Michael addition
/ MY-kul /
The Michael addition is the marriage of two ideas: an enolate as a carbon nucleophile, and conjugate (1,4) addition to an alpha,beta-unsaturated carbonyl. When a C=C is conjugated with a C=O, the far end of the double bond — the beta-carbon — becomes electrophilic, because the carbonyl can pull electron density all the way through the conjugated system. A nucleophile can add there, at the beta-carbon, rather than directly at the carbonyl carbon. That is the '1,4' or conjugate mode.
In the Michael reaction the nucleophile (the 'Michael donor') is a stabilized enolate, classically from a doubly-activated compound like a beta-keto ester or malonic ester, and the electrophile (the 'Michael acceptor') is the alpha,beta-unsaturated carbonyl, such as an enone. The enolate's carbon adds to the beta-carbon; the negative charge that develops flows onto the carbonyl oxygen as an enolate, and a quick protonation finishes the job. The net result is a new carbon-carbon bond joining the two pieces with the donor's carbon now hanging off the beta-position.
Why prefer conjugate over direct addition? Soft, stabilized nucleophiles like enolates favor the 1,4 pathway, which leads to a more stable product (the enolate intermediate is more stable than an alkoxide), whereas hard nucleophiles like organolithiums or hydride often go 1,2 straight at the carbonyl. The Michael addition is one of the most reliable C-C bond-forming reactions in synthesis, and chaining a Michael addition to an intramolecular aldol condensation gives the powerful ring-building Robinson annulation.
Diethyl malonate (the donor) plus methyl vinyl ketone, CH2=CH-CO-CH3 (the acceptor), with base, adds the malonate's central carbon to the beta-carbon, giving (EtO2C)2CH-CH2-CH2-CO-CH3 — a clean 1,4-addition product.
A stabilized enolate adds 1,4 to an enone — the classic Michael addition.
Conjugate (1,4) addition is favored by soft, stabilized nucleophiles like enolates; it is a tendency, not an absolute law. Hard nucleophiles (RLi, RMgX, hydride) often add 1,2 to the carbonyl instead.