Ruthenium catalysts chiral complexes

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... 

Cornejo et al. [65] reported the first immobihzation of pyridine-bis(oxa-zoline) chiral hgands and the use of the corresponding solid ruthenium complex in the model cyclopropanation test. They synthesized vinyl-PyBOx, the vinyl functionahty being introduced in the fourth position of the pyridine ring. This monomer was further homo- or copolymerized in the presence of styrene and divinylbenzene. The corresponding ruthenium catalysts proved... 

These transition-metal catalysts contain electronically coupled hydridic and acidic hydrogen atoms that are transferred to a polar unsaturated species under mild conditions. The first such catalyst was Shvo s diruthenium hydride complex reported in the mid 1980s [41 14], Noyori and Ikatiya developed chiral ruthenium catalysts showing excellent enantioselectivity in the hydrogenation of ketones [45,46]. 

The use of chiral ruthenium catalysts can hydrogenate ketones asymmetrically in water. The introduction of surfactants into a water-soluble Ru(II)-catalyzed asymmetric transfer hydrogenation of ketones led to an increase of the catalytic activity and reusability compared to the catalytic systems without surfactants.8 Water-soluble chiral ruthenium complexes with a (i-cyclodextrin unit can catalyze the reduction of aliphatic ketones with high enantiomeric excess and in good-to-excellent yields in the presence of sodium formate (Eq. 8.3).9 The high level of enantioselectivity observed was attributed to the preorganization of the substrates in the hydrophobic cavity of (t-cyclodextrin. 

End and Pfalz have reported that chiral ruthenium-bisamide (26) complex is a useful catalyst for epoxidation of E-olefins with sodium periodate as a terminal oxidant, although enantiose-lectivity is moderate (Scheme 6B.25) [70]. It is noteworthy that tranT-(I-mcthylstyrene is a better substrate than m-P-methylstyrene for this reaction in terms of enantioselectivity. Competitive... 

Functionalized P-hydroxy esters 64 e,f,g,h were obtained quantitatively with excellent enantiomeric excesses (> 98 %) by hydrogenation of p-ketoesters in the presence of chiral ruthenium catalysts. This convenient methodology gives both optical antipodes with equal ease using (/ ) or (5) atropoisomer ligand for the metal complex. 

Apparently the first asymmetric hydrogenation with a chiral ruthenium catalyst was that reported by Hirai and Furuta (46a,b) using a ruthenium(III) complex of poly-L-methylethylenimine (PLMI)(VIII). The complex was not isolated, but a catalyst solution was prepared in situ by mixing RuC13 3H20... 

Analogues of chiral Pybox have been reported by other chemists and have been applied to ACP with ruthenium catalysts [37,38]. For example, Pybox substituted by a vinyl group at the 4-position of the pyridine skeleton was polymerized with styrene and divinylbenzene to give immobilized ligands, the ruthenium complexes of which were used to give 85% ee for ACP with EDA and styrene [38]. 

In addition to epoxides, three-membered nitrogen heterocycles, aziridines, can be obtained by means of catalytic asymmetric aziridinations (Eq. 30). To this aim, chiral ruthenium(salen) complexes 67 [56] and 68 [57] were useful (Fig. 1). The former phosphine complexes 67 gave the aziridine from two cy-cloalkenes with 19-83% ee [56]. On the other hand, terminal alkenes selectively underwent aziridination in the presence of the latter carbonyl complex 68 with 87-95% ee [57]. In these examples, N-tosyliminophenyliodinane or N-tosyl azide were used as nitrene sources. Quite recently, catalytic intramolecular ami-dation of saturated C-H bonds was achieved by the use of a ruthenium(por-phyrin) complex (Eq. 31) [58]. In the presence of the ruthenium catalyst and 2 equiv iodosobenzene diacetate, sulfamate esters 69 were converted into cyclic sulfamidates 70 in moderate-to-good yields. 

The simpler architecture is the 1,1 -biphenyl scaffold, likewise introduced by Hoveyda and coworkers [19]. The synthesis of the imidazolium salt starts with a chiral diamine and a substituted, achiral biphenyl [82-84], Subsequent introduction of a Mes substituent on the remaining primary amino end and ring closure reaction yields the chiral saturated imidazolium salt after hydrolysation of the methoxy group to liberate the phenolic hydroxy group (see Figure 4.22). Reaction with silver(I) oxide and carbene transfer to a Grubbs (Hoveyda) catalyst sets up the ruthenium catalyst complex. 

The chiral ruthenium(II) carbene complex 8, prepared from diazo(trimethylsilyl)methane, (p-cymene)2ruthenium(II) chloride, and 2,6-bis(4-isopropyloxazolinyl)pyridine, has been introduced as catalyst for the enantioselective cyclopropanation of alkenes with ethyl diazoacetate. The carbene complex 8 also serves as a transfer reagent for trimethylsilylcarbene and cyclopro-panates styrene in 34% yield. This reaction demonstrates the similarities between catalytic and stoichiometric cyclopropanations and between in situ generated and isolated transition metal carbenes. 

---