Conformationally Constrained Diphosphines Derived from (η6-(S)-1-(dimethylamino)indane)Cr(CO)3:  Synthesis and Application in Enantioselective Hydrogenation

Three new enantiopure diphosphine ligands have been prepared starting from [(η6-(1-dimethylamino)indane)Cr(CO)3] by means of a stereoselective synthetic strategy involving highly diastereoselective complexation of the Cr(CO)3 moiety to (S)-(1-dimethylamino)indane, regioselective substitution in the 7-position with the PPh2 group, and, after exchange of the amino group for a chloro substituent with chloroformic esters, introduction of a PR2 group (R = Ph, t-Bu, Cy) in the benzylic position. The stereochemical course of the synthesis has been confirmed by the X-ray determination of the molecular structure of one intermediate and of one of the three ligands. The ligands have been tested in the rhodium-promoted enantioselective hydrogenation of methyl (Z)-N-acetamidocinnamate and dimethyl itaconate. Enantiomeric excesses ranging from 9 to 88% ee have been obtained, depending on the nature of the R substituent on the ligand, with the donor group combination o-PPh2/α-PCy2 (S,Rp)-6c outperforming the other two. The new ligands, which bear the coordinating teeth on the stiff backbone provided by the indane framework, compare well with the parent conformationally unlocked “Daniphos” ligands:  in the hydrogenation of dimethyl itaconate the new ligand (S,Rp)-6c provides better results as to conversion and enantioselectivity compared to the analogous acyclic ligand.