Effect of ligand bulk in ruthenium-catalyzed olefin metathesis : IPr* vs IPr

Novel ruthenium catalysts, [RuCl2(IPr*)(PPh3)(3-phenylindenylidene)] (1) and [RuCl2(IPr*)(Py)(3-phenylindenylidene)] (2), bearing the very bulky ligand IPr* are reported. The complexes were tested in ring closing and cross-metathesis, and their activities were compared to that displayed by [RuCl2(IPr)(PPh3)(3-phenylindenylidene)] (3) and the new [RuCl2(IPr)(Py)(3-phenylindenylidene)] (4), in order to assess how the steric hindrance of the N-heterocyclic carbene influences the catalytic activity of the resulting ruthenium complexes