Hydrogenation with ruthenium complexes

Miyashita, H. Takaya, T. Souchi and R. Noyori (1984) Tetrahedron Lett. 40,1245. 

Substrate Main product Catalyst % e.e. Configuration Reference 

X HOjC NHCOMe NHCOMe lmol% [S,S-DIOP]jRujCl4 DMA 1 atm H, 60°C 59 S e 

4 Carbon-carbon bond formation through o-oss-coupling 

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Scheme 5.5. Enantioselective Hydrogenation with Ruthenium Complex Catalysts... 

Catalytic hydrogenations with ruthenium complexes have been reviewed. Another article deals with the intramolecular stereocontrol of hydrogenation reactions by functional groups in the substrate, and concentrates on the reduction of unsaturated alcohols, ketones, and esters by cationic rhodium and iridium complexes. A review on Enantioselective Synthesis with Optically Active Transition... 

ENANTIOSELECTIVITY IN CATALYTIC ASYMMETRIC SYNTHESIS Table 4.4 Hydrogenations with ruthenium complexes... 

Figure 8.6 Reduction of ketone with ruthenium complex and alcohol dehydrogenase using molecular hydrogen as a hydrogen source [5c],...

More recently, these authors have reported the synthesis of a new thiophene-based analogue of (I ,i )-Me-DuPHOS called UlluPHOS. The facial recognition and enantioselection associated with ruthenium complexes of UlluPHOS and Me-DuPHOS were shown to be similarly high in various hydrogenations of p-keto esters (Scheme 8.32). The most important difference between these two ligands was found by comparing the reaction rates. Indeed, the authors have observed that the use of UlluPHOS considerably increased the activity of the complexes. 

Besides the electrochemical application, the (Cp )Rh(bpy)-complex 9 can also be used to reduce cofactors with hydrogen. In a recent study it was compared with ruthenium complex 13 [RuC12(TPPTS)2]2 (TPPTS tris(w-sulfonatophenyl)-phosphine Scheme 43.5). Both complexes were used to regenerate the cofactors in the reduction of 2-heptanone to (S)-2-heptanol, catalyzed by an ADH from Thermoanaerobium brockii (TfrADH) [46, 47]. The TON for both catalysts was 18. 

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... 

Steines, S. Wasserscheid, P. Driessen-Holscher, B. An Ionic Liquid as Catalyst Medium for Stereoselective Hydrogenations of Sorbic Acid with Ruthenium Complexes, J. Prakt. Chem. 2000, 342, 348-354. 

Transfer hydrogenation is another method widely used in ketone reduction. As a source of hydrogen, secondary alcohols like /PrOH are preferred, and the reaction is carried out in presence of KOH at the boiling point of the solvent. Reactions catalyzed with ruthenium complexes working at higher reaction temperatures were often performed in cyclohexanol or benzylalcohol, but, as a consequence of hydrogen transfer from the secondary alcohols produced, the reaction becomes reversible. 

Alkenes. There are only a few reports about enantioseleetive hydrogenation of alkenes with ruthenium complexes bearing / -stereogenic ligands. The substrates are the same as those used in rhodium-catalysed hydrogenations. Most of the results are Ksted in Table 7.7. 

When investigating the aqueous-phase bicarbonate hydrogenation with ruthenium and rhodium complexes, Benyei and J06 observed certain activity for the reverse reaction, that is, formate decomposition. [RuCl2(mTPPMS)2]2 (mTPPMS = meta-monosulfonated triphenylphosphine) decomposed sodium formate and formic acid (41), while RhCl(mTPPMS)3 slowly decomposed calcium formate and promoted calcium carbonate precipitation (42). 

Prince and Raspin [18] reported that decarbonylation of aldehydes with ruthenium complexes proceeded more slowly compared to that with rhodium. Moreover, the main product was the olefin and not the alkane. Evolving hydrogen may reduce the starting aldehyde depending of the substrate used. For instance, the decarbonylation of w-butanal with a binuclear PPhj-modified Ru complex gave propylene in 80% yield. In strong contrast, when an excess of butanal was employed, butanol was formed in excess. 

Refs. 5b and 5c and AUmang, G., Grass, J.M., Grosselin, J.M. and Mercier, C., Catalytic hydrogenation of all-traus-retinal to all-fraus-retinol with ruthenium complexes, /. MoL CataL, 1991,66, L27-L31. 

Fisher, J.M. Fulford, A. Bennett, P.S. (1992) Catalytic alkene epoxidation with ruthenium complexes and hydrogen-peroxide, /. Mol Catal, 11,229-34. 

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... 

A ruthenium complex [RuCl2(TPPTS)2]2 was used for regeneration of NADP+ to NADPH withhydrogen. Thus, 2-heptanonewas reduced with alcohol dehydrogenase from Thermoanaerobacter brockii in the presence of the mthenium complex, NAD P, and hydrogen at 60°C to (S)-2-heptanol in 40 % ee. Turnover number was reported to be 18 (Figure 8.6) [5cj. 

One other study of group 14 heteroallenes involving transition metals was reported in 1995. Jones et al. described the isolation of a ruthenium complex of a 1-silaallene (132—Scheme 32). The 1-silaallene also interacts with a hydrogen atom as well as the ruthenium metal center. Jones et al. describe this view... 

Reduction of acetophenone by PrOH/H has been studied with the ruthenium complexes [Ru(H)(ri2-BH )(CO)L(NHC)], (L = NHC, PPh3, NHC = IMes, IPr, SIPr). The activity of the system is dependent on the nature of the NHC and requires the presence of both PrOH and H, implying that transfer and direct hydrogenation mechanisms may be operating in parallel [15]. 

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