Elimination anti-periplanar transition state

We can conclude that anti elimination is generally favored in the E2 mechanism, but that steric (inability to form the anti-periplanar transition state), conformational, ion pairing, and other factors cause syn elimination to intervene (and even predominate) in some cases. 

E2 eliminations have anti-periplanar transition states... 

In the next example, there is only one proton that can take part in the elimination. Now there is no choice of anti-periplanar transition states. Whether the product is E or Z> the E2 reaction has only one course to follow. And the outcome depends on which diastereoisomer of the starting material is used. When the first diastereoisomer is drawn with the proton and bromine anti-periplanar, as required, and in the plane of the page, the two phenyl groups have to lie one in front and one behind the plane of the paper. As the hydroxide attacks the C-H bond and eliminates Br , this arrangement is preserved and the two phenyl groups end up trans (the alkene is E). This is perhaps easier to see in the Newman projection of the same conformation. 

A single-step E2 elimination would have to go via an anti-periplanar transition state and would be stereospecific. You will be able compare this stereoselective Julia olefination with the stereospecific Peterson elimination shortly. 

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. 

E2 ELIMINATIONS HAVE ANTI-PERIPLANAR TRANSITION STATES... 

In the next example there is only one proton that can take part in the elimination. Now there is no choice of anti-periplanar transition states. Whether the product is E or Z, the E2 reaction has only one course to follow. And the outcome depends on which diastereoisomer of the starting material is used. When the first diastereoisomer is drawn with the proton and... 

A is called anti-periplanar, and this type of elimination, in which H and X depart in opposite directions, is called anti elimination. Conformation B is syn-periplanar, and this type of elimination, with H and X leaving in the same direction, is called syn elimination. Many examples of both kinds have been discovered. In the absence of special effects (discussed below) anti elimination is usually greatly favored over syn elimination, probably because A is a staggered conformation (p. 139) and the molecule requires less energy to reach this transition state than it does to reach the eclipsed transition state B. A few of the many known examples of predominant or exclusive anti elimination follow. 

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