The doping effect of fluorinated aromatic solvents on the rate of ruthenium-catalysed olefin metathesis

A study concerning the effect of using a fluorinated aromatic solvent as the medium for olefin metathesis reactions catalysed by ruthenium complexes bearing N-heterocyclic carbene ligands is presented. The use of fluorinated aromatic hydrocarbons (FAH) as solvents for olefin metathesis reactions catalysed by standard commercially available ruthenium pre-catalysts allows substantially higher yields of the desired products to be obtained, especially in the case of demanding polyfunctional molecules, including natural and biologically active compounds. Interactions between the FAH and the second-generation ruthenium catalysts, which apparently improve the efficiency of the olefin metathesis transformation, have been studied by X-ray structure analysis and computations, as well as by carrying out a number of metathesis experiments. The optimisation of reaction conditions by using an FAH can be regarded as a complementary approach for the design of new improved ruthenium catalysts. Fluorinated aromatic solvents are an attractive alternative medium for promoting challenging olefin metathesis reactions.     

Synthesis of phosphorus containing medium ring heterocycles by sequential Claisen rearrangement and ring closing metathesis

An efficient method for the synthesis of novel medium ring phosphorus containing heterocycles starting from phenol derivatives by ruthenium catalyzed ring closing metathesis is described. This work deals with a sequential aromatic Claisen-rearrangement, coupling of an allyl/vinyl phosphonate, and ring closing metathesis reaction. All of these reactions were carried out at ambient temperature to afford the medium-sized phosphorus heterocycles in excellent yields.     

Tandem Catalysis Utilizing Olefin Metathesis Reactions

Since olefin metathesis transformation has become a favored synthetic tool in organic synthesis, more and more distinct non‐metathetical reactions of alkylidene ruthenium complexes have been developed. Depending on the conditions applied, the same olefin metathesis catalysts can efficiently promote isomerization reactions, hydrogenation of C=C double bonds, oxidation reactions, and many others. Importantly, these transformations can be carried out in tandem with olefin metathesis reactions. Through addition of one portion of a catalyst, a tandem process provides structurally advanced products from relatively simple substrates without the need for isolation of the intermediates. These aspects not only make tandem catalysis very attractive from a practical point of view, but also open new avenues in (retro)synthetic planning. However, in the literature, the term “tandem process” is sometimes used improperly to describe other types of multi‐reaction sequences. In this Concept, a number of examples of tandem catalysis involving olefin metathesis are discussed with an emphasis on their synthetic value.     

1,7-Octadiene-Assisted Tandem Multicomponent Cross-Enyne Metathesis (CEYM)-Diels-Alder Reactions: A Useful Alternative to Mori's Conditions

The use of 1,7-octadiene as an in situ source of ethylene led us to develop a novel multicomponent tandem cross-enyne metathesis (CEYM)-Diels-Alder reaction. The process can be considered a relay metathesis, in which the ethylene liberated in the ring-closing metathesis (RCM) of 1,7-octadiene initiates the tandem sequence. Aliphatic, aromatic, and fluorinated alkynes and several dienophiles are compatible with the process, which is particularly efficient with aromatic alkynes. This methodology constitutes a useful variant of Mori's conditions in CEYM-related reactions.     

Z‐Selective Cross Metathesis with Ruthenium Catalysts: Synthetic Applications and Mechanistic Implications

Olefin cross metathesis is a particularly powerful transformation that has been exploited extensively for the formation of complex products. Until recently, however, constructing Z‐olefins using this methodology was not possible. With the discovery and development of three families of ruthenium‐based Z‐selective catalysts, the formation of Z‐olefins using metathesis is now not only possible but becoming increasingly prevalent in the literature. In particular, ruthenium complexes containing cyclometalated NHC architectures developed in our group have been shown to catalyze various cross metathesis reactions with high activity and, in most cases, near perfect selectivity for the Z‐isomer. The types of cross metathesis reactions investigated thus far are presented here and explored in depth.     

