Formation of Enantiomers

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Chapter: Organic Chemistry : Stereochemical and Conformational Isomerism

Since chiral centers are most commonly tetrahedral, the conversion of trigo-nal centers to tetrahedral centers by some type of addition process is the most common way in which new chiral centers are created.


FORMATION OF ENANTIOMERS

Since chiral centers are most commonly tetrahedral, the conversion of trigo-nal centers to tetrahedral centers by some type of addition process is the most common way in which new chiral centers are created. The reaction of carbonyl groups with nucleophiles is a classic example. If substituents on the carbonyl group and the nucleophile are all different, then a new chiral center is pro-duced, as in the reaction of acetophenone with sodium borohydride to produce 1-phenylethanol.


The carbonyl group is trigonal and planar and can be thought of as having two faces. Addition of hydride to one face gives one enantiomer while addition to the opposite face gives the opposite enantiomer. As rewritten below, attack from above gives the R enantiomer while attack from below gives the S enantiomer. The faces are stereochemically nonequivalent since different stereoiso-mers are produced.


To differentiate the faces of a carbonyl group, the Re – Si nomenclature has been developed. The groups around the carbonyl carbon are given priorities by the same rules used in the Cahn – Ingold – Prelog system for R,S nomenclature. Then going from the group of highest priority to the group of lowest priority around the face of a carbonyl group, proceeding in the clockwise direction defines the Re face and proceeding in the counterclockwise direction defines the Si face.


The Re – Si nomenclature enables the faces of a carbonyl group to be differen-tiated stereochemically; however, the carbonyl group itself is achiral. Moreover, the Re – Si designation is not indicative of the stereochemistry of the chiral center produced by addition. In the above example hydride addition to the Si face gives the R enantiomer while hydride addition to the Re face gives the S enantiomer. If ethyl lithium were added, the stereochemistry would be reversed, that is, Si S and Re R.

New chiral centers are produced by addition reactions to other trigonal centers as well. Hydrogenation of 3-methyl-3-hexene gives 3-methylhexane. Clearly the addition of hydrogen to one face of the planar olefinic system gives one enan-tiomer and addition to the opposite face gives the opposite enantiomer. Likewise reaction of styrene with chlorine or bromine (X2) or potassium permanganate produces products with a new chiral center. Formation of the two possible enan-tiomers results from addition to either face of the olefin.


Reactive intermediates which are planar can also produce enantiomers. The acid-catalyzed addition of water to 1-pentene proceeds via a secondary carboca-tion. Because the carbocation is a trigonal, planar intermediate, water can add to either face to give the R or S enantiomers.


In reactions in which neither the reactants (C=O, C =C, C+) nor the reagents (BH4, EtMgBr, Br2, H2O, etc.) are chiral, there is no possibility for controlling which face undergoes addition (in fact, addition to either face is equivalent); thus a racemic mixture will be produced. Such processes are described as having no enantioselectivity.

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