Enantioface differentiation by chiral polymers having (+)‐5,6‐exo‐bornanediol moieties. II. Asymmetric reduction of prochiral ketones by chiral polymers containing (+)‐5,6‐exo‐bornanediol derivatives

The asymmetric reduction of prochiral aromatic ketones with modified reagents prepared from sodium borohydride and carboxylic acids in the presence of both chiral polymers and relating low molecular weight compounds having (+)‐5,6‐exo‐dihydroxybornyl derivatives was carried out. The enantioface differentiation took place effectively by raising the reaction temperature, and the highest enantiomeric excess was achieved at 10°C (24.3%) in the presence of the chiral polymers. A higher optical yield (87.8%) can be obtained in the asymmetric reduction by using the low molecular weight (+)‐5,6‐exo‐diol compounds. The effect of the reaction temperature, solvents, and the advantages of the chiral polymer‐bound reagents were also discussed.