Asymmetric Hydrogenation of Unsaturated Acids and Esters

Ll-Rh complex was employed for the enantioselective synthesis of (S)-2-(4-fluorophe-nyl)-3-mefhylbutanoic acid (98% ee) [107], while the Pr-DuPhos-Rh complex was utilized for the enantioselective hydrogenation of a,/ -unsaturated carboxylic acids, as exemplified by tiglic acid [29]. 

In contrast to the many examples of asymmetric hydrogenation of the parent itaconic acid or its dimethyl ester, very few ligands have been reported for the enantioselec- 

Chiral l,T-diphosphetanylferrocene Et-FerroTANE serves as an effective ligand for the rhodium-catalyzed hydrogenation of y9-aryl- and /9-alkyl-substituted monoamido ita-conates (Eqs. 19 and 20) [54]. The Et-DuPhos-Rh catalyst was utihzed for the asymmetric hydrogenation of the trisubstituted olefin derivative in the preparation of an important intermediate for the drug candoxatril ( 99% ee) [110]. 

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