The mechanism proceeds in two stages: β-nucleophilic addition to the unsaturated carbonyl compound, followed by electrophilic substitution at the α-carbon of the resulting enolate.
When the nucleophile is an organometallic reagent, the mechanisms of the first step can vary. Whether reactions take place by ionic or radical mechanisms is unclear in some cases. Research has shown that the second step may even proceed via single-electron transfers when the reduction potential of the electrophile is low. A general scheme involving ionic intermediates is shown below.
Lithium organocuprates undergo oxidative addition to enones to give, after reductive elimination of an organocopper(III) species, β-substituted lithium enolates.
In any case, the second step is well described in all cases as the reaction of an enolate with an electrophile. The two steps may be carried out as distinct experimental operations if the initially formed enolate is protected after β-addition. If the two steps are not distinct, however, the counterion of the enolate is determined by the counterion of the nucleophilic starting material and can influence the reactivity of the enolate profoundly.