Bimolecular Coupling in Olefin Metathesis: Correlating Structure and Decomposition for Leading and Emerging Ruthenium-Carbene Catalysts

The Impact of Ligand Structure and Reaction Temperature on Ethenolysis of Fatty Acid Methyl Esters Catalyzed by Spirocyclic Alkyl Amino Carbene Ru Complexes

Spirocyclic alkyl amino carbene (SCAAC) Ru complexes demonstrate outstanding activity and selectivity in ethenolysis of methyl oleate (MO) or fatty acid methyl esters (FAMEs), and 5,6-dimethoxyindane derivative was the most active catalyst to date. For the further catalyst design, we proposed modifying the spirocyclic fragment by fusion of saturated carbo- or heterocycle, linked to the 5,6-positions of indane or 6,7- positions of tetralin. Another suggested way of the modification of SCAAC complex was the insertion of chromane fragment to the carbene ligand. Using an alternative approach to SCAAC ligand precursors, based on hydroformylation of indenes, dihydronaphthalenes and their analogs, new SCAAC complexes were synthesized, their cis-configuration was confirmed by XRD. Comparative study of new and known selected complexes in ethenolysis of FAMEs (84 wt% MO) revealed that each of SCAAC catalysts has a temperature optimum of activity. At 60 °C 0.5 ppm of the complex containing 1,2,3,4,5,6,7,8-octahydroanthracene spirocyclic fragment provided 56 % conversion of FAMEs with TON=1.1⋅106; 0.25 ppm of this complex in ethenolysis of high-purity MO demonstrated the TON ~2⋅106, leading among the catalysts under study. In ethenolysis of FAMEs chromane derivative showed TON of 4-6⋅105 and unprecedented temperature-independent 99.7-99.9 % selectivity at 15-60 °C.

Synthesis of Linear and Cyclic Poly(allenamer)s by Powerful Cyclic-Alkyl-Amino-Carbene (CAAC) Ruthenium Catalysts and Facile Post-modification

Cyclic polymers are very attractive due to their unique properties; however, so far, they have simple and less reactive backbone structures due to synthetic limitations, restricting their further post-modification. Notably, allenes present a potentially useful platform in polymer chemistry due to their well-established toolbox in organic chemistry. Nevertheless, the biggest challenge remains in synthesizing poly(allenamer)s with high allene contents or polymerization efficiency, as well as synthesizing different types of cyclic poly(allenamer)s. Herein, we synthesized linear and cyclic poly(allenamer)s via ring-opening metathesis polymerization (ROMP) and ring-expansion metathesis polymerization (REMP), employing highly efficient cyclic-alkyl-amino-carbene (CAAC) ruthenium catalysts. Mechanistic studies suggested CAAC ligands enhanced stability of propagating Ru vinylidene, enabling various linear and cyclic poly(allenamer)s with turnover number up to 1360 and molecular weight reaching 549 kDa. Their cyclic architecture was thoroughly characterized by multiangle light scattering size-exclusion chromatography (MALS SEC) with viscometer. Moreover, controlled ROMP of a highly reactive α-substituted cyclic allene was achieved using third-generation Grubbs' catalyst. Finally, we demonstrated highly efficient and selective post-modifications on poly(allenamer)s with primary and secondary alcohols. This broadens the scope of cyclic polymers with improved efficiency and structural control, affording a practical platform for diverse macromolecules.

Eliminating Bimolecular Decomposition to Address Sustainability in Cross-Coupling: Supported Pd-PEPPSI-IPentCl

Fine-chemical manufacturing, with its dismal E-factors, has been known for decades as being one of the worst contributors to the well-being of the environment. Further, mining practices that pursue precious metals used in catalysis lead to considerable destruction of the environment. Further contributing to this is the necessity for high catalyst loads due to the limited mortality of organometallic complexes in solution. Bimolecular decomposition (BD), in particular, is a significant contributor to this problem. Assisting in the sustainability of chemical synthesis is flow chemistry, whose "just-in-time" nature produces chemicals as needed, eliminating vast stockpiles of chemicals associated with batch manufacturing. In this work, Pd-PEPPSI-IPentCl, a high-reactivity, high-selectivity Pd catalyst, has been mounted onto the surface of silica, of which the spacing has eliminated BD. This material has been loaded into packed beds and used in Negishi coupling and Buchwald-Hartwig amination, where the active catalyst has shown tremendous resiliency while producing valuable small-molecule products with deft selectivity and speed with residence time in the order of minutes under mild conditions (e.g., Negishi couplings conducted at room temperature).

