Ruthenium-Based Heterocyclic Carbene-Coordinated Olefin Metathesis Catalysts

The fascinating story of olefin (or alkene) metathesis (eq 1) began almost five decades ago, when Anderson and Merckling reported the first carbon-carbon double-bond rearrangement reaction in the titanium-catalyzed polymerization of norbornene. Nine years later, Banks and Bailey reported “a new disproportionation reaction . . . in which olefins are converted to homologues of shorter and longer carbon chains...”. In 1967, Calderon and co-workers named this metal-catalyzed redistribution of carbon-carbon double bonds olefin metathesis, from the Greek word “μετάθεση”, which means change of position. These contributions have since served as the foundation for an amazing research field, and olefin metathesis currently represents a powerful transformation in chemical synthesis, attracting a vast amount of interest both in industry and academia

Novel ruthenium indenylidene catalysts : from homogeneous to heterogeneous

Nowadays a number of ruthenium metathesis catalysts have been developed owing to their accessibility, remarkable activity and selectivity, connected with good tolerance towards functional groups, air and moisture. Innovative development in the class of ruthenium metathesis catalysts coordinated with NHC has been experienced which mainly directed toward tuning their catalytic activity and selectivity through altering both steric and electronic properties. The unsymmetrical NHC ligands in particular, have been introduced to induce dissymmetry, a key for achieving higher level of selectivity in different reactions. A great number of the ruthenium complexes bearing unsymmetrical NHC ligands have been developed up to this moment. The bis-coordinated ruthenium indenylidene developed in this work showed moderate activity at higher temperature in RCM, ROMP and other kind of reactions such as isomerization of allylic alcohols and isomerization of alkenes. Failure of these catalysts to work at room temperature has been attributed to the lack of labile ligand.This call for further research which will focus on tuning of unsymmetrical NHC ligands to achieve more active and selective ruthenium complexes coordinated with non-labile NHC ligand with the labile one. The research should go in hand with design and synthesis of heterogeneous catalysts that can be recovered from the reaction mixture and be recycled. Although the support materials used in this study proved to be not suitable for metathesis, the obtained results can be considered as a challenge in the journey toward designing stable and active heterogeneous ruthenium indenylidene catalysts. Up to now a number of solid materials have been developed and successfully utilized in the immobilization of ruthenium benzylidene complexes. It is expected that the same materials can act as the suitable supports for ruthenium indenylidene and therefore, a study about development heterogeneous ruthenium indenylidene analogs would be of great interest

Effects of NHC-Backbone Substitution on Efficiency in Ruthenium-Based Olefin Metathesis

series of ruthenium olefin metathesis catalysts bearing N-heterocyclic carbene (NHC) ligands with varying degrees of backbone and N-aryl substitution have been prepared. These complexes show greater resistance to decomposition through C−H activation of the N-aryl group, resulting in increased catalyst lifetimes. This work has utilized robotic technology to examine the activity and stability of each catalyst in metathesis, providing insights into the relationship between ligand architecture and enhanced efficiency. The development of this robotic methodology has also shown that, under optimized conditions, catalyst loadings as low as 25 ppm can lead to 100% conversion in the ring-closing metathesis of diethyl diallylmalonate

Probing Catalyst Degradation in Metathesis of Internal Olefins: Expanding Access to Amine-Tagged ROMP Polymers

Ruthenium-promoted ring-opening metathesis polymerization (ROMP) offers potentially powerful routes to amine-functionalized polymers with antimicrobial, adhesive, and self-healing properties. However, amines readily degrade the methylidene and unsubstituted ruthenacyclobutane intermediates formed in metathesis of terminal olefins. Examined herein is the relevance of these decomposition pathways to ROMP (i.e., metathesis of internal olefins) by the third-generation Grubbs catalyst. Primary alkylamines rapidly quench polymerization via fast adduct formation, followed by nucleophilic abstraction of the propagating alkylidene. Bulkier, Brønsted-basic amines are less aggressive: attack competes only for slow polymerization or strong bases (e.g., DBU). Added HCl limits degradation, as demonstrated by the successful ROMP of an otherwise intractable methylamine monomer.publishedVersio

