Comparative investigation of ruthenium-based metathesis catalysts bearing N-heterocyclic carbene (NHC) ligands

Exchange of one PCy3 unit of the classical Grubbs catalyst 1 by N-heterocyc lic carbene (NHC) ligands leads to "second-generation" metathesis catalysts of superior reactivity and increased stability. Several complexes of this type have been prepared and fully characterized, six of them by X-ray cryst allography. These include the unique chelate complexes 13 and 14 in which t he NHC-and the Ru=CR entities are tethered to form a metallacycle. A partic ularly favorable design feature is that the reactivity of such catalysts ca n be easily adjusted by changing the electronic and steric properties of th e NHC ligands. The catalytic activity also strongly depends on the solvent used; NMR investigations provide a tentative explanation of this effect. Ap plications of the "second-generation" catalysts to ring closing alkene meta thesis and intramolecular enyne cycloisomerization reactions provide insigh ts into their catalytic performance. From these comparative studies it is d educed that no single catalyst is optimal for different types of applicatio ns. The search for the most reactive catalyst for a specific transformation is facilitated by IR thermography allowing a rapid and semi-quantitative r anking among a given set of catalysts.