nucleophilic addition reaction of α, β–unsaturated carbonyl compounds

,4-Addition, also known as conjugate addition, is a nucleophilic addition reaction of α, β–unsaturated carbonyl compounds and α, β–unsaturated nitriles.

eg:  Two nucleophilic addition reaction could occur between methylvinyl ketone (1) and methanethiol in basic medium.

mechanism of Reaction 1:

The net reaction from 1 to 4 is the addition of two ligands to atoms 1 and 4 in 1. Hence, the reaction is called 1,4-addition, or conjugate addition, and its product (2) 1,4-adduct.

mechanism of Reaction 2:

The net reaction from 1 to 3 is the addition of two ligands to atoms 1 and 2 in 1.  Hence, the reaction is called 1,2-addition, or direct addition, and the product (3) a 1,2-adduct.

The overall reaction between an α, β–unsaturated compound and a nucleophile is regioselective. Whether the dominant process is 1,4-addition or 1,2-addition depends on several factors, such as the alkene-containing reactant (the α, β–unsaturated compound), nucleophile, solvent, concentration, temperature, reaction time, and catalyst, if any, making if difficult to make a generalization. Most resonance-stabilized carbon nucleophiles, such as enolate ions and enamines overwhelmingly prefer 1,4-addition to 1,2-addition.