Ruthenium complexes mechanism with

Ruthenium compounds are widely used as catalysts for hydrogen-transfer reactions. These systems can be readily adapted to the aerobic oxidation of alcohols by employing dioxygen, in combination with a hydrogen acceptor as a cocatalyst, in a multistep process. For example, Backvall and coworkers [85] used low-valent ruthenium complexes in combination with a benzoquinone and a cobalt Schiff s base complex. The proposed mechanism is shown in Fig. 14. A low-valent ruthenium complex reacts with the alcohol to afford the aldehyde or ketone product and a ruthenium dihydride. The latter undergoes hydrogen transfer to the benzoquinone to give hydroquinone with concomitant... 

Reaction of the dinuclear ruthenium complexes, XXV, with propene gives n-allylruthenium complexes, XXVI. A mechanism is shown in Scheme 3. 

As carboxylic acid additives increased the efficiency of palladium catalysts in direct arylations through a cooperative deprotonation/metallation mechanism (see Chapter 11) [45], their application to ruthenium catalysis was tested. Thus, it was found that a ruthenium complex modified with carboxylic acid MesC02H (96) displayed a broad scope and allowed for the efficient directed arylation of triazoles, pyridines, pyrazoles or oxazolines [44, 46). With respect to the electrophile, aryl bromides, chlorides and tosylates, including ortho-substituted derivatives, were found to be viable substrates. It should be noted here that these direct arylations could be performed at a lower reaction temperatures of 80 °C (Scheme 9.34). 

The three steps 32-34 have been suggested77 to be equilibria, and the overall equilibrium must lie far to the left because no adduct 23 is found in the reaction mixture when the reaction of sulfonyl chloride with olefin is carried out in the absence of a tertiary amine. A second possible mechanism involving oxidative addition of the arenesulfonyl halide to form a ruthenium(IV) complex and subsequent reductive elimination of the ruthenium complex hydrochloride, [HRulvCl], was considered to be much less likely. 

Dendrimers can be constructed from chemical species other than purely organic monomers. For example, they can be built up from metal branching centres such as ruthenium or osmium with multidentate ligands. The resulting molecules are known as metallodendrimers. Such molecules can retain their structure by a variety of mechanisms, including complexation, hydrogen bonding and ionic interactions. 

Reduction of acetophenone by PrOH/H has been studied with the ruthenium complexes [Ru(H)(ri2-BH )(CO)L(NHC)], (L = NHC, PPh3, NHC = IMes, IPr, SIPr). The activity of the system is dependent on the nature of the NHC and requires the presence of both PrOH and H, implying that transfer and direct hydrogenation mechanisms may be operating in parallel [15]. 

A related study with a similar ruthenium catalyst led to the structural and NMR characterization of an intermediate that has the crucial Ru—C bond in place and also shares other features with the BEMAP-ruthenium diacetate mechanism.33 This mechanism, as summarized in Figure 5.4, shows the formation of a metal hydride prior to the complexation of the reactant. In contrast to the mechanism for acrylic acids shown on p. 378, the creation of the new stereocenter occurs at the stage of the addition of the second hydrogen. 

Barrau and coworkers have synthesized a series of iron and ruthenium complexes by irradiation of Me2HGe(CH)KGeMe2H and Me2HGe(CH)K SiMe2H (n = 1, 2) in the presence of Fe(CO)5 and Ru3(CO)i293. In each case irradiation causes CO loss, with the formation of the M(CO)4 species (reaction 43). When n = 2 the products are photostable with n = 1 (65) a mixture of products (66-69) are obtained due to secondary photolysis (reaction 44). The mechanism, outlined in Scheme 23, is presented to explain these observations. 

Allyl methylcarbonate reacts with norbornene following a ruthenium-catalyzed carbonylative cyclization under carbon monoxide pressure to give cyclopentenone derivatives 12 (Scheme 4).32 Catalyst loading, amine and CO pressure have been optimized to give the cyclopentenone compound in 80% yield and a total control of the stereoselectivity (exo 100%). Aromatic or bidentate amines inhibit the reaction certainly by a too strong interaction with ruthenium. A plausible mechanism is proposed. Stereoselective CM-carboruthenation of norbornene with allyl-ruthenium complex 13 followed by carbon monoxide insertion generates an acylruthenium intermediate 15. Intramolecular carboruthenation and /3-hydride elimination of 16 afford the -olefin 17. Isomerization of the double bond under experimental conditions allows formation of the cyclopentenone derivative 12. 

