conjugate addition

Conjugate addition is the trick of adding something not directly across a double bond, but across a whole conjugated system, so the two pieces end up at the far ends — the 1 and 4 positions — with the double bond shifting into the middle. The name simply emphasizes that the reaction works through conjugation; another common name for it is 1,4-addition. It is the more remote, often more thermodynamically favourable sibling of straightforward 1,2-addition.

The most important version involves not a plain diene but an alpha,beta-unsaturated carbonyl — a C=C conjugated with a C=O, as in an enone. Here conjugation makes the carbon two away from the carbonyl (the beta carbon) electron-poor and therefore electrophilic. A nucleophile can attack that beta carbon, and after the dust settles the nucleophile sits at the beta position while the oxygen has picked up the hydrogen, with the net result looking like 1,4-addition across the O=C-C=C system. When the nucleophile is a carbon nucleophile (an enolate), this exact reaction has its own famous name, the Michael addition, a workhorse for stitching carbon skeletons together.

Conjugate addition matters because it gives chemists a choice. A nucleophile facing an enone can attack the carbonyl carbon directly (1,2-addition) or the beta carbon (1,4-conjugate addition), and which path it takes depends on the nucleophile: soft, stabilized nucleophiles tend to add 1,4, while hard, reactive ones (like organolithiums or hydride) often add 1,2. Controlling that choice is a core skill in synthesis, and conjugate addition — especially the Michael reaction — is one of the most-used carbon-carbon bond-forming reactions in the whole field.