Cations ruthenium

Fig. 5.4. Schematic mechanism for enantioselective hydrogenation of methyl acetamidocinnamate (MAC) over a cationic ruthenium catalyst. Reproduced...

It has been postulated that these cycloheptenes must be formed via a 7r-allylruthenium intermediate (Scheme 59). The cyclization is initiated by activation of the allylic C-H bond to form the 7r-allylruthenium 234. The 1-exo-dig carboruthenation of the alkynoate 234 produces the hydrido-ruthenium enolate 235. Equilibration of 235 followed by reductive elimination gives the corresponding cycloheptenes 237 and regenerates the cationic ruthenium complex. 

Finally, Shibasaki and co-workers reported the use of acetonitrile as a C-nucleophile by cooperative activation with a soft cationic ruthenium catalyst, DBU as base, and a sodium salt (Scheme 76). 

Ruthenium catalysts have also been used in this context.200,201 In particular, the cationic ruthenium complex, CpRu(CH3CN)3PF6, in conjunction with carboxylic acid ligand 3, has been used to achieve the remarkably chemoselective allylation of a variety of alcohols via dehydrative condensation with allyl alcohol (Equation (50)).202 It is worth noting that this transformation proceeds with 0.05 mol% catalyst loading and does not require the use of excess allyl alcohol. 

Cationic ruthenium complexes of the type [Cp Ru(MeCN)3]PF6 have been shown to provide unique selectivities for inter- and intramolecular reactions that are difficult to reconcile with previously proposed mechanistic routes.29-31 These observations led to a computational study and a new mechanistic proposal based on concerted oxidative addition and alkyne insertion to a stable ruthenacyclopropene intermediate.32 This proposal seems to best explain the unique selectivities. A similar mechanism in the context of C-H activation has recently been proposed from a computational study of a related ruthenium(ll) catalyst.33... 

Table 6 Selective synthesis of a-vinylsilanes with a cationic ruthenium complex...

RCM was also employed as a key design element for the synthesis of tricolorin A (106). This novel strategy demonstrated the efficiency of macrocyclizations using the standard ruthenimum catalysis either by the classical Grubbs carbene complex or the recently developed cationic ruthenium aUenylidene complex as the... 

Since the first discovery of transition metal allenylidene complexes (M=G=C=C<) in 1976, " these complexes have attracted a great deal of attention as a new type of organometallic intermediates. Among a variety of such complexes, cationic ruthenium allenylidene complexes Ru =C=C=GR R, readily available by dehydration of propargylic alcohols coordinated to an unsaturated metal center, can be regarded as stabilized propargylic cation equivalents because of the extensive contribution of the ruthenium-alkynyl resonance form... 

Reactions of ruthenium catalyst precursors in carboxylic acid solvents with various salt promoters have also been described (170-172, 197) (Table XV, Expt. 7). For example, in acetic acid solvent containing acetate salts of quaternary phosphonium or cesium cations, ruthenium catalysts are reported to produce methyl acetate and smaller quantities of ethyl acetate and glycol acetates (170-172). Most of these reactions also include halide ions the ruthenium catalyst precursor is almost invariably RuC13 H20. The carboxylic acid is not a necessary component in these salt-promoted reactions as shown above, nonreactive solvents containing salt promoters also allow production of ethylene glycol with similar or better rates and selectivities. The addition of a rhodium cocatalyst to salt-promoted ruthenium catalyst solutions in carboxylic acid solvents has been reported to increase the selectivity to the ethylene glycol product (198). 

Fiedler, D., Pagliero, D., Brumaghim, J.L., Bergman, R.G. and Raymond, K.N. (2004) Encapsulation of cationic ruthenium complexes into a chiral self-assembled cage. Inorg. Chem., 43 (3), 846-848. 

The cationic ruthenium complex [(Cy3)2P(CO)(Cl)Ru=CHCH=C(CH3)2]+ BF4 has been identified as an effective catalyst for the coupling reaction of aniline and... 

With this experimental set-up, highly active, cationic ruthenium-carbene catalysts are used in ring-opening metathesis polymerization (ROMP). Four different structural features of the catalyst [ R2P(CH2) PR2-k2P XRu=CHR]+ (the halogen... 

The reaction of the cationic ruthenium complex [(CO)2(PhMe2P)Ru(773-allyl)]+ with sodium borohydride produces the corresponding ruthenacyclobutane complex among the principal products <2003JCD2603>. 

Because of its potential in synthesis, the propargylation of arenes is a focus of current interest. According to Yamamoto et al. the dinuclear cationic ruthenium complex 15 is exceptionally active in this type of reaction when applied at in 5 %... 

A ruthenium based catalytic system was developed by Trost and coworkers and used for the intermolecular Alder-ene reaction of unactivated alkynes and alkenes [30]. In initial attempts to develop an intramolecular version it was found that CpRu(COD)Cl catalyzed 1,6-enyne cycloisomerizations only if the olefins were monosubstituted. They recently discovered that if the cationic ruthenium catalyst CpRu(CH3CN)3+PF6 is used the reaction can tolerate 1,2-di- or tri-substituted alkenes and enables the cycloisomerization of 1,6- and 1,7-enynes [31]. The formation of metallacyclopentene and a /3-hydride elimination mechanism was proposed and the cycloisomerization product was formed in favor of the 1,4-diene. A... 

The moderate Lewis acidity of ruthenium complexes was used to promote catalytic Diels-Alder reaction of dienes and acrolein derivatives [21-23]. The enantioselective Diels-Alder reaction of methacrolein with dienes was catalyzed with cationic ruthenium complexes containing an arene or cyclo-pentadienyl (Cp) ligand and a chiral ligand such as phosphinooxazoline, pyridyl-oxazoline, monoxidized 2,2 -bis(diphenylphosphino)-1, T-binaphthyl (BINPO)or l,2-bis[bis(pentafluorophenyl)phosphanyloxy]-l,2-diphenylethane (BIPHOP-F). The reaction gave the cycloadduct in high yields with excellent... 

The synthesis of unsaturated compounds by C-C bond formation can also be carried out by coupling of alkenes with allenes, intermolecularly or intra-molecularly. Thus, 1,3-dienes were selectively obtained by coupling of allenes and vinyl ketones [28-30]. The reaction was catalyzed by the complex CpRuCl(COD) and with CeCl3 as a cocatalyst (Eq. 19). This cocatalyst is expected to decrease the chloride ion concentration to keep the active cationic ruthenium complex coordinatively unsaturated. 

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. 

A cyclobutene was recently obtained by a related reaction of dimethyl acetylenedicarboxylate with ethylene in the presence of a cationic ruthenium alkylidene catalyst precursor [43] (Eq. 35). 

Recently, complex Q [103] and other neutral and cationic ruthenium allenylidene complexes (U, V) [104] have been reported as efficient catalyst precursors for the ROMP of cyclic olefins such as cyclooctene and norbor-nene. 

The cationic ruthenium complex [CpRu(MeCN)3]+(PF6) catalyses the hydro-silylation of alkyne with triethylsilane in good yield and with very limited formation of Unear 1,2-disubstituted products (Eq. 17, Table 4). 

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