Alkylidene Complexes Research Articles

Metathesis reactions of rapeseed oil‐derived fatty acid methyl esters induced by monometallic and homobimetallic ruthenium complexes

In this study, homobimetallic ruthenium alkylidene complexes were used for the self‐metathesis and ethenolysis reactions of methyl oleate and rapeseed oil‐derived FAME mixtures. The activity and selectivity of homobimetallic ruthenium alkylidene complexes were also compared with their monometallic analogs. It has been determined that homobimetallic complexes bearing phosphine and N‐heterocyclic carbene ligands are more active and selective catalysts than their monometallic analogs for self‐metathesis and ethenolysis reactions of rapeseed FAME mixture. Homobimetallic ruthenium complexes showed superior selectivity at 25°C toward these reactions. Conversion values of 99% with 96% selectivity toward ethenolysis products were observed by using homobimetallic ruthenium complexes. In addition to these superiorities, relatively lower amounts of double bond isomerization side‐products were obtained by using homobimetallic complexes. This catalytic approach represents a unique example for metathesis transformation of FAME mixtures at relatively low temperature (25°C) and ethylene pressure (1 atm) with high selectivity using homobimetallic ruthenium alkylidene complexes.Practical applications: This study reports the selective transformation of rapeseed oil‐derived FAME by ethenolysis reactions catalyzed by homobimetallic ruthenium alkylidene complexes. Industrially important terminal olefins as well as new polymer precursors can be synthesized in a selective manner using these catalytic systems.Ethenolysis reactions of rapeseed oil‐derived fatty acid methyl ester mixture, catalyzed by homobimetallic ruthenium alkylidene complexes exhibit high selectivity at RT.

Bulky Aryloxide Ligand Stabilizes a Heterogeneous Metathesis Catalyst

AbstractThe reaction of [W(O)(CHCMe2Ph)(dAdPO)2], containing bulky 2,6‐diadamantyl aryloxide ligands, with partially dehydroxylated silica selectively yields a well‐defined silica‐supported alkylidene complex, [(SiO)W(O)(CHCMe2Ph)(dAdPO)]. This fully characterized material is a very active and stable alkene metathesis catalyst, thus allowing loadings as low as 50 ppm in the metathesis of internal alkenes. [(SiO)W(O)(CHCMe2Ph)(dAdPO)] also efficiently catalyzes the homocoupling of terminal alkenes, with turnover numbers exceeding 75 000 when ethylene is constantly removed to avoid the formation of the less reactive square‐based pyramidal metallacycle resting state.

Bulky aryloxide ligand stabilizes a heterogeneous metathesis catalyst.

The reaction of [W(=O)(=CHCMe2Ph)(dAdPO)2], containing bulky 2,6-diadamantyl aryloxide ligands, with partially dehydroxylated silica selectively yields a well-defined silica-supported alkylidene complex, [(≡SiO)W(=O)(=CHCMe2Ph)(dAdPO)]. This fully characterized material is a very active and stable alkene metathesis catalyst, thus allowing loadings as low as 50 ppm in the metathesis of internal alkenes. [(≡SiO)W(=O)(=CHCMe2Ph)(dAdPO)] also efficiently catalyzes the homocoupling of terminal alkenes, with turnover numbers exceeding 75,000 when ethylene is constantly removed to avoid the formation of the less reactive square-based pyramidal metallacycle resting state.

Synthesis of Molybdenum and Tungsten Alkylidene Complexes That Contain Sterically Demanding Arenethiolate Ligands.

Imido alkylidene complexes of Mo and W and oxo alkylidene complexes of W that contain thiophenoxide ligands of the type S-2,3,5,6-Ph4C6H (STPP) and S-2,6-(mesityl)2C6H3 (SHMT = S-hexamethylterphenyl) have been prepared in order to compare their metathesis activity with that of the analogous phenoxide complexes. All thiolate complexes were significantly slower (up to ∼10× slower) for the metathesis homocoupling of 1-octene or polymerization of 2,3-dicarbomethoxynorbornene, and none of them was Z-selective. The slower rates could be attributed to the greater σ-donating ability of a thiophenoxide versus the analogous phenoxide and consequently a higher electron density at the metal in the thiophenoxide complexes.

Synthesis of Tungsten Imido Alkylidene Complexes that Contain an Electron-Withdrawing Imido Ligand.

Tungsten NArR alkylidene complexes have been prepared that contain the electron-withdrawing ArR groups 2,4,6-X3C6H2 (ArX3, X = Cl, Br), 2,6-Cl2-4-CF3C6H2 (ArCl2CF3), and 3,5-(CF3)2C6H3 (Ar(CF3)2). Reported complexes include W(NArR)2Cl2(dme) (dme = 1,2-dimethoxyethane), W(NArR)2(CH2CMe3)2, W(NArR)(CHCMe3)(OTf)2(dme), and W(NArR)(CHCMe3)(ODBMP)2 (DBMP = 4-Me-2,6-(CHPh2)C6H2). The W(NArR)(CHCMe3)(ODBMP)2 complexes were explored as initiators for the polymerization of 2,3-dicarbomethoxynorbornadiene (DCMNBD).

