Enamines are convenient stable enol equivalents for conjugate addition
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص608
2025-06-29
560
If you want a more reliable way of doing a conjugate addition of an aldehyde or ketone without having a second anion-stabilizing group, you need some stable and relatively unreactive enol equivalent. you saw how enamines, particularly those derived from cyclic secondary amines, are useful in alkylation reactions. These neutral species are also perfect for conjugate addition as they are soft nucleophiles but are more reactive than enols and can be prepared quantitatively in advance. The reactivity of enamines is such that heating the reactants together, sometimes neat, is all that is required. Acid catalysis can also be used to catalyse the reaction at lower temperature.
The mechanism is rather like enol addition. The differences are that the enamine is more nucleophilic because of the nitrogen atom and that the product is also an enamine, which can be converted into the corresponding carbonyl by mild acidic hydrolysis. This is usually performed during the work-up and so does not really constitute an extra step. The amine is washed out as the hydrochloride salt so isolation is straightforward. After conjugate addition the resulting enolate-iminium ion undergoes proton transfer rapidly to produce the more stable carbonyl-enamine tautomer. This is shown as an intramolecular process but it could just as easily be drawn with an external base and source of protons. The resulting enamine is then stable until aqueous acid is added at the end of the reaction. Hydrolysis occurs via the iminium ion to reveal the second carbonyl group and release the secondary amine.
In these two examples enamines from cyclohexanone formed with pyrrolidine and mor pholine add in good yield to an α,β-unsaturated carbonyl compound with an extra electron withdrawing methylthio or phenylsulfonyl group.
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