Antiperiplanar effect

It is also important to note that trans-alkenes are often more stable than cis-alkenes due to diminished steric hindrance (p. 190), but this is not always the case. It is known, for example, that c -l,2-difluoroethene is thermodynamically more stable than tra j-l,2-difluoroethene. This appears to be due to delocalization of halogen lone-pair electrons and an antiperiplanar effect between vicinal antiperiplanar bonds. 

Normally, additions depicted by model C lead to the highest asymmetric induction. The antiperiplanar effect of OR substituents can be very efficient in the Houk model B ( , , , , ) however it plays no role in model C. Furthermore, the Houk model B must be considered in all cycloaddition-like reactions. The Felkin-Anh model A is operative for nucleophilic additions other than cuprate additions ( ). The epoxidation reactions are unique as they demonstrate the activation of one diastereoface by a hydroxy group which forms a hydrogen bridge to the reagent ( Henbest phenomenon ). The stereochemical outcome may thus be interpreted in terms of the reactive conformations 1 and 2 where the hydroxy function is perpendicular to the olefinic plane and has an optimal activating effect. 

The face-selectivity of hydride reductions of the conformationally-rigid ketone series (100) has been examined for pure axial and equatorial isomers with four different R groups, viz. Me, Cl, OMe, and SMe.162 The reactivities show Taft correlations with the inductive effects of the substituents. Only through-bond and -space electrostatic interactions are used to explain the results neither Cieplak nor Anh antiperiplanar effects are invoked. 

Recently, a new pictorial representation of the antiperiplanar effect has been put forward. Inagaki et al. (1986) proposed that antiperiplanar effects of both bonds and lone pairs were brought about by the continuity of phase of the bonding or nonbonding orbital, the antibonding orbital of the antiperiplanar bond, and the bonding orbital of the intervening bond. They supported their approach with extended Hiickel calculations, a fairly low level of theory, so the status of their proposal is not clear. 

In 1986 Inagaki et al. (266,267) suggested that gauche and anomeric effects (antiperiplanar effects) are brought about by the interaction of three bonds A, B, and C (see Figure 35) represented by appropriate bonding a, b, and c and antibonding a, b and c orbitals which are linear combinations of relevant... 

A. P. Kozikowski and A. K. Ghosh, Diastereoselection in intennolecular nitrile oxide cycloaddition (NOC) reactions Confirmation of the antiperiplanar effect through a simple synthesis of 2-deoxy-D-ribose, y. Am. Chem. Soc., 104, 5788 (1982). 

The Bohimann effect is also observed in C-H bonds antiperiplanar to lone pairs in systems where the lone pair containing heteroatom is donbly bonnd to the C-H carbon (i.e. aldehydes, imines, etc.). The additional stereoelectronic feature of imines is the presence of geometric isomers that allow clear distinction between syn- and antiperiplanarity effects (Figure 12.2). The stretching IR frequencies for the C-H bonds antiperiplanar to the nitrogen lone pair are noticeably red-shifted. 

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