Enantioselective olefin metathesis with cyclometalated ruthenium complexes.

The success of enantioselective olefin metathesis relies on the design of enantioenriched alkylidene complexes capable of transferring stereochemical information from the catalyst structure to the reactants. Cyclometalation of the NHC ligand has proven to be a successful strategy to incorporate stereogenic atoms into the catalyst structure. Enantioenriched complexes incorporating this design element catalyze highly Z- and enantioselective asymmetric ring opening/cross metathesis (AROCM) of norbornenes and cyclobutenes, and the difference in ring strain between these two substrates leads to different propagating species in the catalytic cycle. Asymmetric ring closing metathesis (ARCM) of a challenging class of prochiral trienes has also been achieved. The extent of reversibility and effect of reaction setup was also explored. Finally, promising levels of enantioselectivity in an unprecedented Z-selective asymmetric cross metathesis (ACM) of a prochiral 1,4-diene was demonstrated.

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.

Ring-opening metathesis polymerization of vinylnorbornene and following polymer modifications

Ring-opening metathesis polymerization of vinylnorbornene with molybdenum (VI) complex or with Hoveyda-Grubbs type Ru alkylidene complex provided soluble polymers, poly (VNBE)s of Mn about 10 000, having vinyl pendant groups on cyclopentenylenevinylene main chains. Post-polymerization modification via cross-metathesis of pendant vinyl groups with cis-1,4-diacetoxybut-2-ene, 5-hexenyl acetate, allyl acetoacetate, and allyltrimethylsilane was accomplished. Functionalization of poly (VNBE) was also performed via ene-yne cross-metathesis of vinyl pendants with (4-fluorophenyl) acetylene and (2,4-difluorophenyl) acetylene.

Stability and activity of cis-dichloro ruthenium olefin metathesis precatalysts bearing chelating sulfur alkylidenes

S-Chelated Grubbs type complexes are usually found in their stable cis-dichloro conformation. These precatalysts are usually latent and necessitate external stimuli to promote olefin metathesis reactions. Herein we report the synthesis of new S-chelated ruthenium complexes, by a simple exchange of the benzylidene to an alkylidene chelating ligand. The structure of the complexes was studied by NMR spectroscopy, single crystal X-ray diffraction and DFT calculations. The new complexes were tested in a series of olefin metathesis reactions and were found to be the first cis-dichloro ruthenium precatalysts that are active at room temperature. The observed differences in reactivity and structure between the alkylidene complexes and their benzylidene counterparts highlight the important influence aromaticity may have on the stability and activity of chelated Grubbs type complexes. The findings may also have implications to develop alternative strategies to stabilize, or destabilize, other organometallic complexes bearing relevant chelating ligands.

18-Electron Ruthenium Phosphine Sulfonate Catalysts for Olefin Metathesis

The first instances of ruthenium alkylidene complexes based on chelating phosphine sulfonates are presented. Although these complexes are formally 18-electron complexes bearing cis phosphines and cis one-electron donors (sulfonates and chlorides), they are surprisingly active for ring-closing metathesis, cross-metathesis, and ring-opening metathesis polymerization, thus highlighting the unique potential of the sulfonate ligand in the design of a ruthenium metathesis catalyst.

Synthesis of stereoregular polymers through ring-opening metathesis polymerization.

Some of the most readily available and inexpensive monomers for ring-opening metathesis polymerization (ROMP) are norbornenes or substituted norbornadienes. Polymers made from them have tacticities (the stereochemical relationship between monomer units in the polymer chain) that remain after the C═C bonds in the polymer backbone are hydrogenated. Formation of polymers with exclusively a single structure (one tacticity) was rare until approximately 20 years ago, when well-defined ROMP catalysts based on molybdenum imido alkylidene complexes that contain a chiral biphenolate or binaphtholate ligand were shown to yield cis,isotactic-poly(2,3-dicarbomethoxynorbornadiene) and related polymers through addition of the monomer to the same side of the M═C bond in each step. Over the past few years, molybdenum and tungsten monoaryloxide pyrrolide (MAP) imido alkylidene initiators have been found to produce cis,syndiotactic polynorbornenes and substituted norbornadienes through addition of the monomer to one side of the M═C bond in one step followed by addition to the other side of the M═C bond in the next step. This "stereogenic metal control" is possible as a consequence of the fact that the configuration of the stereogenic metal center switches with each step in the polymerization. Stereogenic metal control also allows syndiotactic polymers to be prepared from racemic monomers in which enantiomers of the monomer are incorporated alternately into the main chain. Because pure trans polymers have not yet been prepared through some predictable mechanism of stereochemical control, it seems unlikely that all four basic polymer structures from a single given monomer can be prepared simply by choosing the right initiator. However, because tactic, and relatively oxygen-stable, hydrogenated polymers are often a desirable goal, the ability to form pure cis,isotactic polymers (through enantiomorphic site control) and cis,syndiotactic polymers (through stereogenic metal control) is sufficient for preparing hydrogenated polymers with a single structure. It is hoped that the principles of forming polymers that have a single structure through ring-opening metathesis polymerization will be general for a relatively large number of monomers and that some important problems in ROMP polymer chemistry can benefit from knowledge of polymer structure at a molecular level. With an increase in knowledge concerning the mechanistic details of polymerization by well-defined initiators, more elaborate ROMP polymers and copolymers with stereoregular structures may be possible.

