Acyclic E alkenes are usually more stable than acyclic Z alkenes because they are less sterically hindered. Yet Z alkenes do not spontaneously convert to E alkenes because the π bond prevents free rotation: the energy required to break the π bond is about 260 kJ mol⁻¹ (rotation about a σ bond requires about 10 kJ mol⁻¹). You may therefore find the following result surprising. Dimethyl maleate is easily made by refluxing maleic anhydride in methanol with an acid catalyst. If the product is isolated straight away, a liquid boiling at 199–202°C is obtained. This is dimethyl maleate. However, if the product is left to stand, crystals of dimethyl fumarate (the E isomer of dimethyl maleate) form. How has the geometry been inverted so easily?

A clue is that the process is accelerated enormously by a trace of amine. Michael addition of this amine, or of methanol, or any other nucleophile, provides a chemical mechanism by which the π bond can be broken. There is free rotation in the intermediate, and re-elimination of the nucleophile can give either E or Z alkene. The greater stability and crystallinity of the E alkene means that it dominates the equilibrium. Michael addition therefore provides a mechanism for the equilibration of Z alkenes to E alkenes.

Similar mechanisms account for the double bond geometry obtained in aldol reactions followed by dehydration to give α,β-unsaturated carbonyl compounds. Any Z alkene that is formed is equilibrated to E by reversible Michael addition during the reaction. The two examples which follow illustrate how effective this method is.

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مواضيع ذات صلة

Predominantly E alkenes can be formed by stereoselective elimination reactions

Addition of nucleophiles to alkynes

E-selective reduction of alkynes using sodium in liquid ammonia

Using light to make Z alkenes from E alkenes

Exploiting cyclic compounds

Lewis acids promote couplings via oxonium ions

Allyl silanes are more reactive than vinyl silanes but also react through β-silyl cations

Allyl silanes as nucleophiles

Vinyl silanes offer a regio- and stereoselective route to alkenes

Aryl silanes undergo ipso substitution with electrophiles

Silicon stabilizes a positive charge on the β carbon

Alkynyl silanes are used for protection and activation