Antiperiplanar rules

Butanes are chosen as the simplest models for the normal and branched isomers. Both branched and normal isomers contain a C-C bond (2 ) interacting with the terminal C-H bonds (2 and 2 ) (Scheme 26a). The cyclic -aj-a2 -a3 a2- interaction (Scheme 26b) occurs in the polarization of the middle C-C a-bond by the interactions with the antiperiplanar C-H a-bonds. The orbital phase is continuous in the branched isomer and discontinuous in the normal isomer (cf Scheme 4). The branched isomer is more stable. The basic rule of the branching effects on the stability of alkanes is ... 

The part of the Sequence Rule (4) that deals with conformations provides terms that are either fully chiral (MIP = minus/plus) or fully achiral (ap/sp = antiperiplanar/synperiplanar), but none that are suitable if the difference is either only graphochiral or only pherochiral. The torsional difference between 29a and... 

If this interpretation is correct, then in any reaction with asymmetric induction, a search for antiperiplanarity between the incipient bond (Nu-Cl) and an adjacent sigma bond (C2-L) should lead to the most favourable transition states, all other things being equal. Let us apply this rule to the so-called product development control problem. Consider a conformationally fixed cyclohexanone, for example 14. 

On the other hand, puckering the ring will destroy it equatorial attack cannot approach antiperiplanarity to the C2—C3 and C5—C6 bonds. We are thus led to the rule the more flattened the ring, the more axial attack. This flattening rule may help to rationalize experimental results quite difficult to understand otherwise. 

Studies of Felkin s model have shown that the transition state for nucleophilic addition to a carbonyl compound is strongly stabilized when the C2-X and Nu- Cj bonds are antiperiplanar.41 Let us apply this rule to a configurationally rigid cyclohexanone. 

Pentanone is fairly flexible, so an antiperiplanar attack can occur at either face. The o cc orbital lies lower in energy than o CH so frontier orbital control favors the anti transition state by 3.73 kcal mol-1. Therefore, the flattening rule may be generalized as follows antiperiplanar attack and frontier orbital control in general are only important for reasonably flexible ketones. 

As demonstrated by the spectra of 1,2-dichloroethane shown in Fig. 4.1-11C, two halogen atoms in 1,2-position also show in-phase as well as out-of-phase vibrations. The antiperiplanar conformation is subject to the exclusion rule op. Raman 750 cm Oa . IR 708 cm. In the case of the synclinal conformation, on the other hand, the in-phase vibration at 645 cm is stronger both in the IR and in the Raman spectrum than the out-of-phase vibration at 677 cm. Freely rotating 1,2-dihalogen compounds therefore show four different C-Cl stretching vibrations (Fig. 4.1-11C). 

Two of the factors that determine the reactivity of tethered ir-nucleophiles in Mannich-type cycliza-tions have been emphasized stereoelectronic effects and reaction medium effects. The stereoelectronics of orbital overlaps between the ir-nucleophile and the iminium electrophile are best evaluated by considerations such as antiperiplanar addition trajectories and Baldwin s rules for ring formation. The critical importance of the reaction medium has received serious attention only recently. However, it already appears clear that Tr-nucleophiles that would lead, upon cyclization, to relatively unstable carbocations can have their reactivity markedly increased by carrying out the cyclization in the presence of a nucleophilic solvent or additive which, by nucleophilic participation, can obviate the formation of high energy cyclic carbenium ion intermediates. 

While the relative importance of these effects is highly substrate dependant, the preferential migration of an antiperiplanar er-bond is the general rule, as demonstrated by the rearrangement of bond a of tricyclo[4.3.0.03,8]non-7-yl benzenesulfonate 1s. Of note in this system is the formation of the tricyclic exo-4, resulting from a second [1,2] shift which results in a thermodynamic partitioning of the intermediate carbocations 3 and 5. The observed 2 1 ratio of 2/(4 + 6) provides an indication of the relative importance of the migrating bond orientation for the brendyl system, while the ratio of 4/6 reflects the relative thermodynamic stability of cationic intermediates. 

As a rule, elimination occurs preferentially from the lower energy antiperiplanar conformation. 

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.

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. 

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. 

The most common crown ethers are shown in Fig. 22. Since application of lUPAC rules to polyethers leads to somewhat cumbersome designations, we will follow the simple crown nomenclature proposed by Pedersen Polyethers are built up from 1,4-dioxa units, O—CH —CH —O. The minimum energy conformation of these units is staggered with torsion angles about C-C bonds being synclinal (60°) and about C-O bonds being antiperiplanar ( 180 °C) (for definitions see Fig. 23). These preferences, however, do not preclude deviations if required by ring formation or cation complexation. 

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