Complexes chiral ruthenium

Zhang et al. [49] prepared a chiral ruthenium complex coordinated by a pyridine-bis(imine) ligand (structure 43 in Scheme 21). 

The first example of an asymmetric reduction of C=N bonds proceeding via DKR was reported in 2005 by Lassaletta et al. In this process, the transfer hydrogenation of 2-substituted bicyclic and monocyclic ketimines could be accomplished via DKR by using a HCO2H/TEA mixture as the hydrogen source and a chiral ruthenium complex including TsDPEN ligand,... 

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. 

The isomerization of the dioxepins83 was investigated using some chiral ruthenium complexes, and these catalysts gave moderate (up to 61% ee) enantioselectivities (Equation (24)).84... 

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]. 

Ruthenium complexes are capable of catalyzing halogen atom transfer reactions to olefins. This has been illustrated in the enantioselective atom transfer reactions of alkane and arene-sulfonyl chlorides and bro-motrichloromethanes to olefins using chiral ruthenium complexes. Moderate ee s up to 40% can be achieved for these transformations [74-77]. These specific reactions are believed to follow a radical redox transfer chain process. 

Cyclizations of dihydroxystilbene 256 using 4 mol % of chiral ruthenium complexes under photolytic conditions were investigated by Katsuki et al. (Scheme 65) [167]. Coordination of alcohols/phenols to Ru(IV) species generates a cation radical with concomitant reduction of metal to Ru(III). Cycli-zation of this oxygen radical followed by another cyclization provides the product 257. Catalyst 259 provided 81% ee of the product in chlorobenzene solvent. Optimization of the solvent polarity led to a mixture of toluene and f-butanol in 2 3 ratio as the ideal solvent. Substituents on the phenyl rings led to a decrease in selectivity. Low yields were due to the by-product 258. 

Chiral ruthenium complexes, with luminescence characteristics indicative of binding modes, and stereoselectivities that may be tuned to the helix topology, may be useful molecular probes in solution for nucleic acid secondary structure36). 

S. Cheng, H. Wan, K. Tsai, andT. Ikariya, Asymmetric transfer hydrogenation of prochiral ketones catalyzed by chiral ruthenium complexes with aminophos-phine ligands, J. Mol. Cat. A Chemical 1999, 147, 105-111. 

A number of methods have been developed for the asymmetric reduction of acetylpyridines. Asymmetric hydrogenation of 2- and 3-acetylpyridine occurs in high yield and >94% ee to give the (A)-alcohol product using 2.5 mol% chiral ruthenium complex 71 in 2-propanol in an autoclave pressurized with 50 atm of hydrogen gas <20000L1749>. 

Noyori, R. and S. Hashiguchi, Asymmetric Transfer Hydrogenation Catalyzed by Chiral Ruthenium Complexes, Accounts of Chemical Research, 30, 97-102 (1997). 

Fig. 11 Structures of chiral ruthenium complexes used as catalysts in epoxidations using a variety of oxygen donors...

Consiglio and Morandini and co-workers (67) have investigated the stereochemistry involved in the addition of acetylenes to chiral ruthenium complexes. Reaction of propyne with the separated epimer of the chiral ruthenium phosphine complex 34 at room temperature results in the chemo- and stereospecific formation of the respective propylidene complex 64. An X-ray structure of the product (64) proves that the reaction proceeds with retention of configuration at the ruthenium center. The identical reaction utilizing the epimer with the opposite configuration at ruthenium (35) also proceeded with retention of configuration at the metal center, proving that the stereospecificity of the reaction in not under thermodynamic control [Eq. (62)]. 

The main methodologies developed until now for enantioselective oxidation of sulfides are effective only in the oxidation of alkyl aryl sulfoxides. Dialkyl sulfoxides on the other hand are generally oxidized with only poor selectivity. In an attempt to solve this problem, Schenk s group69 recently reported a stereoselective oxidation of metal-coordinated thioethers with DMD. The prochiral thioether is first coordinated to a chiral ruthenium complex by reaction with the chloride complexes [CpRu[(S,S)-chiraphos]Cl], 36. Diastereoselective oxygen transfer from DMD produces the corresponding sulfoxides in high yield and selectivity. The chiral sulfoxides 37 are liberated from the complexes by treatment with sodium iodide. Several o.p. aryl methyl sulfoxides have been obtained by this method in moderate to high ee (Scheme 12). 

