Ru Carbenes Research Articles

Stereocontrolled acyclic diene metathesis polymerization.

The cis/trans geometry of olefins is known to dramatically influence the thermal and mechanical properties of polyalkenamers. Yet, polymerization methods that efficiently control this parameter are scarce. Here we report the development of a stereoretentive acyclic diene metathesis polymerization that uses the reactivity of dithiolate Ru carbenes combined with cis monomers. These Ru catalysts exhibit exquisite retention of the cis geometry and tolerate many polar functional groups, enabling the synthesis of all-cis polyesters, polycarbonates, polyethers and polysulfites. The stereoretentive acyclic diene metathesis polymerization is also characterized by low catalyst loadings and tolerance towards trans impurities in the monomer batch, which should facilitate large-scale implementation. Modulation of the reaction temperature and time leads to an erosion of stereoretention, permitting a stereocontrolled synthesis of polyalkenamers with predictable cis:trans ratios. The impact of the stereochemistry of the repeating alkenes on the thermal properties is clearly demonstrated through differential scanning calorimetry and thermogravimetric analysis.

Cyclododecene in Olefin Metathesis: Polymerization and Macromolecular Cross-Metathesis with Polynorbornene

The reaction of macromolecular cross-metathesis of polynorbornene and polydodecenamer catalyzed by the first-generation Grubbs Ru-carbene complex is studied for the first time. Polydodecenamer with high-molecular-weight characteristics (Mw = 406 × 103, Ð = 2.5) and a yield of 96% is synthesized by the ring-opening metathesis polymerization of cyclododecene mediated by the second-generation Grubbs catalyst. The polymer has a melting temperature of 62–83°C, depending on the content of trans double bonds, and shows poor solubility in organic solvents at room temperature. Its cross-metathesis with polynorbornene at 40°С gives rise to new statistical multiblock copolymers of norbornene and cyclododecene with different degree of blockiness. NMR, GPC, and DSC were used to investigate the effect of reaction conditions on the structure and thermal properties of the copolymers. At the initial step of the reaction, along with a high-molecular-weight peak, the GPC chromatograms exhibit a peak due to the low-molecular-weight fraction (M = (1–2) × 103), suggesting the formation of cyclooligomers. With increasing degree of interchain exchange, the proportion of the oligomeric fraction decreases appreciably. The kinetics of the cross-metathesis of polynorbornene with polydodecenamer and polyoctenamer at 40°С is studied by in situ 1Н NMR spectroscopy and ex situ 13С NMR spectroscopy. The rates of elementary reactions in these blends differ insignificantly, except for the conversion stage of the initial carbenes into polymer ones. The formation rate for carbene [Ru]=polyoctenamer is three times higher than the formation rate for the [Ru]=polydodecenamer carbene. In both cases, the fraction of Ru carbenes attached to norbornene units is extremely small throughout the process.

Living Polymerization Caught in the Act: Direct Observation of an Arrested Intermediate in Metathesis Polymerization.

Understanding the stability and reactivity of the propagating species is critical in living polymerization. Therefore, most living olefin metathesis polymerizations require the stabilization of the catalyst by coordination of external ligands containing Lewis basic heteroatoms, e.g., phosphines and pyridines. However, in some cases, chemists postulated that the propagating metal carbene could also be stabilized by olefin chelation. Here, we disclose that stable 16-electron olefin-chelated Ru carbenes play a key role in previously reported living/controlled ring-opening metathesis polymerization of endo-tricyclo[4.2.2.02,5]deca-3,9-diene and cyclopolymerization of 1,8-nonadiynes using Grubbs catalysts. We successfully isolated these propagating species during polymerization and confirmed their olefin-chelated structures using X-ray crystallography and NMR analysis. DFT calculations and van 't Hoff plots from the equilibrium between olefin-chelated Ru carbenes and 3-chloropyridine (Py)-coordinated carbenes revealed that entropically favored olefin chelation overwhelmed enthalpically more stable Py-coordinated Ru carbenes at room temperature. Therefore, olefin chelation stabilized the propagating species and slowed down the propagation relative to initiation, thereby lowering polydispersity. This finding provides a deeper understanding of the olefin metathesis polymerization mechanism using Grubbs catalysts and offers clues for designing new controlled/living polymerizations.

Toward Olefin Metathesis with Iron Carbene Complexes: Benefits of Tridentate σ-Donating Ligands

Nowadays the homogeneous olefin metathesis reaction is performed using Mo, W, or Ru carbenes that show outstanding activities and selectivities. However, the use of an iron complex instead of the existing catalysts is a desired goal in terms of catalyst cost, toxicity and environmental impact. DFT(OPBE)-D2 calculations have been used to identify the requirements that could favor the design of a L3Fe═CH2 iron carbene with activity in alkene metathesis. Results show that strong σ-donating ligands are essential for favoring a singlet ground state of the carbene. However, they do not favor the singlet ground state for the metallacyclobutane per se. In fact, since the geometry around the metal center in the metallacycle is different for the singlet (trigonal bipyramid) and triplet (square-based pyramid) states, the stabilization of the singlet state requires disfavoring the latter coordination. This is achieved by using tridentate pincer ligands with the strongest σ-donor group in central position. The here in...

