Scheme 3
In the case of addition reactions, multiple bonds may be reduced to single bonds giving rise to chiral centres (Scheme 4). In these cases, there are no prochiral centres but rather prochiral faces. Thus, the term prochirality also applies to an achiral molecule or entity which contains a trigonal system and which can be made chiral by the addition to the trigonal system of a new atom or achiral group. The addition of hydrogen to one of the faces of the prochiral methyl ethyl ketone gives one of the enantiomers of the chiral alcohol. However, the same faces may regarded as diastereotopic if the addition of a nucleophile generates a diastereomeric species. Thus, addition of cyanide anion to one of the diastereotopic faces of the chiral aldehyde shown below converts it into one of the diastereoisomers of the cyanohydrin. The two faces of the trigonal system may be described as Re and Si. The allotment of the descriptor follows a rule similar to the allotment of R and descriptors. First, the substituents are assigned priority according to the CIP rules. Next, consider the molecule in the plane of a paper. Then, looking from the top, an arrow from the first-priority group, through the second, to the third. If the arrow points clockwise, the face is called (Re). If the arrow points counter-clockwise, the face is called (Si).
Scheme 4
In the next few pages, some of the reactions discussed in various chapters will be discussed from a perspective of stereochemistry (Scheme 5). SN2 reactions of cyclohexane derivatives present a nice case. If the conformation of the molecule is fixed by a locking group, the inversion mechanism of the SN2 reaction, means that, if the leaving group is axial, then the incoming nucleophile will end up equatorial and vice versa.
Scheme 5