Corey–Fuchs reaction

By suitable choice of base, it is often possible to stop the reaction at the 1-bromoalkyne, a useful functional group for further transformation.

Reaction mechanism

The Corey–Fuchs reaction is based on a special case of the Wittig reaction, where two equivalents of triphenylphosphine are used with carbon tetrabromide to produce the triphenylphosphine-dibromomethylene ylide.

This ylide undergoes a Wittig reaction when exposed to an aldehyde. Alternatively, using a ketone generates a gem-dibromoalkene.

The second part of the reaction converts the isolable gem-dibromoalkene intermediate to the alkyne. Deuterium-labelling studies show that this step proceeds through a carbene mechanism. Lithium-bromide exchange is followed by α-elimination to afford the carbene. 1,2-shift then affords the deuterium-labelled terminal alkyne. The 50% H-incorporation could be explained by deprotonation of the (acidic) terminal deuterium with excess BuLi.