The Ruthenium Allenylidene Precursors

These initial studies revealed that alkene metathesis activity was favored by the presence of (i) an electron-donating and bulky phosphine (PR3 PCy3 P Pr3 PPh3) (ii) a weakly coordinating anion (TfO PFg BF ) (iii) an easily displaceable arene (p-cymene CgMeg) and (iv) the addition of a small amount of acid [13,14, 22]. The resulting systems were shown to be efficient for the RCM of diallylamides, the formation of macrocycles [13, 14], and the preparation of cyclic, fluorinated amino acids [23]. 

The allenylidene complex [RuCl(=C=C=CPh2)(PCy3)(p-cymene)] PFg la was observed to be a very active enyne metathesis catalyst, particularly under reaction conditions utilizing UV irradiation (300 nm) at 80 °C, where reactions were found to be complete in only 0.5 h [24, 25]. The allenylidene complex la was also shown to be an active catalyst for the ROMP of unstrained, cycUc olefins after its exposure to either heat (80 °C) or UV irradiation [26]. Heat or light transforms the allenylidene complex into the catalyst, which was later 

From Allenylidene to Indenylidene Ruthenium Complexes and Catalysts 

From these studies, it was demonstrated that the alkene metathesis activity was not due to the allenylidene precursor, but due to the indenylidene ruthenium catalyst 6, which has a structure analogous to the Grubbs I catalyst [15, 17]. Both complexes generate the same RuCl2(=CFl2) intermediate upon reaction with a terminal alkene. 

A few weeks later, Nolan [16] reported the indenylidene-ruthenium complexes 7-10, which contain a NHC ligand analogous to the Grubbs II catalyst. Using catalysts 6 and 7 -10, excellent activities were shown in the RCM reaction of various 

---