need for careful chemical practice, but they distinguish the thianthrenium approach from reactions dependent on more hazardous materials.

The authors’ chemistry also reaches beyond cyclopropane formation. Thianthrenium ylides are presented as a broader class for metal–carbene reactivity, with applications that include insertion into sigma bonds and sigmatropic rearrangements. These are different forms of molecular reorganisation, but they share the central feature of transferring carbene reactivity from the thianthrenium system to another part of a molecule.

Taken together, the findings link three goals that have often been difficult to achieve simultaneously: efficient carbene generation, freedom to vary both reaction partners and a safety profile compatible with scale-up. Nature’s report frames thianthrenium ylides as a platform for pursuing all three, with cyclopropanation serving as the clearest demonstration of the approach.