An Amplificative Detection Approach for Autocatalytic Sensing of Ethylene

Amplified sensing offers the potential for high sensitivity; however, the vast majority of molecular strategies involve stoichiometric detection and signal transduction, including numerous recent examples of systems inspired by transition-metal-catalyzed reactions. Activation of latent precatalysts by a target analyte represents an attractive strategy for detecting low-concentration species. Analyte amplification represents another attractive approach, akin to PCR-based assays. Here we report an autocatalytic detection system based on the ethylene activation of Ru-I2 olefin metathesis precatalysts. Signal transduction is amplified by both catalytic ring closing metathesis of profluorescent substrates and ethylene propagation to activate additional units of catalyst. High sensitivity is observed as a result of this dual-mode amplified detection of ethylene. Detection of endogenous ethylene from fruit and oxidation-decomposition of polyunsaturated fatty acids via lipid peroxides is demonstrated.

Olefin Metathesis in Water: Speciation of a Leading Water-Soluble Catalyst Pinpoints Challenges and Opportunities for Chemical Biology

The metathetical modification of biomolecules in aqueous environments holds great promise for advances at the interface of chemistry, biology, and medicine. However, rapid degradation of the metathesis catalysts necessitates their use in large stoichiometric excess, resulting in undesired side-reactions promoted by the ruthenium products. Although water is now known to play a central role in catalyst decomposition, the elusive nature of the intermediates has hampered insight into the pathways involved. We describe the detailed speciation in water of AquaMet (AM), the dominant ruthenium catalyst used for aqueous metathesis, and implications for catalysis. Potentiometric and spectroscopic speciation studies reveal that only trace AM is present under the pH-neutral, salt-free conditions routinely employed in synthetic applications of aqueous metathesis. Instead, metathesis-inactive hydroxide species dominate. Even at pH 3, Ru-H2O complexes dominate in 0.01 M NaCl(aq), and the water ligands are readily deprotonated as the pH is increased. Raising NaCl(aq) concentrations to 1 M suppresses deprotonation events below pH 8, stabilizing AM as the dominant solution species at neutral pH, and significantly expanding the metathesis-compatible regime. Hitherto unrecognized catalyst solubility issues are also revealed, pointing toward avenues for advance. More broadly, the capacity to directly link catalyst environment to structure and performance opens new opportunities for olefin metathesis in complex, water-rich settings.

Air and Thermally Stable Cyclic (Alkyl)(amino)carbene Ruthenium Complexes for Efficient Ring Expansion Metathesis Polymerization

Ring expansion metathesis polymerization (REMP) has emerged as a potent strategy for obtaining cyclic polymers over the past two decades. The scope of monomers, however, remains limited due to the poor functional group tolerance and stability of the catalyst, necessitating a rational catalyst design to address this constraint. Here, we present ruthenium complexes featuring tethered cyclic (alkyl)(amino)carbene ligands for REMP, aiming to deepen our understanding of the structure-property relationship in newly designed catalysts. Notably, these ruthenium catalysts exhibit remarkable thermal stability even in air, as confirmed through monitoring the air-exposed solution at 80 °C. In addition, the initiation rate of the catalysts was effectively modulated by tuning the steric hindrance of the N-aryl substituent, adjusting tethered chain lengths, or introducing a Blechert-type ligand. Based on systematic studies of catalysts, we successfully demonstrate challenging REMP of a cyclic allene (2,8-dimethylnona-4,5-diene) for the first time, as well as methyl-5-norbornene-2-carboxylate, resulting in cyclic polymers. We also note that the exceptional stability of the catalyst enables REMP under air. This study provides valuable insights into the rational design of catalysts and introduces a novel, user-friendly platform for the synthesis of cyclic polymers.

Synthesis and Application of Robust Spiro [Fluorene-9] CAAC Ruthenium Alkylidene Complexes for the "One-Pot" Conversion of Allyl Acetate to Butane-1,4-diol

A series of a novel CAAC ligands featuring a spiro-fluorene group have been synthesized and complexed with ruthenium alkylidenes, yielding the corresponding Hoveyda-type derivatives as a new family of olefin metathesis catalysts. The novel complexes have been characterized by XRD, HRMS and NMR measurements. The synthetised complexes were tested in catalysis and showed good activity in olefin metathesis, as demonstrated on diethyl diallylmalonate and allyl acetate substrates. The unique backbone in the ligand with the large, yet inflexible condensed system renders interesting properties to the catalyst, exemplified by the good catalytic performance and improved Z-selectivity. In addition, the complex can also serve as a hydrogenation catalyst in a consecutive (one-pot) reaction. The latter reaction can convert allyl acetate to butane-1,4-diol, a valuable chemical intermediate for biodegradable polybutylene succinate (PBS).

