Chiral Borate Esters in Asymmetric Synthesis. Part 2
- 23 September 2004
- journal article
- research article
- Published by Wiley in Helvetica Chimica Acta
- Vol. 87 (9) , 2310-2317
- https://doi.org/10.1002/hlca.200490206
Abstract
Asymmetric catalytic activity of the chiral spiroborate esters 1–9 with a O3BN 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.Keywords
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