A Synthetic Approach to Sterically Hindered P‐Chiral Phosphine Ligands and Their Applications for Asymmetric Gold Catalysis

In contrast to wide‐ranging applications of axially and planar chiral bidentate phosphine ligands, the development of P‐chiral monophosphine ligands is much less advanced. Here a practical method is outlined for the synthesis of P‐chiral biaryl or ortho ‐((diphenyl)methyl)phenyl phosphines by desymmetrizing alkyl(diphenyl)phosphine oxides by directed ortho ‐lithiation and electrophilic quenching of the aryllithium intermediates. The biaryl subunit is introduced by Suzuki‐Miyaura coupling reactions of ortho ‐iodophenyl phosphine oxides in very good yields. Enantiomerically highly enriched phosphine oxides are obtained by crystallization. A widely unsolved problem consists of the asymmetric reduction of P‐chiral phosphine oxides to the corresponding phosphines. Herein, mild and highly selective Ti(O i Pr) 4 /hydrosilane‐mediated stereoretentive reduction reactions of biarylphosphine oxides or stereoinvertive reductions of ortho ‐(diphenylmethyl)phenyl phosphine oxides to the corresponding phosphines are described. The mechanism of this reduction method is investigated and most likely proceeds by a Ti(II)‐mediated PO bond cleavage in the phosphine oxide. The optically highly enriched P‐stereogenic monophosphine ligands form stable gold(I) complexes. Their initial application in an intramolecular asymmetric [2 + 2] cycloaddition of an allenyl indole reveals good catalytic activity and promising asymmetric induction.