Chiral Mono- and Bidentate Ligands Derived from d-Mannitol and Their Application in Rhodium(I)-Catalyzed Asymmetric Hydrogenation Reactions

摘要

The new monodentate phosphoramidites 8a-g and bidentate phospholanes 13a-e are prepared in an ex-chiral-pool synthesis from d-mannitol. Chiral diols 7-ag, obtained via nucleophilic ring opening of bis(epoxides) 6a-b, are the key intermediates for the production of both classes of ligands. Treatment of 8a-g or 13a-e with [PdCl_2(COD)] or [Rh(COD)_2][SO_3CF_3] yield the corresponding Pd (10a, 10f, 15a) and Rh compounds (9a-g and 14a-e), respectively. The C_2 symmetry of the complexes in the solid state is demonstrated by X-ray structure investigations performed on 10a, 10f and 15a. Surprisingly high enantioselectivities in the asymmetric hydrogenation of itaconic acid (94% ee) and a-acetamidocinnamic acid (89% ee) are observed on using the Rh complex 9g bearing two monodentate phosphoramidite ligands. Although the chelating bis(phospholanes) described herein are more effective, the adjustable synthesis of the monodentate phosphoramidites may permit the optimization of asymmetric catalytic reactions.