Highly efficient ruthenium catalysts for the formation of tetrasubstituted olefins via ring-closing metathesis.

[reaction: see text] A series of ruthenium-based metathesis catalysts with N-heterocyclic carbene (NHC) ligands have been prepared in which the N-aryl groups have been changed from mesityl to mono-ortho-substituted phenyl (e.g., tolyl). These new catalysts offer an exceptional increase in activity for the formation of tetrasubstituted olefins via ring-closing metathesis (RCM), while maintaining high levels of activity in ring-closing metathesis (RCM) reactions that generate di- and trisubstituted olefins.