Olefin hydrogenation ruthenium-catalyzed

Asymmetric epoxidation of olefins with ruthenium catalysts based either on chiral porphyrins or on pyridine-2,6-bisoxazoline (pybox) ligands has been reported (Scheme 6.21). Berkessel et al. reported that catalysts 27 and 28 were efficient catalysts for the enantioselective epoxidation of aryl-substituted olefins (Table 6.10) [139]. Enantioselectivities of up to 83% were obtained in the epoxidation of 1,2-dihydronaphthalene with catalyst 28 and 2,6-DCPNO. Simple olefins such as oct-l-ene reacted poorly and gave epoxides with low enantioselectivity. The use of pybox ligands in ruthenium-catalyzed asymmetric epoxidations was first reported by Nishiyama et al., who used catalyst 30 in combination with iodosyl benzene, bisacetoxyiodo benzene [PhI(OAc)2], or TBHP for the oxidation of trons-stilbene [140], In their best result, with PhI(OAc)2 as oxidant, they obtained trons-stilbene oxide in 80% yield and with 63% ee. More recently, Beller and coworkers have reexamined this catalytic system, finding that asymmetric epoxidations could be perfonned with ruthenium catalysts 29 and 30 and 30% aqueous hydrogen peroxide (Table 6.11) [141]. Development of the pybox ligand provided ruthenium complex 31, which turned out to be the most efficient catalyst for asymmetric... 

Incorporation of rhodium triphenylphosphine moieties into carboranes has led to HRh(C2B9Hn)(PPh3)2 complexes, which are formally hydri-dorhodium(III) dicarbollides and which catalyze olefin hydrogenation under mild conditions (527). Iridium and ruthenium analogs are also known, including complexes with carboranylphosphine ligands, e.g., HRuCl(PPh3)(l-P(CH3)2-l,2-C2B, Hn]2 (,527-530). 

Taking advantage of the slow hydrogenation of carbon-carbon double bonds at room temperature in the presence of platinum dioxide, it was possible to perform the ruthenium-catalyzed cross coupling reaction of electron-deficient olefins such as conjugated enones and acrylic derivatives with allyl silanes in the presence of Pt(>2 under hydrogen (Scheme 46) [99]. Prolonged... 

The use of catalysts based on polymers with inverse temperature solubility, often copolymers of TV-isopropy-lacrylamide, to allow recovery by raising the temperature to precipitate the polymer for filtration,9 was mentioned in Chap. 5. The opposite, if the catalyst is soluble hot, but not cold, has also been used in ruthenium-catalyzed additions to the triple bonds of acetylenes (7.1).10 The long aliphatic tail of the phosphine ligand caused the catalyst to be insoluble at room temperature so that it could be recovered by filtration. There was no loss in yield or selectivity after seven cycles of use. A phosphine-modified poly(A-iso-propylacrylamide) in 90% aqueous ethanol/heptane has been used in the hydrogenation of 1-olefins.11 The mixture is biphasic at 22°C, but one phase at 70°C, at which the reaction takes place. This is still not ideal, because it takes energy to heat and cool, and it still uses flammable solvents. 

Transition metal complexes, zeolites, biomimetic catelysts have been widely used for various oxidation reactions of industrial and environmental importance [1-3]. However, few heterogenized polymeric catalysts have also been applied for such purpose. Mild condition oxidation catalyzed by polymer anchored complexes is attractive because of reusability and selectivity of such catalysts. Earlier we have reported synthesis of cobalt and ruthenium-glycine complex catalysts and their application in olefin hydrogenation [4-5]. In present study, we report synthesis of the palladium-glycine complex on the surface of the styrene-divinylbenzene copolymer by sequential attachment of glycine and metal ions and investigation of oxidation of toluene to benzaldehyde which has been widely used as fine chemicals as well as an intermidiate in dyes and drugs. 

Iridium-catalyzed hydrogenations of olefins and ketmies have also been reported. These reactions have not yet been studied computationally at the level of detail of the rhodium- and ruthenium-catalyzed reactions, and comparisons to experiments have been less clear. As a result, only a very general description of the mechanism and rationalization of the stereoselective outcome is available at this time. The results do, however, clearly demonstrates that amidst such mechanistic diversity of the iridium- and the ruthenium-catalyzed reactions, they prefer either the direct transfer after migratory insertion, or the concerted transfer. 

For example (a) Weissman, H., Song, X. and Mdstein, D. (2001) Ru-catalyzed oxidative coupling of arenes with olefins using O2. /. Am. Chem. Soc., 123, 337-8 (b) Kakiuchi, R, Sato, T., Yamauchi, M. et al. (1999) Ruthenium-catalyzed coupling of aromatic carbon-hydrogen bonds in aromatic imidates with olefins. Chem. Lett., 28, 19-20. 

The selective domino dienyne metathesis of 94 with Grubbs catalyst 2 to give the fused 5/9-bicyclic substructure embedded in 95 controlled by olefin substitution was coupled in a sequential manner with a chemoselective ruthenium-catalyzed hydrogenation of the disubstituted alkene of 95 to yield the tricyclic protected amino ketone 96 in a one-pot process (Scheme 2.35) [18h]. Only two further steps were required to convert 96 into the Lycopodium alkaloid (+)-lycoflexine. 

The intramolecular insertion of a hydride into a coordinated olefin is a crucial step in olefin hydrogenation catalyzed by late transition metal complexes, such as those of iridium, rhodium, and ruthenium (Chapter 15), in hydroformylation reactions catalyzed by cobalt, rhodium, and platinum complexes (Chapter 16), and in many other reactions, including the initiation of some olefin polymerizations. The microscopic reverse, 3-hydride elimination, is the most common pathway for the decomposition of metal-alkyl complexes and is a mechanism for olefin isomerizations. 

Ruthenium catalysts are now widely used for olefin hydrogenation, and many examples of enantioselective ruthenium-catalyzed hydrogenation are discussed in Section 15.7. Here, before addressing the issues of stereoselectivity, the elementary steps of ruthenium-catalyzed hydrogenation are discussed. These catalysts react through monohydride species containing a second anionic ligand. 

All of the olefins discussed so far contain a functional group, other than the C=C bond, that binds to the metal to create a defined structure. The asymmetric hydrogenation of olefins that lack this second functional group has been a major challenge. Few complexes of any type catalyze the hydrogenation of tri-substituted and tetra-substituted olefins, let alone catalyze asymmetric hydrogenation of these olefins. Recall from Section 15.3 on achiral catalysts for olefin hydrogenation that Wilkinson s catalyst and ruthenium-hydride complexes display little reactivity for the reduction of tri-substituted alkenes, and no reactivity for... 

Tse, M., Dobler, C., Bhor, S., et al (2004). Development of a Ruthenium-Catalyzed Asymmetric Epoxidation Procedure with Hydrogen Peroxide as the Oxidant, Angew. Chem. Int. Ed., 43, pp. 5255-5260 Tse, M., Bhor, S., Klawonn, M., et al, (2006). Ruthenium-Catalyzed Asymmetric Epoxidation of Olefins Using H2O2. Part 11 catalytic Activities and Mechanism, Chem. Eur.J., 12, pp. 1875-1888. 

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