Grubbs-Hoveyda catalysts conjugated to a β-barrel protein: Effect of halide substitution on aqueous olefin metathesis activity

The effect of halide substitution in Grubbs-Hoveyda II catalysts (GHII catalysts) embedded in the engineered β-barrel protein nitrobindin (NB4exp) on metathesis activity in aqueous media was studied. Maleimide tagged dibromido and diiodido derivates of the GHII catalyst were synthesized and covalently conjugated to NB4exp. The biohybrid catalysts were characterized spectroscopically confirming the structural integrity. When the two chloride substituents at ruthenium center were exchanged against bromide and iodide, the diiodo derivative was found to show significantly higher catalytic activity in ring-closing metathesis of α,ω-diolefins, whereas the dibromido derivative was less efficient when compared with the parent dichlorido catalyst. Using the diiodido catalyst, high turnover numbers of up to 75 were observed for ring-closing metathesis (RCM) yielding unsaturated six- and seven-membered N-heterocycles.

"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.

Cyclic (amino)(barrelene)carbene Ru-complexes: synthesis and reactivity in olefin metathesis

The synthesis of ruthenium-complexes with cyclic (amino)(barrelene)carbenes (namely CABCs) as ligands is reported. Isolated in moderate to good yields, these new complexes showed impressive thermal stability at 110 °C over several days. Good catalytic performances were demonstrated in various ring-closing metathesis (RCM), macrocyclic-RCM, ring-closing enyne metathesis (RCEYM), cross-metathesis (CM), and ring-opening cross metathesis (ROCM) reactions.

Inhibition of the Decomposition Pathways of Ruthenium Olefin Metathesis Catalysts: Development of Highly Efficient Catalysts for Ethenolysis

Ruthenium-based Hoveyda-type olefin metathesis catalysts bearing novel rigid spirocyclic alkyl amino carbenes (CAACs) have been developed. They are characterized by exceptional stability toward decomposition through β-elimination and bimolecular pathways, thus enabling unprecedented efficiency in the cross-metathesis of seed oil-derived fatty acid esters with ethylene (ethenolysis). Catalyst loading as low as 100 ppb was applied to the ethenolysis of the model substrate methyl oleate, leading to a remarkable turnover number (TON) of 2.6 million, significantly higher than previously reported (TON 340 000 at 1 ppm and 744 000 at 0.5 ppm catalyst loading). Ethenolysis of methyl esters derived from high oleic sunflower oil and rapeseed oil, readily available on an industrial scale, inexpensive, and renewable feedstocks, was for the first time effectively carried out with 0.5 ppm catalyst loading with TON as high as 964 000.

Mesomeric Acceleration Counters Slow Initiation of Ruthenium-CAAC Catalysts for Olefin Metathesis (CAAC = Cyclic (Alkyl)(Amino) Carbene)

Ruthenium catalysts bearing cyclic (alkyl)(amino)carbene (CAAC) ligands can attain very high productivities in olefin metathesis, owing to their resistance to unimolecular decomposition. Because the propagating methylidene species RuCl2(CAAC)(=CH2) is extremely susceptible to bimolecular decomposition, however, turnover numbers in the metathesis of terminal olefins are highly sensitive to catalyst concentration, and hence loadings. Understanding how, why, and how rapidly the CAAC complexes partition between the precatalyst and the active species is thus critical. Examined in a dual experimental-computational study are the rates and basis of initiation for phosphine-free catalysts containing the leading CAAC ligand C1 Ph , in which a CMePh group α to the carbene carbon helps retard degradation. The Hoveyda-class complex HC1 Ph (RuCl2(L)(=CHAr), where L = C1 Ph , Ar = C6H3-2-O i Pr-5-R; R = H) is compared with its nitro-Grela analogue (nG-C1 Ph ; R = NO2) and the classic Hoveyda catalyst HII (L = H2IMes; R = H). t-Butyl vinyl ether (tBuVE) was employed as substrate, to probe the reactivity of these catalysts toward olefins of realistic bulk. Initiation is ca. 100× slower for HC1 Ph than HII in C6D6, or 44× slower in CDCl3. The rate-limiting step for the CAAC catalyst is cycloaddition; for HII, it is tBuVE binding. Initiation is 10-13× faster for nG-C1 Ph than HC1 Ph in either solvent. DFT analysis reveals that this rate acceleration originates in an overlooked role of the nitro group. Rather than weakening the Ru-ether bond, as widely presumed, the NO2 group accelerates the ensuing, rate-limiting cycloaddition step. Faster reaction is caused by long-range mesomeric effects that modulate key bond orders and Ru-ligand distances, and thereby reduce the trans effect between the carbene and the trans-bound alkene in the transition state for cycloaddition. Mesomeric acceleration may plausibly be introduced via any of the ligands present, and hence offers a powerful, tunable control element for catalyst design.

