Prochiral relationships

Substitutiofi products are superimpouble. There is a plane of symmetry defined by the atoms H-C(2)-D. 

If a similar process occurred involving the two protons at C-1, a stereochemically different situation will result. Substitution at C-1 produces a chiral product, -deuterio-, i-propanediol  

The two protons at C-1 are topologically nonequivalent, since substitution of one produces a product tiiat is stereochemically distinct fiom that produced by substitution of the other. Ligands of this type are termed heterotopic, and, because the products of substitution are enantiomers, the more precise term enantiotopic also applies. If a chiral assembly is generated when a particular ligand is replaced by a new ligand, the original assembly is prochiral. Both C-1 and C-3 of 1,3-propanediol are prochiral centers. 

Westheimer, in Steric Effects in Organic Chemistry, M. S. Newman, ed., John Wiley Sons, New 3 rk, 1956, Chapter 12. 

The enzyme-catalyzed interconversion of acetaldehyde and ethanol serves to illustrate a second important feature of prochiral relationships, that ofprochiral faces. Addition of a fourth ligand, different from the three already present, to the carbonyl carbon of acetaldehyde will produce a chiral molecule. The original molecule presents to the approaching reagent two faces which bear a mirror-image relationship to one another and are therefore enantiotopic. The two faces may be classified as re (from rectus) or si (from sinister), according to the sequence rule. If the substituents viewed from a particular face appear clockwise in order of decreasing priority, then that face is re if coimter-clockwise, then si. The re and si faces of acetaldehyde are shown below. 

Reaction of an achiral reagent with a molecule exhibiting enantiotopic faces will produce equal quantities of enantiomers, and an optically inactive product will 

Fumaric acid is converted to L-malic acid by hydration in the presence of the enzyme fumarase. From the structure of the substrate and the configuration of the product, it is apparent that the hydroxyl group has been added to the si face of one of the carbon atoms of the double bond. Each of the trigonal carbon atoms of an alkene has its face specified separately. The molecule of fumaric acid is viewed from the re-re face as written. 

Chemical differences between diastereotopic ligands are readily observable. The protons adjacent to sulfoxide groups undergo base-catalyzed hydrogen deuterium exchange in deuterated solvents easily. In benzyl methyl sulfoxide, the 

It is frequently necessary to distinguish between identical ligands, that, even though bonded to the same atom, may be topologically nonequivalent. Let us consider 1,3-propanediol as an example. If a process occurs in which a proton at C-2 is substituted by another ligand, say, deuterium, the two possible substitution modes generate identical products. The two protons at C-2 are therefore toj)olo ically equivalent and are termed homotopic ligands. 

Reaction of an achiral reagent with a molecule exhibiting enantiotopic faces will produce equal quantities of enantiomers, and a racemic mixture will result. The achiral reagent sodium borodeuteride, for example, will produce racemic l-deuterio ethanol. Chiral reagents can discriminate between the prochiral faces, and an optically active product can result. Enzymatic reduction of acetaldehyde-l-d produces R-l-deuterio-ethano that is optically pure.  

It is frequently necessary, particularly in enzymic processes, to distinguish between like ligands that, even though they are bonded to the same atom, may be 

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