Scandium-Catalyzed Asymmetric Reduction with Potassium Borohydride

Key words

scandium - 1,2-reduction - chiral allylic alcohols - metal borohydrides

Significance

As an extension on previous work using chiral N,N′-dioxide–metal complexes for asymmetric catalysis (see Review), the authors now describe the scandium-catalyzed asymmetric reaction of enones and ketones with KBH₄. The resulting chiral alcohols are obtained with good yield and enantioselectivity.

Review

X. Liu, L. Lin, X. Feng Acc. Chem. Res. 2011, 44, 574–587.

Comment

Chiral allylic alcohols are important motifs widely present in natural products and biologically active molecules. The enantioselective reduction of enones is known as the most straight-forward access to such motifs. Herein, the first example of catalytic enantioselective reduction of enones and ketones by using KBH₄ is reported. The utilization of an aqueous solution of KBH₄ was found to be crucial for obtaining high yield and enantioselectivity as the presence of water is believed to benefit proton transfer to accelerate the catalytic cycle. In this case, the reaction was performed in a homogeneous catalyst system. The HRMS spectra experiments indicated that the initial reducing species is KBH₃OH.