lisäyseliminaatiomekanismini

Lisäyseliminaatiomekanismina (EU: addition–elimination mechanism) is a term used in organic chemistry to describe a reaction pathway in which a nucleophilic addition step is immediately followed by an elimination step, generally resulting in the net removal of a leaving group and formation of a new π‑bond or a substituted product. The mechanism is typically classified as an SN2′ (nucleophilic substitution at a position adjacent to the site of attack) or an SN1‑like mechanism that gives an ylide intermediate, which then undergoes β‑elimination.

In most addition–elimination reactions the reagents first undergo a concerted or stepwise addition of a nucleophile to a multiple bond or an activated carbon, such as a Michael addition to a α,β‑unsaturated carbonyl. Following the addition, the intermediate rearranges and expels a leaving group, forming a new unsaturated system. A classic example is the Claisen rearrangement, in which an allyl vinyl ether undergoes a [3,3] sigmatropic shift that can be interpreted as a sequential addition of the vinyl group to the allylic system and subsequent elimination of the oxygen lone pair.

The significance of the addition–elimination mechanism lies in its ability to create stereochemically defined products and functional groups that would be difficult to access via simple substitution or elimination routes. The term is often applied in the discussion of organometallic catalysis, where metal–alkyl complexes add to unsaturated substrates and eliminate alkyl groups, as well as in the synthesis of heterocycles, where tandem addition–elimination steps construct complex ring systems.