Schrock Catalyst Research Articles

Ring-Opening Metathesis Polymerization of Thianorbornenes.

A series of exo-7-thiabicyclo[2.2.1]hept-5-ene-2,3-dicarboximides were synthesized and polymerized using Schrock's catalyst, 2,6-diisopropylphenylimidoneophylidene molybdenum(VI) bis(hexafluoro-tert-butoxide). The ring-opening metathesis polymerization (ROMP) reactions were found to proceed in a controlled manner, enabling chain extensions and tuning of polymer molecular weight. The polymers were characterized using size exclusion chromatography (SEC) as well as spectroscopic (NMR, FT-IR), thermal (TGA, DSC), and optical techniques. The physical, chemical, and optical properties of the polymers were found to be affected by the embedded sulfur atoms and the pendant substituents. Copolymers with norbornene were also synthesized and characterized. Treatment of a poly(thianorbornene) with potassium hydroxide led to ring-opening hydrolysis and afforded a derivative that was soluble in aqueous media.

Genetic algorithm-based re-optimization of the Schrock catalyst for dinitrogen fixation

This study leverages a graph-based genetic algorithm (GB-GA) for the design of efficient nitrogen-fixing catalysts as alternatives to the Schrock catalyst, with the aim to improve the energetics of key reaction steps. Despite the abundance of nitrogen in the atmosphere, it remains largely inaccessible due to its inert nature. The Schrock catalyst, a molybdenum-based complex, offered a breakthrough but its practical application is limited due to low turnover numbers and energetic bottlenecks. The genetic algorithm in our study explores the chemical space for viable modifications of the Schrock catalyst, evaluating each modified catalyst’s fitness based on reaction energies of key catalytic steps and synthetic accessibility. Through a series of selection and optimization processes, we obtained fully converged catalytic cycles for 20 molecules at the B3LYP level of theory. From these results, we identified three promising molecules, each demonstrating unique advantages in different aspects of the catalytic cycle. This study offers valuable insights into the potential of generative models for catalyst design. Our results can help guide future work on catalyst discovery for the challenging nitrogen fixation process.

Air-stable 18-electron adducts of Schrock catalysts with tuned stability constants for spontaneous release of the active species

Schrock alkylidenes are highly versatile, very active olefin metathesis catalysts, but their pronounced sensitivity to air still hinders their applications. Converting them into more robust but inactive 18-electron adducts was suggested previously to facilitate their handling. Generating the active species from the inactive adducts, however, required a high-temperature Lewis acid treatment and resulted in an insoluble by-product, limiting the practicality of the methodology. Herein, we introduce an approach to circumvent the inconvenient, costly, and environmentally taxing activation process. We show that 18-electron adducts of W- and Mo-based Schrock catalysts with finite stability constants (typically K = 200–15,000 M−1) can readily be prepared and isolated in excellent yields. The adducts display enhanced air-stability in the solid state, and in solution they dissociate spontaneously, hence liberating the active alkylidenes without chemical assistance.

183 W NMR Spectroscopy Guides the Search for Tungsten Alkylidyne Catalysts for Alkyne Metathesis.

Triarylsilanolates are privileged ancillary ligands for molybdenum alkylidyne catalysts for alkyne metathesis but lead to disappointing results and poor stability in the tungsten series. 1H,183W heteronuclear multiple bond correlation spectroscopy, exploiting a favorable 5 J‐coupling between the 183W center and the peripheral protons on the alkylidyne cap, revealed that these ligands upregulate the Lewis acidity to an extent that the tungstenacyclobutadiene formed in the initial [2+2] cycloaddition step is over‐stabilized and the catalytic turnover brought to a halt. Guided by the 183W NMR shifts as a proxy for the Lewis acidity of the central atom and by an accompanying chemical shift tensor analysis of the alkylidyne unit, the ligand design was revisited and a more strongly π‐donating all‐alkoxide ligand prepared. The new expanded chelate complex has a tempered Lewis acidity and outperforms the classical Schrock catalyst, carrying monodentate tert‐butoxy ligands, in terms of rate and functional‐group compatibility.

