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.
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