We report a computational mechanistic study explaining the low stability of Hoveyda–Grubbs catalyst in the presence of acrylonitrile. We show the atomistic and energetic basis of why recently synthesized cyclic alkyl amino carbene (CAAC) ruthenium catalyst is much more stable in the presence of acrylonitrile than the Hoveyda–Grubbs catalyst and the CAAC catalyst bearing phenyl group and how it affects the metathesis reaction.

Hoveyda–Grubbs catalysts can perform metathesis with mono- and disubstituted olefins bearing chloro and methoxy moieties, but are prone to decomposition with methoxyethene and do not form stable complexes with (Z)-1,2-dichloroethene.

Ruthenium (II) complexes with N-heterocyclic carbenes (NHC) are commonly used as efficient catalysts in hydrogenation of olefins with simultaneous intramolecular C–H activation. Using the DFT approach, we have investigated the entire hydrogenation reaction pathway for four new potential catalysts and ethylene, a model substrate. Our calculations imply that the dissociation of phosphine is the rate-limiting step of hydrogenation, contrary to recent computational results. We also found that catalysts bearing NHCs with aliphatic and aromatic side groups are energetically favorable over other aliphatic cyclohexyl-substituted NHC. To examine how electronic properties of various catalysts influence the energetic barrier in the crucial steps of the reaction, we applied the Noncovalent Interaction analysis, which allowed us to reveal crucial interactions which stabilize/destabilize important intermediates and transition states in the hydrogenation reaction.

Hoveyda–Grubbs metathesis catalysts undergo a relatively fast decomposition in the presence of olefins. Using a computational density functional theory approach, we show that positively charged derivatives of N-heterocyclic carbenes have little impact on the degradation/deactivation rates of such catalysts with respect to neutral carbenes. On the other hand, the hypothetical anionic Hoveyda–Grubbs-like catalysts are predicted to less likely undergo degradation in the presence of the olefin, while being as active as standard, neutral Hoveyda–Grubbs catalysts.

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.