Unexpected Reactivity Patterns of Ruthenium Alkylidenes with N-Phosphino-Functionalized N-Heterocyclic Carbene Ligands (NHCPs)

N-phosphino-functionalized N-heterocyclic carbene (NHCP) ligands have been evaluated as potential supporting ligands in ruthenium-catalyzed olefin metathesis. Initial density functional theory (DFT) calculations suggested that these NHCP ligands may allow access to neutral 14 valence electron (VE) species—equivalents of the active 14 VE species formed by phosphine dissociation from Grubbs II precatalysts—via facile decoordination of the NHCP phosphino donor of the strained four-membered [RuPNC] chelate systems. Their attempted synthesis from NHCPs and Grubbs-type Ru carbenes revealed addition of an NHCP donor atom (P or C) to the alkylidene fragment, forming a new C–P or C–C bond in five-membered chelate structures. DFT investigations showed that these reactions are controlled kinetically and must not be neglected as important possible deactivation routes in olefin metathesis.

ChemInform Abstract: Evolution of Catalytic Stereoselective Olefin Metathesis: From Ancillary Transformation to Purveyor of Stereochemical Identity

AbstractReview: [121 refs.

Evolution of catalytic stereoselective olefin metathesis: from ancillary transformation to purveyor of stereochemical identity.

There have been numerous significant advances in catalytic olefin metathesis (OM) during the past two decades. Such progress has transformed this important set of reactions to strategically pivotal processes that generate stereochemical identity while delivering molecules that cannot be easily prepared by alternative routes. In this Perspective, an analysis of the origin of the inception of bidentate benzylidene ligands for Ru-based OM catalysts is first presented. This is followed by an overview of the intellectual basis that culminated in the development of Mo-based diolates and stereogenic-at-Ru complexes for enantioselective OM. The principles accrued from the study of the latter Ru carbenes and Mo alkylidenes and utilized in the design of stereogenic-at-Mo, -W, and -Ru species applicable to enantioselective and Z-selective OM are then discussed. The influence of the recently introduced catalytic OM protocols on the design of synthesis routes leading to complex organic molecules is probed. The impact of a better understanding of the mechanistic nuances of OM toward the discovery of stereoselective catalysts is reviewed as well.

The Influence of Anionic Ligands on Stereoisomerism of Ru Carbenes and Their Importance to Efficiency and Selectivity of Catalytic Olefin Metathesis Reactions

Investigations detailed herein provide insight regarding the mechanism of stereochemical inversion of stereogenic-at-Ru carbene complexes through a nonolefin metathesis-based polytopal rearrangement pathway. Computational analyses (DFT) reveal that there are two key factors that generate sufficient energy barriers that are responsible for the possibility of isolation and characterization of high-energy, but kinetically stable, intermediates: (1) donor-donor interactions that involve the anionic ligands and the strongly electron donating carbene groups and (2) dipolar effects arising from the syn relationship between the anionic groups (iodide and phenoxide). We demonstrate that a Brønsted acid lowers barriers to facilitate isomerization, and that the positive influence of a proton source is the result of its ability to diminish the repulsive electronic interactions originating from the anionic ligands. The implications of the present studies regarding a more sophisticated knowledge of the role of anionic units on the efficiency of Ru-catalyzed olefin metathesis reactions are discussed. The electronic basis for the increased facility with which allylic alcohols participate in olefin metathesis processes will be presented as well. Finally, we illustrate how a better understanding of the role of anionic ligands has served as the basis for successful design of Ru-based Z-selective catalysts for alkene metathesis.

ChemInform Abstract: Correlation Between Functionality Preference of Ru Carbenes and exo/endo Product Selectivity for Clarifying the Mechanism of Ring‐Closing Enyne Metathesis.

Functionality preferences of metathesis Ru carbenes to various alkenes and alkynes with electronic and steric diversity were determined by using time-dependent fluorescence quenching. The functionality preferences depend not only on the properties of multiple bonds but also on the ligands on Ru. There was a good correlation between functionality preference and the metathesis reaction outcome. The correlation between functionality preference and exo/endo product ratio offers a solution to resolve the mechanistic issue related with alkene- vs alkyne-initiated pathway in ring-closing enyne metathesis. The correlation indicates the preference is likely to dictate the reaction pathway and eventually the outcome of the reaction. The Ru catalyst favoring alkyne over alkene provides more endo product, indicating that the reaction mainly initiates at the alkyne. By changing the substitution pattern, the preference can be reversed to give an exclusive exo product.

Correlation between Functionality Preference of Ru Carbenes and exo/endo Product Selectivity for Clarifying the Mechanism of Ring-Closing Enyne Metathesis

Functionality preferences of metathesis Ru carbenes to various alkenes and alkynes with electronic and steric diversity were determined by using time-dependent fluorescence quenching. The functionality preferences depend not only on the properties of multiple bonds but also on the ligands on Ru. There was a good correlation between functionality preference and the metathesis reaction outcome. The correlation between functionality preference and exo/endo product ratio offers a solution to resolve the mechanistic issue related with alkene- vs alkyne-initiated pathway in ring-closing enyne metathesis. The correlation indicates the preference is likely to dictate the reaction pathway and eventually the outcome of the reaction. The Ru catalyst favoring alkyne over alkene provides more endo product, indicating that the reaction mainly initiates at the alkyne. By changing the substitution pattern, the preference can be reversed to give an exclusive exo product.