Fluorescent organometallic dyads and triads: establishing spatial relationships

FRET pairs involving up to three different Bodipy dyes are utilized to provide information on the assembly/disassembly of organometallic complexes. Azolium salts tagged with chemically robust and photostable blue or green or red fluorescent Bodipy, respectively, were synthesized and the azolium salts used to prepare metal complexes [(NHC_blue)ML], [(NHC_green)ML] and [(NHC_red)ML] (ML = Pd(allyl)Cl, IrCl(cod), RhCl(cod), AuCl, Au(NTf₂), CuBr). The blue and the green Bodipy and the green and the red Bodipy, respectively, were designed to allow the formation of efficient FRET pairs with minimal cross-talk. Organometallic dyads formed from two subunits enable the transfer of excitation energy from the donor dye to the acceptor dye. The blue, green and red emission provide three information channels on the formation of complexes, which is demonstrated for alkyne or sulfur bridged digold species and for ion pairing of a red fluorescent cation and a green fluorescent anion. This approach is extended to probe an assembly of three different subunits. In such a triad, each component is tagged with either a blue, a green or a red Bodipy and the energy transfer blue →green → red proves the formation of the triad. The tagging of molecular components with robust fluorophores can be a general strategy in (organometallic) chemistry to establish connectivities for binuclear catalyst resting states and binuclear catalyst decomposition products in homogeneous catalysis.

Water-Accelerated Decomposition of Olefin Metathesis Catalysts

Water is ubiquitous in olefin metathesis, at levels ranging from contaminant to cosolvent. It is also non-benign. Water-promoted catalyst decomposition competes with metathesis, even for "robust" ruthenium catalysts. Metathesis is hence typically noncatalytic for demanding reactions in water-rich environments (e.g., chemical biology), a challenge as the Ru decomposition products promote unwanted reactions such as DNA degradation. To date, only the first step of the decomposition cascade is understood: catalyst aquation. Here we demonstrate that the aqua species dramatically accelerate both β-elimination of the metallacyclobutane intermediate and bimolecular decomposition of four-coordinate [RuCl(H₂O)_n(L)(=CHR)]Cl. Decomposition can be inhibited by blocking aquation and β-elimination.

Preparation of Dienyl Boronates by Tandem Ene-Yne Metathesis/Dienyl Isomerization: Ready Access to Diene Building Blocks for the Synthesis of Polyenes

The ene-yne metathesis of alkenyl boronates with terminal alkynes is reported. These challenging metatheses were accomplished using a Grubbs catalyst bearing the cyclic alkyl amino carbene (CAAC) ligand, whereas N-heterocyclic carbene (NHC) derived catalysts gave lower yields. Subsequent dienyl isomerization via a cobalt-catalyzed hydrogen atom transfer (HAT) furnished the more substituted dienyl boronate with high EE/EZ ratios. Finally, the resulting dienyl boronate products were successfully used in Suzuki-Miyaura cross-coupling reactions and in a Diels-Alder cycloaddition.

Theoretical Perspectives in Organocatalysis

It is clear that the field of organocatalysis is continuously expanding during the last decades. With increasing computational capacity and new techniques, computational methods have provided a more economic approach to explore different chemical systems. This review offers a broad yet concise overview of current state-of-the-art studies that have employed novel strategies for catalyst design. The evolution of the all different theoretical approaches most commonly used within organocatalysis is discussed, from the traditional approach, manual-driven, to the most recent one, machine-driven.

