Air-Stable Chiral Primary Phosphines: A Gateway to MOP Ligands with Previously Inaccessible Stereoelectronic Profiles
2015-12-17T05:58:38Z (GMT) by
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.