Decomposition of Ruthenium Metathesis Catalysts: Unsymmetrical N-Heterocyclic Carbenes versus Cyclic Alkyl Amino Carbenes

Abstract

Catalysts bearing cyclic (alkyl)(amino) carbenes (CAACs) and unsymmetrical N-heterocyclic carbenes (uNHCs) demonstrate high productivity in metathetical chemical transformations. Despite their high durability, they undergo decomposition reactions, which may hamper their catalytic activity. With the help of computational density functional theory, we show that the bimolecular coupling mechanism is the energetically favored degradation pathway for both types of catalysts compared to the β-hydride mechanism. Moreover, all investigated catalysts were predicted to be less prone to decomposition than the well-known Hoveyda–Grubbs catalyst. For the β-hydride/van Rensburg mechanism, we considered two model substrates, ethylene and allylbenzene, and determined which of them expedites the decomposition of the investigated catalysts. We also determined, based on the Gibbs free energies and partial charges, the preferred way of inactivation of CAACs and uNHCs.

Inhibition of the Decomposition Pathways of Ruthenium Olefin Metathesis Catalysts: Development of Highly Efficient Catalysts for Ethenolysis

“…The latest studies showed that the remarkable performance of the metathesis catalysts bearing a CAAC ligand, as demonstrated in the olefin metathesis reactions compared to the NHC-ligated counterpart, was attributed to the higher stability of the ruthenacyclobutane to the β-elimination decomposition pathway. − Despite the bimolecular catalyst decomposition pathway remains operative and is even faster for CAAC-ligated complexes than for NHC-ligated ones, , the overall performance is higher for the former (Scheme ).…”