For some eliminations only one product is possible. For others, there may be a choice of two (or more) alkene products that differ either in the location or stereochemistry of the double bond. We shall now move on to discuss the factors that control the stereochemistry (geometry—cis or trans) and regiochemistry (that is, where the double bond is) of the alkenes, starting with E1 reactions.

E and Z alkenes

You met the idea that alkenes can exist as geometrical cis and trans isomers , and now that you have read we can be more precise with our definitions. cis and trans are rather loosely defined terms (like syn and anti), although no less useful for it. But for formal assignment of geometry, we use the stereochemical descriptors E and Z. For disubstituted alkenes, E corresponds to trans and Z corresponds to cis. To assign E or Z to tri-or tetrasubstituted alkenes, the groups at either end of the alkene are given an order of priority according to the same rules as those outlined for R and S in Chapter 15. If the two higher priority groups are cis, the alkene is Z; if they are trans the alkene is E. Of course, molecules don’t know these rules, and sometimes (as in the second example here) the E alkene is less stable than the Z.

E alkenes (and transition states leading to E alkenes) are usually lower in energy than Z alkenes (and the transition states leading to them) for steric reasons: the substituents can get further apart from one another. A reaction that can choose which it forms is therefore likely to favour the formation of E alkenes. For alkenes formed by E1 elimination, this is exactly what happens: the less hindered E alkene is favoured. Here is an example.

The geometry of the product is determined at the moment that the proton is lost from the intermediate carbocation. The new π bond can only form if the vacant p orbital of the carbo cation and the breaking C–H bond are aligned parallel. In the example shown there are two possible conformations of the carbocation with parallel orientations, but one is more stable than the other because it suffers less steric hindrance. The same is true of the transition states on the route to the alkenes—the one leading to the E alkene is lower in energy, and more E alkene than Z alkene is formed. The process is stereoselective because the reaction chooses to form predominantly one of two possible stereoisomeric products.

Tamoxifen is an important drug in the fight against breast cancer, one of the most common forms of cancer. It works by blocking the action of the female sex hormone oestrogen. The tetrasubstituted double bond can be introduced by an E1 elimination: there is no ambiguity about where the double bond goes, although the two stereoisomers form in about equal amounts. Tamoxifen is the Z isomer.

مواضيع ذات صلة

Distinguishing acid derivatives by carbon NMR is difficult

E1cB eliminations in context

Anion-stabilizing groups allow another mechanism—E1cB

The regioselectivity of E2 eliminations

E2 elimination from vinyl halides: how to make alkynes

E2 eliminations from cyclohexanes

E2 eliminations can be stereospecific

E2 eliminations have anti-periplanar transition states

The role of the leaving group

Substrate structure may allow E1

E1 and E2 mechanisms

Size