Ruthenium complex catalysts asymmetric

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... 

The significant potential of the ruthenium complex 65 was further underlined in the catalytic asymmetric ring-opening/cross metathesis of the cyclic alkene 70 (Scheme 44). This transformation is catalyzed by 5% mol of 65 at room temperature, in air, and with undistilled and nondegassed THF to deliver the corresponding diene 71 in 96% ee and 66% isolated yield. In standard conditions (distilled and degassed THF), the alkene 70 reacts in 75 min to give the diene in 95% ee and 76% yield, with only 0.5 mol % of catalyst. 

In 2008, Grisi et al. reported three ruthenium complexes 65-67 bearing chiral, symmetrical monodentate NHC ligands with two iV-(S)-phenylethyl side chains [74] (Fig. 3.26). Three different types of backbones were incorporated into the AT-heterocyclic moiety of the ligands. When achiral triene 57 was treated with catalysts 65-67 under identical reaction conditions, a dramatic difference was observed. As expected, the absence of backbone chirality in complex 65 makes it completely inefficient for inducing enantioselectivity in the formation of 58. Similarly, the mismatched chiral backbone framework of complex 66 was not able to promote asymmetric RCM of 57. In contrast, appreciable albeit low selectivity (33% ee) was observed when the backbone possessed anti stereochemistry. 

In 2000, these authors also developed a very efficient diphosphine-bithiophene ligand, tetraMe-BITIOP, which is depicted in Scheme 8.29. The ruthenium complex of this electron-rich diphosphine was used as the catalyst in asymmetric hydrogenation reactions of prostereogenic carbonyl functions of a-... 

Manufacture of ruthenium precatalysts for asymmetric hydrogenation. The technology in-licensed from the JST for the asymmetric reduction of ketones originally employed BINAP as the diphosphine and an expensive diamine, DAIPEN." Owing to the presence of several patents surrounding ruthenium complexes of BINAP and Xylyl-BINAP, [HexaPHEMP-RuCl2-diamine] and [PhanePHOS-RuCl2-diamine] were introduced as alternative catalyst systems in which a cheaper diamine is used. Compared to the BINAP-based systems both of these can offer superior performance in terms of activity and selectivity and have been used in commercial manufacture of chiral alcohols on multi-100 Kg scales. 

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. 

These reports sparked off an extensive study of metalloporphyrin-catalyzed asymmetric epoxidation, and various optically active porphyrin ligands have been synthesized. Although porphyrin ligands can make complexes with many metal ions, mainly iron, manganese, and ruthenium complexes have been examined as the epoxidation catalysts. These chiral metallopor-phyrins are classified into four groups, on the basis of the shape and the location of the chiral auxiliary. Class 1 are C2-symmetric metalloporphyrins bearing the chiral auxiliary at the... 

Besides ruthenium porphyrins (vide supra), several other ruthenium complexes were used as catalysts for asymmetric epoxidation and showed unique features 114,115 though enantioselectivity is moderate, some reactions are stereospecific and treats-olefins are better substrates for the epoxidation than are m-olcfins (Scheme 20).115 Epoxidation of conjugated olefins with the Ru (salen) (37) as catalyst was also found to proceed stereospecifically, with high enantioselectivity under photo-irradiation, irrespective of the olefmic substitution pattern (Scheme 21).116-118 Complex (37) itself is coordinatively saturated and catalytically inactive, but photo-irradiation promotes the dissociation of the apical nitrosyl ligand and makes the complex catalytically active. The wide scope of this epoxidation has been attributed to the unique structure of (37). Its salen ligand adopts a deeply folded and distorted conformation that allows the approach of an olefin of any substitution pattern to the intermediary oxo-Ru species.118 2,6-Dichloropyridine IV-oxide (DCPO) and tetramethylpyrazine /V. V -dioxide68 (TMPO) are oxidants of choice for this epoxidation. 

Noyori and coworkers reported well-defined ruthenium(II) catalyst systems of the type RuH( 76-arene)(NH2CHPhCHPhNTs) for the asymmetric transfer hydrogenation of ketones and imines [94]. These also act via an outer-sphere hydride transfer mechanism shown in Scheme 3.12. The hydride transfer from ruthenium and proton transfer from the amino group to the C=0 bond of a ketone or C=N bond of an imine produces the alcohol or amine product, respectively. The amido complex that is produced is unreactive to H2 (except at high pressures), but readily reacts with iPrOH or formate to regenerate the hydride catalyst. 

Carpentier and coworkers studied the asymmetric transfer hydrogenation of /f-keloeslers using chiral ruthenium complexes prepared from [(// -p-cyrriene)-RuC12]2 and chiral aminoalcohols based on norephedrine. During this study, these authors became aware of substrate inhibition when ketoesters carrying 4-halo-substituents were used. It transpired that this was caused by formation of a complex between the substrate and the catalyst [28]. 

Asymmetric hydrogenation of ketones is one of the more common reduction methods, with ruthenium complexes often used as catalysts, a topic which has been reviewed.305... 

A review of asymmetric hydrogenation of ketones with rhodium complexes as catalysts has been presented.330 A review of the developments in the asymmetric hydrogenation of ketones with ruthenium complexes as homogenous catalysts of hydrogenation, with particular emphasis on the work of Halpern, has been presented.331... 

A highly regio- and enantio-selective hydroformylation of alkenes, such as PhCH= CH2, CH2=CHCH2CN, and CH2=CHOAc, catalysed by ruthenium complexes with (g) 2,5-disubstituted phospholane ligands has been reported. With (83) as the ligand, the turnover rates over 4000 h-1 at 80 °C, have been attained.108 (Acac)Rh(CO)2-TangPhos [Tangphos = (84)] has been developed as a new enantioselective catalyst for asymmetric hydroformylation of norbornene and other [2.2.1]-bicyclic alkenes (55-92% ee).109... 

The first transition metal catalysis using BINAP-ruthenium complex in homogeneous phase for enantioselective hydrogenation of P-ketoesters was developed by Noyori and co-workers [31]. Genet and co-workers described a general synthesis of chiral diphosphine ruthenium(II) catalysts from commercially available (COD)Ru(2-methylallyl)2 [32]. These complexes preformed or prepared in situ have been found to be very efficient homogeneous catalysts for asymmetric hydrogenation of various substrates such as P-ketoesters at atmospheric pressure and at room temperature [33]. 

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