Alkyne Metathesis with d2 Re(V) Alkylidyne Complexes Supported by Phosphino-Phenolates: Ligand Effect on Catalytic Activity and Applications in Ring-Closing Alkyne Metathesis

A family of d2Re(V) alkylidyne complexes bearingtwo decorated phosphino-phenolates (POs) and a labile pyridineligand were prepared that can efficiently promote alkyne metathesisreactions in toluene. The relative activity of these complexes varieswith the PO ligands. Complexes with an electron-rich metal centerhave a higher activity. Ligand exchange experiments suggest thatthe pyridine ligands of the Re(V) alkylidynes with more electron-donating PO ligands are more labile and are more easily released togenerate catalytically active species. However, complexes withelectron-withdrawing PO ligands are more air-stable than thosewith electron-donating PO ligands. These Re(V) alkylidynecatalysts can promote the homometathesis of functionalizedinternal alkyl- and aryl-alkynes, as well as ring-closing alkyne metathesis (RCAM) of methyl-capped diynes, forming macrocycleswith a ring size >= 12 efficiently for concentrations <= 5 mM. These reactions represent thefirst examples of RCAM mediated by non-d0alkylidyne complexes. The Re(V) alkylidyne catalysts tolerate a wide range of functional groups including ethers, esters, ketones,aldehydes, alcohols, phenols, amines, amides, and heterocycles. Moreover, the catalytic RCAM reactions promoted by robust Re(V)alkylidyne catalysts could also proceed normally in wet toluene.