Anti Elimination in E2 Reactions Stereoelectronic Effects

Further insight into the E2 mechanism comes from stereochemical studies. One such experiment compares the rates of elimination of the cis and trans isomers of 4-tert-butyl-cyclohexyl bromide. 

Although both stereoisomers yield 4-ferf-butylcyclohexene as the only aUcene, they do so at quite different rates. The cis isomer reacts over 500 times faster than the trans. 

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. 

Anti periplanar orbitals aligned and bonds are staggered 

The peri- in periplanar means almost or nearly. Although coplanarity of the p orbitals is the best geometry for the E2 process, modest deviations from this ideal can be tolerated. 

Because adjacent bonds are eclipsed when the H—C—C—X unit is syn periplanar, a transition state having this geometry is less stable than one that has an anti periplanar relationship between the proton and the leaving group. 

Anti coplanar Staggered conformation C—H and C—X bonds aligned 

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The first example of a stereoelectronic effect in this text concerned anti elimination in E2 reactions of alkyl halides (Section 5.16). 

In the previous subsection, it was shown that the Ferrier reaction offers an opportunity to convert glycal derivatives into unsaturated sugar derivatives, which have an isolated double bond between C(2) and C(3). The Tipson-Cohcn reaction is another important reaction for the introduction of isolated double bonds.29 In this procedure, a cis or tram diols are converted into disulfonates (mesylates or tosylates) which are reductively eliminated with sodium iodide and zinc in refluxing DMF (Scheme 3.6a). In this reaction, the C(3) sulfonate is substituted by an iodide, which then is reductively removed by zinc with concomitant elimination of the second sulfonate moiety, introducing a double bond. Stereoelectronic effects make nucleophilic substitutions at C(3) more favourable than similar reactions at C(2) (see Section 3.2.3). Probably, the elimination proceeds through a boat conformation. In this case, the iodide and tosylate are in a syn relation. In most cases, E2 elimination proceeds via a transition state involving an anti orientation. Nevertheless, syn elimination becomes the dominant mode of reaction when structural features prohibit an anti orientation. 

Many examples of stereoelectronic effects have been proposed in numerous areas of organic chemistry. The textbook example is perhaps the requirement for the anti conformation of the electrons of the scissile C—H bond with the leaving group in the E2 elimination reaction. However, over the past decade, the term stereoelectronic effect has become synonymous with an effect otherwise termed the kinetic anomeric effect or the antiperiplanar lone-pair hypothesis. While it is quite erroneous to label this hypothesis as the stereoelectronic effect , the fact that this situation has come about does serve to emphasize the ascendency of this hypothesis in the minds of many organic chemists. 

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