Diastereoselective hydrogenation of folic acid esters with the Daniphos ligand

Folic acid dimethylester benzenesulfonate was hydrogenated homogeneously in a rhodium-catalyzed diastereoselective reaction employing a set of the previously published planar-chiral “Daniphos” ligands, which are based on an arene chromium tricarbonyl scaffold. Diastereoselectivities of up to 42% de were achieved, almost matching the benchmark ligand BINAP. An X-ray structure of the most successful ligand P(i-Pr2)/PPh2 is presented and discussed.