Alkylidene ruthenium complexes

Table 3 Ruthenium alkylidene complexes with JV-heterocyclic carbene ligands...

More recently, a new metathesis catalyst involving a ruthenium-alkylidene complex with a sterically bulky and electron-rich phosphine ligand has been synthesized and applied to RCM in aqueous media (Figure 3.5).197 This catalyst has the benefit of being soluble in almost... 

Scheme 6. Synthesis of ruthenium-alkylidene complexes starting at the azolium salt without isolating the NHCs.

In situ deprotonation combined with a substitution of a phosphine ligand was reported as a convenient way for the synthesis of ruthenium-alkylidene complexes (Scheme For imidazolidin-2-ylidenes, this is the only way... 

In order to investigate this point more fully, the rates of reaction of the two complexes with ethyl vinyl ether (EVE) were studied. This alkene was chosen as it is rather reactive towards ruthenium alkylidene complexes and forms an inert alkoxyalkylidene product in an essentially irreversible manner. This alkene, therefore, should rapidly capture any nascent complex from which a Cy3P ligand has dissociated (27 and 30 in Scheme 12.21). The two complexes displayed very different kinetics. The rate of reaction of the first generation pro-catalyst complex 24c with EVE was found to be dependent on EVE concentration (over a range of 30-120 equivalents of EVE) and did not reach pseudo-first-order conditions... 

Scheme 12.21 Contrasting kinetics for the irreversible reactions of first and second generation ruthenium alkylidene complexes 24c and 28 with ethyl vinyl ether (EVE).

The nucleophilic ruthenium alkylidene complex Cl2(PPh3)2Ru=CHCH= CPh2 also appeared to catalyse the polymerisation of norbornene the catalyst has been shown to be living with the norbornene monomer [63], Another highly strained monomer, bicyclo[3.2.0]hept-6-ene, has been polymerised in a living system with this ruthenium alkylidene catalyst [92] ... 

The activity of the ruthenium alkylidene complex can be greatly increased by exchanging triphenylphosphine ligands for the more electron-donating bulky tricyclohexylphosphine ligands a complex with the latter ligands is active for the polymerisation of only slightly strained cyclopentene [93,94]. 

Conjugated dienes were thus selectively obtained by hydrovinylation of alkynes catalyzed by a cationic ruthenium alkylidene complex [43] (Eq. 31). This reaction is thought to be promoted by the ruthenium hydride species resulting from the deprotonation of the <5-methyl group of the metallic precursor, followed by the sequential insertion of alkyne and ethylene into the metal-hydride and metal-vinyl bonds. 

It has been reported that activities of the ruthenium alkylidene complexes, which contain mesityl groups at the N atoms, are highly influenced by solvent [41]. Reactions in toluene occur substantially faster than those in CH2CI2. While treatment of diene 81 with 1.2 mol% of 71 in toluene led to essentially complete consumption of the starting material in 6.5 h, the same reaction took over 20 h in CH2CI2 by use of 4 mol% of 71 (Eq. 12.33) [42]. 

Olefin isomerization catalyzed by ruthenium alkylidene complexes can be applied to the deprotection of allyl ethers, allyl amines, and synthesis of cyclic enol ethers by the sequential reaction of RCM and olefin isomerization. Treatment of 70 with allyl ether affords corresponding vinyl ether, which is subsequently converted into alcohol with an aqueous HCl solution (Eq. 12.37) [44]. In contrast, the allylic chain was substituted at the Cl position, and allyl ether 94 was converted to the corresponding homoallylic 95 (Eq. 12.38). The corresponding enamines were formed by the reaction of 70 with allylamines [44, 45]. Selective deprotection of the allylamines in the presence of allyl ethers by 69 has been observed (Eq. 12.39), which is comparable with the Jt-allyl palladium deallylation methodology. This selectivity was attributed to the ability of the lone pair of the nitrogen atom to conjugate with a new double bond of the enamine intermediate. 

Copper(I) triflate was used as a co-catalyst in a palladium-catalyzed carbonylation reaction (Sch. 27). The copper Lewis acid was required for the transformation of homoallylic alcohol 118 to lactone 119. It was suggested that the CuOTf removes chloride from the organopalladium intermediate to effect olefin complexation and subsequent migratory insertion [60]. Copper(I) and copper(II) chlorides activate ruthenium alkylidene complexes for olefin metathesis by facilitating decomplexation of phosphines from the transition metal [61]. 

Substituted vinylphosphonates (195) and allylphosphonates (196) with E-olefin stereochemistry have been prepared for the first time via intermolecular olefin cross-metathesis (CM) using ruthenium alkylidene complex (197) in good yield. A variety of terminal olefins, styrenes and geminally substituted olefins has been successfully employed in these reactions (Scheme 49). ... 

