Homobimetallic Ruthenium-Ethylene,Vinylidene, Allenylidene,and Indenylidene Catalysts for Olefin Metathesis

Summary: Following the discovery of bimetallic ruthenium scaffold ( 1 ), two new homobimetallic ruthenium-N-heterocyclic carbene complexes ( 2 , 3 ) were synthesized and found highly suitable for promoting ROMP, RCM, and CM reactions. Results from this study indicated that the ethylene ligand was highly labile and that adding a small amount of a terminal alkyne to the reaction media had a beneficial influence on the metathetical activity. These observations prompted us to further investigate the role of the alkyne co-catalyst. Thus, homobimetallic ruthenium-arene complexes bearing vinylidene ( 4 , 5 ), allenylidene ( 6 ), and indenylidene ( 7 ) ligands were prepared from complex 1 and propargyl alcohol derivatives. Their catalytic activities were probed in several types of olefin metathesis reactions, and they were found valuable intermediates for the safe and efficient one-pot synthesis of the Hoveya–Grubbs isopropoxybenzylidene catalyst ( 8 ).