Reactants, molecules containing chiral centers

Molecules Containing Chiral Centers as Reactants or Products... 

Preparation of enantiomerically enriched materials by use of chiral catalysts is also based on differences in transition-state energies. While the reactant is part of a complex or intermediate containing a chiral catalyst, it is in a chiral environment. The intermediates and complexes containing each enantiomeric reactant and a homochiral catalyst are diastereomeric and differ in energy. This energy difference can then control selection between the stereoisomeric products of the reaction. If the reaction creates a new stereogenic center in the reactant molecule, there can be a preference for formation of one enantiomer over the other. 

In the enantioselective synthesis, the asymmetry (i.e., the stereoselectivity) is induced by the external chiral catalyst, while the diastereoselective synthesis does not require a chiral catalyst. The stereogenic center already present in the molecule is able to induce stereoselectivity, assuming that the synthesis starts with a single enantiomer. For instance, imagine that an a,/ -substituted product is formed, and that the reactant already contains a stereogenic carbon at a. If the reaction of (aS) leads, e.g., largely to (aS, / R) and hardly to the (aS, /IS) diastereomer (i.e., stereoisomers that are not mirror-images of each other), the reaction is diastereoselective (Scheme 14.2). 

Similarly, the two faces at a trigonal carbon in a molecule containing a chiral center are diastereotopic. Both chiral and achiral reactants can distinguish between these diastereotopic faces. Many examples of diastereoselective transformations of such compounds are known. One of the cases which has been examined closely is addition reactions at a trigonal center adjacent to a chiral carbon. Particular attention has been given to the case of nucleophilic addition to carbonyl centers. 

---