Ruthenium Metathesis Catalysts Bearing Anionic N‑Heterocyclic Carbenes: A Computational Study on Failed Approaches to Their Synthesis

Despite recent efforts, the synthesis of ruthenium metathesis catalysts bearing anionic N-heterocyclic carbenes has not been successful. Using a computational density functional theory approach, we show that previously synthesized anionic N-heterocyclic carbenes are, with one exception, too weak Lewis bases to replace a chloride ion in the structure of common ruthenium metathesis catalysts. On the other hand, the transmetalation from silver or copper complexes is not feasible because of the very high affinity of anionic N-heterocyclic carbenes to these transition metals in comparison to ruthenium. We also show that the heterobimetallic Ag/Ru and Cu/Ru complexes obtained during such transmetalation are unlikely to catalyze olefin metathesis but may be good candidates for other catalysts as a result of facile dissociation of the phosphine moiety to produce an active Ru complex

Structural and Mechanistic Basis of the Fast Metathesis Initiation by a Six-Coordinated Ruthenium Catalyst

The N-heterocyclic carbene coordinated ruthenium benzylidene complex [(H₂IMes)­(PCy₃)­(Cl)₂RuCHPh], a highly active olefin metathesis catalyst, is known to convert into a very fast initiator in the presence of pyridine or 3-bromopyridine. Computational studies presented in this work reveal the mechanistic basis of this phenomenon. Depending on the size of the olefin, the reaction follows either a dissociative or associative mechanism

Ruthenium Metathesis Catalysts Bearing Anionic N‑Heterocyclic Carbenes: A Computational Study on Failed Approaches to Their Synthesis

Despite recent efforts, the synthesis of ruthenium metathesis catalysts bearing anionic N-heterocyclic carbenes has not been successful. Using a computational density functional theory approach, we show that previously synthesized anionic N-heterocyclic carbenes are, with one exception, too weak Lewis bases to replace a chloride ion in the structure of common ruthenium metathesis catalysts. On the other hand, the transmetalation from silver or copper complexes is not feasible because of the very high affinity of anionic N-heterocyclic carbenes to these transition metals in comparison to ruthenium. We also show that the heterobimetallic Ag/Ru and Cu/Ru complexes obtained during such transmetalation are unlikely to catalyze olefin metathesis but may be good candidates for other catalysts as a result of facile dissociation of the phosphine moiety to produce an active Ru complex

An Anthracene-Thiolate-Ligated Ruthenium Complex: Computational Insights into Z‑Stereoselective Cross Metathesis

Stereoselective control of the cross metathesis of olefins is a crucial aspect of synthetic procedures. In this study, we utilized density functional theory methods to calculate thermodynamic and kinetic descriptors to explore the stereoselectivity of cross metathesis between allylbenzene and 2-butene-1,4-diyl diacetate. A ruthenium-based complex, characterized primarily by an anthracene-9-thiolate ligand, was designed in silico to completely restrict the E conformation of olefins through a bottom-bound mechanism. Our investigation of the kinetics of all feasible propagation routes demonstrated that Z-stereoisomers of metathesis products can be synthesized with an energy cost of only 13 kcal/mol. As a result, we encourage further research into the synthetic strategies outlined in this work

Olefin Metathesis Catalyzed by a Hoveyda–Grubbs-like Complex Chelated to Bis(2-mercaptoimidazolyl) Methane: A Predictive DFT Study

Although highly selective complexes for the cross-metathesis of olefins, particularly oriented toward the productive metathesis of Z-olefins, have been reported in recent years, there is a constant need to design and prepare new and improved catalysts for this challenging reaction. In this work, guided by density functional theory (DFT) calculations, the performance of a Ru-based catalyst chelated to a sulfurated pincer in the olefin metathesis was computationally assessed. The catalyst was designed based on the Hoveyda–Grubbs catalyst (SIMes)­Cl2Ru­(CH–o–OiPrC6H4) through the substitution of chlorides with the chelator bis­(2-mercaptoimidazolyl)­methane. The obtained thermodynamic and kinetic data of the initiation phase through side- and bottom-bound mechanisms suggest that this system is a versatile catalyst for olefin metathesis, as DFT predicts the highest energy barrier of the catalytic cycle of ca. 20 kcal/mol, which is comparable to those corresponding to the Hoveyda–Grubbs-type catalysts. Moreover, in terms of the stereoselectivity evaluated through the propagation phase in the metathesis of propene–propene to 2-butene, our study reveals that the Z isomer can be formed under a kinetic control. We believe that this is an interesting outcome in the context of future exploration of Ru-based catalysts with sulfurated chelates in the search for high stereoselectivity in selected reactions

An Anthracene-Thiolate-Ligated Ruthenium Complex: Computational Insights into Z‑Stereoselective Cross Metathesis

Stereoselective control of the cross metathesis of olefins is a crucial aspect of synthetic procedures. In this study, we utilized density functional theory methods to calculate thermodynamic and kinetic descriptors to explore the stereoselectivity of cross metathesis between allylbenzene and 2-butene-1,4-diyl diacetate. A ruthenium-based complex, characterized primarily by an anthracene-9-thiolate ligand, was designed in silico to completely restrict the E conformation of olefins through a bottom-bound mechanism. Our investigation of the kinetics of all feasible propagation routes demonstrated that Z-stereoisomers of metathesis products can be synthesized with an energy cost of only 13 kcal/mol. As a result, we encourage further research into the synthetic strategies outlined in this work

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

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

Molecular Modeling of Mechanisms of Decomposition of Ruthenium Metathesis Catalysts by Acrylonitrile

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