Synthesis of Unsaturated Macrocycles by Ru‐Catalyzed Ring‐Closing Metathesis: A Comparative Study

The activity and stereoselectivity of phosphane‐ and N‐heterocyclic carbene (NHC)‐containing ruthenium benzylidene complexes have been evaluated in macrocyclic ring‐closing olefin metathesis to produce unsaturated lactones and lactams. The success of the macrocyclization depends on the nature of the ligand (phosphane or N‐heterocyclic carbene) on the ruthenium center and on the NHC properties. As for stereoselectivity, E / Z ratios seem to be influenced not only by the nature of the ruthenium catalyst but also by the thermodynamic stabilities of the resulting unsaturated macrocycles, as confirmed by theoretical results.