Removing ruthenium residues from olefin metathesis reaction products

Olefin metathesis has revolutionized the way chemists design and synthesize molecules, mostly due to the development of well-defined ruthenium catalysts with high oxygen-, moisture-, and functional-group tolerance. However, the complete removal of residual ruthenium after the end of a metathesis reaction often imposes significant challenges. This Minireview summarizes the strategies for the sequestration of ruthenium impurities from olefin metathesis post-reaction mixtures, thus comprising a practical guide for synthetic chemists employing ruthenium-catalyzed metathesis reactions in the synthesis of organic or polymeric materials.     

Ruthenium dihydride complexes as enyne metathesis catalysts

Ruthenium–catalyzed enyne metathesis is a reliable and efficient method for the formation of 1,3-dienes, a common structural motif in synthetic organic chemistry. The development of new transition-metal complexes competent to catalyze enyne metathesis reactions remains an important research area. This report describes the use of ruthenium (IV) dihydride complexes with the general structure RuH₂Cl₂(PR₃)₂ as new catalysts for enyne metathesis. These ruthenium (IV) dihydrides have been largely unexplored as catalysts in metathesis-based transformations. The reactivity of these complexes with 1,6 and 1,7-enynes was investigated. The observed reaction products are consistent with the metathesis activity occurring through a ruthenium vinylidene intermediate.     

1,7‐Octadiene‐Assisted Tandem Multicomponent Cross‐Enyne Metathesis (CEYM)‐Diels–Alder Reactions: A Useful Alternative to Mori’s Conditions

The use of 1,7‐octadiene as an in situ source of ethylene led us to develop a novel multicomponent tandem cross‐enyne metathesis (CEYM)‐Diels–Alder reaction. The process can be considered a relay metathesis, in which the ethylene liberated in the ring‐closing metathesis (RCM) of 1,7‐octadiene initiates the tandem sequence. Aliphatic, aromatic, and fluorinated alkynes and several dienophiles are compatible with the process, which is particularly efficient with aromatic alkynes. This methodology constitutes a useful variant of Mori’s conditions in CEYM‐related reactions.     

Carbon monoxide-promoted carbene insertion into the aryl substituent of an N-heterocyclic carbene ligand: Buchner reaction in a ruthenium carbene complex

In the presence of carbon monoxide, ruthenium carbenes give a net insertion/ring expansion (Buchner reaction) into one of the aromatic rings of the N-heterocyclic carbene ligand. In alkene metathesis applications, the N-heterocyclic carbene ligand is both robust and typically inert to reactions with the metal-bound carbene. This unique reaction is completely regioselective. The complexes obtained through ring expansion were fully characterized in the solid state using X-ray crystallography and in solution using NMR and IR spectroscopy.     

Synthesis and Catalytic Activity of a Two-core Ruthenium Carbene Complex: a Unique Catalyst for Ring Closing Metathesis Reaction

The reaction of a ruthenium carbide complex RuCl2(C:)(PCy3)2 with [H(Et2O)x]+[BF4]– at a molar ratio of 1:2 produced a two-core ruthenium carbene complex, {[RuCl(=CHPCy3)(PCy3)]2(μ-Cl)3}+·[BF4]–, in the form of a yellow-green crystalline solid in a yield of 94%. This two-core ruthenium complex is a selective catalyst for ring closing metathesis of unsubstituted terminal dienes. More importantly, no isomerized byproduct was observed for N-substrates when the two-core ruthenium complex was used as the catalyst at an elevated temperature(137 °C), indicating that the complex is a chemo-selective catalyst for ring closing metathesis reactions.     

Synthesis and Catalytic Study of Ruthenium Carbene Catalyst Containing a Zn‐Porphyrin Ligand

A ruthenium carbene complex containing a Zn‐porphyrin ligand has been developed. The complex was characterized by ¹H NMR, IR, HRMS and elemental analysis. The catalytic activity of the ruthenium carbene complex for olefin metathesis reactions was also investigated. The complex exhibited excellent performance for both ring‐closing and cross metathesis reactions at 35°C.     

