Ligand-Controlled Chemoselectivity in the Rhodium-Catalyzed Synthesis of Pentafulvenes via (2 + 2 + 1) Alkyne Cyclotrimerization.

The synthesis of pentafulvenes with varied substituents has been efficiently achieved using novel rhodium-based catalysts via (2 + 2 + 1) alkyne cyclotrimerization. A rational design of the catalyst structure, including pyridonato, NHC, and CO ligands, ensures the alkyne chemoselectivity and prevents the formation of robust rhodium-fulvene species. Furthermore, the judicious choice of acidity and steric properties of different alkynes enables the preparation of cross-coupled fulvene derivatives. Stoichiometric and deuteration experiments, as well as DFT calculations, shed light on the reaction mechanism, showing that it includes an initial alkyne deprotonation, two successive alkyne insertions, cyclization, and protonolysis, the first insertion being the rate-determining step.