Preparation of chiral phosphorus(V) reagents and their uses with borane in the enantioselective reduction of ketones

Chiral pentavalent phosphorus reagents 5-17 were prepared from POCl3 and ephedrine (or the related alpha-amino alcohols), followed by substitution with nucleophiles of phenylmagnesium bromide, alcohols, amines, diamines and triamines. The substitution reactions occurred in a stereospecific manner with retention of the configuration at the phosphorus center. The structures of these phosphorus(V) reagents were determined by spectral methods and verified by X-ray diffraction in several instances. These phosphorus(V) reagents were used with borane/dimethylsulfide complex in the enantioselective reduction of aromatic, aliphatic and heterocyclic ketones. The phosphorus(V) reagents likely functioned as Lewis bases to react with borane, forming in situ zwitterionic species as the chiral reducing agents. The reactions were generally carried out at 0 degrees C in tetrahydrofuran solution with a molar ratio of ketone/borane/phosphoramidate = 1:1:0.2 to afford secondary alcohols with modest enantioselectivity, up to 67% ee in the reduction of 4'-methylacetophenone.