Highly Selective Olefin Metathesis with CAAC-Containing Ruthenium Benzylidenes

"Inverted" Cyclic(Alkyl)(Amino)Carbene (CAAC) Ruthenium Complex Catalyzed Isomerization Metathesis (ISOMET) of Long Chain Olefins to Propylene at Low Ethylene Pressure

Isomerization Metathesis (ISOMET) reaction is an emerging tool for "open loop" chemical recycling of polyethylene to propylene. Novel, latent N-Alkyl substituted Cyclic(Alkyl)(Amino)Carbene (CAAC)-ruthenium catalysts (5a-Ru, 3b-Ru - 6c-Ru) are developed rendering "inverted" chemical structure while showing enhanced ISOMET activity in combination with (RuHCl)(CO)(PPh3)3 (RuH) double bond isomerization co-catalyst. Systematic investigations reveal that the steric hindrance of the substituents on nitrogen and carbon atom adjacent to carbene moiety in the CAAC ligand have significantly improved the catalytic activity and robustness. In contrast to the NHC-Ru and CAAC-Ru catalyst systems known so far, these systems show higher isomerization metathesis (ISOMET) activity (TON: 7400) on the model compound 1-octadecene at as low as 3.0 bar optimized pressure, using technical grade (3.0) ethylene. The propylene content formed in the gas phase can reach up to 20% by volume.

Plasmonic visible-near infrared photothermal activation of olefin metathesis enabling photoresponsive materials

Light-induced catalysis and thermoplasmonics are promising fields creating many opportunities for innovative research. Recent advances in light-induced olefin metathesis have led to new applications in polymer and material science, but further improvements to reaction scope and efficiency are desired. Herein, we present the activation of latent ruthenium-based olefin metathesis catalysts via the photothermal response of plasmonic gold nanobipyramids. Simple synthetic control over gold nanobipyramid size results in tunable localized surface plasmon resonance bands enabling catalyst initiation with low-energy visible and infrared light. This approach was applied to the ROMP of dicyclopentadiene, affording plasmonic polymer composites with exceptional photoresponsive and mechanical properties. Moreover, this method of catalyst activation was proven to be remarkably more efficient than activation through conventional heating in all the metathesis processes tested. This study paves the way for providing a wide range of photoinduced olefin metathesis processes in particular and photoinduced latent organic reactions in general by direct photothermal activation of thermally latent catalysts.

Influence of Substituents in a Six-Membered Chelate Ring of HG-Type Complexes Containing an N→Ru Bond on Their Stability and Catalytic Activity

An efficient approach to the synthesis of olefin metathesis HG-type catalysts containing an N→Ru bond in a six-membered chelate ring was proposed. For the most part, these ruthenium chelates can be prepared easily and in high yields based on the interaction between 2-vinylbenzylamines and Ind II (the common precursor for Ru-complex synthesis). It was demonstrated that the increase of the steric volume of substituents attached to the nitrogen atom and in the α-position of the benzylidene fragment leads to a dramatic decrease in the stability of the target ruthenium complexes. The bulkiest ⁱPr substituent bonded to the nitrogen atom or to the α-position does not allow the closing of the chelate cycle. N,N-Diethyl-1-(2-vinylphenyl)propan-1-amine is a limiting case; its interaction with Ind II makes it possible to isolate the corresponding ruthenium chelate in a low yield (5%). Catalytic activity of the synthesized complexes was tested in RCM reactions and compared with α-unsubstituted catalysts obtained previously. The structural peculiarities of the final ruthenium complexes were thoroughly investigated using XRD and NMR analysis, which allowed making a reliable correlation between the structure of the complexes and their catalytic properties.

Highly selective ethenolysis with acyclic-aminooxycarbene ruthenium catalysts

Acyclic carbene–ruthenium catalysts were developed for the ethenolysis. Remarkable catalytic efficiency (turnover numbers of 100 000) and excellent α-olefin selectivity (up to 98%) were exhibited.

A Review on Olefin Metathesis Reactions as a Green Method for the Synthesis of Organic Compounds

Olefin metathesis is a metal-mediated C-C bond exchange by which the two fragments within the olefin precursor are redistributed as a result of breaking the double bond to obtain a new product. Currently, most of the synthetic organic compounds, polymers, drugs, plastics, and other synthetic materials are synthesized via the application of olefin metathesis reactions. In this review, different types of olefin metathesis reactions with their plausible mechanisms and their application in synthetic organic chemistry have been discussed.

