BINAPS: An Axially Chiral P,S-Heterodonor Ligand for Asymmetric Catalysis Based on Binaphthalene Backbone

The chelating P,S-heterodonor ligand 2-diphenylphosphanyl-1,1′-binaphthalene-2′-thiol (11) (BINAPS), which features a chiral axis as the unique stereogenic element, has been prepared in both racemic and enantiopure form through a multistep reaction sequence using 2,2′-dihydroxy-1,1′-binaphthalene (BINOL) as the starting material. The reaction sequence is completely stereoconservative and (S)-11 is obtained with no loss of enantiopurity from pure (S)-BINOL. (R)-11 can be alternatively obtained by resolution of racemic 11 using the chiral (S)-benzylaminato Pd(II)-complex 19 as the resolving agent. The S-methyl or the S-i-propyl derivatives 14 have been used as chiral ligands in the Rh(I)-catalyzed asymmetric hydroformylation of styrene and in the hydrogen transfer reduction of acetophenone with modest success (up to 20% ee). In the presence of suitable Pd-complexes the same ligands provide higher ees in the hydrosilylation of styrene (50% ee) and in the allylic alkylation of 1,3-diphenylprop-2-enyl acetate (60% ee).