Decomposition of Ruthenium Metathesis Catalysts: Unsymmetrical <i>N</i>-Heterocyclic Carbenes versus Cyclic Alkyl Amino Carbenes

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.