Evaluation of Catalysts for the Metathesis of Ethene and 2-Butene to Propene

Different metathesis catalysts were evaluated regarding their activity for propene production from ethene and trans-butene feedstocks. Nickel, molybdenum, rhenium and tungsten, along with bimetallic nickel-rhenium systems were applied with commercial supports and self-synthesized MCM-41. For the latter support the Si/Al ratio was adjusted as an additional optimization parameter (Si/Al = 60). Attractive activities were observed using Re and NiRe based catalysts at moderate temperatures of 200–250 °C. In contrast, the tungsten-based catalysts were only active above 450 °C. Three catalysts, namely Re/AlMCM-41(60), NiRe/mix (1:1) and W/SiO2 offered propene selectivity’s exceeding 40% at attractive conversion rates. These catalysts were characterized by BET, powder XRD, NH3-TPD and TPR-TPO-TPR cycles. At specific reaction temperatures, reaction-regeneration cycles were performed, which revealed that for the Re and W catalysts the initial reactant conversions and propene selectivity can be recovered. In contrast, for the NiRe catalyst, a continuous, gradual and irreversible decrease of activity was observed. Even though the tungsten catalyst was operated at the highest temperature, no irreversible decrease in conversion and propene selectivity occurred. Therefore, this catalyst has potential as a promising candidate for the synthesis of propene

Development and exploration of Schiff base ruthenium carbene catalysts for olefin metathesis

The PhD contains to parts. In the first part, an extensive overview is presented on the history of metathesis catalysts, the current ideas in organometallic thinking and the theoretical accounts on Ru-metathesis catalysts. The second part deals with the synthesis and activity of Schiff base substituted second generation Grubbs catalysts, bimetallic Schiff base substituted Grubbs catalysts, the development of new NHCs (N-Heterocyclic Carbene) for the 2nd generation Grubbs catalysts and a detailed investigation of secondary metathesis events. The Schiff base substituted second generation Grubbs catalysts were synthesized, proving a previous synthesis was incorrect. The new synthesis was confirmed with NMR and crystal structure analysis. A series of ten of these catalysts was synthesized and their activity was screened for several metathesis reactions. The main advantages lie in the stability of the catalysts which is expressed in challenging reactions, such as the RCM (Ring Closing Metathesis) in methanol and the RCM of disubstituted olefines. We also attempted to synthesize the bimetallic Schiff base substituted Grubbs catalysts, though we showed that here also the previous account of these catalysts was incorrect. In fact, the catalysts can not be synthesized because a disproportionation destroys the complex. Using the extensive analysis made in the theoretical piece on organometallic chemistry and theoretical accounts on Ru-metathesis catalysts, a strategy was proposed for new NHC ligands. We focused on the installation of a pi-acceptor function in the NHC, which is also interesting as a fundamental research in organometallic chemistry. We achieved in preparing an (amino)(amido)NHC which showed this characteristic and when this ligand was substituted on the Grubbs catalyst, the resulting catalysts showed faster initiation than the classical second generation catalysts for the ROMP (Ring Closing Metathesis Polymerization) of COD (Cyclooctadiene). Also the secondary metathesis was intensively investigated. We showed that the second generation Grubbs catalysts were able to lead any form of the COD-polyCOD-CDT (Cyclododecatriene) mixture to equilibrium. The first generation catalysts were unable to perform transformations concerning COD. It was also shown that the catalyst leaves a fingerprint on all polymers in the beginning of ROMP, which disappears after time due to secondary metathesis

Development of O,N-bidentate ruthenium catalysts for isomerization and kinetic studies of ruthenium carbenes for C=C coupling reactions

The fountainhead to develop the chemical science is the demand of various compounds by human being. While most of the requirements concerns organic molecules, organometallic compounds have paved the way of homogeneous catalysis in producing bulk, fine chemicals, and even natural products. During the last decade, ruthenium catalysts have provided new indispensable synthetic methods that cannot be promoted by other catalysts and applied in wide range of chemical reactions. This work deals with the development of O,N-bidentate ruthenium homogeneous catalysts for isomerization and kinetic studies of ruthenium cabenes for C=C coupling reactions. Specifically designed ligands are the key in optimizing the efficiency of catalysts. Schiff bases are known to strongly enhance the thermal and moisture stability of the corresponding complexes and isomerization is important in many chemical processes. The above reasons promote the first part of the work to concentrate on the synthesis of a novel class of homogeneous ruthenium complexes containing Schiff bases as O,N-bidentate ligands catalyzing the isomerization reaction. By a proper choice of the Schiff base, the new ruthenium complexes showed improved reactivity, selectivity and stability toward air and moisture during the isomerization reaction. The temperature and solvent tolerance was likewise substantially improved. Later, another series of analogue Schiff bases ruthenium complexes has been synthesized as isomerization catalysts and showed even better reactivity, selectivity and stability. The formation of carbon-carbon bonds is one of the most fundamental chemical processes. In this context, C=C coupling reactions (metathesis, cyclopropanation and etc.) make a significant contribution. Despite the recent advances, the search for commercially relevant catalyst systems remains challenging. The kinetic studies of ruthenium carbenes are useful for fundamental insight to design new catalysts or to improve existing catalytic systems. The second part of this work estabilished a pre-research for the future outlook

