Conformationally Constrained Diphosphines Derived from (η<sup>6</sup>-(<i>S</i>)-1-(dimethylamino)indane)Cr(CO)<sub>3</sub>:  Synthesis and Application in Enantioselective Hydrogenation

Three new enantiopure diphosphine ligands have been prepared starting from [(η<sup>6</sup>-(1-dimethylamino)indane)Cr(CO)<sub>3</sub>] by means of a stereoselective synthetic strategy involving highly diastereoselective complexation of the Cr(CO)<sub>3</sub> moiety to (<i>S</i>)-(1-dimethylamino)indane, regioselective substitution in the 7-position with the PPh<sub>2</sub> group, and, after exchange of the amino group for a chloro substituent with chloroformic esters, introduction of a PR<sub>2</sub> group (R = Ph, <i>t</i>-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 (<i>Z</i>)-<i>N</i>-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 <i>o</i><i>-</i>PPh<sub>2</sub>/α-PCy<sub>2</sub> <b>(</b><b><i>S</i></b><b>,</b><b><i>R</i></b><b>p)-6c</b> 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 <b>(</b><b><i>S</i></b><b>,</b><b><i>R</i></b><b>p)-6c</b> provides better results as to conversion and enantioselectivity compared to the analogous acyclic ligand.