Mechanism of Coupling of Methylidene to Ethylene Ligands in Dimetallic Assemblies; Computational Investigation of a Model for a Key Step in Catalytic C1 Chemistry

Methylidene complexes often couple to ethylene complexes, but the mechanistic insight is scant. The path by which two cations [(η⁵-C₅H₅)Re(NO)(PPh₃)(═CH₂)]⁺ (5⁺) transform (CH₂Cl₂/acetonitrile) to [(η⁵-C₅H₅)Re(NO)(PPh₃)(H₂C═CH₂)]⁺ (6⁺) and [(η⁵-C₅H₅)Re(NO)(PPh₃)(NCCH₃)]⁺ is studied by density functional theory. Experiments provide a number of constraints such as the second-order rate in 5⁺; no prior ligand dissociation/exchange; a faster reaction of (S)-5⁺ with (S)-5⁺ than with (R)-5⁺ ("enantiomer self-recognition"). Although dirhenium dications with Re(μ-CH₂)₂Re cores represent energy minima, they are not accessible by 2 + 2 cycloadditions of 5⁺. Transition states leading to ReCH₂CH₂Re linkages are prohibitively high in energy. However, 5⁺ can give non-covalent S_Re/S_Re or S_Re/R_Re dimers with π interactions between the PPh₃ ligands but long ReCH₂···H₂CRe and H₂CRe···H₂CRe distances (3.073-3.095 Å and 3.878-4.529 Å, respectively). In rate-determining steps, these afford [(η⁵-C₅H₅)Re(NO)(PPh₃)(μ-η²:η²-H₂C···CH₂)(Ph₃P)(ON)Re(η⁵-C₅H₅)]²⁺ (13²⁺), in which one rhenium binds the bridging ethylene more tightly than the other (2.115-2.098 vs 2.431-2.486 Å to the centroid). In the S_Re/R_Re adduct, Dewar-Chatt-Duncanson optimization leads to unfavorable PPh₃/PPh₃ contacts. Ligand interactions are further dissected in the preceding transition states via component analyses, and ΔΔG^‡ (1.2 kcal/mol, CH₂Cl₂) favors the S_Re/S_Re pathway, in accordance with the experiment. Acetonitrile then displaces 6⁺ from the more weakly bound rhenium of 13²⁺. The formation of similar μ-H₂C···CH₂ intermediates is found to be rate-determining for varied coordinatively saturated M═CH₂ species [M = Fe(d⁶)/Re(d⁴)/Ta(d²)], establishing generality and enhancing relevancy to catalytic CH₄ and CO/H₂ chemistry.

Synthesis and catalytic olefin metathesis activity of amberlyst-15 supported cyclic and bicyclic alkyl amino carbene ruthenium complexes

Amberlyst-15 supported cyclic alkyl amino carbene and bicyclic alkyl amino carbene ruthenium olefin metathesis catalysts for sustainable catalytic applications have been synthesized by the well-known wet impregnation method utilizing ionic complex/support interaction. Surface coverages are as high as 4 and 7 wt% were achieved in the case of the significantly higher pore volume Amberlyst-15, compared to Amberlyst-36. These phase separable catalysts show high activity in cross metathesis, ring closing metathesis and ethenolysis reactions compared to the reported heterogenized olefin metathesis catalysts. Leeching tests revealed no more than 1.5 ppm ruthenium content for the investigated metathesis reactions, which is well below the accepted 10 ppm limit in case of consumer products.

Catalytic Decomposition of Long-Chain Olefins to Propylene via Isomerization-Metathesis Using Latent Bicyclic (Alkyl)(Amino)Carbene-Ruthenium Olefin Metathesis Catalysts

One of the most exciting scientific challenges today is the catalytic degradation of non-biodegradable polymers into value-added chemical feedstocks. The mild pyrolysis of polyolefins, including high-density polyethylene (HDPE), results in pyrolysis oils containing long-chain olefins as major products. In this paper, novel bicyclic (alkyl)(amino)carbene ruthenium (BICAAC-Ru) temperature-activated latent olefin metathesis catalysts, which can be used for catalytic decomposition of long-chain olefins to propylene are reported. These thermally stable catalysts show significantly higher selectivity to propylene at a reaction temperature of 75 °C compared to second generation Hoveyda-Grubbs or CAAC-Ru catalysts under ethenolysis conditions. The conversion of long-chain olefins (e.g., 1-octadecene or methyl oleate) to propylene via isomerization-metathesis is performed by using a (RuHCl)(CO)(PPh₃ )₃ isomerization co-catalyst. The reactions can be carried out at a BICAAC-Ru catalyst loading as low as 1 ppm at elevated reaction temperature (75 °C). The observed turnover number and turnover frequency are as high as 55 000 and 10 000 mol_propylene  mol_catalyst ^-1  h^-1 , respectively.

