Allenylidene-to-indenylidene rearrangement in arene-ruthenium complexes: a key step to highly active catalysts for olefin metathesis reactions.

The allenylidene-ruthenium complexes [(eta6-arene)RuCl(=C=C=CR2)(PR'3)]OTf (R2 = Ph; fluorene, Ph, Me; PR'3 = PCy3, P(i)Pr3, PPh3) (OTf = CF3SO3) on protonation with HOTf at -40 C are completely transformed into alkenylcarbyne complexes [(eta6-p-cymene)RuCl([triple bond]CCH=CR2)(PR3)](OTf)2. At -20 degrees C the latter undergo intramolecular rearrangement of the allenylidene ligand, with release of HOTf, into the indenylidene group in derivatives [(eta6-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.