Improved Chiral Olefin Metathesis Catalysts: Increasing the Thermal and Solution Stability via Modification of a C1-Symmetrical N-Heterocyclic Carbene Ligand

Four new ruthenium‐based olefin metathesis catalysts that possess an N‐heterocyclic carbene (NHC) ligand with benzyl (Bn) or or n‐propyl (n‐Pr) N‐alkyl groups have been prepared. The synthetic routes developed for the synthesis of the required dihydroimidazolium salts are general. Catalysts bearing larger NHC ligands with larger N‐alkyl groups displayed improved thermal and solution state stability up to 80 °C. The reactivity of the new catalysts in ring‐closing metathesis is directly related to the increased steric bulk of the NHC ligand. The new catalysts have been evaluated in desymmetrization reactions and the nature of the N‐alkyl group of the NHC ligands has been shown to have an important effect on the observed enantioselectivities.