Tungsten Oxo Alkylidene Complexes as Initiators for the Stereoregular Polymerization of 2,3-Dicarbomethoxynorbornadiene

We have employed 2,3-dicarbomethoxynorbornadiene (DCMNBD) as a monomer to explore new tungsten oxo alkylidene complexes as initiators for stereoregular ROMP (ring-opening metathesis polymerization). The initiators include MAP (monoaryloxide pyrrolide) oxo alkylidene complexes with the general formula W(O)(CHCMe2Ph)(Me2Pyr)(OAr) (Me2Pyr = 2,5-dimethylpyrrolide, OAr = an aryloxide) and W(O)(CHCMe2Ph)(OR)2 (OR = an aryloxide or OC(CF3)3), or PPh2Me or CH3CN adducts thereof. We have found that MAP initiators yield cis,syndiotactic-poly(DCMNBD) as a consequence of stereogenic metal control. In contrast, W(O)(CHCMe2Ph)(OR)2(L) initiators (where L = PPh2Me or acetonitrile) are strongly biased toward formation of cis,isotactic structures, while W(O)(CHCMe2Ph)(OR)2 initiators are strongly biased toward formation of cis,syndiotactic structures. Addition of B(C6F5)3 to W(O)(CHCMe2Ph)(Me2Pyr)(OR) species leads to a dramatic increase in the rate of polymerization and to an increase in the cis,syndiotacticity of the po...

Enforcing Z-selectivity in olefin metathesis through use of catalysts grafted on well-defined phenolic hybrid material

We have developed a new hybrid material, featuring single-site phenolic functions on a silica surface. This new material was used for the grafting, by surface organometallic chemistry, of tungsten perhydrocarbyl and molybdenum bispyrrolide alkylidene complexes. Dramatic improvements in terms of activity and stability were observed in propylene metathesis, when compared to the silica counterpart. Introducing a phenolic spacer between the active molybdenum center and the surface enhances the stereoselectivity towards the Z isomer.

ChemInform Abstract: [2 + 2 + 2] Cyclotrimerization of Alkynes and Isocyanates/Isothiocyanates Catalyzed by Ruthenium—Alkylidene Complexes.

AbstractThis method uses established metathesis catalysts for the cyclotrimerization of diynes with isocyanates, isothiocyanates, or carbon disulfide to yield bicyclic heterocyclic products.

Thermally stable pseudo-third-generation Grubbs ruthenium catalysts with pyridine–phosphinimine ligand

The ligand precursors 2-(R3PN)CH2Py (R=Ph(1a), Cy(2a)) were prepared from reaction of pyridine azide with various phosphine ligands. Reaction of 1a or 2a with RuCl2(CHPh)(Py)2(H2IMes) (Py=pyridine) afforded the ruthenium alkylidene complex RuCl2(CHPh)(PyCH2(NPR3))(H2IMes) (R=Ph(1), Cy(2)). Both catalysts showed good thermal stability and latent behavior toward RCM and ROMP reactions.

DFT study on the reaction mechanism of the ring closing enyne metathesis (RCEYM) catalyzed by molybdenum alkylidene complexes

