Prochiral Molecules, Enantiotopic Groups and Faces

This chapter is concerned in part with prochirality. In order to define terms we consider sp3 and sp2 hybridized carbons separately. 

A molecule such as 1 is said to be prochiral, and the and H5 atoms in 4 are called pro-R and pro-S, respectively. The same analysis can be pursued for any molecule of type 5 (with A not the same as B) in which the X groups are enantiotopic. Other examples of prochirality include 

In a molecule such as CHaCl2, replacement of either hydrogen by deuterium gives the same molecule. Hydrogens in molecules such as CHjCI2 are homotopic 

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. 

Reduction of acetophenone (9) with LiAIH4 gives an alcohol with a stereogenic carbon the alcohol Is racemic. The sp3-hybrid carbon is prochiral, and the faces of the carbonyl carbon are enantiotopic. 

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