The coordination chemistry of oxide and nanocarbon materials

Understanding how a ligand affects the steric and electronic properties of a metal is the cornerstone of the inorganic chemistry enterprise. What happens when the ligand is an extended surface? This question is central to the design and implementation of state-of-the-art functional materials containing transition metals. This perspective will describe how these two very different sets of extended surfaces can form well-defined coordination complexes with metals. In the Green formalism, functionalities on oxide surfaces react with inorganics to form species that contain X-type or LX-type interactions between the metal and the oxide. Carbon surfaces are neutral L-type ligands; this perspective focuses on carbons that donate six electrons to a metal. The nature of this interaction depends on the curvature, and thereby orbital overlap, between the metal and the extended π-system from the nanocarbon.

The Janus face of high trans-effect carbenes in olefin metathesis: gateway to both productivity and decomposition

Ruthenium–cyclic(alkyl)(amino)carbene (CAAC) catalysts, used at ppm levels, can enable dramatically higher productivities in olefin metathesis than their N-heterocyclic carbene (NHC) predecessors. A key reason is the reduced susceptibility of the metallacyclobutane (MCB) intermediate to decomposition via β-H elimination. The factors responsible for promoting or inhibiting β-H elimination are explored via density functional theory (DFT) calculations, in metathesis of ethylene or styrene (a representative 1-olefin) by Ru–CAAC and Ru–NHC catalysts. Natural bond orbital analysis of the frontier orbitals confirms the greater strength of the orbital interactions for the CAAC species, and the consequent increase in the carbene trans influence and trans effect. The higher trans effect of the CAAC ligands inhibits β-H elimination by destabilizing the transition state (TS) for decomposition, in which an agostic MCB C_β–H bond is positioned trans to the carbene. Unproductive cycling with ethylene is also curbed, because ethylene is trans to the carbene ligand in the square pyramidal TS for ethylene metathesis. In contrast, metathesis of styrene proceeds via a ‘late’ TS with approximately trigonal bipyramidal geometry, in which carbene trans effects are reduced. Importantly, however, the positive impact of a strong trans-effect ligand in limiting β-H elimination is offset by its potent accelerating effect on bimolecular coupling, a major competing means of catalyst decomposition. These two decomposition pathways, known for decades to limit productivity in olefin metathesis, are revealed as distinct, antinomic, responses to a single underlying phenomenon. Reconciling these opposing effects emerges as a clear priority for design of robust, high-performing catalysts.

In ruthenium catalysts for olefin metathesis, carbene ligands of high trans influence/effect suppress decomposition via β-H elimination, but increase susceptibility to bimolecular decomposition.

Routes to High-Performing Ruthenium-Iodide Catalysts for Olefin Metathesis: Ligand Lability Is Key to Efficient Halide Exchange

Clean, high-yielding routes are described to ruthenium-diiodide catalysts that were recently shown to enable high productivity in olefin metathesis. For the second-generation Grubbs and Hoveyda catalysts (GII: RuCl2(H2IMes)(PCy3)(=CHPh); HII: RuCl2(H2IMes)(=CHAr), Ar = C6H4-2-O i Pr), slow salt metathesis is shown to arise from the low lability of the ancillary PCy3 or ether ligands, which retards access to the four-coordinate intermediate required for efficient halide exchange. To exploit the lability of the first-generation catalysts, the diiodide complex RuI2(PCy3)(=CHAr) HI-I 2 was prepared by treating "Grubbs I" (RuCl2(PCy3)2(=CHPh), GI) with NaI, H2C=CHAr (1a), and a phosphine-scavenging Merrifield iodide (MF-I) resin. Subsequent installation of H2IMes or cyclic (alkyl)(amino)carbene (CAAC) ligands afforded the second-generation iodide catalysts in good to excellent yields. Given the incompatibility of the nitro group with a free carbene, the iodo-Grela catalyst RuI2(H2IMes)(=CHAr') (nG-I 2 : Ar' = C6H3-2-O i Pr-4-NO2) was instead accessed by sequential salt metathesis of GI with NaI, installation of H2IMes, and finally cross-metathesis with the nitrostyrenyl ether H2C=CHAr' (1b), with MF-I as the phosphine scavenger. The bulky iodide ligands improve the selectivity for macrocyclization in ring-closing metathesis.