Overlap anti-periplanar

In an E2 elimination, the new 7t bond is formed by overlap of the C-H a bond with the C-X a antibonding orbital. The two orbitals have to lie in the same plane for best overlap, and now there are two conformations that allow this. One has H and X syn-periplanar, the other anti-periplanar. The anti-periplanar conformation is more stable because it is staggered (the syn-periplanar conformation is eclipsed) but, more importantly, only in the anti-periplanar conformation are the bonds (and therefore the orbitals) truly parallel. 

In Chapter 19 you saw that anti-periplanar transition states are usually preferred for elimination reactions because this alignment provides the best opportunity for good overlap between the orbitals involved. Syn-periplanar transition states can, however, also lead to elimination—and this particular case should remind you of the Wittig reaction (Chapter 14) with a four-membered cyclic intermediate. 

The difference in reaction rate results from different degrees of tt bond development in the E2 transition state. Since tt overlap of p orbitals requires their axes to be parallel, tt bond formation is best achieved when the four atoms of the H—C—C—X unit lie in the same plane at the transition state. The two conformations that permit this relationship are termed syn periplanar and anti periplanar. 

Syn-coplanar and Anti-periplanar Overlap. In the discussion about the anomeric effect, the lone pair has been oriented, without comment, anti to the C—X bond. The lone pair and the C X bond are able to overlap in this orientation 2.91 since they are coplanar, but at first sight they could equally easily have overlapped had they been syn 2.92. Undoubtedly, coplanarity is the single most important constraint for good overlap, but what about the choice between syn and anti One answer, immediately apparent even in these simplified drawings, is that the syn arrangement 2.92 carries with it at least one eclipsing... 

The only difference between the compounds is stereochemistry and, if we look at the orbitals involved in the reactions, we can see why this is so important. As the N2 leaving group departs, electrons in the bond to the migrating group have to flow into the C-N a orbital— we discussed this on p. 949. But what we didn t talk about then was the fact that best overlap between these two orbitals (a and o ) occurs if they are anti-periplanar to one another—just as in an E2 elimination reaction. 

Experimental evidence indicates that the five atoms involved in the E2 elimination reaction must lie in the same plane the anti-periplanar conformation is preferred. This conformation is necessary for the orbital overlap that must occur for the TT bond to be generated in the alkene. The sp -hybridized atomic orbitals on carbon that comprise the C—H and C—a bonds broken in the reaction develop into the p orbitals comprising the -ir bond of the alkene formed ... 

Both arrangements allow n overlap of indpient parallel 2p orbitals as the C bonds are broken. The more common anti periplanar geometry corresponds to the staggered anticonformation, which is easily achieved in conformationally flexible molecules. The syn periplanar geometry corresponds to an eclipsed conformation, which is important only in some rigid, cyclic compounds. 

The stereoelectronic effect reflects the geometry of the developing 7i bond in the transition state for the reaction. The anti periplanar arrangement is favored because it aligns the O orbitals of the sp -hybridized C—H and C—X bonds so, they can partially overlap as they become the 2p n orbitals in the product (Figure 9.8). A similar argument holds for the syn periplanar transition state. 

Partial overlap of the developing ti orbitals in the transition state for an E2 reaction in which the leaving group and the proton are in an anti periplanar relationship. 

One of the steric requirements of E2 elimination is the need for periplanar geometry, which optimizes orbital overlap in the transition state leading to alkene product. Two types of periplanar arrangements of substituents are possible — syn and anti. 

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