Alkene Metathesis – A Tool for the Synthesis of Conjugated Polymers

Alkene metathesis is a superb methodology. We report the progress using alkene metathesis in the synthesis of polymeric organic semiconductors. Three classes of polymers have been synthesized using acyclic diene metathesis (ADMET) or ring opening metathesis polymerization (ROMP), viz., poly(acetylene)s (PA), poly(arylene-vinylene)s (PAV), and organometallic polymers. For PAs, ROMP of cyclooctatetraenes is best, whereas for PAV, both ADMET and indirect and direct ROMP are viable. Metathesis performs flawlessly with the correct monomers, as molybdenum and particularly the robust Ru carbenes demonstrate. When performing ROMP, one is often rewarded with structurally uniform polymers that can display very low polydispersities. Overall, metathesis is a powerful tool for the preparation of semiconducting polymers.

ChemInform Abstract: endo‐Selective Enyne Ring‐Closing Metathesis Promoted by Stereogenic‐at‐Mo Monoalkoxide and Monoaryloxide Complexes. Efficient Synthesis of Cyclic Dienes Not Accessible Through Reactions with Ru Carbenes.

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Endo-selective enyne ring-closing metathesis promoted by stereogenic-at-Mo monoalkoxide and monoaryloxide complexes. Efficient synthesis of cyclic dienes not accessible through reactions with Ru carbenes.

Stereogenic-at-Mo monoalkoxide and monoaryloxide complexes promote enyne ring-closing metathesis (RCM) reactions, affording the corresponding endo products with high selectivity (typically >98:<2 endo:exo). All catalysts can be prepared and used in situ. Five-, six-, and seven-membered rings are obtained through reactions with enyne substrates that bear all-carbon tethers as well as those that contain heteroatom substituents. The newly developed catalytic protocols complement the related exo-selective Ru-catalyzed processes. In cases where Ru-based complexes deliver exo and endo products nondiscriminately, such as when tetrasubstituted cyclic alkenes are generated, Mo-catalyzed reactions afford the endo product exclusively. The efficiency of synthesis of N- and O-containing endo diene heterocycles can be improved significantly through structural modification of Mo catalysts. The modularity of Mo-based monopyrrolides is thus exploited in the identification of the most effective catalyst variants. Through alteration of O-based monodentate ligands, catalysts have been identified that promote enyne RCM with improved efficiency. The structural attributes of three Mo complexes are elucidated through X-ray crystallography. The first examples of catalytic enantioselective enyne RCM reactions are reported (up to 98:2 enantiomer ratio and >98% endo).

Feasibility of associative mechanism in enyne metathesis catalyzed by grubbs complexes

The mechanism of enyne metathesis catalyzed by first and second generation Grubbs complexes has been computationally explored at the DFT level. The relative reactivity and the regioselectivity for the reaction of differently substituted alkenes and alkynes with model Ru complexes has been studied. The usually accepted dissociative mechanism for the alkene metathesis has been explored for alkynes, and compared with an associative pathway involving initial coordination of the alkyne to the 16-electron catalyst. Our results show that an associative mechanism would be the preferred pathway for the reaction of phosphine-based (first generation) Ru carbenes, at least for small phosphines such as PMe(3), whereas for the more reactive complexes containing a heterocyclic carbene as ligand (second generation catalysts), the dissociative process is far more favourable.

Regarding the Mechanism of Olefin Metathesis with Sol−Gel-Supported Ru-Based Complexes Bearing a Bidentate Carbene Ligand. Spectroscopic Evidence for Return of the Propagating Ru Carbene

Two isotopically and structurally labeled Ru-based carbenes (2-d4 and 13) have been prepared and attached to the surface of monolithic sol-gel glass. The resulting glass-supported complexes (18-dn and 19) exhibit significant catalytic activity in promoting olefin metathesis reactions and provide products of high purity. Through analysis of the derivatized glass pellets used in a sequence of catalytic ring-closing metathesis reactions mediated by various supported Ru carbenes, it is demonstrated that free Ru carbene intermediates in solution can be scavenged by support-bound styrene ether ligands prior to the onset of competing transition metal decomposition. The observations detailed herein provide rigorous evidence that the initially proposed release/return mechanism is, at least partially, operative. The present investigations shed light on a critical aspect of the mechanism of an important class of Ru-based metathesis complexes (those bearing a bidentate styrene ether ligand).

Ruthenium Carbenes Bonded to a Macrocycle: Carbon Monoxide Induced Carbene Migration Pathways from the Metal to the Ligand

Activated by π-acceptors like CO though otherwise very stable, Ru–carbenes 1–3 undergo a migration of the carbene unit to the ancillary macrocyclic ligand in the presence of CO. This mechanism is reminiscent of the crucial step in metatheses mediated by ruthenium carbenes.