Hydrido-ruthenium catalysts

Depending on the substrate, the enallenes 213 react with a ruthenium-hydrido catalyst to give either the initial product the methylenecyclopentanes 214 with a 1,4-diene substructure or to the conjugated vinylcyclopentenes 215. The latter are formed by a subsequent ruthenium-catalyzed isomerization of the initial cycization product 214 (Scheme 15.69) [136]. 

A number of ruthenium-based catalysts for syn-gas reactions have been probed by HP IR spectroscopy. For example, Braca and co-workers observed the presence of [Ru(CO)3l3]", [HRu3(CO)ii]" and [HRu(CO)4] in various relative amounts during the reactions of alkenes and alcohols with CO/H2 [90]. The hydrido ruthenium species were found to be active in alkene hydroformylation and hydrogenation of the resulting aldehydes, but were inactive for alcohol carbonylation. By contrast, [Ru(CO)3l3]" was active in the carbonylation of alcohols, glycols, ethers and esters and in the hydrogenation of alkenes and oxygenates. 

Recently, Grubbs and coworkers were able to isolate a ruthenium-hydrido complex, formed as a thermal degradation product of catalyst 2 which could be made responsible for double bond migration Hong, S.H., Day, M.W., Grubbs, R.H., J. Am. Chem. Soc. 2004,126 7414. 

One of the most successful embodiments of this concept combined a metathetical step with the selective isomerisation of a terminal C=C double bond, and found applications in natural product synthesis. As discussed in Sections 7.3.2.2 and 7.3.2.3, it is indeed possible to convert the metathetically active benzylidene initiator 20 into hydrido complex 43, simply by adding trimethyl(vinyloxy)silane or methanol to the reaction mixture, thereby triggering a consecutive catalytic isomerisation process. Other methods for the decomposition of ruthenium metathesis catalysts for use in tandem with olefin isomerisation reactions include treatment with hydrogen, formic acid, sodium borohydride, or sodium hydroxide in isopropanol. ... 

The catalyst used for these mechanistic studies has been characterized by X-ray crystallography, as shown in Figure 5.6. It is obtained as a hydrido ruthenium(II) species that is also coordinated by a [BH4 anion. The catalyst is prepared by exposing the DINAP-diamine RuC12 complex to excess NaBH 54... 

This finding is the consequence of the distribution of various ruthenium(II) hydrides in aqueous solutions as a function of pH [RuHCl(mtppms)3] is stable in acidic solutions, while under basic conditions the dominant species is [RuH2(mtppms)4] [10, 11]. A similar distribution of the Ru(II) hydrido-species as a function of the pH was observed with complexes of the related p-monosulfo-nated triphenylphosphine, ptpprns, too [116]. Nevertheless, the picture is even more complicated, since the unsaturated alcohol saturated aldehyde ratio depends also on the hydrogen pressure, and selective formation of the allylic alcohol product can be observed in acidic solutions (e.g., at pH 3) at elevated pressures of H2 (10-40 bar [117, 120]). (The effects of pH on the reaction rate of C = 0 hydrogenation were also studied in detail with the [IrCp (H20)3]2+ and [RuCpH(pta)2] catalyst precursors [118, 128].)... 

With ruthenium catalysts the same products are formed as in palladium catalysis, i.e. the Cg-6-lactone and traces of esters (compare Equation 6). Ruthenium(II)-hydrido-phosphine complexes such as RuH(OAc)-(PPh3)3, RuH2(PPh3)4, RuH(OAc)(CO)(PPh3)2 or RuH (C0)(PPh3)3 can be used as catalyst precursors. When triisopropyl phosphine is added as ligand yields up to 7 5l> are obtained. Of course, the yields are rather low, however, this is the first proof that both rhodium and ruthenium are active catalyst metals in diene/C02 chemistry. 

Cobalt, nickel, iron, ruthenium, and rhodium carbonyls as well as palladium complexes are catalysts for hydrocarboxylation reactions and therefore reactions of olefins and acetylenes with CO and water, and also other carbonylation reactions. Analogously to hydroformylation reactions, better catalytic properties are shown by metal hydrido carbonyls having strong acidic properties. As in hydroformylation reactions, phosphine-carbonyl complexes of these metals are particularly active. Solvents for such reactions are alcohols, ketones, esters, pyridine, and acidic aqueous solutions. Stoichiometric carbonylation reaction by means of [Ni(CO)4] proceeds at atmospheric pressure at 308-353 K. In the presence of catalytic amounts of nickel carbonyl, this reaction is carried out at 390-490 K and 3 MPa. In the case of carbonylation which utilizes catalytic amounts of cobalt carbonyl, higher temperatures (up to 530 K) and higher pressures (3-90 MPa) are applied. Alkoxylcarbonylation reactions generally proceed under more drastic conditions than corresponding hydrocarboxylation reactions. 

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