Syn periplanar geometry

We concluded in Problem 11.61 that E2 elimination in compounds of this bicyclic structure occurs with syn periplanar geometry. In compound A, -H and -Cl can be eliminated via the syn-periplanar route. Since neither syn nor anti periplanar elimination is possible for B, elimination occurs by a slower, El route. 

Zaitsev s rule E2 reaction syn periplanar geometry anti periplanar geometry deuterium isotope effect El reaction ElcB reaction... 

Anti periplanar geometry for E2 eliminations has specific stereochemical consequences that provide strong evidence for the proposed mechanism. To take just one example, meso-l,2-dibromo-l,2-diphenylethane undergoes E2 elimination on treatment with base to give only the pure E alkene. None of the isomeric Z alkene is formed because the transition state leading to the Z alkene would have to have syn periplanar geometry. 

Formation and reactivity of o-benzynes o-benzynes serve as illustrative stereoelectronic models due to two features a) structural rigidity that restricts orbital interactions exclusively to syn-periplanar geometries and b) distortion of the in-plane x-system. These features are translated in the interplay of two related stabilizing effects controlling the structure of substituted benzynes and their reactivity towards nucleophiles 1) interaction of o-acceptors with bent bonds and 2) interaction of o-acceptors with anionic centers. 

Although dehydrohalogenation occurs with anti periplanar geometry, some eliminations have syn periplanar geometry. Examine the starting material and product of each elimination, and state whether the elimination occurs with syn or anti periplanar geometry. 

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. 

---