Carbenes ruthenium

Acyclic diene molecules are capable of undergoing intramolecular and intermolec-ular reactions in the presence of certain transition metal catalysts molybdenum alkylidene and ruthenium carbene complexes, for example [50, 51]. The intramolecular reaction, called ring-closing olefin metathesis (RCM), affords cyclic compounds, while the intermolecular reaction, called acyclic diene metathesis (ADMET) polymerization, provides oligomers and polymers. Alteration of the dilution of the reaction mixture can to some extent control the intrinsic competition between RCM and ADMET. 

X(A1C13) = 0.5) to immobilize a ruthenium carbene complex for biphasic ADMET polymerization of an acyclic diene ester (Figure 7.4-2). The reaction is an equilibrium processes, and so removal of ethylene drives the equilibrium towards the products. The reaction proceeds readily at ambient temperatures, producing mostly polymeric materials but also 10 % dimeric material. 

A benzannulation reaction yielding the naphthoquinone 61 could also be performed with the ruthenium carborane-stabilised carbene 60 and 1-hexyne [56] (Scheme 36). The ruthenium carbene unit can be regarded as an 18-electron fragment containing a formal Ru(II) centre coordinated to a dianionic six-electron-donor cobaltacarborane ligand. 

Scheme 8 Ruthenium carbene complexes from propargyl halides [16]...

Scheme 9 Ruthenium carbene complexes from alkynes and application [17]...

Van der Schaaf et al. described a synthesis of the 14-electron complex [RuHCl(PPr13)2] (32) from [RuCl2(COD)]A.,PPr31,isopropanol,and abase.Compound 32 is a suitable precursor for ruthenium carbene complex 33, as outlined in Scheme 10. Although 32 was isolated and structurally characterized, it may also be generated in situ for the preparation of the carbene complex 33 [18]. 

Ruthenium hydride complexes, e.g., the dimer 34, have been used by Hofmann et al. for the preparation of ruthenium carbene complexes [19]. Reaction of 34 with two equivalents of propargyl chloride 35 gives carbene complex 36 with a chelating diphosphane ligand (Eq. 3). Complex 36 is a remarkable example because its phosphine ligands are, in contrast to the other ruthenium carbene complexes described so far, arranged in a fixed cis stereochemistry. Although 36 was found to be less active than conventional metathesis catalysts, it catalyzes the ROMP of norbornene or cyclopentene. 

Scheme 10 Ruthenium carbene complexes from ruthenium hydride species prepared in situ [18]...

As a final example in this section, a contribution by Grubbs et al. is discussed. The chloride-free ruthenium hydride complex [RuH2(H2)2(PCy3)2] (37) is believed to react, in the presence of alkenes, to form an unidentified ruthenium(O) species which undergoes oxidative additions with dihalo compounds, e.g., 38, to give the corresponding ruthenium carbene complex 9 (Eq. 4) [20]. 

Eq. 14) [81]. Although this transformation does not appear to be a metathesis reaction, it is thought to proceed via the formation of ruthenium carbene species and not via classical [2+2+2]-cycloaddition pathways. A rationale for the strong preference of the meta isomer 99 was provided on the basis of a metathesis-type mechanism. 

While diene metathesis or diyne metathesis are driven by the loss of a (volatile) alkene or alkyne by-product, enyne metathesis (Fig. 2) cannot benefit from this contributing feature to the AS term of the reaction, since the event is entirely atom economic. Instead, the reaction is driven by the formation of conjugated dienes, which ensures that once these dienes have been formed, the process is no longer a reversible one. Enyne metathesis can also be considered as an alkylidene migration reaction, because the alkylidene unit migrates from the alkene part to one of the alkyne carbons. The mechanism of enyne metathesis is not well described, as two possible complexation sites (alkene or alkyne) exist for the ruthenium carbene, leading to different reaction pathways, and the situation is further complicated when the reaction is conducted under an atmosphere of ethylene. Despite its enormous potential to form mul-... 

