Air-Stable Chiral Primary Phosphines: A Gateway to MOP Ligands with Previously Inaccessible Stereoelectronic Profiles

The air-stable chiral primary phosphines <b>1a</b>,<b>b</b> facilitate the synthesis of previously inaccessible or hard-to-access chiral MOP-type ligands <b>2a</b>,<b>b</b>–<b>5a</b>,<b>b</b>, which can be prepared in one-pot reactions. These derivatives have been prepared to allow for a unique comparison of their differing structural and electronic profiles, determined here by a number of experimental and theoretical studies. Phosphiranes <b>2a</b>,<b>b</b> and phosphonites <b>5a</b>,<b>b</b> are electron-poor compounds, with the former possessing exceptional thermal stability. Conversely, the dimethylarylphosphines <b>3a</b>,<b>b</b> and bis­(dimethylamino)­arylphosphines <b>4a</b>,<b>b</b> are good electron donors, and, in contrast to earlier reports, the dialkylarylphosphines were found to be remarkably air-stable. The ligands were coordinated to platinum­(II), and the weak <i>trans</i>-influence of the highly strained phosphiranes <b>2a</b>,<b>b</b> was revealed both in solution and in the solid state. The steric parameters of the ligands were investigated by the allyl rotation of their methallylpalladium­(II) complexes, which showed subtle differences in exchange rates. Aryl side-on coordination of the MOP-backbone to palladium­(II) was observed for complexes with a non-coordinating counterion and structurally analyzed in the case of ligand <b>4b</b>. The asymmetric induction and catalytic activity of <b>2a</b>,<b>b</b>–<b>5a</b>,<b>b</b> were tested in the hydrosilylation of styrene as well as the allylic alkylation of (<i>rac</i>)-(<i>E</i>)-1,3-diphenylallyl acetate. Major differences in reactivity were related back to the electronic parameters of the ligands.