183W NMR Spectroscopy Guides the Search for Tungsten Alkylidyne Catalysts for Alkyne Metathesis

AbstractTriarylsilanolates are privileged ancillary ligands for molybdenum alkylidyne catalysts for alkyne metathesis but lead to disappointing results and poor stability in the tungsten series. 1H,183W heteronuclear multiple bond correlation spectroscopy, exploiting a favorable 5J‐coupling between the 183W center and the peripheral protons on the alkylidyne cap, revealed that these ligands upregulate the Lewis acidity to an extent that the tungstenacyclobutadiene formed in the initial [2+2] cycloaddition step is over‐stabilized and the catalytic turnover brought to a halt. Guided by the 183W NMR shifts as a proxy for the Lewis acidity of the central atom and by an accompanying chemical shift tensor analysis of the alkylidyne unit, the ligand design was revisited and a more strongly π‐donating all‐alkoxide ligand prepared. The new expanded chelate complex has a tempered Lewis acidity and outperforms the classical Schrock catalyst, carrying monodentate tert‐butoxy ligands, in terms of rate and functional‐group compatibility.

Concise Seven-Membered Oxepene/Oxepane Synthesis – Structural Motifs in Natural and Synthetic Products

This work outlines a suitable method for the synthesis of oxepane skeleton using iterative C-glycoside technology on the oxepene intermediate, which was synthesized utilizing Wilkinson’s catalyst [Rh(PPh3)3Cl] to generate the isomerized product in a linear synthetic manner. The central core of the oxepene motif was achieved via an olefin metathesis reaction using the Grubbs second-generation and Schrock catalysts. The synthesis of the functionalized oxepane having the desired adriatoxin E-ring relative stereochemistry was achieved starting from commercially available homopropargylic alcohol.

Grubbs' and Schrock's Catalysts, Ring Opening Metathesis Polymerization and Molecular Brushes-Synthesis, Characterization, Properties and Applications.

In this review, molecular brushes and other macromolecular architectures bearing a bottlebrush segment where the main chain is synthesized by ring opening metathesis polymerization (ROMP) mediated by Mo or Ru metal complexes are considered. A brief review of metathesis and ROMP is presented in order to understand the problems and the solutions provided through the years. The synthetic strategies towards bottlebrush copolymers are demonstrated and each one discussed separately. The initiators/catalysts for the synthesis of the backbone with ROMP are discussed. Syntheses of molecular brushes are presented. The most interesting properties of the bottlebrushes are detailed. Finally, the applications studied by different groups are presented.

Mono- and Bisionic Mo- and W-Based Schrock Catalysts for Biphasic Olefin Metathesis Reactions in Ionic Liquids.

An extensive series of the first ionic Mo- and W-based Schrock-type catalysts based on pyridinium and phosphonium tagged aryloxide ligands were prepared. Bisionic complexes of the general formula Mo(Imido)(CHR)(OR')2 (OTf)2 and monoionic monoaryloxide pyrrolide (pyr) (MAP-type) catalysts [M(Imido)(CHR)(OR')(pyr)+ ][A- ] were successfully employed and tested in various olefin metathesis benchmark reactions under both, homogeneous and biphasic conditions using pyrrole and, for the first time with Schrock-type catalysts, ionic liquids as the polar phase. Productivities under biphasic conditions up to several thousand turn overs were achieved and are comparable to those obtained in reactions carried out in chlorobenzene or toluene. Metal contamination in the nonpolar product-containing heptane phase was <2 ppm.

Mechanistic Consequences of Chelate Ligand Stabilization on Nitrogen Fixation by Yandulov–Schrock-Type Complexes

The Yandulov–Schrock catalyst, a mononuclear molybdenum complex with a tetra-coordinate triamidoamine chelate ligand with hexa-iso-propyl-terphenyl groups at the amide nitrogen atoms, catalyzes the reduction of dinitrogen to ammonia. Its turnover number is very low, which may be attributed to the (partial) loss of the chelate ligand. Protonation of an amide nitrogen atom of the ligand and subsequent reduction leads to the formation of a labile amine ligand. We find that this equatorial amine group can detach from the molybdenum center of the Yandulov–Schrock complex with a comparatively small barrier. This decomposition reaction is in direct competition with reactions producing intermediates of the Chatt–Schrock cycle. Clamping the substituents on the amide nitrogen atoms by a calix[6]arene unit (replacing the hexa-iso-propyl-terphenyl groups) successfully suppresses the detachment of a generated equatorial amine group from the molybdenum center. We discuss dinitrogen reduction according to the Chatt–Schr...

Highly β-Selective Cyclopolymerization of 1,6-Heptadiynes and Ring-Closing Enyne Metathesis Reaction Using Grubbs Z-Selective Catalyst: Unprecedented Regioselectivity for Ru-Based Catalysts.

