Enantioselective hydrogenation using

Scheme 36.10 Enantioselective hydrogenation using libraries of peptide-based rhodium catalysts.

For recent syntheses of chiral allenes via enantioselective hydrogenation using (S)-BINAL-H, see K. M. Brummond,... 

Table 2. Enantioselective hydrogenation using BINAP complexes [12]...

Enantioselective hydrogenations using palladium on carbon as the catalyst in the presence of (,S )-proline have been reported40,41. This system is quite different from other heterogeneous enantioselective catalysts since the chiral agent is in solution together with the substrate, not merely on the surface of a previously prepared catalyst. This method has been effective in the hydrogenation of a./Tunsaturated ketones. A chiral iminium salt is considered as an intermediate, its C=N double bond is reduced with up to 60% optical purity. 

Achiwa et al. [46,47,48,49] further developed the approach to access the monobutyrolactone skeleton by creating a very efficient catalytic asymmetric method. Aryldensuccinic acid mono-methyl esters (23), obtained by Stobbe condensation of dimethyl succinate and the corresponding substituted aldehydes, were enantioselectively hydrogenated using a neutral rhodium (I) complex of (4[Pg.550]

Among the chemical methods, the catalytic epoxidation method proposed by Sharpless [84-87] and enantioselective hydrogenation using Noyori catalysts attract the most attention [88] for the kinetic resolution of drug enantiomers. 

The scope of the ruthenium-catalysed asymmetric hydrogenation is relatively wide and a diverse array of alkenes has been reduced with high enan-tioselectivity. For example, diketene (2.94), and unsaturated lactone (2.96) imdergo highly enantioselective hydrogenation using BINAP as ligand. The... 

Since, up to now, in the asymmetric version only poor enantiomeric excess values could be realized, the alternative, based on the enantioselective hydrogenation using the corresponding unsaturated ester as substrate (up to 77% ee), followed by reduction to the alcohol and final oxidation seems to be superior [162]. 

In Summary Hydrogenation of the double bond in alkenes requires a catalyst. This transformation occurs stereospecilically by syn addition and, when there is a choice, from the least hindered side of the molecule. This principle underlies the development of enantioselective hydrogenation using chiral catalysts. 

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