Cationic ruthenium allenylidene complexes as catalysts for ring closing olefin metathesis

A series of well accessible cationic ruthenium allenylidene complexes of th e general type [(eta(6)-arene)(R3P)RuCl(=C=C-CR'(2))](+) X- is described wh ich constitute a new class of pre-catalysts for ring closing olefin metathe sis reactions (RCM) and provide an unprecedented example for the involvemen t of metal allenylidenes in catalysis. They effect the cyclization of vario us functionalized dienes and enynes with good to excellent yields and show a great tolerance towards an array of functional groups. Systematic variati ons of their basic structural motif have provided insights into the essenti al parameters responsible for catalytic activity which can be enhanced furt her by addition of Lewis or Bronsted acids, by irradiation with UV light, o r by the adequate choice of the "non-coordinating" counterion X-. The latte r turned out to play a particularly important role in determining the rate and selectivity of the reaction. A similarly pronounced influence is exerte d by remote substituents on the allenylidene residue which indicates that t his ligand (or a ligand derived thereof) may remain attached to the metal t hroughout the catalytic process. X-ray crystal structures of the catalytica lly active allenylidene complexes 3b . PF6 and 15 . OTf as well as of the c helate complex 10 required for the preparation of the latter catalyst are r eported.