Ene-Yne Metathesis of Allylphosphonates and Allylphosphates: Synthesis of Phosphorus-Containing 1,3-Dienes

A variety of ene-yne cross metathesis reactions were performed using unsaturated phosphonate and phosphate reagents, affording the corresponding phosphorylated 1,3-diene products in good to excellent yields. These difficult ene-yne metatheses employed a Grubbs catalyst bearing a cyclic amino alkyl carbene ligand. A variety of terminal alkynes of varying substitution underwent the reaction, and different phosphorus-containing alkenes were found to give the conjugated diene products in high yields. The resulting dienes were further transformed by Horner-type Wittig reactions and a Diels-Alder cycloaddition.

4-Methyltetrahydropyran as a Convenient Alternative Solvent for Olefin Metathesis Reaction: Model Studies and Medicinal Chemistry Applications

A number of metathesis reactions were successfully conducted in 4-methyltetrahydropyran, including both standard model dienes, as well as more complex substrates, such as analogues of biologically active compounds and active pharmaceutical ingredients. To place this solvent in a context of pharmaceutical R + D, larger-scale syntheses of SUAM 1221, a prolyl endopeptidase inhibitor with potential application in Alzheimer disease treatment, and a derivative of sildenafil, an analogue of the popular Viagra drug, were executed. In the latter case, despite all the setup being made in air, the metathesis reaction at a 33 g scale proceeded very well with relatively low catalyst loading and without need of aqueous workup or column chromatography.

Light-Activated Olefin Metathesis: Catalyst Development, Synthesis, and Applications

The most important means for tuning and improving a catalyst's properties is the delicate exchange of the ligand shell around the central metal atom. Perhaps for no other organometallic-catalyzed reaction is this statement more valid than for ruthenium-based olefin metathesis. Indeed, even the simple exchange of an oxygen atom for a sulfur atom in a chelated ruthenium benzylidene about a decade ago resulted in the development of extremely stable, photoactive catalysts. This Account presents our perspective on the development of dormant olefin metathesis catalysts that can be activated by external stimuli and, more specifically, the use of light as an attractive inducing agent.The insight gained from a deeper understanding of the properties of cis-dichlororuthenium benzylidenes opened the doorway for the systematic development of new and efficient light-activated olefin metathesis catalysts and catalytic chromatic-orthogonal synthetic schemes. Following this, ways to disrupt the ligand-to-metal bond to accelerate the isomerization process that produced the active precatalyst were actively pursued. Thus, we summarize herein the original thermal activation experiments and how they brought about the discoveries of photoactivation in the sulfur-chelated benzylidene family of catalysts. The specific wavelengths of light that were used to dissociate the sulfur-ruthenium bond allowed us to develop noncommutative catalytic chromatic-orthogonal processes and to combine other photochemical reactions with photoinduced olefin metathesis, including using external light-absorbing molecules as "sunscreens" to achieve novel selectivities. Alteration of the ligand sphere, including modifications of the N-heterocyclic carbene (NHC) ligand and the introduction of cyclic alkyl amino carbene (CAAC) ligands, produced more efficient light-induced activity and special chemical selectivity. The use of electron-rich sulfoxides and, more prominently, phosphites as the agents that induce latency widened the spectrum of light-induced olefin metathesis reactions even further by expanding the colors of light that may now be used to activate the catalysts, which can be used in applications such as stereolithography and 3D printing of tough metathesis-derived polymers.

Challenging Metathesis Catalysts with Nucleophiles and Brønsted Base: Examining the Stability of State-of-the-Art Ruthenium Carbene Catalysts to Attack by Amines

