Olefin hydrogenation ruthenium catalysts

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... 

Hydrogenation of Functionalized Olefins with Ruthenium Catalysts... 

Several S/N ligands have also been investigated for the asymmetric hydrogenation of prochiral olefins. Thus, asymmetric enamide hydrogenations have been performed in the presence of S/N ligands and rhodium or ruthenium catalysts by Lemaire et al., giving enantioselectivities of up to 70% ee. Two... 

In recent years, the asymmetric hydrogenation of prochiral olefins have been developed in the presence of various chiral sulfur-containing ligands combined with rhodium, iridium or more rarely ruthenium catalysts. The best results have been obtained by using S/P ligands, with enantioselectivities of up to 99% ee in... 

Another possible reason that ethylene glycol is not produced by this system could be that the hydroxymethyl complex of (51) and (52) may undergo preferential reductive elimination to methanol, (52), rather than CO insertion, (51). However, CO insertion appears to take place in the formation of methyl formate, (53), where a similar insertion-reductive elimination branch appears to be involved. Insertion of CO should be much more favorable for the hydroxymethyl complex than for the methoxy complex (67, 83). Further, ruthenium carbonyl complexes are known to hydro-formylate olefins under conditions similar to those used in these CO hydrogenation reactions (183, 184). Based on the studies of equilibrium (46) previously described, a mononuclear catalyst and ruthenium hydride alkyl intermediate analogous to the hydroxymethyl complex of (51) seem probable. In such reactions, hydroformylation is achieved by CO insertion, and olefin hydrogenation is the result of competitive reductive elimination. The results reported for these reactions show that olefin hydroformylation predominates over hydrogenation, indicating that the CO insertion process of (51) should be quite competitive with the reductive elimination reaction of (52). 

J. Wolf, W. Stuer, C. Grunwald, H. Werner, P. Schwab, and M. Schulz, Ruthenium Trichloride, Tricyclohexylphosphane, 1-Alkynes, Magnesium, Hydrogen, and Water-Ingredients of an Efficient One-Pot Synthesis of Ruthenium Catalysts for Olefin Metathesis, Angew. Chem. Int. Ed. 37, 1124-1126 (1998). 

Recent reviews on olefin metathesis [1, 2], nonmetathesis [3], asymmetric hydrogenation [4] and organic synthesis reactions [5] have shown the potential of selected ruthenium catalysts. Among the emerging topics in which ruthenium catalysts play a crucial role are the selective transformations of multiple carbon-carbon bonds. 

Because ruthenium catalysts are relatively unreactive for alkene hydrogenation and they are poor for double-bond isomerization, these catalysts are particularly effective for the selective hydrogenation of monosubstituted alkenes in the presence of di- and tri-substituted olefins at ambient temperature under 2-3 atmospheres of hydrogen (Eqn. 15.28). Water in the reaction medium... 

The RuCl4 system is only an effective catalyst for the hydrogenation of activated olefins. Unactivated olefins form ruthenium adducts, but are not hydrogenated. 

In aqueous hydrochloric add solutions, ruthenium(II) chloride catalyzed the hydrogenation of water-soluble olefins such as maleic and fumaric acids [6], After learning so much of so many catalytic hydrogenation reactions, the kinetics of these simple Ru(II)-catalyzed systems still seem quite fascinating since they display many features which later became established as standard steps in the mechanisms of hydrogenation. The catalyst itself does not react with hydrogen, however, the mthenium(II)-olefin complex... 

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. 

Ruthenium-hydridophosphine complexes are well-known homogeneous hydrogenation catalysts . In some cases stable >/ -olefin complexes can be isolated. Thus ethylene and styrene react with RuH2(PPh3)4 to yield the Ru(0) complexes, Ru(PPh3)3(olefin). These reactions are accompanied by olefin hydrogenation " ... 

Ruthenium catalysts on carriers are specific for the hydrogenation of the carbonyl group in aliphatic aldehydes and ketones at atmospheric conditions. They reduce preferentially the carbonyl group first in the presence of an olefinic linkage in the compound so that in certain instances the olefinic bond can be preserved. Ruthenium is specifically active for the reduction of sugars to polyhydroxy alcohols. 

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