Noyori type catalyst

On the other hand, Backvall, Privalov and co-workers proposed an inner-sphere reaction mechanism based on experimental studies and DFT calculations for the reduction of imines with Shvo s catalyst (370,371). The same group demonstrated that imines have to be protonated before being reduced by ruthenium Noyori s catalyst (372). But kinetic studies on the reduction of cyclic imine I (Fig. 94) reported by Blackmond and coworkers (373) conclude that the un-protonated imine is reduced by a Rh(III) catalyst of the Noyori-type catalyst (Fig. 95), using formic acid as the hydrogen transfer agent. 

Recently, Wills and co-workers (344) studied the application of ruthenium catalysts of the Noyori-type catalyst containing W-alkylated TsDPEN derivatives bearing alkyl groups of different size as the chiral ligand for the reduction of cyclic imine I (Fig. 94). The authors suggest that a viable model for the reduction of this imine (Fig. 96) could involve a protonated imine with a transition state allowing the CH-jt interaction between the aryl group of the iminium cation and the arene coordinated to the ruthenium. 

Synthetically readily accessible, robust, organo-osmium ATH catalysts were most recently developed by Wills and co-workers [170]. The complexes 181 (Fig. 58) are osmium analogues of Noyori-type catalysts synthesised by a... 

Ding s group also extended this approach to the generation of self-supported Noyori-type catalysts 45 through hetero-coordination of an achiral bridged diphosphine 44 and a chiral bridged diamine ligand 40 with Ru(II) ions. The... 

Asymmetric hydrogenation has been achieved with dissolved Wilkinson type catalysts (A. J. Birch, 1976 D. Valentine, Jr., 1978 H.B. Kagan, 1978). The (R)- and (S)-[l,l -binaph-thalene]-2,2 -diylblsCdiphenylphosphine] (= binap ) complexes of ruthenium (A. Miyashita, 1980) and rhodium (A. Miyashita, 1984 R. Noyori, 1987) have been prepared as pure atrop-isomers and used for the stereoselective Noyori hydrogenation of a-(acylamino) acrylic acids and, more significantly, -keto carboxylic esters. In the latter reaction enantiomeric excesses of more than 99% are often achieved (see also M. Nakatsuka, 1990, p. 5586). 

Fig. 12 (a) DKR polymerization of 1,4-diol and dimethyl adipate, (b) Chemical structure of Noyori-type racemization catalyst 1 and Shvo s racemization catalyst 2... 

The applied catalytic system consisted of a Ru-Noyori-type racemization catalyst 1 (Fig. 12b) and Novozym 435. This catalyst combination tolerates a wide range of acyl donors, and it was expected that it would allow the use of bifunctional acyl donors for the formation of polycondensates. Before the start of the reaction, the monomer mixture showed the expected diastereomer ratio of (S,S) R,R) R,S) of 1 1 2 of the 1,4-diol employed. After 30 h of reaction the (5,5)-enantiomer almost completely disappeared, whereas the ratio of [R,R)- to (/ , 5)-monomer was ca. 3 1 (R S ca. 7 1). At a hydroxyl group conversion of 92% after 70h, no further conversion was observed and a final ratio of R,R) to R,S) of 16 1 (R S ca. 33 1) was obtained. Unfortunately, the molecular weights of the polymer were moderate at best (Mw = 3.4kDa) and Novozym 435 had to be added every few hours to compensate for the activity loss of the lipase. This suggests that Ru-catalyst 1 and Novozym 435 are not fully compatible. 

Hydrogenation of olefins, enols. or enamines with chiral fVilkinson type catalysts, e.g., Noyori hydrogenation. Hydroboration of olefins with chiral boranes. Sharpless epoxi-dation of allylic alcohols. 

Up to the mid-1980s the field of enantioselective hydrogenation had been dominated by the Rh-based Wilkinson-type catalysts. Then, Noyori et al. introduced a new family of Ru-based catalysts, which showed a wider applicability than the Rh catalysts. a, ff-Unsaturated acids other than dehydroamino acids became substrates which could be hydrogenated with high enantioselectivity... 

Racemization can be achieved with a variety of homogeneous catalysts. The Noyori type Ru-racemization catalyst 1 was first selected as a suitable candidate (Figure 11.3b). In fact, this was the first example in which DKR was combined with an enzyme-catalyzed polymerization reaction. It appeared, however, that polymerization with the Novozym 435/1 catalytic system was problematic only oligomers were obtained in two-step reactions because the catalysts were incompatible under the reaction conditions employed. 

Ligand-metal bifunctional catalysis provides an efficient method for the hydrogenation of various unsaturated organic compounds. Shvo-type [83-85] Ru-H/OH and Noyori-type [3-7] Ru-H/NH catalysts have demonstrated bifimctionality with excellent chemo- and enantioselectivities in transfer hydrogenations and hydrogenations of alkenes, aldehydes, ketones, and imines. Based on the isoelectronic analogy of H-Ru-CO and H-Re-NO units, it was anticipated that rhenium nitrosyl-based bifunctional complexes could exhibit catalytic activities comparable to the ruthenium carbonyl ones (Scheme 29) [86]. 

Parallel to this, another pathway is proposed. In a similar fashion to Noyori-Morris-type catalysts [37, 111, 112], H-H heterolytic cleavage of the polarized H2 ligand across the Re-N bond could occur. This step demonstrates a novel cooperative function of the bent nitrosyl ligand especially suited for H2 catalyses with polar reactivity characteristics [88]. The N-H moiety is then thought to be deprotonated by a labile c -positioned phosphine ligand affording 14e Re (III) hydrides, which seem to be more activated than the related 16e complexes. 

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