Chiral Oxazaborolidine‐catalyzed Asymmetric Borane Reduction of Alkyl 4‐Dialkylaminophenyl Ketones

Abstract

A series of alkyl 4‐dialkylaminophenyl ketones were prepared and reduced asymmetrically by borane under the chiral oxazaborolidine catalysis. The results indicate that the ketones show a more obvious substituent effect on the enantioselectivity than the corresponding 4‐alkyl/alkoxy/alkylthiophenyl ketones in the asymmetric reduction because of the existence of a strong coordinate nitrogen atom with the boron atom in the catalyst and borane.

A series of new C2‐symmetric amino alcohols with multicoordination groups: Promising catalysts for synthesis of both enantiomers in the borane‐mediated reduction of prochiral ketones

“…p-Methoxyacetophenone gave a much lower enantioselectivity, which could be rationalized to the coordination of the heteroatom with the boron atom in the catalyst and borane (Table 2, entry 5). [28][29][30] Probably because of steric effects, 1-acetonaph- thone and ketones containing substituents at the ortho position gave lower enantioselectivities ( Table 2, entries 6-8). The symmetric reduction of the cyclic ketone a-tetralone (entry 9) and the aliphatic ketone benzylacetone (entry 10) produced a moderate degree of enantioselectivity.…”