For metathesis, the 00’s was the decade of the second generation ruthenium catalysts. Besides the well-known 2nd generation Grubbs and Hoveyda-Grubbs catalysts, hundreds of catalysts featuring heterocyclic carbene ligands have been published in the past ten years. There are actually so many that it has become nearly impossible to have a clear idea of what has been tried or done. Fortunately, two very complete and up-to-date reviews recently published by Grela and Grubbs help us keep track of everything!
Grela’s review focuses primarily on NHC-Ru complexes that contain either tricyclohexylphosphine or Hoveyda-type chelating ligands (which encompasses most metathesis catalysts). What I really liked about the review is that the authors not only present the catalysts and their syntheses, but also compare various complexes in several metathesis reactions, putting an emphasis on the influence of the heterocyclic carbene on the activity of the Ru catalysts.
The most recent review published by Vougioukalakis and Grubbs is more complete. With its 400 catalyst entries, it covers almost all the second generation complexes made to date containing variations of the heterocyclic carbene ligand, the anionic ligands, the alkylidene moiety and the neutral ligand. This review is great if you want to know if a catalyst has been made, because if it is not in there, it has probably not been done (yet)!