The proposed mechanisms are similar in both cases and involve in particular an (aryl)(hydrido)ruthenium intermediate in which the ruthenium is additionally coordinated by an in. ( ////-generated /V-phenylimine moiety tethered to the same Ru-bound aromatic ring. The C-C bond-forming step for the construction of the corresponding heterocyclic framework proceeds via insertion of the C=N double bond into the C-Ru bond with transfer of the (hydrido) ruthenium complex to the now phenylamine nitrogen. The desired heterocycles 158 and 159 were obtained after successive reductive elimination, deamination, and dehydrogenation. 

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... 

Ruthenium complexes do not have an extensive history as alkyne hydrosilylation catalysts. Oro noted that a ruthenium(n) hydride (Scheme 11, A) will perform stepwise alkyne insertion, and that the resulting vinylruthenium will undergo transmetallation upon treatment with triethylsilane to regenerate the ruthenium(n) hydride and produce the (E)-f3-vinylsilane in a stoichiometric reaction. However, when the same complex is used to catalyze the hydrosilylation reaction, exclusive formation of the (Z)-/3-vinylsilane is observed.55 In the catalytic case, the active ruthenium species is likely not the hydride A but the Ru-Si species B. This leads to a monohydride silylmetallation mechanism (see Scheme 1). More recently, small changes in catalyst structure have been shown to provide remarkable changes in stereoselectivity (Scheme ll).56... 

Although the molybdenum and ruthenium complexes 1-3 have gained widespread popularity as initiators of RCM, the cydopentadienyl titanium derivative 93 (Tebbe reagent) [28,29] can also be used to promote olefin metathesis processes (Scheme 13) [28]. In a stoichiometric sense, 93 can be also used to promote the conversion of carbonyls into olefins [28b, 29]. Both transformations are thought to proceed via the reactive titanocene methylidene 94, which is released from the Tebbe reagent 93 on treatment with base. Subsequent reaction of 94 with olefins produces metallacyclobutanes 95 and 97. Isolation of these adducts, and extensive kinetic and labeling studies, have aided in the eluddation of the mechanism of metathesis processes [28]. 

In the transition metal-catalyzed reactions described above, the addition of a small quantity of base dramatically increases the reaction rate [17-21]. A more elegant approach is to include a basic site into the catalysts, as is depicted in Scheme 20.13. Noyori and others proposed a mechanism for reactions catalyzed with these 16-electron ruthenium complexes (30) that involves a six-membered transition state (31) [48-50]. The basic nitrogen atom of the ligand abstracts the hydroxyl proton from the hydrogen donor (16) and, in a concerted manner, a hydride shift takes place from the a-position of the alcohol to ruthenium (a), re-... 

When aldehydes, with or without a hydrogen, are treated with aluminum ethoxide, one molecule is oxidized and another reduced, as in 9-69, but here they are found as the ester. The process is called the Tishchenko reaction. Crossed Tishchenko reactions are also possible. With more strongly basic alkoxides, such as magnesium or sodium alkoxides, aldehydes with an a hydrogen give the aldol reaction. Like 9-69, this reaction has a mechanism that involves hydride transfer.751 The Tishchenko reaction can also be catalyzed752 by ruthenium complexes.753 by boric acid,754 and, for aromatic aldehydes, by disodium tetracarbonylferrate Na2Fe(CO)4,755 OS I, 104. 

Despite acetonitrile s feeble acidity (pATa ca 29) compared with enolizable aldehydes (67, pA s 16-17), the combination of a simple ruthenium complex, [RuCp(PPli3)2]+, and diazabicycloundecane (DBU) brings about a nitrile-selective deprotonation to give /I-hydroxynitriles (68).274 A mechanism is proposed in which DBU, aldehyde, and acetonitrile can displace triphenylphosphines, with the metal centre activating acetonitrile to convert it to an NC-CFU- ligand (proposed intermediate, 69). A nickel-diarylamidodiphosphine complex (70) also catalyses this transformation in the presence of DBU.275... 

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