Copper(l) promoted allylic nucleophilic substitutions: a synthetic and mechanistic study

Allylic alkylations and substitutions promoted by copper(l) have been studied. These reactions are very useful from the synthetic point of view, since a wide variety of allylic substrates such as halides, esters, carbonates and even alcohols are activated towards substitutions. They can be carried out under acidic conditions and hence complement the widely used palladium catalysed reaction. The driving force for the reaction is the exchange of the poorly coordinating anion on the copper, usually perchlorate or tetrafluoroborate, for a good ligating anion from the allylic substrate. The reaction proceeds through a highly reactive, symmetrical intermediate. The various possibilities include an $\eta^3-\pi$ allylic complex of copper, a fluxional $\eta^2$ complex of the allylic cation and a free cation. Experiments designed to distinguish between the possibilities are described