A new concept for the noncovalent binding of a ruthenium-based olefin metathesis catalyst to polymeric phases: preparation of a catalyst on Raschig rings

A new concept for noncovalent immobilization of a ruthenium olefin metathesis catalyst is presented. The 2-isopropoxybenzylidene ligand of a Hoveyda-Grubbs carbene is further modified by an additional amino group (7) and immobilization is achieved by treatment with sulfonated polystyrene forming the corresponding ammonium salt. In this novel strategy for the immobilization of ruthenium-based metathesis catalysts, the amino group plays a two-fold role, being first an active anchor for immobilization and second, after protonation, activating the catalysts (electron donating to electron withdrawing activity switch). The polymeric support was prepared by precipitation polymerization which led to small bead sizes (0.2-2 microm) and large surface areas. Compared to commercial resins this tailor-made phase showed superior properties in immobilization of complex 7. This concept of immobilization was applied to glass-polymer composite megaporous Raschig rings. Ru catalyst 7 on Raschig rings was used under batch conditions in various metathesis reactions, including ring-closing (RCM), cross- (CM) and enyne metathesis, to give products of high chemical purity with very low ruthenium contamination levels (21-102 ppm). The same ring can be used for up to 6 cycles of metathesis.     

Sustainable concepts in olefin metathesis

Ruthenium-catalyzed olefin metathesis reactions represent an attractive and powerful transformation for the formation of new carbon-carbon double bonds. This area is now quite familiar to most chemists as numerous catalysts are available that enable a plethora of olefin metathesis reactions. Nevertheless, with the exception of uses in polymerization reactions, only a limited number of industrial processes use olefin metathesis. This is mainly due to difficulties associated with removing ruthenium from the final products. In this context, a number of studies have been carried out to develop procedures for the removal of the catalyst or the products of catalyst decomposition, however, none are universally attractive so far. This situation has resulted in tremendous activity in the area dealing with supported or tagged versions of homogeneous catalysts. This Review summarizes the numerous studies focused on developing cleaner ruthenium-catalyzed metathesis processes.     

Unprecedented Selectivity of Ruthenium Iodide Benzylidenes in Olefin Metathesis Reactions

The development of selective olefin metathesis catalysts is crucial to achieving new synthetic pathways. Herein, we show that cis‐diiodo/sulfur‐chelated ruthenium benzylidenes do not react with strained cycloalkenes and internal olefins, but can effectively catalyze metathesis reactions of terminal dienes. Surprisingly, internal olefins may partake in olefin metathesis reactions once the ruthenium methylidene intermediate has been generated. This unexpected behavior allows the facile formation of strained cis‐cyclooctene by the RCM reaction of 1,9‐undecadiene. Moreover, cis‐1,4‐polybutadiene may be transformed into small cyclic molecules, including its smallest precursor, 1,5‐cyclooctadiene, by the use of this novel sequence. Norbornenes, including the reactive dicyclopentadiene (DCPD), remain unscathed even in the presence of terminal olefin substrates as they are too bulky to approach the diiodo ruthenium methylidene. The experimental results are accompanied by thorough DFT calculations.     

A rapid and simple cleanup procedure for metathesis reactions

[structure: see text]. A new method for easy removal of ruthenium from metathesis reactions by using a polar isocyanide is reported. This protocol removed most ruthenium byproducts from a variety of synthetically useful metatheses. Moreover, the isocyanide-promoted carbene insertion results in rapid destruction of carbene reactivity, demonstrated in the commonly used first- and second-generation Grubbs' carbenes.     

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.     

Aqueous olefin metathesis

According to popular belief, oxygen and water are the natural enemies of organometallic reactions and therefore must be excluded rigorously from the reaction vessel. This belief is founded in the case of the highly reactive nucleophilic metal alkylidene complexes that were used in early catalytic olefin metathesis. However, owing to the high stability of the ruthenium carbene complexes introduced by Grubbs, metathesis in water has become reality.     

