Ruthenium Benzylidene Research Articles

Ruthenium–Benzylidenes and Ruthenium–Indenylidenes as Efficient Catalysts for the Hydrogenation of Aliphatic Nitriles into Primary Amines

The catalytic reduction of alkyl nitriles into primary amines with ruthenium-benzylidene and ruthenium-indenylidene catalysts allowed the efficient hydrogenation of aliphatic nitrile esters that were derived from plant oils.

A New Family of Halogen‐Chelated Hoveyda–Grubbs‐Type Metathesis Catalysts

Coordination, not insertion: New ruthenium benzylidenes with a chelating halogen atom were easily prepared and showed excellent stability and activity as metathesis catalysts (see figure). Structure-activity studies reveal that strength of the ruthenium-halogen interaction can be tuned across a wide range to set up a family of latent to active catalysts.

Halide Exchange in Second-Generation cis-Dihalo Ruthenium Benzylidene Complexes

The current article discusses the lability of the chloride ligands in cis-dichloro ruthenium benzylidene complexes featuring the NHC ligand N,N′-bis[2,4,6-(trimethyl)phenyl]imidazolin-2-ylidene] (SIMes) and an ester-chelating benzylidene ligand. A simple halide exchange reaction with iodide salts was used to assess the halide's lability in this type of complex. The starting compound bearing the cis-dichloro stereochemistry (cis-1) underwent rapid and selective exchange of the chloride trans to the SIMes ligand (cis-2). The substitution of the chloride cis to the NHC ligand for an iodide was found to be much slower, and the corresponding cis-diiodo compound could be isolated and characterized (cis-3). The latter compound undergoes fast and selective exchange of the iodide trans to the NHC ligand when an excess of a chloride source is added, and the fourth member of the family, bearing a chloride trans to the NHC ligand and a iodide cis to the NHC ligand (cis-4), was obtained. The four compounds have been fully characterized, and their catalytic activity was accessed using ring-opening metathesis polymerization of a norbornene derivative as the benchmark reaction. The activity of the initiators decreased within the series cis-4 > cis-1 > cis-2 > cis-3. The high activity of cis-4 could be traced back to the formation of the highest amounts of the corresponding trans-dihalo species, which are the actual active initiators.

Synthesis of Unsaturated Macrocycles by Ru‐Catalyzed Ring‐Closing Metathesis: A Comparative Study

AbstractThe activity and stereoselectivity of phosphane‐ and N‐heterocyclic carbene (NHC)‐containing ruthenium benzylidene complexes have been evaluated in macrocyclic ring‐closing olefin metathesis to produce unsaturated lactones and lactams. The success of the macrocyclization depends on the nature of the ligand (phosphane or N‐heterocyclic carbene) on the ruthenium center and on the NHC properties. As for stereoselectivity, E/Z ratios seem to be influenced not only by the nature of the ruthenium catalyst but also by the thermodynamic stabilities of the resulting unsaturated macrocycles, as confirmed by theoretical results.

On the isomerization of a trans-dichloro to a cis-dichloro amide-chelated ruthenium benzylidene complex and the catalytic scope of these species in olefin metathesis

Upon comparative NMR spectroscopy studies, the isomerization of newly disclosed dichloro [κ2(C,O)-2-(N-propylaminocarbonyl)benzylidene][1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene]ruthenium from trans-dichloro configuration (SPY-5-31) to its thermodynamically more favored cis-dichloro derivative (with SPY-5-34 stereochemistry) was found to proceed via dissociation of one chloride ligand to form an intermediate cationic species. Furthermore, the performance of the trans- and the cis-dichloro derivatives as catalysts in ring-closing metathesis and as initiators in ring-opening metathesis polymerization was studied.

Acyclic Triene Metathesis Polymerization of Plukenetia Conophora Oil: Branched Polymers by Direct Polymerization of Renewable Resources

AbstractOil from the Plukenetia conophora seed, a tropical oilseed consisting mainly of linolenic acid, is polymerized via acyclic triene metathesis using different ruthenium benzylidene and indenylidene catalysts to yield highly branched and functionalized polyesters. The influence of the synthesis conditions on the structure and molar mass characteristics of the obtained hyperbranched polyesters as well as an in‐depth melt and solution material property investigation are presented.

Catalytic enantioselective synthesis of azacycloalkenes via intermolecular rhodium carbenoid C–H insertion/ring-closing metathesis sequence

Enantiomerically enriched cyclopropanes and products of C–H insertion reactions were obtained in excellent combined yields and enantioselectivities as a consequence of rhodium-catalyzed decomposition of vinyl diazoacetate in the presence of tert-butoxycarbonyl-(Boc)-protected amines as trapping agents. A series of enantiomerically enriched six- to eight-membered nitrogen-containing heterocycles were subsequently prepared via ring-closing metathesis of the dienes catalyzed by ruthenium benzylidene complex.