Critical to advancing the uptake of olefin metathesis in leading contexts, including pharmaceutical manufacturing, is identification of highly active catalysts that resist decomposition. Amines constitute an aggressive challenge to ruthenium metathesis catalysts. Examined here is the impact of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), morpholine, n-butylamine, and triethylamine on Ru metathesis catalysts that represent the current state of the art, including cyclic alkyl amino carbene (CAAC) and N-heterocyclic carbene (NHC) complexes. Accordingly, the amine-tolerance of the nitro-Grela catalyst RuCl₂(H₂IMes)(=CHAr) (nG; Ar = C₆H₄-2-O ⁱ Pr-5-NO₂) is compared with that of its CAAC analogues nGC1 and nGC2, and the Hoveyda-class catalyst RuCl₂(C2)(=CHAr') HC2 (Ar' = C₆H₄-2-O ⁱ Pr). In C1, the carbene carbon is flanked by an N-2,6-Et₂C₆H₃ group and a CMePh quaternary carbon; in C2, by an N-2- ⁱ Pr-6-MeC₆H₃ group and a CMe₂ quaternary carbon. The impact of 1 equiv amine per Ru on turnover numbers (TONs) in ring-closing metathesis of diethyl diallylmalonate was assessed at 9 ppm Ru, at RT and 70 °C. The deleterious impact of amines followed the trend NEt₃ ∼ NH₂ ⁿ Bu ≪ DBU ∼ morpholine. Morpholine is shown to decompose nGC1 by nucleophilic abstraction of the methylidene ligand; DBU, by proton abstraction from the metallacyclobutane. Decomposition was minimized at 70 °C, at which nGC1 enabled TONs of ca. 60 000 even in the presence of morpholine or DBU, vs ca. 80 000 in the absence of base. Unexpectedly, H₂IMes catalyst nG delivered 70-90% of the performance of nGC1 at high temperatures, and underwent decomposition by Brønsted base at a similar rate. Density functional theory (DFT) analysis shows that this similarity is due to comparable net electron donation by the H₂IMes and C1 ligands. Catalysts bearing the smaller C2 ligand were comparatively insensitive to amines, owing to rapid, preferential bimolecular decomposition.

Tuning thermal properties of cross-linked DCPD polymers by functionalization, initiator type and curing methods

ROMP derived poly-dicyclopentadiene (poly-DCPD) is a relatively new engineering material with high impact resistance, high chemical corrosion resistance and high deflection temperature.

Integrating Activity with Accessibility in Olefin Metathesis: An Unprecedentedly Reactive Ruthenium-Indenylidene Catalyst

Access to leading olefin metathesis catalysts, including the Grubbs, Hoveyda, and Grela catalysts, ultimately rests on the nonscaleable transfer of a benzylidene ligand from an unstable, impure aryldiazomethane. The indenylidene ligand can be reliably installed, but to date yields much less reactive catalysts. A fast-initiating, dimeric indenylidene complex (Ru-1) is reported, which reconciles high activity with scaleable synthesis. Each Ru center in Ru-1 is stabilized by a state-of-the-art cyclic alkyl amino carbene (CAAC, C1) and a bridging chloride donor: the lability of the latter elevates the reactivity of Ru-1 to a level previously attainable only with benzylidene derivatives. Evaluation of initiation rate constants reveals that Ru-1 initiates >250× faster than indenylidene catalyst M2 (RuCl₂(H₂IMes)(PCy₃)(Ind)), and 65× faster than UC (RuCl₂(C1)₂(Ind)). The slow initiation previously regarded as characteristic of indenylidene catalysts is hence due to low ligand lability, not inherently slow cycloaddition at the Ru=CRR' site. In macrocyclization and "ethenolysis" of methyl oleate (i.e., transformation into α-olefins via cross-metathesis with C₂H₄), Ru-1 is comparable or superior to the corresponding, breakthrough CAAC-benzylidene catalyst. In ethenolysis, Ru-1 is 5× more robust to standard-grade (99.9%) C₂H₄ than the top-performing catalyst, probably reflecting steric protection at the quaternary CAAC carbon.

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[4.2.2.0^2,5]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.

Highly Ambiphilic Room Temperature Stable Six-Membered Cyclic (Alkyl)(amino)carbenes

Cyclic (alkyl)(amino)carbenes with a six-membered backbone were prepared. Compared to their five-membered analogues, they feature increased % V_bur and enhanced donor and acceptor properties, as evidenced by the observed n → π* transition trailing into the visible region. The high ambiphilic character even allows for the intramolecular insertion of the carbene into an unactivated C(sp³)-H bond. When used as ligands, they outcompete the five-membered analogues in the palladium-mediated α-arylation of ketones with aryl chlorides.