Stereochemistry of Eliminations— Orbital Considerations

A contact ion pair formed in the El reaction of cryf/iro-3-D-2-butyl tosylate results in products only from S)m elimination. 

Stereoelectronically-controlled E2 path-ways. A. Antiperiplanar, and B. Synperiplanar. 

Let s look at a few examples of anti- and s)mperiplanar eliminations. As you might anticipate, anti elimination occurs for the vast majority of systems, even when the less stable isomer is produced, whereas syn elimination requires some special circumstances. When stereochemistry is possible in the product, the two paths typically give opposite results. For example, when the very weak base tetrabutylammonium chloride reacts with the brosylate compound (Bs = S02C6H4Br) shown in Eq. 10.85, only the less stable product is produced, that from anti elimination (see the Connections highlight on page 593 for another example). 

As an example of ion pairing, consider the elimination of meso-l,2-dichloro-l,2-diphe-nylethane in Eq. 10.87. Here, syn elimination occurs 13% of the time. Addition of 18-crown-6 completely wipes out any syn elimination. Apparently the small percent of syn elimination arises from an ion pair where the potassium cation bridges the base and the leaving group (see margin). Addition of the crown ether negates the ability of the K to act in this manner. 

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