Organometall‐Reaktionen in Wasser

Water has attracted significant attention as an alternative solvent for organometallic reactions because it is nontoxic, nonflammable, and inexpensive, and is easily separated from organic products. Organometallic reactions, like the palladium‐catalyzed couplings of organic halides with organoboron compounds (Suzuki) and organotin reagents (Stille), are among the most widely used reactions for the formation of carbon‐carbon bonds. Owing to the discovery of water‐soluble, sulfonated phosphane derivatives and particularly the design of water‐soluble palladium‐catalysts it was possible to import these reactions into aqueous media. Another efficient, metal‐catalyzed, carbon‐carbon bond‐forming process that is nowadays possible in aqueous media is the olefin metathesis. The approaches so far include the use of water‐soluble ruthenium‐catalysts, surfactants and additives, ultrasonication, the introduction of polar quaternary ammonium groups or the incorporation of PEG as a water solubilizing moiety. The last point bears also a great potential for further developments in the removal of ruthenium‐containing byproducts. Additionally, water is the ideal reaction environment for polar, water soluble substrates such as natural product or pharmaceuticals.     

ESIMS Studies and Calculations on Alkali‐Metal Adduct Ions of Ruthenium Olefin Metathesis Catalysts and Their Catalytic Activity in Metathesis Reactions

Electrospray ionization mass spectrometry (ESIMS) and subsequent tandem mass spectrometry (MS/MS) analyses were used to study some important metathesis reactions with the first‐generation ruthenium catalyst 1 , focusing on the ruthenium complex intermediates in the catalytic cycle. In situ cationization with alkali cations (Li⁺, Na⁺, K⁺, and Cs⁺) using a microreactor coupled directly to the ESI ion source allowed mass spectrometric detection and characterization of the ruthenium species present in solution and particularly the catalytically active monophosphine–ruthenium intermediates present in equilibrium with the respective bisphosphine–ruthenium species in solution. Moreover, the intrinsic catalytic activity of the cationized monophosphine–ruthenium complex 1 a ?K⁺ was directly demonstrated by gas‐phase reactions with 1‐butene or ethene to give the propylidene Ru species 3 a ?K⁺ and the methylidene Ru species 4 a ?K⁺, respectively. Ring‐closing metathesis (RCM) reactions of 1,6‐heptadiene ( 5 ), 1,7‐octadiene ( 6 ) and 1,8‐nonadiene ( 7 ) were studied in the presence of KCl and the ruthenium alkylidene intermediates 8 , 9 , and 10 , respectively, were detected as cationized monophosphine and bisphosphine ruthenium complexes. Acyclic diene metathesis (ADMET) polymerization of 1,9‐decadiene ( 14 ) and ring‐opening metathesis polymerization (ROMP) of cyclooctene ( 18 ) were studied analogously, and the expected ruthenium alkylidene intermediates were directly intercepted from reaction solution and characterized unambiguously by their isotopic patterns and ESIMS/MS. ADMET polymerization was not observed for 1,5‐hexadiene ( 22 ), but the formation of the intramolecularly stabilized monophosphine ruthenium complex 23 a was seen. The ratio of the signal intensities of the respective with potassium cationized monophosphine and bisphosphine alkylidene Ru species varied from [I _4a ]/[I ₄ ]=0.02 to [I _23a ]/[I ₂₃ ]=10.2 and proved to be a sensitive and quantitative probe for intramolecular π‐complex formation of the monophosphine–ruthenium species and of double bonds in the alkylidene chain. MS/MS spectra revealed the intrinsic metathesis catalytic activity of the potassium adduct ions of the ruthenium alkylidene intermediates 8 a , 9 a , 10 a , 15 a , and 19 a , but not 23 a by elimination of the respective cycloalkene in the second step of RCM. Computations were performed to provide information about the structures of the alkali metal adduct ions of catalyst 1 and the influence of the alkali metal ions on the energy profile in the catalytic cycle of the metathesis reaction.     

以N-杂环卡宾为配体的金属络合物催化有机合成的反应

综述了近几年来以N-杂环卡宾为配体的金属络合物催化有机合成的反应。