Explain the mechanism of the Michael reaction?
The Michael reaction is an addition reaction, usually of an enolate to an
The case you're familiar with is this one:
where I've highlighted the molecular fragment that gets attached. The mechanism is a bit visually challenging, but it's not too bad:
You can think of it as a variation on the aldol addition, where we've replaced the aldehyde with a conjugated enone.
- A base deprotonates the
#bb(alpha)#-carbon on the ketone, which forms the enolate. Sometimes, LDA, lithiumdiisopropylamide, is used instead to give a better yield (though it is too reactive for aldehydes).
- The enolate then acts as a good nucleophile to attack the
#beta#position on the enone. Draw the resonance structure, and you should find that the carbonyl oxygen, which is electron-withdrawing and #delta^(-)#, causes the #beta#-carbon to be #delta^(+)#.
- The reaction finishes by tautomerizing the enolate back into the ketone by deprotonating the water molecule that was made in step 1.
This is usually the first step in a Robinson Annulation. Here is an example of an annotated Robinson Annulation (notice the heat added in step 7):