Why do sodium borohydride and lithium tri-sec-butylborohydride produce the resul

Question

Why do sodium borohydride and lithium tri-sec-butylborohydride produce the results seen in the table on page 46? Sterically large hydric reducing agents are forced to make an equatorial attack due to steric factors. If they were to make an axial attack, these hydride reagents would sterically clash with the 1, 3- diaxial hydrogen atoms. Now, the satirically result has reversed; this esult in the C is isometer being the major product. Notice that the highly bulky reagent lithium tri-sec-butylborohydride delivers its hydride equatorially and gives predominantly the cis product. The following is a tabulation of some laboratory results of production of t-butylclohexanone with differrnt hydride reducing agents. 4-t-Butylclohexanone will be reduced by sodium borohydride to form a mixture of diastereomeric alcohols, which will be analyzed by^1H NMR.

Explanation / Answer

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

Now the smaller , less bulkier hydrides like sodiumborohydride or lithium aluminium hydride , the hydride can approch axially (due to the small size) thus the oxygen of keto group goes to the equatorial position. This leads to foramtion of trans isomer.

When a 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 axial side ( the 1,3 hydrogens rpel the bulky groups). thus the reagent has to approach from the equtorial side where there is no such steric restrictions, thus yielding cis isomer as the major product.

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