Shvo’s catalyst

Scheme 5.9 DKR using Shvo s catalyst as racemization catalyst and immobilized lipase TL to yield...

DKR of a-hydroxy ketones with Shvo s catalyst (racemization) and lipase TL (esterification) Lipase encapsulated by silicone elastomer spheres... 

A variety of ketones were hydrogenated using Shvo s catalyst at 100 °C using excess formic acid rather than H2 as the source of hydrogen [74], Excellent yields (> 90%) of alcohols were generally obtained in 6 h or less, with total turnovers in the range of 6000-8000. The unsaturated 16-electron Ru complex that results after hydrogen is delivered to the substrate is proposed to react with for-... 

Casey has suggested that the hydrogenation of alkenes by Shvo s catalyst may proceed by a mechanism involving loss of CO from the Ru-hydride complex, and coordination of the alkene. Insertion of the alkene into the Ru-H bond would give a ruthenium alkyl complex that can be cleaved by H2 to produce the alkane [75], If this is correct, it adds further to the remarkable chemistry of this series of Shvo complexes, if the same complex hydrogenates ketones by an ionic mechanism but hydrogenates alkenes by a conventional insertion pathway. 

Nevertheless, the mechanism of the Shvo s catalyst has been one of the most controversial regarding the nature of the hydrogen-transfer process (84). The analysis of this reaction mechanism served as an example of comparison of both the inner- and outer-sphere reaction pathways for hydrogenation of polar, C=0 (85-87) and C=N (88—95) and unpolar bonds (95). In the next subsections are presented the mechanistic studies we carried out for the hydrogenation of ketones, imines, alkenes, and alkynes (29,87,95). 

Fig. 4. Energy profiles in THF for both concerted pathways at B3LYP level for the hydrogenation of ketones by the Shvo s catalyst. Inner-sphere mechanism dashed fine outer-sphere mechanism solid line.

Scheme 20. Concerted outer-sphere mechanism for carbonyl hydrogenation by the Shvo s catalyst.

Fig. 5. Transition-states of the concerted outer-sphere mechanism for the hydrogenation of ketones in both the model (left) and complete (right) Shvo s catalysts.

An additional advantage of the use of AB monomers over AA-BB monomers is the lack of sensitivity on stoichiometric issues polymerization of the monomers (Scheme 11.14) employing Novozym 435 and Shvo s catalyst 2 was slow (reaction time = 170 h) but did result in chiral polymers of good molecular weigth and ee. For example, DKRP of Me-7HO resulted in a polymer with an Mp of 16.3 kg mol 1 and an ee of 92% (determined after acid-catalyzed methanolysis). 

A mechanism for reactions catalyzed by the [Ru(BINAP)(diamine)(H)J complex is shown in Scheme 15.15. " Related mechanisms have been proposed for transfer hydrogenations catalyzed by [Ru(arene)(NHjCHRCHRNHTs)H] complexes and by Shvo s catalyst, except for the portion of the catalytic cycle involving the regeneration of the dihydride. The mechanism deduced by many experiments and calculations involves... 

A widely-reported method for the DKR of secondary alcohols and a- and p-hydroxy acid esters involves ruthenium catalysed hydrogenation. No additional base is required as a cocatalyst (and consequently base-catalysed transesterification can be avoided) because one of the ligand s oxygen atoms can act as a basic centre. A robust ruthenium complex (named Shvo s catalyst) along with a p-chlorophenylacetate was developed by the BackvaU group. The metal catalyst must be used in combination with thermostable enzymes because it is activated by heat (Scheme 4.26). This system (with CALB) has been successfully used for the DKR of many secondary alcohols and diols (Scheme 4.27) [52, 63, 64]. 

Scheme 4.26 Racemization mechanism using Shvo s catalyst 1, dissociation of 1 into la and lb, which is the active component for racem-ization of an alcohol (step b).

Scheme 4.27 DKR of secondary alcohols using Shvo s catalyst.

As another application of this methodology, an efficient route to chiral epoxides was reported in 2002 by the same group based on the DKR of S-haloalcohols (Scheme 4.3). In this case, Shvo s catalyst was associated with a Pseudomonas species lipase, such as PS-C Pseudomonas cepacia lipase immobilised on ceramic particles). 

In 2002, Backvall s group also demonstrated that a combination of Pseudomonas cepacia (PS-C) lipase and Shvo s catalyst allowed the DKR of 6- or y-hydroxyacid derivatives to be achieved, as shown in Scheme 4.4. ... 

Scheme 4.1 DKRs of various alcohols with Shvo s catalyst.

Concerted catalyrtic reactions for the conversion of enol acetates or ketones to acetates catalysed by Shvo s catalyst. 

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