Ruthenium carbene complexes with N,N '-bis(mesityl) imidazol-2-ylidene ligands: RCM catalysts of extended scope

The ruthenium carbene complexes 3a,b bearing imidazol-2-ylidene ligands con stitute excellent precatalysts for ring-closing metathesis (RCM) reactions allowing the formation of tri- and tetrasubstituted cycloalkenes. They also apply to annulations that are beyond the scope of the standard Grubbs carb ene 1 as well as to ring-closing reactions of acrylic acid derivatives even if the resulting alpha,beta-unsaturated lactones (or lactams) are tri- or tetrasubstituted. The reactivity of 3a was found to be highly dependent on the reaction medium: particularly high reaction rates are observed in tolue ne, although this solvent also leads to an increased tendency of the cataly st to isomerize the double bonds of the substrates.