It is well-known that Ru-based Grubbs catalysts undergo a highly selective α-addition to alkynes to promote exo-cyclization during ring-closing enyne metathesis (RCEYM) or to produce conjugated polyenes containing five-membered rings during the cyclopolymerization (CP) of 1,6-heptadiynes. There are a few reports of β-selective addition to alkynes using Schrock catalysts based on Mo but none for readily accessible and easy-to-use Ru-based catalysts. We report the first example of β-selective addition to alkynes using Grubbs Z-selective catalyst, which produces only endo products during the RCEYM reaction of terminal enynes and promotes the CP of 1,6-heptadiyne derivatives to give conjugated polyenes containing a six-membered ring as a major repeat unit. This unique preference for β-selectivity originated from the side-bound approach of alkynes to the catalyst, where the steric hindrance between the chelating N-heterocyclic carbene ligand of the catalyst and the alkynes disfavored α-addition. To enhance the β-selectivity for CP further, one could increase the size of the substrates on the monomers and lower the reaction temperature to obtain conjugated polyenes containing up to 95% six-membered rings. Moreover, the physical properties of the resulting polymer were analyzed in detail and compared with those of the conjugated polyenes containing only five-membered rings prepared from the same monomer but with a conventional Grubbs catalyst.

N-heterocyclic carbene, high oxidation state molybdenum alkylidene complexes: functional-group-tolerant cationic metathesis catalysts.

We synthesized the first N-heterocyclic carbene (NHC) complexes of Schrock's molybdenum imido alkylidene bis(triflate) complexes. Unlike existing bis(triflate) complexes, the novel 16-electron complexes represent metathesis active, functional-group-tolerant catalysts. Single-crystal X-ray structures of two representatives of this novel class of Schrock catalysts are presented and reactivity is discussed in view of their structural peculiarities. In the presence of monomer (substrate), these catalysts form cationic species and can be employed in ring-closing metathesis (RCM), ring-opening metathesis polymerization (ROMP), as well as in the cyclopolymerization of α,ω-diynes. Monomers containing functional groups, which are not tolerated by the existing variations of Schrock's catalyst, e.g., sec-amine, hydroxy, and carboxylic acid moieties, can be used. These catalysts therefore hold great promise in both organic and polymer chemistry, where they allow for the use of protic monomers.

Role of Electronegative Substituents on the Bond Energies in the Grubbs Metathesis Catalysts for M = Fe, Ru, Os

Coupled cluster theory [CCSD(T)] with the aug-cc-pVDZ/aug-cc-pVDZ-PP basis sets is used to predict the thermodynamic properties of models of the Grubbs catalyst, H2ImM(PH3)(CRR′) and H2ImM(C2H4)(CRR′) for M = Fe, Ru, and Os and CRR′ = CH2, CHF, and CF2. The PH3 and C2H4, imidazolinium carbene (H2Im), and CRR′ bond dissociation energies (BDEs) are reported. Because of low metal-carbene BDEs, the M = Fe complexes are unlikely to form, so they will not be good catalysts for olefin metathesis. The metal–carbene BDE is an important component in metathesis catalyst design and correlates with the singlet–triplet splitting in the carbene. The two metallacyclobutane intermediates (cis and trans to the imidazolinium carbene) formed by reaction of the CRR′═CRR′ with the 14-electron active species (H2ImM(CRR′)) (R and R′ either H or F) are investigated at the same level of theory. The metallacycles cis to the nitrogen heterocyclic carbene are lower in energy than the trans conformer with the exception of four M = Fe metallacycles in the gas phase and in CH2Cl2 solution at 298 K. The olefin π complex for the simplest CH2 ligand plus C2H4 reactant combination is more stable in the gas phase, but in CH2Cl2 solution at 298 K, the cis metallacycles are more stable than the π complexes or the trans metallacycles on the free energy scale as a result of the large dipole moments in the cis metallacycles. The results show that the best energy balance is achieved with M = Ru, a CH2 carbene substituent, and a C2H4 reactant. The energetics for the Grubbs catalysts are shown to differ from those of the Schrock catalysts.

