Phosphite-Thiother Ligands Derived from Carbohydrates allow the Enantioswitchable Hydrogenation of Cyclic β-Enamides by using either Rh or Ir Catalysts.

Abstract

Phosphite-thioether ligands with a simple modular architecture, derived from inexpensive l-(+)-tartaric acid and d-mannitol, have been for the first time successfully applied (ee values up to 99 %) in the synthesis of 2-aminotetralines and 3-aminochromanes by metal-catalyzed asymmetric hydrogenation of cyclic β-enamides. The ligands have the advantages of the robustness of the thioether/phosphite moieties and the extra control provided by the flexibility of the chiral pocket through the presence of a biaryl phosphite group and a modular carbohydrate-derived backbone. Moreover, they are solid and stable to air and they are therefore easy to handle, manipulate, and store. Usefully, both enantiomers of the hydrogenated products were obtained by simply switching from Rh to Ir. Low hydrogen pressure and environmentally friendly propylene carbonate can be used, with no loss of selectivity.