Ruthenium systems

Allylic alkylations of cinnamyl carbonate by sodium malonate have been studied with a series of ruthenium catalysts, obtained from the azohum salts 126-128 and the ruthenium complex 129 (Scheme 2.25) in MeCN or THF to give moderate yields of mixtures of alkylated products in the allylic and ipi o-carbons (90 10 to 65 35). The observed regioselectivity is inferior to similar ruthenium systems with non-NHC co-ligands. The stereoelectronic factors which govern the observed regioselectivity were not apparent [102]. 

Another stndy on binding to NHC complexes, that combined experiments and DFT (density functional theory) calculations was recently reported on a ruthenium system. This study shows the reversible binding of oxygen to the tetra-NHC complex [Ru(NHC) H)][BAr/] 6 (BAr/ = B (3,5-CF3) C H3 ), which leads to complex 7 (Scheme 10.2) [12]. Unexpectedly, the chemical shift of the hydride ligand undergoes a large downfield shift upon coordination to (from -41.2 ppm... 

In the late 1960s it was discovered (Entina, 1968 Binder et al., 1972) that a strong synergy effect exists in the platinum-ruthenium system. Alloys of these two metals are two to three orders of magnitude more active catalytically for the anodic oxidation of methanol than pure platinum, whereas pure ruthenium is altogether inactive for this reaction. Prolonged exploitation of such anodes is attended by a gradual decrease in catalytic activity of the alloys because of slow anodic dissolution of ruthenium from the surface layer. A similar simation is seen for platinum-tin alloys, which... 

Joachim C, Launay JP (1986) Bloch effective Hamiltonian for the possibility of molecular switching in the ruthenium-bipyridylbutadiene-ruthenium system. Chem Phys 109 93... 

Knowledge of the active site allows for speculation on the mechanism of H2-D20 exchange which these Fe4 systems catalyze 473,483). Ruthe-nium(III) systems catalyze such an exchange via a ruthenium(III) hydride intermediate (7, p. 73 Section II,A), as exemplified in reactions (82) and (83), and iron hydrides must be involved in the hydrogenase systems. Ruthenium(III) also catalyzes the H2 reduction of ruthenium(IV) via reaction (82), followed by reaction (84) (3), and using these ruthenium systems as models, a very tentative scheme has been proposed 473) for... 

Several ruthenium systems catalyze the hydrogenation of aromatic rings, and this topic is detailed in Chapter 16. An early example reported by Bennett and coworkers was that of RuHCl( 76-C6Me6)(PPh3), which catalyzed the hydrogenation of benzene to cyclohexane at 25 °C, 1 bar H2 [69]. Since ruthenium colloids are very active for this reaction under certain conditions, there is evidence that at least some of the reported catalysts are heterogeneous [70]. 

An interesting catalytic ruthenium system, Ru(7/5-C5Ar4OH)(CO)2H based on substituted cyclopentadienyl ligands was discovered by Shvo and coworkers [95— 98]. This operates in a similar fashion to the Noyori system of Scheme 3.12, but transfers hydride from the ruthenium and proton from the hydroxyl group on the ring in an outer-sphere hydrogenation mechanism. The source of hydrogen can be H2 or formic acid. Casey and coworkers have recently shown, on the basis of kinetic isotope effects, that the transfer of H+ and TT equivalents to the ketone for the Shvo system and the Noyori system (Scheme 3.12) is a concerted process [99, 100]. 

The concerted delivery of protons from OH and hydride from RuH found in these Shvo systems is related to the proposed mechanism of hydrogenation of ketones (Scheme 7.15) by a series of ruthenium systems that operate by metal-ligand bifunctional catalysis [86]. A series of Ru complexes reported by Noyori, Ohkuma and coworkers exhibit extraordinary reactivity in the enantioselective hydrogenation of ketones. These systems are described in detail in Chapters 20 and 31, and mechanistic issues of these hydrogenations by ruthenium complexes have been reviewed [87]. 

Similar mechanistic arguments can be applied to the ruthenium system mentioned above. 

A similar range of reactions has also been reported for the ruthenium carbonyl-triphenylphosphine systems (148). In these systems, a high percentage of the products were dinuclear, reflecting the weaker bonding in the ruthenium system, and as for some of the osmium complexes discussed above, some contain orthometallated phenylphos-phine groups (see Fig. 29, structures I, IV, X). 

