New olefin metathesis catalysts with fluorinated unsymmetrical imidazole-based ligands

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Chelating alkylidene ligands as pacifiers for ruthenium catalysed olefin metathesis

Olefin metathesis catalysed by ruthenium has emerged at the frontier of modern synthetic chemistry. The desire to enhance catalyst stability, gain control over the catalytic process and deepen the understanding of the mechanisms of metathesis has yielded a class of latent ruthenium precatalysts of delayed initiation and with switchable activity. One of the main methodologies developed for this purpose has been the introduction of tethered carbene ligands. Herein we track the evolution of ruthenium based metathesis catalysts bearing chelated alkylidenes, from the early oxygen Hoveyda type benzylidenes to the latent sulphur containing complexes.     

Allenyl esters as quenching agents for ruthenium olefin metathesis catalysts

In the attempt to synthesize substituted allenyl esters through a metathesis coupling of unsubstituted allenyl esters and alkenes using a variety of ruthenium catalysts, it was discovered that allenyl esters themselves cleanly arrested the activity of the catalysts. Further studies suggests possible utility of allene esters as general quenching agents for metathesis reactions. To explore this idea, several representative olefin metathesis reactions, including ring closing, were successfully terminated by the addition of simple allenyl esters for more convenient purification.     

Thermomorphic polyethylene-supported olefin metathesis catalysts

The preparation of polyethylene-oligomer (PE(olig))-supported N-heterocyclic carbene ligands (NHCs) and their Ru complexes is described. These complexes are structurally analogous to their low molecular weight counterparts and can serve as thermomorphic, recoverable/recyclable ring-closing metathesis (RCM) catalysts. Because of the insolubility of PE(olig)-supported species at 25 °C, such complexes can perform homogeneous RCM reactions at 65 °C and, upon cooling, precipitate as solids. This allows for their quantitative separation from solutions of products.     

Improved ruthenium catalysts for Z-selective olefin metathesis

Several new C-H-activated ruthenium catalysts for Z-selective olefin metathesis have been synthesized. Both the carboxylate ligand and the aryl group of the N-heterocyclic carbene have been altered and the resulting catalysts evaluated using a range of metathesis reactions. Substitution of bidentate with monodentate X-type ligands led to a severe attenuation of metathesis activity and selectivity, while minor differences were observed between bidentate ligands within the same family (e.g., carboxylates). The use of nitrato-type ligands in place of carboxylates afforded a significant improvement in metathesis activity and selectivity. With these catalysts, turnover numbers approaching 1000 were possible for a variety of cross-metathesis reactions, including the synthesis of industrially relevant products.     

Highly efficient Ru-pseudohalide catalysts for olefin metathesis

Ruthenium alkylidene complexes containing one aryloxide "pseudohalide" ligand catalyze ring-closing metathesis of diene and ene-yne substrates with exceptionally high efficiency. Chromatographic removal of Ru residues is unexpectedly facile, offering a convenient means of isolating pure organic products in high yields.     

Asymmetric catalysts for stereocontrolled olefin metathesis reactions

Since the discovery of metathesis as an instrument to reorganize olefinic double bonds, substantial progress has been attained, establishing this method as a versatile and efficient tool for C-C-bond formation. In the last decade fundamental achievements were accomplished in the field of chiral Ru- and Mo-based olefin metathesis, providing an asymmetric access to structures, which are difficult to obtain by alternative routes. The reader is taken behind the scenes of catalyst development, important areas of application are described up to the current state of research; this tutorial review deals with the question, how metathesis is connected to enantioselective synthesis.     

Chelated ruthenium catalysts for Z-selective olefin metathesis

We report the development of ruthenium-based metathesis catalysts with chelating N-heterocyclic carbene (NHC) ligands that catalyze highly Z-selective olefin metathesis. A very simple and convenient procedure for the synthesis of such catalysts has been developed. Intramolecular C-H bond activation of the NHC ligand, promoted by anion ligand substitution, forms the appropriate chelate for stereocontrolled olefin metathesis.     

