Enantiomers alcohol-containing reagents

The stereochemistiy of carbonyl reduction follows the same principles we have previously learned. Reduction converts a planar sp hybridized carbonyl carbon to a tetrahedral sp hybridized carbon. What happens when a new stereogenic center is formed in this process With an achiral reagent like NaBITt or LiAlHi. a racemic product is obtained. For example, NaBH4 in CH3OH solution reduces 2-butanone, an achiral ketone, to 2-butanol, an alcohol that contains a new stereogenic center. Both enantiomers of 2-butanol are formed in equal amounts. 

Organic molecules that contain sp2 hybridized carbons can also be prochiral if the substituent pattern is correct. An example is provided by phenylethanone (acetophenone, 9), which has three different groups attached to the carbonyl carbon. After reduction with, say, LiAlH4, the product is the chiral alcohol 10 and a 50 50 mixture of enantiomers is observed. The same racemic mixture is obtained from the reaction of the Grignard reagent MeMgBr with benzaldehyde (11). In contrast, the product of reduction of propanone (12) with LiAlH4 is propan-2-ol (13), which is not chiral. 

This provides the chemist with a means for separating, or resolving, enantiomers. If a compound exists as a pair of enantiomers and the mixture is reacted with an enantiomerically pure material, the products will be diastereomers and can therefore be separated, for example, by crystallisation or distillation. Reversal of the reaction then produces enantiomerically pure forms of the original material. For example, if we have a terpenoid alcohol as a racemic mixture and wish to obtain the two pure enantiomers, we could esterify it with a single enantiomer of an optically active acid, such as lactic acid. The ester mixture will then contain diastereomers which can be separated. After hydrolysis of the separated esters and recovery of the alcohols, we will have each alcohol in enantiomerically pure form. Such a process is referred to as resolution. Treatment of a mixture of enantiomers with a chiral reagent or catalyst may result in a reaction of one enantiomer only. For example, treating a racemic acetate ester of a terpenoid alcohol with a chiral lipase enzyme might result in the hydrolysis of only one enantiomer. The alcohol thus... 

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