Second-generation ruthenium-carbene complex, (tricyclohexylphosphine-[l,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene][benzylidene]ruthenium (IV) dichloride),4e was purchased from Strem Chemicals. 

Ru—C(carbene) bond distances are shorter than Ru—P bond lengths, but this can simply be explained by the difference in covalent radii between P and The variation of Ru—C(carbene) bond distances among ruthenium carbene complexes illustrates that nucleophilic carbene ligands are better donors when alkyl, instead of aryl, groups are present, with the exception of 6. This anomaly can be explained on the basis of large steric demands of the adamantyl groups on the imidazole framework which hinder the carbene lone pair overlap with metal orbitals. Comparison of the Ru—C(carbene) bond distances among the aryl-substituted carbenes show... 

Several examples of carbene complexes have been structurally characterized (Fig. 5), and selected data for Ru(TPP)(=C(C02Et)2)(Me0H). Os(TTP)-(=C(p-C(,H4Me)2)(THF), Os(TTP)(=CHSiMe2)(THF), Os(TTP)(=SiEt2THF)-(THF) and a /x-carbido phthalocyanine complex, Ru(Pc)(py)]2C, are given in Table The ruthenium carbene complex has a Ru=C bond signifi-... 

NHC-bearing ruthenium carbene complexes are used to design latent and switch-able initiators. In this research field emphasis is given to the development of initiators, which are inactive at room temperature in the presence of the corresponding monomer and can be activated upon a proper stimulus such as heat [130] or light [131]. Once activated, a high polymerisation activity is desired which is provided by NHC co-ligands [132]. 

For the last 2 decades ruthenium carbene complexes (Grubbs catalyst first generation 109 or second generation 110, Fig. 5.1) have been largely employed and studied in metathesis type reactions (see Chapter 3) [31]. However, in recent years, the benefits of NHC-Ru complexes as catalysts (or pre-catalysts) have expanded to the area of non-metathetical transformations such as cycloisomerisation. 

Further improvements in activity of the ruthenium carbene complexes were achieved by incorporation of methyl groups in 3,4-position of imidazol-2-ylidene moiety. Introduction of sulfur in the trara-position to the N-heterocyclic carbene leads to increased stability of the resulting ruthenium complexes. The synthesis and the first applications of these new rathenium complexes are described herein. 

Formation of a trinuclear ruthenium carbene complex via the olefin scission reaction has also been noted (68) ... 

H-NMR studies of oligocarbene Ru(II) complexes indicate a substantial barrier to rotation about the metal-carbene carbon and nitrogen-R bonds. This restricted rotation is thought to arise as a consequence of intramolecular non-bonding cis interactions of the carbene nitrogen-R substituents, and not because of any significant double bond character in ruthenium-carbene carbon (76). 

Metatheses of 1,7-octadienes containing various functional groups are catalysed by ruthenium carbene complexes of the type 248. For instance, the alcohol 249 (R = CH2OH), the aldehyde 249 (R = CHO) and the carboxylic acid 249 (R = CO2H) are all converted into the corresponding cyclohexenes 250 in 82-88% yields (equation 127) and the heterocycles 252 (n = 0, 1 or 2) are efficiently produced from the amides 251 (equation 128)123. 

Thermolysis of ruthenium carbene complexes leads to intramolecular r/> -bcnzylic C-H functionalization in the presence of a hydrogen-accepting olefin (Equation (35)).44,44a. 

Quite recently, ruthenium carbene complexes more typically known as olefin metathesis catalysts have been shown to act as alkyne hydrosilylation catalysts.78,79 7Vzz r-addition is the major product with trialkylsilanes, even in a single example with an internal alkyne.78 This result represents one of the very few examples of fra r-hydrosilylation of internal alkynes. 

Keywords Ruthenium-carbenes, Ruthenium-allenylidenes, Ring closing metathesis, Natural product synthesis, Fine chemicals. 