A Brief Examination of the Latest ADMET Chemistry

Acyclic diene metathesis polymerization (ADMET) has taken a long path from its beginnings with Schrock catalyst and unfunctionalized dienes to use of Grubbs catalysts with nearly unlimited choices for pendant groups and chain composition. This review covers synthesis of model linear polyethylene via ADMET polymerization, synthesis of polymers from green feed stocks, and the effect of spacer length on morphology and polymer properties. By taking advantage of ADMET’s functional group tolerance, it can be used in conjunction with a variety of other organic transformations and in-chain functional groups to yield unique architectures and properties. These topics will demonstrate the utility of ADMET in the future of polymer science.

Functional Monolithic Materials for Boronate‐Affinity Chromatography via Schrock Catalyst‐Triggered Ring‐Opening Metathesis Polymerization

Monolithic polymeric materials are prepared via ring-opening metathesis copolymerization of norborn-2-ene with 1,4,4a,5,8,8a-hexahydro-1,4,5,8-exo,endo-dimethanonaphthalene in the presence of macro- and microporogens, that is, of n-hexane and 1,2-dichloroethane, using the Schrock catalyst Mo(N-2,6-(2-Pr)(2) -C(6) H(3) )(CHCMe(2) Ph)(OCMe(3) )(2) . Functionalization of the monolithic materials is accomplished by either terminating the living metal alkylidenes with various functional aldehydes or by post-synthesis grafting with norborn-5-en-2-ylmethyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzoate. Finally, boronate-grafted monolithic columns (100 × 3 mm i.d.) are successfully applied to the affinity chromatographic separation of cis-diol-based biomolecules.

ChemInform Abstract: Practical New Silyloxy‐Based Alkyne Metathesis Catalysts with Optimized Activity and Selectivity Profiles.

AbstractModified Schrock catalysts (I)(fully air‐stable and user‐friendly), (II)(air‐sensitive but highly active and selective) and (III)(similar potent as the etherate; can be handled in air but needs to be stored under argon) are prepared and tested under optimized conditions A)‐C).

Ring opening metathesis polymerization-derived block copolymers bearing chelating ligands: synthesis, metal immobilization and use in hydroformylation under micellar conditions.

Norborn-5-ene-(N,N-dipyrid-2-yl)carbamide (M1) was copolymerized with exo,exo-[2-(3-ethoxycarbonyl-7-oxabicyclo[2.2.1]hept-5-en-2-carbonyloxy)ethyl]trimethylammonium iodide (M2) using the Schrock catalyst Mo(N-2,6-Me2-C6H3)(CHCMe2Ph)(OCMe(CF3)2)2 [Mo] to yield poly(M1-b-M2). In water, poly(M1-b-M2) forms micelles with a critical micelle-forming concentration (cmc) of 2.8 × 10−6 mol L−1; Reaction of poly(M1-b-M2) with [Rh(COD)Cl]2 (COD = cycloocta-1,5-diene) yields the Rh(I)-loaded block copolymer poly(M1-b-M2)-Rh containing 18 mg of Rh(I)/g of block copolymer with a cmc of 2.2 × 10−6 mol L−1. The Rh-loaded polymer was used for the hydroformylation of 1-octene under micellar conditions. The data obtained were compared to those obtained with a monomeric analogue, i.e. CH3CON(Py)2RhCl(COD) (C1, Py = 2-pyridyl). Using the polymer-supported catalyst under micellar conditions, a significant increase in selectivity, i.e. an increase in the n:iso ratio was accomplished, which could be further enhanced by the addition of excess ligand, e.g., triphenylphosphite. Special features of the micellar catalytic set up are discussed.

Polymer‐Supported, Carbon Dioxide‐Protected N‐Heterocyclic Carbenes: Synthesis and Application in Organo‐ and Organometallic Catalysis

