Highly active chiral ruthenium catalysts for asymmetric ring-closing olefin metathesis

The synthesis of olefin metathesis catalysts containing chiral, monodentate N-heterocyclic carbenes and their application to asymmetric ring-closing metathesis (ARCM) are reported. These catalysts retain the high levels of reactivity found in the related achiral variants (1a and 1b). Using the parent chiral catalysts 2a and 2b and derivatives that contain steric bulk in the meta positions of the N-bound aryl rings (catalysts 3-5), five- through seven-membered rings were formed in up to 92% ee. The addition of sodium iodide to catalysts 2a-4a (to form 2b-4b in situ) caused a dramatic increase in enantioselectivity for many substrates. Catalyst 5a, which gave high enantiomeric excesses for certain substrates without the addition of NaI, could be used in loadings of < or =1 mol %. Mechanistic explanations for the large sodium iodide effect as well as possible mechanistic pathways leading to the observed products are discussed.     

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.     

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.     

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.     

Green and efficient synthesis of bidentate Schiff base Ru catalysts for olefin metathesis

A simple, green synthetic protocol to bidentate Schiff base substituted ruthenium carbene complexes is described. These complexes are potent catalysts for olefin metathesis, but the original synthetic route employs the Schiff base ligands in the form of highly toxic thallium(I) salts. The new protocol involves a two-step, quasi-one-pot approach and silver(I) carbonate as base. The target ruthenium carbene complexes are obtained in yields that are similar to, or better than, those of the original protocol.     

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.     

Highly active water-soluble olefin metathesis catalyst

A novel water-soluble ruthenium olefin metathesis catalyst supported by a poly(ethylene glycol) conjugated saturated 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene ligand is reported. The catalyst displays improved activity in ring-opening metathesis polymerization, ring-closing metathesis, and cross-metathesis reactions in aqueous media.     

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.     

Phosphites as ligands in ruthenium-benzylidene catalysts for olefin metathesis

The use of phosphites in second generation, ruthenium-based olefin metathesis pre-catalysts leads to an improvement in catalyst stability and activity at low catalyst loadings.     

Highly efficient ruthenium catalysts for the formation of tetrasubstituted olefins via ring-closing metathesis

[reaction: see text] A series of ruthenium-based metathesis catalysts with N-heterocyclic carbene (NHC) ligands have been prepared in which the N-aryl groups have been changed from mesityl to mono-ortho-substituted phenyl (e.g., tolyl). These new catalysts offer an exceptional increase in activity for the formation of tetrasubstituted olefins via ring-closing metathesis (RCM), while maintaining high levels of activity in ring-closing metathesis (RCM) reactions that generate di- and trisubstituted olefins.     

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.     

Highly selective ruthenium metathesis catalysts for ethenolysis

N-Aryl,N-alkyl N-heterocyclic carbene (NHC) ruthenium metathesis catalysts are highly selective toward the ethenolysis of methyl oleate, giving selectivity as high as 95% for the kinetic ethenolysis products over the thermodynamic self-metathesis products. The examples described herein represent some of the most selective NHC-based ruthenium catalysts for ethenolysis reactions to date. Furthermore, many of these catalysts show unusual preference and stability toward propagation as a methylidene species and provide good yields and turnover numbers at relatively low catalyst loading (<500 ppm). A catalyst comparison showed that ruthenium complexes bearing sterically hindered NHC substituents afforded greater selectivity and stability and exhibited longer catalyst lifetime during reactions. Comparative analysis of the catalyst preference for kinetic versus thermodynamic product formation was achieved via evaluation of their steady-state conversion in the cross-metathesis reaction of terminal olefins. These results coincided with the observed ethenolysis selectivities, in which the more selective catalysts reach a steady state characterized by lower conversion to cross-metathesis products compared to less selective catalysts, which show higher conversion to cross-metathesis products.     

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.     

Oxygen‐chelated indenylidene ruthenium catalysts for olefin metathesis

Olefin metathesis is a transition metal‐mediated transformation that rearranges the carbon atoms of the carbon–carbon double bond of olefins. This reaction has become one of the most important and powerful reactions. Therefore development of new, well‐defined, highly active and selective catalysts is very desirable and a valuable goal. This mini‐review mainly introduces the development of ruthenium catalysts in olefin metathesis highlighting oxygen‐chelated indenylidene ruthenium catalysts. Applying an alkoxyl group on the indenylidene ligand fragment can generate the Ru ? O chelating bond. Additionally, various modifications of the ligand as well as the catalytic activity for ring‐closing metathesis reaction and selectivity of cross metathesis reaction are overviewed. Finally, the perspectives on the development of new catalysts are summarized. Copyright © 2015 John Wiley & Sons, Ltd.     

Simply assembled and recyclable polymer-supported olefin metathesis catalysts

Polymer-supported ruthenium catalysts (PCy(3))(2)Ru(=C(H)Ph)Cl(2), (PCy(3))Ru(IMes)(=C(H)Ph)Cl(2), and (PCy(3))Ru(SIMes)(=C(H)Ph)Cl(2), where IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene and SIMes = 1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene, have been prepared and found to be effective "boomerang" catalysts for ring-closing metathesis. They are recyclable, show comparable or better reactivity than their homogeneous counterparts, tolerate functional groups, and perform very well with dienes and moderately well with highly hindered substrates.     

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.