Silica-immobilized N,O-prolinate ruthenium benzylidene complexes for catalytic applications

The immobilization of N,O-prolinate ruthenium benzylidene complexes as hybrid silicas is reported according to three routes: (i) co-condensation of a silylated precursor, 10, with TEOS via the sol–gel process (material M1); (ii) grafting of the same precursor on a preformed mesoporous silica M2 (material M3); entrapment of a parent non-silylated compound, 8, in the preformed mesoporous silica M2 (material M4). They have been characterized by several techniques. Solid state 29Si NMR measurements ensure the anchorage of the Ru-complexed prolinate fragment in the silica matrix of M1 and M3 by the presence of T and Q sites, while only Q sites are observed as expected for M4. Elemental analyses provide the N/Si ratios and TGA experiments give the weight loss of the organics during decomposition and hence indicating the amount of organics inserted in the materials. Adsorption–desorption analyses show that these solids had surface areas ranging from 310 to 620 m2g−1.These materials were tested as solid catalysts for two different reactions: ring-closing olefin metathesis and hydrosilylation of phenylacetylene with HSiEt3. Their recyclability has also been investigated. While M1 is the less active for the first reaction giving low conversions and selectivity compared to M3 and M4, it is by far the most efficient catalyst for the second reaction.

Synthesis and ionic transport of sulfonated ring-opened polynorbornene based copolymers

The N-pentafluorophenyl-exo-endo-norbornene-5,6-dicarboximide (1a) and N-phenyl-exo-endo-norbornene-5,6-dicarboximide (1b) monomers were synthesized and copolymerized via ring opening metathesis polymerization (ROMP) using bis(tricyclohexylphosphine)benzylidene ruthenium(IV) dichloride (I) and tricyclohexylphosphine[1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene][benzylidene]ruthenium dichloride (II). Experiments, at distinct monomer molar ratios, were carried out using catalyst I in order to determine the copolymerization reactivity constants by applying the Mayo-Lewis and Fineman-Ross methods. Moreover, both catalysts were used to produce random and block high molecular weight copolymers of 1a with 1b and 1a with norbornene (NB) which were further hydrogenated using a Wilkinson’s catalyst. Then, the saturated copolymers underwent a nucleophilic aromatic substitution by reacting with sodium 4-hydroxybenzenesulfonate dihydrate to generate new polynorbornene ionomers bearing fluorinated pendant benzenesulfonate groups. A thorough study on the electrochemical characteristics involving electromotive forces of concentration cells and proton conductivity of cation-exchange membranes based on a block copolymer of norbornene dicarboximides containing structural units with phenyl and fluorinated pendant benzenesulfonate moieties is reported. The study of electromotive forces (emf) of concentration cells with the sulfonated membrane of copolymer 8 separating electrolyte solutions of different concentration indicate that the membranes exhibit high permselectivity to protons and sodium ions at moderately low concentrations. In principle, these results suggest that the membranes can be considered candidates for ionic separation applications.

Widening the Latency Gap in Chelated Ruthenium Olefin Metathesis Catalysts

The synthesis of novel sulfur-chelated ruthenium benzylidenes afforded latent catalysts with a wider range of activities and new isomeric forms. A ruthenium complex with a tridentate ligand displayed latency for even one of the most reactive ROMP monomers, dicyclopentadiene, while a room temperature latent trifluoromethyl-substituted thioether derivative was shown to be the most active sulfur-chelated precatalyst to date in several metathesis reactions at higher temperatures. These new complexes widen the spectrum of activity for this family of catalysts, enabling several practical applications and enhancing the understanding for the mechanisms of activation in strongly chelated ruthenium alkylidenes.

Ligand Isomerization in Sulfur-Chelated Ruthenium Benzylidenes

cis-Dichloro−trans-dichloro isomerization was studied in sulfur-chelated ruthenium benzylidenes. The effect of solvents and additives on the isomerization process was analyzed. Notably, π-acid ligand molecules, dichloromethane, and polar solvents were found to significantly accelerate the isomerization rate. As expected, the cis−trans isomer equilibria had a strong effect on the olefin metathesis reactivity; the bulkier cis-dichloro catalysts, which show a larger amount of trans isomer in the equilibrium, were more reactive than their less bulky counterparts. This tendency was generally reversed in the isolated trans isomer series, where both steric factors and faster isomerization play a role in dictating the olefin metathesis reactivity. A better understanding of the trans−cis isomerization and the mechanism that governs reactivity in strongly chelated cis-dichloro ruthenium benzylidenes was obtained.

