Main stereoelectronic rule

The effects of directionality and symmetry are intertwined in stereoelectronic effects. A particularly simple, yet surprisingly powerful generalization - often referred to as the main stereoelectronic rule - is the pronounced preference for antiperiplanarity of the vicinal donor and acceptor orbitals (Figure 5.3). The rule can be expressed in the following way There is a stereoelectronic preference for conformations in which the best donor lone pair or bond is antiperiplanar to the best acceptor bond. ... 

Figure 5.3 Newman projections showing the possible conformations In a donor/acceptor substituted ethane molecule. The "main stereoelectronic rule" favors the antiperiplanar conformation.

The relative importance of different eclipsed conformations in substituted propenes can be controlled stereoelectronically by variations in the donor and acceptor properties of aUyUc C-X bonds and the alkene (Figure 6.31). For example, the difference between the two conformations of aUyl fluoride is small (the -0.2-0.8 kcal/mol gas phase preference for the conformation with the C-F eclipsed bond). This is an apparent violation of the main stereoelectronic rule (the best acceptor, p, is orthogonal to the best donor in this... 

However, for each of these patterns, there is a unifying stereochemical preference. It can be summarized in the following short definition which is sometimes referred to as the main stereoelectronic rule The most interacting pair of orbitals (the best donor and the best acceptor) prefer an antiperiplanar geometry . We have summarized the key manifestations of this rule for the six important classes of organic compounds in Figure 6.131. 

Figure 6.131 The "main stereoelectronic rule" is a common foundation for conformational preferences in a...

---