The ring closing enyne metathesis reaction (RCEYM) catalyzed by molybdenum based monoalkoxy pyrrolyl Schrock type catalysts has been studied by means of DFT (B3LYP-D) calculations. The two potential active alkylidene species as well as the three proposed reaction mechanisms (ene-then-yne, endo-yne-then-ene and exo-yne-then-ene) have been taken into account. Moreover, the influence on the exo- and endo- selectivity of the reactant substituents has also been explored. Results show that, in contrast to what is found for RCEYM processes catalyzed by Ru-based catalysts, the metallacyclobutene is a very short-living reaction intermediate that can be present in two isomeric forms (trigonal bipyramid (TBP) coordination around the metal center and square based pyramid (SPY) coordination). These two isomers are directly involved in the reaction mechanism, and the ring opening takes place from the SPY species. Moreover and regardless of the nature of the reacting metal-alkylidene, the yne-then-ene pathways (endo- or exo-) are computed to present significantly lower energy barriers than the ene-then-yne pathway and thus the latter is computed not to take place. Finally, the exo-/endo- selectivity is predicted to highly depend on the sterics of the two carbon ends of the alkyne fragment. In this way, the carbon bearing the largest group prefers to interact with the carbon end of the metal-alkylidene. This places the bulkiest groups as far away as possible from the metal fragment and overall leads to a generally lower energy barrier for the metallacyclobutene formation, the key step in defining the exo-/endo- selectivity.

Synthesis and Characterization of Tungsten Alkylidene and Alkylidyne Complexes Supported by a New Pyrrolide-Centered Trianionic ONO3– Pincer-Type Ligand

Synthetic protocols for a pyrrolide-centered ONO3– trianionic pincer-type ligand are presented. Treating (tBuO)3W≡CtBu with the proligand [pyr-ONO]H3 (2) results in the formation of the trianionic pincer alkylidene complex [pyr-ONO]W═CHtBu(OtBu) (3). Addition of a mild base to complex 3 provides the trianionic pincer alkylidyne complex {MePPh3}{[pyr-ONO]W≡CtBu(OtBu)} (4). All new compounds were characterized by NMR spectroscopy, combustion analysis, and, in the case of complex 4, single-crystal X-ray crystallography. DFT calculations performed on 4 provide insight into its electronic structure and indicate that the HOMO is ligand-based and localized on the pyrrolide π orbitals.

Theoretical Investigations of Heterogeneous Olefin Metathesis Catalysts

Olefin metathesis catalysed by solid systems has found a number of industrial applications. During the last decade, heterogeneous catalysts for olefin metathesis were comprehensively studied by experimental and theoretical methods. A great progress has been achieved in the field of both conventional and well-defined catalytic systems. The computational approach, taking advantage of growing computer power, enabled for the development of advanced models representing surface species relevant to olefin metathesis. In the case of the conventional ill-defined heterogeneous catalysts, the main achievements concern detailed description of the mechanism of the propagation steps and understanding the role of the support. The initiation stage of the olefin metathesis process was also explored, however, further investigations of the active sites generation, and, especially, catalyst deactivation are still required. Quantum chemistry methods, being complementary to experimental techniques, have become a standard tool for the characterization of the well-defined heterogeneous systems. The propagation and deactivation routes for olefin metathesis catalysed by various silicasupported Re, Mo and W alkylidene complexes were recognized in detail and the structure-activity relationships are now well established. Computational studies of alkane metathesis confirmed that olefin metathesis steps play a key role in the mechanism of this process. Keywords: Alkylidene complexes, Cluster model, DFT, Heterogeneous catalyst, Mechanism, Olefin metathesis, Periodic model.

A thermally robust ruthenium phosphonium alkylidene catalyst — the effect of more bulky N-heterocyclic carbene ligands on catalyst performance in olefin metathesis reactions

Three new ruthenium phosphonium alkylidene complexes incorporating N-heterocyclic carbene ligands with bulky N-aryl groups (2,6-diethyl, L = 1,3-bis(2,6-diethylphenyl)imidazolin-2-ylidene (H2IDEP) and 2,6-diisopropyl, L = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene (H2ID-i-PP)) were synthesized and characterized. The H2ID-i-PP supported complex was found to exhibit excellent thermal stabilities relative to the parent N-mesityl (N-Mes) complexes as well as the H2IDEP supported complexes. All three phosphonium alkylidenes were evaluated in comparison to the N-Mes derivative and Grubbs second generation catalyst using standard olefin metathesis reactions and conditions. The complex containing the bulky H2ID-i-PP ligand was found to have excellent activity and longevity in comparison to the other catalysts. Although initiation rates were slow for this sterically bulky precatalyst, its superior activity led to the best overall efficiency in test reactions.

A New Route to Ruthenium Thiolate Alkylidene Complexes

(Im(OMe)2)(SIMes)(PPh3)RuHCl (Im(OMe)2 = (C3H2(NCH2CH2OMe)2) reacts with aryl vinyl sulfides (PhSCHCH2 and (C6F5)SCHCHPh) to give the Ru thiolate alkylidene complexes (Im(OMe)2)(SIMes)(PhS)RuCl(═CHCH3) and (Im(OMe)2)(SIMes)(F5C6S)RuCl(═CHCH2Ph), which are shown to be effective olefin metathesis catalysts upon activation with BCl3.