Cationic ruthenium allenylidene complexes as catalysts for ring closing olefin metathesis

A series of well accessible cationic ruthenium allenylidene complexes of the general type [(eta6-arene)(R3P)RuCl(=C=CR'2)]+ X- is described which constitute a new class of pre-catalysts for ring closing olefin metathesis reactions (RCM) and provide an unprecedented example for the involvement of metal allenylidenes in catalysis. They effect the cyclization of various functionalized dienes and enynes with good to excellent yields and show a great tolerance towards an array of functional groups. Systematic variations of their basic structural motif have provided insights into the essential parameters responsible for catalytic activity which can be enhanced further by addition of Lewis or Bronsted acids, by irradiation with UV light, or by the adequate choice of the "non-coordinating" counterion X-. The latter turned out to play a particularly important role in determining the rate and selectivity of the reaction. A similarly pronounced influence is exerted by remote substituents on the allenylidene residue which indicates that this ligand (or a ligand derived thereof) may remain attached to the metal throughout the catalytic process. X-ray crystal structures of the catalytically active allenylidene complexes 3b.PF6 and 15.OTf as well as of the chelate complex 10 required for the preparation of the latter catalyst are reported.     

Chelated amines as ligands in olefin hydrogenation catalysts

Activities of catalysts obtained in the reaction of [MCl(C₈H₁₄)₂]₂, (M=Rh, Ir) with amines NH₂(CH₂)_nNH₂, (N=2–5), (CH₃)₂NC₂H₄(CH₃)₂ and 1.8=diaminonaphthalene have been examined. The most efficient catalyst was obtained with 1.3-diaminopropane.

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, [MCl(C₈H₁₄)₂]₂. (M=Rh, Ir) NH₂(CH₂)_nNH₂, (n=2–5). (CH₃)₂NC₂H₄(CH₃)₂ 1,8-. , 1,3-.

     

New olefin metathesis catalysts bearing polyether clamp in N-heterocyclic carbenes ligands

Synthesis of new type of the Hoveyda–Grubbs catalysts containing modified N-heterocyclic carbene ligands is described herein. New catalysts bear different in size polyether clamp embracing N,N′-2,4-dimethylphenyl substituents in N-heterocyclic carbene. New complexes were tested in model RCM, enyne and CM reactions. They showed comparable activity to that of commercially available Grubbs second generation and Hoveyda–Grubbs second generation complexes. Complex with larger polyether clamp proved Z-stereoselective in a macrocycle formation and yielded more Z isomers than commercial complexes in CM reactions. The catalysts are stable and easy to purify.     

Supported chiral Mo-based complexes as efficient catalysts for enantioselective olefin metathesis

Syntheses and catalytic activities of seven new polymer-supported chiral Mo-based complexes are disclosed. Four of the complexes are polystyrene-based, and three involve polynorbornene supports. Studies concerning the ability of the polymer-bound chiral complexes to promote an assortment of asymmetric ring-closing (ARCM) and ring-opening (AROM) metathesis reactions are detailed. In many instances, levels of reactivity and enantioselectivity are competitive with those of the analogous homogeneous catalysts. The positive effect of lower cross-linking within the polymer backbone on reaction efficiency and asymmetric induction is detailed. The optically enriched products obtained through the use of the supported complexes, after simple filtration and removal of the supported Mo catalysts, contain significantly lower levels of metal impurities as compared to products synthesized with the corresponding homogeneous catalysts.     

Decomposition of ruthenium olefin metathesis catalysts

The decomposition of a series of ruthenium metathesis catalysts has been examined using methylidene species as model complexes. All of the phosphine-containing methylidene complexes decomposed to generate methylphosphonium salts, and their decomposition routes followed first-order kinetics. The formation of these salts in high conversion, coupled with the observed kinetic behavior for this reaction, suggests that the major decomposition pathway involves nucleophilic attack of a dissociated phosphine on the methylidene carbon. This mechanism also is consistent with decomposition observed in the presence of ethylene as a model olefin substrate. The decomposition of phosphine-free catalyst (H2IMes)(Cl)2Ru=CH(2-C6H4-O-i-Pr) (H2IMes = 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene) with ethylene was found to generate unidentified ruthenium hydride species. The novel ruthenium complex (H2IMes)(pyridine)3(Cl)2Ru, which was generated during the synthetic attempts to prepare the highly unstable pyridine-based methylidene complex (H2IMes)(pyridine)2(Cl)2Ru=CH2, is also reported.     

