Synthesis of Phenol-Tagged Ruthenium Alkylidene Olefin Metathesis Catalysts for Robust Immobilisation Inside Metal–Organic Framework Support

Two new unsymmetrical N-heterocyclic carbene ligand (uNHC)-based ruthenium complexes featuring phenolic OH function were obtained and fully characterised. The more active one was then immobilised on the metal–organic framework (MOF) solid support (Al)MIL-101-NH2. The catalytic activity of such a heterogeneous system was tested, showing that, while the heterogeneous catalyst is less active than the corresponding homogeneous catalyst in solution, it can catalyse selected olefin metathesis reactions, serving as the proof-of-concept for the immobilisation of catalytically active complexes in MOFs using a phenolic tag.

Electrophilicity of Hoveyda-Grubbs Olefin Metathesis Catalysts as the Driving Force that Controls Initiation Rates

The dissociative mechanism of initiation for a series of Hoveyda-Grubbs type metathesis catalysts modified at the para and meta positions in the isopropoxybenzylidene ligand is investigated by means of DFT calculations. The electron donating/withdrawing capacity of the ligand was screened through the incorporation of various substituents such as halogens, nitro, alkoxides, ketones, esters, amines, and amides. Variations in structural parameters, energy barriers for the Ru-O bond dissociation, and Ru-O bond strength were examined as a function of the Hammett constant. It was found that electronic properties of the catalysts such as chemical potential, hardness, and electrophilicity correlate linearly with the dissociative energy barriers. These findings enable a systematic rationalization and prediction of rate of precatalyst initiation through the calculation of only the HOMO-LUMO gap of catalysts, as the faster the initiation, the more electrophilic the catalyst.

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.

Ruthenium Complexes Featuring Unsymmetrical N-Heterocyclic Carbene Ligands-Useful Olefin Metathesis Catalysts for Special Tasks

This review describes a distinct class of ruthenium olefin metathesis catalysts featuring unsymmetrical N-heterocyclic carbene (uNHC) ligands, from its historical beginning to the present state of the art. Thanks to advantageous traits, such as pronounced thermodynamic stability, chemical latency, outstanding selectivity, and compatibility with green solvents, these catalysts led to good results in a number of specialized metathesis transformations. Therefore, while being a niche, the uNHC complexes can potentially be implemented in a number of industrial processes, such as valorization of Fischer-Tropsch olefin fractions, ethenolysis of renewable products, and modern pharmaceutical production.

Impact of the Carbene Derivative Charge on the Decomposition Rates of Hoveyda-Grubbs-like Metathesis Catalysts

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.

Rate-Limiting Steps in the Intramolecular C-H Activation of Ruthenium N-Heterocyclic Carbene Complexes

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.

Looking for the Noncyclic(amino)(alkyl)carbene Ruthenium Catalyst for Ethenolysis of Ethyl Oleate: Selectivity Is on Target

A wide set of 65 diverse Ru metathesis catalysts was investigated in the ethenolysis reaction of biosourced ethyl oleate to allow the comparison between the catalyst structure and its activity and selectivity. Handling of the oleic substrate, weighing of the catalysts, and charging the reactor were done in air, with exclusion of a glovebox or Schlenk techniques. A catalyst bearing the unsymmetrical N-heterocyclic ligand featuring a thiophene fragment (Ru-63) was selected to offer the best combination between high selectivity and sufficient activity under conditions mimicking oil industry practice. A proof-of-concept large-scale ethenolysis experiment was also done with the selected catalyst to prove its high selectivity at the 1 L scale reaction with a 90% pure non-distilled substrate.

Ruthenium-based olefin metathesis catalysts with monodentate unsymmetrical NHC ligands

An overview on the catalytic properties of ruthenium complexes for olefin metathesis bearing monodentate unsymmetrical N-heterocyclic diaminocarbene ligands is provided. The non-symmetric nature of these NHC architectures strongly influences activity and selectivity of the resulting catalysts. The main achievements that have been accomplished in significant areas of olefin metathesis up to the current state of research are discussed.