The well-known olefin metathesis reaction involves a pair of molecules—each with a carbon-carbon double bond—swapping bonding partners. It is widely used for making drugs, polymers, and synthetic natural products, because the reaction forms new, useful C–C double bonds. For the most part, C–C single bonds are left out of the party, because chemists generally consider them too inert for this type of reaction. Guangbin Dong and coworkers at the University of Chicago have uncovered a metathesis reaction that not only works with single-bonded biaryl rings but is reversible (Nat. Chem. 2021, DOI: 10.1038/s41557-021-00757-4). Although alkane metathesis reactions are known, Dong says, either they involve olefin formation or they are not reversible. Here, the group synthesized a series of compounds with two of the same aryl groups, then used a ruthenium catalyst to create biaryl compounds with two different aryl groups (shown). Chemists previously would have used a cross-coupling reaction that required an oxidant or reductant, or an acid or base. Dong’s method works with many functional groups, including rarely tolerated bromides and boronates, and does not need redox or pH-adjusted conditions. It’s more simple and straightforward, Dong says. “You just add a catalyst, then the bonds break and re-form.” Through computational analysis, the team found that the reaction goes through an “olefin-metathesis-like” pathway, Dong says, in which the compound breaks a C–C bond to oxidatively add to the catalyst. A reductive elimination then forms the new C–C bond. This research shows that direct C–C single-bond metathesis is possible, which is potentially useful, Dong says.