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Chiral Borate Esters in Asymmetric Synthesis. Part 2 : Asymmetric Borane Reduction of Prochiral Ketones in the Presence of a Chiral Spiroborate Ester

✍ Scribed by Dejun Liu; Zixing Shan; Yan Zhou; Xiaojun Wu; Jingui Qin

Publisher
John Wiley and Sons
Year
2004
Tongue
German
Weight
212 KB
Volume
87
Category
Article
ISSN
0018-019X

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✦ Synopsis

Abstract

Asymmetric catalytic activity of the chiral spiroborate esters 1–9 with a O~3~BN framework (see Fig. 1) toward borane reduction of prochiral ketones was examined. In the presence of 0.1 equiv. of a chiral spiroborate ester, prochiral ketones were reduced by 0.6 equiv. of borane in THF to give (R)‐secondary alcohols in up to 92% ee and 98% isolated yields (Scheme 1). The stereoselectivity of the reductions depends on the constituents of the chiral spiroborate ester (Table 2) and the structure of the prochiral ketones (Table 1). The configuration of the products is independent of the chirality of the diol‐derived parts of the catalysts. A mechanism for the catalytic behavior of the chiral spiroborate esters (R,S)‐2 and (S,S)‐2 during the reduction is also suggested.

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