Strong agostic-type interactions in ruthenium benzylidene complexes containing 7-azaindole based scorpionate ligands

The complexes [Ru(Tai)Cl{=C(H)Ph}(PCy(3))] (4) and [Ru((Ph)Bai)Cl{=C(H)Ph}(PCy(3))] (5) [where Tai = HB(7-azaindolyl)(3) and (Ph)Bai = Ph(H)B(7-azaindolyl)(2)] have been prepared and structurally characterised. The borohydride unit is located in the coordination site trans to the chloride ligand in both complexes. The degree of interaction between the borohydride group and the metal centre was found to be significantly large in both cases. Thermolysis reactions involving complex 4 led to a dehydrogenation reaction forming [Ru(Tai)Cl{PCy(2)(η(2)-C(6)H(9))}] (6) where the benzylidene group acts as a hydrogen acceptor.

Pyridine as trigger for chloride isomerisation in chelated ruthenium benzylidene complexes: implications for olefin metathesis

The cationic pyridine adduct of a ruthenium complex bearing a chelating benzylidene and an N-heterocyclic carbene was identified as an intermediate during the activation of cis dichloro species and a novel triggering concept for olefin metathesis catalysts based on cationic species was disclosed.

ChemInform Abstract: New Approaches to Olefin Cross-Metathesis.

New methodology for the selective cross-metathesis (CM) of terminal olefins employing ruthenium benzylidene 1 is described.1 CM with symmetric internal olefins was found to provide a useful means for homologating terminal olefins to protected allylic alcohols, amines, and esters. Due to the limited commercial availability of symmetric internal olefins, a two-step CM procedure was developed in which terminal olefins were first homodimerized prior to the CM reaction. Terminal olefins with allylic methyl substituents were observed to provide CM products in diminished yield albeit with markedly improved trans-selectivity. Reaction rates were measured for CM reactions utilizing butenediol and allyl alcohol derivatives, and the results demonstrated distinct advantages in reaction rate and stereoselectivity for reactions employing the disubstituted olefins. In the course of studies of substrates with allylic oxygen substituents, a new CM application was discovered involving the metathesis of acrolein acetal derivatives with terminal olefins. Acrolein acetals, including asymmetric variants derived from tartaric acid, proved to be exceptionally robust and trans-selective CM substrates. In related work, a pinacol-derived vinyl boronate was also found to be a reactive CM partner, providing a novel means for converting terminal olefins into precursors for the Suzuki coupling reaction.

ChemInform Abstract: Synthesis of Eight-Membered Ring Compounds Using Enyne Metathesis.

A novel procedure for synthesizing eight-membered ring compounds was developed using ruthenium-catalyzed enyne metathesis. When a CH2Cl2 solution of enyne connected with catechol, o-amino phenol, or o-phenylenediamine was stirred in the presence of benzylidene ruthenium carbene complex (10 mol %) at room temperature overnight, an eight-membered ring compound was obtained in high yield. In a similar manner, monocyclic 1,4-diaza- or 1-oxa-4-azacyclooctene derivative was obtained in high yield.

Polynorbornene with pentafluorophenyl imide side chain groups: Synthesis and sulfonation

AbstractThe mixtures of exo‐endo‐monomers and isomerically pure endo‐monomers of N‐pentafluorophenyl‐norbornene‐5,6‐dicarboximide (2a) and N‐phenyl‐norbornene‐5,6‐dicarboximide (2b) were synthesized and polymerized via ring opening metathesis polymerization using bis(tricyclohexylphosphine) benzylidene ruthenium (IV) dichloride (I) and tricyclohexylphosphine [1,3‐bis(2,4,6‐trimethylphenyl)‐4,5‐dihydroimidazol‐2‐ylidene][benzylidene] ruthenium dichloride (II). Ring opening metathesis polymerization of mixtures of exo‐endo‐monomers (2a) and (2b) and pure endo‐2b gave the corresponding high molecular weights poly(N‐pentafluorophenyl‐norbornene‐5,6‐dicarboximide) (3a) and poly(N‐phenyl‐norbornene‐5,6‐dicarboximide) (3b). The isomerically pure endo‐2a did not polymerize by I in these conditions, since I is the least active catalyst and endo‐2a is the least active monomer because of the intramolecular complex formation between the Ru active center and the fluorine atom of ring‐opened endo‐2a on the one hand and steric hindrances caused by the pentafluorinated ring on the other. The quantitative hydrogenation of the polymer 3a, at room temperature and 115 bar, was achieved by a Wilkinson's catalyst. The new polynorbornene bearing highly fluorinated sulfonic acid groups (5) was obtained by the reaction of the hydrogenated poly(N‐pentafluorophenyl‐norbornene‐5,6‐dicarboximide) (4) with sodium 4‐hydroxybenzenesulfonate dihydrate. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2925–2933, 2010