In a formal sense, complexes 1 represent pre-catalysts that convert in the first turn of the catalytic cycle (vide infra) into ruthenium methylidene species of type 3 which are believed to be the actual propagating species in solution (Schemes 2,4). The ease of formation of 3 strongly depends on the electronic properties of the original carbene moiety in 1. In addition to complexes la-c with R1=CH=CPh2, ruthenium carbenes with Rx=aryl (e.g. Id, Scheme 3) constitute another class of excellent metathesis pre-catalysts, which afford the methylidene complex 3 after an even shorter induction period [5]. In contrast, any kind of electron-withdrawing (e.g. -COOR) or electron-donating substitu-... 

The olefin binding site is presumed to be cis to the carbene and trans to one of the chlorides. Subsequent dissociation of a phosphine paves the way for the formation of a 14-electron metallacycle G which upon cycloreversion generates a pro ductive intermediate [ 11 ]. The metallacycle formation is the rate determining step. The observed reactivity pattern of the pre-catalyst outlined above and the kinetic data presently available support this mechanistic picture. The fact that the catalytic activity of ruthenium carbene complexes 1 maybe significantly enhanced on addition of CuCl to the reaction mixture is also very well in line with this dissociative mechanism [11] Cu(I) is known to trap phosphines and its presence may therefore lead to a higher concentration of the catalytically active monophosphine metal fragments F and G in solution. 

The ruthenium carbene complexes 1 discussed in the previous chapter have set the standards in the field of olefin metathesis and are widely appreciated tools for advanced organic synthesis [3]. A serious drawback, however, relates to the preparation of these compounds requiring either 3,3-diphenylcyclopropene or diazoalkanes, i.e. reagents which are rather difficult to make and/or fairly hazardous if used on a large scale [60]. Therefore, a need for metathesis catalysts persists that exhibit essentially the same activity and application profile as 1 but are significantly easier to make. 

In particular, ruthenium carbenes 1 are more sensitive to the substitution pattern of the alkenes than the molybdenum catalyst 24 [19]. While the latter reacts readily even with di- and tri-substituted double bonds and is apparently the only catalyst capable of producing tetrasubstituted cycloalkenes (cf. Table 2, en-... 

In turn, the propensity of 1 to respond to steric hindrance can be used to control the site of initiation of an RCM reaction in a polyene substrate (Scheme 9) [20]. Thus, dienyne 25 reacts with the catalyst regioselectively at the least substituted site the evolving ruthenium carbene 26 undergoes a subsequent enyne metathesis leading to a new carbene 27, which is finally trapped by the disubsti-tuted olefin to afford the bicyclo[4.4.0]decadiene product 28. By simply reversing the substitution pattern of the double bonds, the complementary bicyclo [5.3.0] compound 32 is formed exclusively, because the cyclization cascade is then triggered at the other end of the substrate. Note that in both examples tri-substituted olefins are obtained by means of a ruthenium based metathesis catalyst [20] ... 

The ruthenium carbene catalysts 1 developed by Grubbs are distinguished by an exceptional tolerance towards polar functional groups [3]. Although generalizations are difficult and further experimental data are necessary in order to obtain a fully comprehensive picture, some trends may be deduced from the literature reports. Thus, many examples indicate that ethers, silyl ethers, acetals, esters, amides, carbamates, sulfonamides, silanes and various heterocyclic entities do not disturb. Moreover, ketones and even aldehyde functions are compatible, in contrast to reactions catalyzed by the molybdenum alkylidene complex 24 which is known to react with these groups under certain conditions [26]. Even unprotected alcohols and free carboxylic acids seem to be tolerated by 1. It should also be emphasized that the sensitivity of 1 toward the substitution pattern of alkenes outlined above usually leaves pre-existing di-, tri- and tetrasubstituted double bonds in the substrates unaffected. A nice example that illustrates many of these features is the clean dimerization of FK-506 45 to compound 46 reported by Schreiber et al. (Scheme 12) [27]. 

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