Z-Selective Monothiolate Ruthenium Indenylidene Olefin Metathesis Catalysts

Ru-alkylidenes bearing sterically demanding arylthiolate ligands (SAr) constitute one of only two classes of catalyst that are Z-selective in metathesis of 1-alkenes. Of particular interest are complexes bearing pyridine as a stabilizing donor ligand, [RuCl(SAr)(═CHR)(NHC)(py)] (R = phenyl or 2-thienyl, NHC = N-heterocyclic carbene, py = pyridine), which initiate catalysis rapidly and give appreciable yields combined with moderate to high Z-selectivity within minutes at room temperature. Here, we extend this chemistry by synthesizing and testing the first two such complexes (5a and 5b) bearing 3-phenylindenylidene, a ligand known to promote stability in other ruthenium-based olefin metathesis catalysts. The steric pressure resulting from the three bulky ligands (the NHC, the arylthiolate, and the indenylidene) forces the thiolate ligand to position itself trans to the NHC ligand, a configuration different from that of the corresponding alkylidenes. Surprisingly, although this configuration is incompatible with Z-selectivity and slows down pyridine dissociation, the two new complexes initiate readily at room temperature. Although their thermal stability is lower than that of typical indenylidene-bearing catalysts, 5a and 5b are fairly stable in catalysis (TONs up to 2200) and offer up to ca. 80% of the Z-isomer in prototypical metathesis homocoupling reactions. Density functional theory (DFT) calculations confirm the energetic cost of dissociating pyridine from 5a (= M1-Py) to generate 14-electron complex M1. Whereas the latter isomer does not give a metathesis-potent allylbenzene π-complex, it may isomerize to M1-trans and M2, which both form π-complexes in which the olefin is correctly oriented for cycloaddition. The olefin orientation in these complexes is also indicative of Z-selectivity.publishedVersio

Catalytic living ring-opening metathesis polymerization with Grubbs’ second- and third-generation catalysts

In a conventional living ring-opening metathesis polymerization (ROMP), an equal number of ruthenium complexes to the number of polymer chains synthesized are required. This can lead to high loadings of ruthenium complexes when aiming for shorter polymers. Here, a reversible chain-transfer agent was used to produce living ROMP polymers from norbornene derivatives using catalytic amounts of Grubbs’ ruthenium complexes. The polymers obtained by this method showed all of the characteristics of a living polymerization (that is, good molecular weight control, narrow molecular weight dispersities and the ability to form block copolymers). Monomers carrying functional moieties such as ferrocene, coumarin or a triisopropylsilyl-protected primary alcohol could also be catalytically polymerized in a living fashion. The method presented follows a degenerative chain-transfer process and is more economical and environmentally friendly compared with previous living ROMP procedures as it utilizes only catalytic amounts of costly and toxic ruthenium complexes

Effect of Lewis Acids on the Catalyst Activity for Alkene Metathesis, Z-/E- Selectivity and Stability of Tungsten Oxo Alkylidenes

Altres ajuts: Acord transformatiu CRUE-CSICLewis acids increase the catalytic activity of classical heterogeneous catalysts and molecular d tungsten oxo alkylidenes in a variety of olefin metathesis processes. The formation of labile adducts between the metal complex and the Lewis acid has been observed experimentally and suggested to be involved in the catalyst activity increase. In this contribution, DFT (M06) calculations have been performed to determine the role of Lewis acids on catalyst activity, Z-/E- selectivity and stability by comparing three W(E)(CHR)(2,5-dimethylpyrrolide)(O-2,6-dimesithylphenoxide) (E = oxo, imido or oxo-Lewis acid adduct) alkylidenes. Results show that the formation of the alkylidene-Lewis acid adducts influences the reactivity of tungsten oxo alkylidenes due to both steric and electronic effects. The addition of the Lewis acid on the E group increases its bulkiness and this decreases catalyst Z-selectivity. Moreover, the interaction between the oxo ligand and the Lewis acid decreases the donating ability of the former toward the metal. This is important when the oxo group has either a ligand in trans or in the same plane that is competing for the same metal d orbitals. Therefore, the weakening of oxo donating ability facilitates the cycloaddition and cycloreversion steps and it stabilizes the productive trigonal bipyramid metallacyclobutane isomer. The two factors increase the catalytic activity of the complex. The electron donating tuneability by the coordination of the Lewis acid also applies to catalyst deactivation and particularly the key β-hydride elimination step. In this process, the transition states show a ligand in pseudo trans to the oxo. Therefore, the presence of the Lewis acid decreases the Gibbs energy barrier significantly. Overall, the optimization of the E group donating ability in each step of the reaction makes tungsten oxo alkylidenes more reactive and this applies both for the catalytic activity and catalyst deactivation