Question

why do sodium borohydride and lithium tri-sec-butylborohydride produce the results seen in the table on page 46?

2 (pg 46).png

2 (pg 45,a).png

2 (pg 45,b).png

Answer 1

4-t-butylcylohexanone exists in the chair conformation and the bulky t=butyl group is always fixed up in the axial position.

Now the smaller, less bulky hydrides like sodium borohydride or lithium aluminum hydride, hydride can approach axially (due to the small size) thus the oxygen of the keto group goes to the equatorial position. This leads to the formation of a trans isomer.

When bulky reducing agents like lithiumtri-sec-butyl borohydride, the bulky three (sec-butyl) groups do not allow the hydrogen of the reagent to attack from the axial side ( the 1,3 hydrogens repel the bulky groups). thus the reagent has to approach from the equatorial side where there are no such steric restrictions, thus yielding cis isomer as the major product.