AbstractThe synthesis of a resin‐supported, carbon dioxide‐protected N‐heterocyclic carbene (NHC) and its use in organocatalysis and organometallic catalysis are described. The resin‐bound carbon dioxide‐protected NHC‐based catalyst was prepared via ring‐opening metathesis copolymerization of 1,4,4a,5,8,8a‐hexahydro‐1,4,5,8‐exo,endo‐dimethanonaphthalene (DMNH6) with 3‐(bicyclo[2.2.1]hept‐5‐en‐2‐ylmethyl)‐1‐(2‐propyl)‐3,4,5,6‐tetrahydropyrimidin‐1‐ium‐2‐carboxylate (M1), using the well‐defined Schrock catalyst Mo[N‐2,6‐(2‐Pr)2‐C6H3](CHCMe2Ph)(OCMe3)2 and was used for a series of organocatalytic reactions, i.e., for the trimerization reaction of isocyanates, as well as for the cyanosilylation of carbonyl compounds. In the latter reaction, turn‐over numbers (TON) up to 5000 were achieved. In addition, the polymer‐supported, carbon dioxide‐protected N‐heterocyclic carbene served as an excellent progenitor for various polymer‐supported metal complexes. It was loaded with a series of rhodium(I), iridium(I), and palladium(II) precursors and the resulting Rh‐, Ir‐, and Pd‐loaded resins were successfully used in the polymerization of phenylacetylene, in the hydrogen transfer reaction to benzaldehyde, as well as in Heck‐type coupling reactions. In the latter reaction, TONs up to 100,000 were achieved. M1, as a non‐supported analogue of poly‐M1‐b‐DMNH6, as well as the complexes PdCl2[1,3‐bis(2‐Pr)tetrahydropyrimidin‐2‐ylidene]2 (Pd‐1) and IrBr[1‐(norborn‐5‐ene‐2‐ylmethyl)‐3‐(2‐Pr)‐3,4,5,6‐tetrahydropyrimidin‐2‐ylidine](COD) (Ir‐1) were used as homogeneous analogues and their reactivity in the above‐mentioned reactions was compared with that of the supported catalytic systems. In all reactions investigated, the TONs achieved with the supported systems were very similar to the ones obtained with the unsupported, homogeneous ones, the turn‐over frequencies (TOFs), however, were lower by up to a factor of three.

A Stable Six‐Coordinate Intermediate in Ammonia–Dinitrogen Exchange at Schrock's Molybdenum Catalyst

In this work, we investigate the mechanism of the ammonia-dinitrogen exchange reaction, which is the decisive step to close the catalytic cycle of Schrock's dinitrogen reduction sequence under ambient conditions. We identify several viable pathways for the approach of dinitrogen to the five-coordinate molybdenum center of the ammonia complex by means of first-principles molecular-dynamics simulations. These exploratory simulations are then complemented by rigorous quantum-chemical structure optimizations. Our calculations have been performed for the full Schrock catalyst without simplifying the large chelate ligand, and are hence not affected by model assumptions. We show that the reaction obeys an addition-elimination mechanism via a stable six-coordinate intermediate. This intermediate has been fully characterized by stationary quantum-chemical methods. The predicted infrared spectrum of this species features an N[triple bond]N stretching vibration, which is well separated in frequency from all other N[triple bond]N stretching vibrations of N(2)-binding complexes involved in the Schrock cycle. Depending on the life time of this intermediate in the reaction liquor, the production of this intermediate might even be monitored by the absorption of the N[triple bond]N stretching vibration.

Schrock Catalyst Triggered, Ring-Opening Metathesis Polymerization Based Synthesis of Functional Monolithic Materials

The synthesis of monolithic materials via the Schrock catalyst triggered ring-opening metathesis polymerization (ROMP) is described. Mo(N-2,6-(2-Pr)2-C6H3)(CHCMe2Ph)(OCMe3)2 (1) was used as an initiator. Using various ratios of norborn-2-ene (NBE) and 1,4,4a,5,8,8a-hexahydro-1,4,5,8-exo,endo-dimethanonaphthalene (DMN-H6) in different mixtures of micro- with macroporogens, i.e., 1,2-dichloroethane, toluene and THF with hexane or pentane, monolithic polymeric materials with continuous, interconnected pores in the micrometer range as well as with micro- and mesopores in the 1.5−300 nm range could be synthesized within the confines of 3 × 100 mm glass columns. The resulting monoliths were characterized via inverse size exclusion chromatography (ISEC) in terms of the total pore volume (Vp), the volume fraction of the pore porosity (ϵp), the volume fraction of the interstitial porosity (ϵz) and the pore size distribution. Finally, the novel monoliths were successfully used for the fast separation of proteins as well as of 9-fluorenylmethoxycarbonyl (FMOC) protected amino acids.

Stereocontrolled Total Synthesis of Amphidinolide X via a Silicon-Tethered Metathesis Reaction

Two esterifications and an RCM to create the challenging trisubstituted C12-C13 double bond were required in the total synthesis of amphidinolide X (1) reported here. Assembling the three fragments in this order, no RCM occurred or the process yielded mainly isomer Z. However, generating the E double bond first, by a new variant of a Si-tethered metathesis (using Schrock's catalyst), and carrying out the esterification and macrolactonization steps later, 1 was obtained exclusively.