Allenylidene-to-Indenylidene Rearrangement in Arene−Ruthenium Complexes: A Key Step to Highly Active Catalysts for Olefin Metathesis Reactions

The allenylidene−ruthenium complexes [(η⁶-arene)RuCl(CCCR₂)(PR‘₃)]OTf (R₂ = Ph; fluorene, Ph, Me; PR‘₃ = PCy₃, PⁱPr₃, PPh₃) (OTf = CF₃SO₃) on protonation with HOTf at −40 °C are completely transformed into alkenylcarbyne complexes [(η⁶-p-cymene)RuCl(CCHCR₂)(PR₃)](OTf)₂. At −20 °C the latter undergo intramolecular rearrangement of the allenylidene ligand, with release of HOTf, into the indenylidene group in derivatives [(η⁶-arene)RuCl(indenylidene)(PR₃)]OTf. The in situ-prepared indenylidene−ruthenium complexes are efficient catalyst precursors for ring-opening metathesis polymerization of cyclooctene and cyclopentene, reaching turnover frequencies of nearly 300 s^-1 at room temperature. Isolation of these derivatives improves catalytic activity for the ring-closing metathesis of a variety of dienes and enynes. A mechanism based on the initial release of arene ligand and the in situ generation of the active catalytic species RuCl(OTf)(CH₂)(PR₃) is proposed.