DR. SUTIN The cobalt systems that you mention differ from the iron and ruthenium systems I discussed in that the electron transfer is also accompanied by a spin change the cobalt(III) complexes are low-spin and the cobalt(II) complexes are high-spin. Thus, the electron transfer is spin forbidden and should not occur since =0. It becomes allowed through... 

Quinolines can be prepared from the oxidative coupling and cyclation of the 2-aminobenzyl alcohol and ketones (Scheme 4) catalyzed by the system [RhCl(PPh3)3]/KOH [58]. The reactions were carried out in dioxane at 80 °C with 85% yield in 24 h (alcohol/Rh = 100/1). However, better yields are obtained with the related ruthenium system [RuCl2(=CHPh)(PCy3)2 [59]. 

The activity of this ruthenium system is comparable to, or somewhat greater than, that of cobalt catalysts under the same conditions of temperature and pressure. Rhodium catalysts provide substantially higher activity than either of these systems. As will be seen later, however, addition of ionic promoters can greatly increase the activity of ruthenium-based catalysts. 

Studies of ruthenium-catalyzed reactions in carboxylic acid solvents have been reported by Knifton (171,172), but most of these experiments contain added salt promoters which greatly modify the catalytic behavior. These experiments will be considered in Section V, along with other Lewis base-promoted ruthenium systems. 

The observation of glycerol triacetate as a trace product of CO hydrogenation by this ruthenium system in acetic acid solvent (179) suggests that glycolaldehyde (ester) can undergo further chain growth by the process outlined in (26) for the cobalt system. As with formaldehyde, however, a carboxylic acid is apparently necessary to promote formation of the metal-carbon bonded intermediate which can produce the longer-chain product. 

Reaction pressure has a dramatic effect on the rate of product formation in the halide-promoted ruthenium system. As shown in Fig. 18, the dependence of the glycol-forming reaction on total H2/CO pressure is approximately fourth-order, whereas that for the methanol-producing reaction is... 

When compared to the rhodium catalytic system, it can be seen that under identical conditions of temperature and pressure the iodide-promoted ruthenium system produces ethylene glycol at a comparable or somewhat lower rate. However, the rate of methanol formation is substantially higher than for the rhodium system. Thus, the overall activity of this ruthenium system is higher than that of the rhodium-based system, but the selectivity to the two-carbon product is lower. 

A mechanism possibly involving intermolecular hydride transfer in this promoted ruthenium system is thus very different from the reaction pathways presented for the cobalt and unpromoted ruthenium catalysts, where the evidence supports an intramolecular hydrogen atom transfer in the formyl-producing step. Nevertheless, reactions following this step could be similar in all of these systems, since the observed products are essentially the same. Thus, a chain growth process through aldehyde intermediates, as outlined earlier, may apply to this ruthenium system also. 

Recent work by Ford et al. demonstrates that a variety of metal carbonyl clusters are active catalysts for the water-gas shift under the same reaction conditions used with the ruthenium cluster (104a). In particular, the mixed metal compound H2FeRu3(CO)13 forms a catalyst system much more active than would be expected from the activities of the iron or ruthenium systems alone. The source of the synergetic behavior of the iron/ruthenium mixtures is under investigation. The ruthenium and ruthenium/iron systems are also active when piperidine is used as the base, and in solutions made acidic with H2S04 as well. Whether there are strong mechanistic similarities between the acidic and basic systems remains to be determined. 

Hydroxide ion attack, again leading to a nitro complex, has been observed in a ruthenium system. The process is reversed in the presence of H+ (equation 21). 

Both are stable metallocarbene complexes, but they have very different reactivity profiles. The molybdenum catalyst is highly reactive and is effective widi sterically demanding olefins. Its drawbacks are diat it is not highly tolerant of diverse functional groups and has high sensitivity to air, moisture, and solvent impurities. The ruthenium system, on die odier hand, is catalytically active in die presence of water or air, and it exhibits a remarkable functional group tolerance. It is not a reactive as the molybdenum catalyst, particularly toward sterically bulky substrates. However, it is readily available and is die reagent of choice for all but die most difficult substrates. 

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