ABSTRACT
Typical addition of a Grignard reagent to a carbonyl compound leads to an alcohol . For example, reaction at the planar carbonyl carbon of an achiral (not optically active) aldehyde gives two possible stereoisomers, which are nonsuperimposable mirror images. Since two enantiomers or chiral compounds are formed, the carbon atom is described as a stereogenic atom. Before reaction, the carbonyl carbon is described as prostereogenic or prochiral, because it can lead to two different isomers or enantiomers. From a statistical point of view, one would predict that equal amounts of each enantiomer should be formed. However, if one isomer is formed in greater yield, the reaction mixture is described as scalemic, and the enantiomeric purity of the products is the en anti orne ric excess (e.e.) of one i somer over the other. For example, if one enantiomer i s formed in 7 5 % and the second in 25 % , the e.e. i s 50% (75 % - 2 5 % = 50% ). Most of the reactions in this chapter will present e.e. as the indication of asymmetric induction in a reaction. Certainly, for a reaction to be synthetically useful, the e.e. will be very high; an ideal example of asymmetric induction will have an e.e . of 1 00% ( 1 00% of one enantiomer and 0% of the second).