Latent ruthenium olefin metathesis catalysts featuring a phosphine or an N-heterocyclic carbene ligand

The synthesis and characterization of latent 18-electron ruthenium benzylidene complexes (PCy 3)(( κ N, O)-picolinate) 2RuCHPh ( 5) and (H 2IMes)(( κ N, O)-picolinate) 2RuCHPh ( 6) are described. Both complexes appear as two isomers. The ratio between the isomers is dependent on l-type ligand. The complexes are inactive in ring-closing metathesis and ring-opening metathesis polymerization reactions even at elevated temperatures in the absence of stimuli. Upon addition of HCl, complexes 5 and 6 become highly active in olefin metathesis reactions. The advantage of the latent catalysts is demonstrated in the ring-opening metathesis polymerization of dicyclopentadiene, where the latency of 6 assures adequate mixing of catalyst and monomer before initiation. Trapping experiments suggests that the acid converts the 18-electron complexes into their corresponding highly olefin metathesis active 14-electron benzylidenes.

Ring‐opening metathesis polymerization of tetracyclododecene using various catalyst systems

AbstractRing‐opening metathesis polymerization (ROMP) of a polycyclic olefin monomer (tetracyclododecene, TCD) was carried out using various catalyst systems. The catalysts included titanium‐based ternary system (titanium tetrachloride, TiCl4/triethylaluminum, Et3Al/triethylamine, Et3N), tungsten‐based ternary system (tungsten hexachloride, WCl6/triisobutyl aluminum, (i‐Bu)3Al/ethanol), and a ruthenium benzylidene catalyst (Grubbs 1st catalyst). The Grubbs 1st catalyst was most active in ROMP of TCD. The tungsten‐based catalyst was found to be more active than the titanium‐based catalyst. In contrast to the titanium‐based and the Grubbs 1st catalysts, ROMP of TCD with the tungsten‐based catalyst was very sensitive to gelation in the course of reaction. The effects of ethanol, WCl6 concentration, TCD/WCl6 ratio, and 1‐hexane on the polymerization yield and the polymer gel formation were investigated in detail, and optimized ROMP conditions were determined. Chemical structure of synthesized polymer was characterized using one‐ and two‐dimensional 150 MHz 13C and 600 MHz 1H nuclear magnetic resonance spectroscopy, molecular weights were measured using gel‐permeation chromatography, and thermal properties were characterized using differential scanning calorimetry. Number‐average molecular weights (Mn) of the polymer were in the range of 1.5–4.7 × 104 g/mol, and glass transition temperatures were in the range of 185–220°C. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Predicting the Cis−Trans Dichloro Configuration of Group 15−16 Chelated Ruthenium Olefin Metathesis Complexes: A DFT and Experimental Study

Gradient-corrected (BP86) and hybrid (M06-L) density functional calculations were used to study the relative stability of cis and trans-dichloro X-chelated benzylidene ruthenium complexes (X = O, S, Se, N, P). Calculations in the gas phase differed from experimental results, predicting the trans-dichloro configuration as being more stable in every case. The addition of Poisson-Boltzmann (PBF) continuum approximation (dichloromethane) corrected the disagreement and afforded energies consistent with experimental results. Novel N, Se, and P chelated ruthenium olefin metathesis complexes were synthesized to evaluate calculation predictions. These findings reinforce the importance of including solvent corrections in DFT calculations of ruthenium metathesis catalysts and predict that stronger sigma donors as chelating atoms tend to electronically promote the unusual and less active cis-dichloro configuration.

The bis(trifluoroacetate) analogue of the first-generation Grubbs catalyst: Synthesis, X-ray structure, and metathesis activity of [Ru(CF 3CO 2)(η 2-CF 3CO 2)( [dbnd]CHPh)(PCy 3) 2

A simple synthesis of [Ru(CF 3CO 2)(η 2-CF 3CO 2)( CHPh)(PCy 3) 2] from dimeric [Ru 2(CF 3CO 2) 2(μ-CF 3CO 2) 2( CHPh) 2(PCy 3) 2(μ-H 2O)] is described. The new complex crystallizes in monoclinic system ( P2(1)/ c space group) with distorted octahedral coordination. The very low metathesis activity of this new ruthenium benzylidene complex demonstrates that substitution of chlorides with trifluoroacetates in [RuCl 2( CHPh)(PCy 3) 2] results in loss of the catalytic activity.