Michael Addition
Michael Addition: A base deprotonates the acidic C–H between the two ester carbonyls, generating a stabilized enolate; this carbanion then attacks the…
What happens
A base deprotonates the acidic C–H between the two ester carbonyls, generating a stabilized enolate; this carbanion then attacks the β-carbon of the α,β-unsaturated nitrile in a conjugate (1,4) fashion rather than adding directly to the nitrile itself.
Reaction summary
Diethyl malonate (as its enolate) + acrylonitrile → cyanoethylated malonate (1,4-addition)
Reagents and conditions
- Starting material: Diethyl malonate
- Reagents: Acrylonitrile (Michael acceptor); Catalytic base (e.g. NaOEt)
- Products: Cyanoethylated malonate (Michael adduct)
Mechanism
- Enolate formation: Base removes the doubly-activated α-proton of diethyl malonate, giving a resonance-stabilized enolate.
- Conjugate (1,4) addition: The enolate carbon attacks the β-carbon of acrylonitrile; the resulting negative charge is pushed onto the nitrile nitrogen through the conjugated π system rather than landing directly on carbon.
- Protonation: Protonation on workup gives the neutral product.
Selectivity
- Regioselectivity: 1,4- (conjugate) addition dominates over 1,2-addition to the carbonyl/nitrile itself, because the softer, more polarizable enolate nucleophile prefers the softer β-carbon electrophile.
Common mistakes
- Drawing the nucleophile adding directly to the C=O or C≡N carbon (1,2-addition) instead of the β-carbon (1,4-addition).
- Forgetting only a catalytic amount of base is needed, the product's α-proton is reprotonated to regenerate the base at the end of each cycle.
Memory trick
"1,4 not 1,2": Michael donors attack four atoms away from the carbonyl oxygen (the β-carbon), not the carbonyl carbon itself.