Furthermore Grubbs et al. have published water-soluble as well as chiral ruthenium alkylidene complexes based on 16 for ARCM and AROM, whereas Schrock, Hoveyda and coworkers have synthesized a variety of asymmetric molybdenum alkylidene complexes, e.g. (5)-17 17,27 addition Hoveyda et al. have synthesized the achiral ruthenium complex 18 and the chiral complex 19 for ARCM and AROM. 

The reactions of vinyltrisubstituted silanes with allyl substituted heteroorganic compounds proceed in the presence of ruthenium alkylidene complexes (catalyst IV) as well as catalysts including an Ru-H bond (I) according to the following equation, giving two isomeric products (E + Z) and ethylene (Scheme 4). 

Direct arylations of alkenyl pronucleophiles with readily available chlorides as electrophiles occurred with high efficacy and excellent diastereoselectivity using either ruthenium alkylidene complexes or a ruthenium catalyst derived form air-stable SPO preligand (l-Ad)2P(0)H (59) (Scheme 24) [84],... 

The lessons learned from these complexes were eventually applied to the synthesis of well-defined ruthenium alkylidenes 8 and 9. Although they were insoluble in water, these alkylidenes could be used to initiate the living ROMP of functionalized norbornenes and 7-oxanorbomenes in aqueous emulsions. Substitution of the phosphine ligands in 9 for bulky, electron-rich, water-soluble phosphines produced water-soluble alkylidenes 10 and 11, which served as excellent initiators for the ROMP of water-soluble monomers in aqueous solution. These new ruthenium alkylidene complexes are powerful tools in the synthesis of highly functionalized polymers and organic molecules in both organic and aqueous environments. 

A pyrene derivative was also used as an anchor to immobilize a ruthenium alkylidene complex onto SWCNTs [94]. The immobilization of the Ru complex was performed by two different paths (1) adsorption of pyrene derivative precursor onto the sidewall of nanotubes by ti-ti interactions, followed by cross-metathesis with the ruthenium alkylidene complex, and (2) adsorption onto the sidewall of SWCNTs of the pyrene-substituted ruthenium alkylidene prepared previously. 

To date, two types of metallocene complexes have been used in the polymerization of olefins at CNT surfaces by PFT a ruthenium alkylidene complex anchored by two routes onto SWCNTs via a pyrene derivative spacer [94], and zirconium complexes of general formula [(R)2ZrCl2], R being a derivative of cyclopentadienyl or indenyl, anchored onto CNTs modified with MAO [97]. 

Another important mechanistic issue is the thermal decomposition of ruthenium alkylidene catalysts. To understand the decomposition pathways available in these systems, the thermolysis of two ruthenium alkylidene complexes, the propylidene (PCy3)2(Cl)2Ru=CHEt (3) and the methyhdene (PCy3)2(Cl)2Ru=CH2 (4), was examined in detail [93]. These two compounds were chosen because a variety of alkylidenes [as modeled by the propylidene (3)] and the methyhdene (4) are key intermediates in a range of olefin metathesis reachons with terminal alkenes. The studies revealed that the thermal decomposihon of the propylidene... 

Fig. 4.36. Synthesis of mixed NHC-phosphine ruthenium alkylidene complexes.

Although the well-defined water-resistant metal-alkylidene complexes do not dissolve in water, they can be used in emulsion polymerizations. Small size polymer particles and polymer latex can be prepared with this method in aqueous media in the presence of cationic surfactants (e.g., Dode cyltrimethylammonium bromide (DTAB)). Water-soluble, biologically active glycopolymers have been also synthesized with ruthenium-alkylidene complexes in... 

The RCM of <7,a)-dienes was successfully catalyzed with a new-generation ionic ruthenium-alkylidene complex in water (Figure Z9i)P... 

In a later study, bis-NHC ruthenium-alkylidene complex was activated under compressive strain [87] (Fig. 16). In order to initiate Ru-mediated polymerisation of norbomene in solid state, polymer catalyst (34 kg mol ) and a norbomene monomer were incorporated in a high molecular weight poly(tetrahydrofuran) (pTHF) matrix (Mn=170 kDa, PDI=1.3) which provided the physical cross-linking through the crystalline domains and allowed macroscopic forces to be transferred to the metal-ligand bonds. Consecutive compressions showed that up to 25% of norbomene monomer was polymerised after five loading cycles. 

Starting with a dimeric bis(allyl)ruthenium complex, the precatalyst Id was synthesized by complexation with tricyclohexylphosphane. After addition of trimethylsilyldiazomethane, the basic bisallylic ligands were displaced from the coordination sphere of the metal to form the catalytically active ruthenium(Il)-alkylidene species IdTMS [10]. Although the ruthenium-alkylidene complex generated in situ is easy to obtain, the structure of catalytically active species is poorly defined. Therefore, a control over the activity is difficult, and reproducible experimental results are almost impossible to achieve. 

Scheme 5.3 illustrates the most commonly used ruthenium-based olefin metathesis catalysts (I) the first well-defined, metathesis-active ruthenium alkylidene complex... 

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