Chiral Borate Esters in Asymmetric Synthesis. Part 2

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.