Synthesis and activity of ruthenium olefin metathesis catalysts coordinated with thiazol-2-ylidene ligands

A new family of ruthenium-based olefin metathesis catalysts bearing a series of thiazole-2-ylidene ligands has been prepared. These complexes are readily accessible in one step from commercially available (PCy3)2Cl2Ru=CHPh or (PCy3)Cl2Ru=CH(o-iPrO-Ph) and have been fully characterized. The X-ray crystal structures of four of these complexes are disclosed. In the solid state, the aryl substituents of the thiazole-2-ylidene ligands are located above the empty coordination site of the ruthenium center. Despite the decreased steric bulk of their ligands, all of the complexes reported herein efficiently promote benchmark olefin metathesis reactions such as the ring-closing of diethyldiallyl and diethylallylmethallyl malonate and the ring-opening metathesis polymerization of 1,5-cyclooctadiene and norbornene, as well as the cross metathesis of allyl benzene with cis-1,4-diacetoxy-2-butene and the macrocyclic ring-closing of a 14-membered lactone. The phosphine-free catalysts of this family are more stable than their phosphine-containing counterparts, exhibiting pseudo-first-order kinetics in the ring-closing of diethyldiallyl malonate. Upon removing the steric bulk from the ortho positions of the N-aryl group of the thiazole-2-ylidene ligands, the phosphine-free catalysts lose stability, but when the substituents become too bulky the resulting catalysts show prolonged induction periods. Among five thiazole-2-ylidene ligands examined, 3-(2,4,6-trimethylphenyl)- and 3-(2,6-diethylphenyl)-4,5-dimethylthiazol-2-ylidene afforded the most efficient and stable catalysts. In the cross metathesis reaction of allyl benzene with cis-1,4-diacetoxy-2-butene increasing the steric bulk at the ortho positions of the N-aryl substituents results in catalysts that are more Z-selective.     

Supported silica-molybdena catalysts for olefin metathesis activated by photoreduction

Cyclopropane (CP) or methylcyclopropane (MCP) chemisorption on silica-molybdena catalysts photoreduced in CO was found to result in a sharp increase (by one order of magnitude) of specific activities for olefin metathesis (propene, 1-hexene, ethyl oleate). IR and UV-VIS spectroscopic studies revealed that promoting effect of CP and MCP treatment was associated with the formation of the thermally stable carbene complexes Mo = CH₂ and Mo=CH-CH₃, arising from CP and MCP interaction with low coordinated MO⁴⁺ ions in photoreduced Mo/SiO₂ IR data showed fast reversible transformation of Mo=CH₂ to Mo=CH-CH₃ thus providing the first direct spectroscopic confirmation of a chain carbene mechanism of propene metathesis. A general scheme for CP and MCP interaction with photoreduced Mo/SiO₂ was proposed.     

Mechanistically inspired catalysts for enantioselective desymmetrizations by olefin metathesis

In asymmetric olefin metathesis reactions, the addition of halide additives is often required to augment enantioselectivities, despite the fact that the additives result in catalysts with diminished reactivities. The preparation of new chiral Ru-based catalysts was accomplished by exploiting previously reported mechanistic studies. The catalysts possess a high level of reactivity and successfully induce high levels of asymmetry in desymmetrization reactions without the use of halide additives.     

Dipyrrolyl precursors to bisalkoxide molybdenum olefin metathesis catalysts

Addition of 2 equiv of lithium pyrrolide to Mo(NR)(CHCMe2R')(OTf)2(DME) (OTf = OSO2CF3; R = 2,6-i-Pr2C6H3, 1-adamantyl, or 2,6-Br2-4-MeC6H2; R' = Me or Ph) produces Mo(NR)(CHCMe2R')(NC4H4)2 complexes in good yield. All compounds can be recrystallized readily from toluene or mixtures of pentane and ether and are sensitive to air and moisture. An X-ray structure of a 2,6-diisopropylphenylimido species shows it to be an unsymmetric dimer, {Mo(NAr)(syn-CHCMe2Ph)(eta5-NC4H4)(eta1-NC4H4)}{Mo(NAr)(syn-CHCMe2Ph)(eta1-NC4H4)2}, in which the nitrogen in the eta5-pyrrolyl bound to one Mo behaves as a donor to the other Mo. All complexes are fluxional on the NMR time scale at room temperature, with one symmetric species being observed on the NMR time scale at 50 degrees C in toluene-d8. The dimers react with PMe3 (at Mo) or B(C6F5)3 (at a eta5-NC4H4 nitrogen) to give monomeric products in high yield. They also react rapidly with 2 equiv of monoalcohols (e.g., Me3COH or (CF3)2MeCOH) or 1 equiv of a biphenol or binaphthol to give 2 equiv of pyrrole and bisalkoxide or diolate complexes in approximately 100% yield.