Question

Explain the mechanism of the Michael reaction?

Answer 1

The Michael reaction is an addition reaction, usually of an enolate to an `alpha,beta`-unsaturated ketone (a conjugated enone), to form a diketone.

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.

1. 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).
2. 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^(+)`.
3. 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):