Ohkubo and coworkers studied the photooxidative racemic resolution of rac-1, T-bi-2-naphthol (rac-38) with the axially chiral ruthenium complex A-[Ru(menbpy)3]2+ (39, menbpy = 4,4 -dimenthoxycarbonyl-2,2 -bipyridine)... 

Scheme 75 Highly enantioselective imidation of allylic sulfides with chiral ruthenium complex 311...

Preparative Methods by ring-closure of 3-bromobutyric acid with Sodium Carbonate, or by hydrogenation of Diketene. The optically active forms are obtained in the same manner starting from (J )- or (,S)-3-bromobutyric acid, which may be resolved with the (5) form of I-(I-Naphthyl)ethylamine Asymmetric aldol condensation using an enantiopure iron acetyl complex followed by cyclization, or asymmetric hydrogenation of diketene catalyzed by a chiral ruthenium complex, also gives the optically active p-lactone. 

Asymmetric lactonization of prochiral diols has been performed vsdth chiral phosphine complex catalysts (Ru2Cl4((-)-DIOP)3 and [RuCl((S)-BINAP)(QH6)]Cl [17, 18]. Kinetic resolution of racemic secondary alcohol was also carried out with chiral ruthenium complexes 7 and 8 in the presence of a hydrogen acceptor, and optically active secondary alcohols were obtained with >99% e.e. (Eqs. 3.7 and 3.8) [19, 20]. 

Enantioselective allylic substitutions catalyzed by transition-metal complexes are a powerful method for constructing complex organic molecules [4f,55]. Palladium-based catalysts have often given excellent results. To expand the scope of the reaction, a new enantioselective allylic alkylation catalyzed by planar-chiral ruthenium complexes was developed [56]. For example, the reaction of l,3-diphenyl-2-propenyl ethyl carbonate with sodium dimethyl malonate in the presence of 5 mol% of a planar chiral (S)-ruthenium complex (Figure 5.3) at 20 °C for 6 h in THE resulted in the formation of the corresponding chiral allylic alkylated product of dimethyl 2-((2 )(lS)-l,3-diphenylprop-2-enyl)propane-l,3-dioate in 99% yield vsdth 96% e.e. (Eq. 5.33). 

Noyori R, Hashiguchi S, Iwasawa Y. Asymmetric transfer hydrogenation catalyzed by chiral ruthenium complexes. Acc. Chem. 

Better results are obtained with ruthenium complexes of the corresponding thiono-lactones. Reductive ring cleavage with LiAlH4, then methylation and decomplexation, leads to the thioether (76 % ee). The chiral ruthenium complex recovered in this step can be converted back into the starting complex in two steps (Sch. 7) [55]. 

Asymmetric transfer hydrogenation with a chiral ruthenium complex is an alternative option for preparation of substituted phenethyl alcohols, which are important building blocks for the agricultural fungicide, (S)-MA20565 [47]. In the enantioselective synthesis of antidepressant sertraline (50), different chiral secondary alcohols have been proposed as pivotal intermediates (Scheme 14). Reduction of the keto ester 46 catdyzed by oxazaborolidine 45 provides chiral intermediate 47 in 90% ee [48]. Alternatively, reductive fragmentation of C2-symmetric oxa-tricyclic alkene 48 with DIBAL catalyzed by a BINAP-Ni complex generates a novel intermediate 49 in 88 % yield with 91% ee [49]. 

Bianchi, M., Matteoli, U., Menchi, G., Frediani, P., Piacenti, F. Asymmetric synthesis by chiral ruthenium complexes. VIII. The asymmetric Cannizzaro reaction. J. Organomet. Chem. 1982, 240, 65-70. 

Hamada et al. have reported the enantioselective, photocatalytic oxidation of (R)-and (S)-l,l -bi-2-naphthol (2 and 3) with [Ru(menbpy)3] 12 a chiral ruthenium complex [93]. A-12 was photoexcited using filtered visible light (X > 400 nm), and the excited complex oxidized by [Co(acac)3], before returning to its ground state oxidation level through reaction with the diol. The (5)-diol was found to be the most reactive of the two enantiomers, and after 13.8% conversion, 2 was present in 15.2% ee (Scheme 10). However, this value was observed to decrease steadily as the percentage conversion increased. 

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