Stereoelectronic main rule antiperiplanarity

Stereoelectronic preference for conformations in which the best donor lone pair or bond is antiper anar to the best acceptor bond 

Since vicinal hyperconjugation is increased in the antiperiplanar conformation, a number of hyperconjuga-tive stereoelechonic effects are fully displayed in the most favorable geometry, where the best donor and the best acceptor are antiperiplanar to each other (see Chapters 6 and 7 for a number of illustrative examples). Of course, the antiperiplanar preference may be modified by the effects of sterics and electrostatics. In addition, as we have discussed earlier, the main rule is only applicable to intramolecular vicinal interactions. Intermolecular preferences are different. 

Reproduced with permission of John Wiley and Sons.) (b) The antiperiplanar stereoelectronic preference for 

In the following sections, we discuss intrinsic properties of common donor and acceptor orbitals that can provide additional modulations of stereoelectronic interactions. We will start with a general outline of polarity trends. 

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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.

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. 

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