Experimental Evidence Supporting a CuIII Intermediate in Cross-Coupling Reactions of Allylic Esters with Diallylcuprate Species

The reaction between an allylic ester and a magnesium diallylcuprate, or an allylic Grignard reagent in combination with a catalytic amount of a copper salt, has been studied. These reactions yield a mixture of homo‐ and cross‐coupled 1,5‐diene products. The product ratios obtained are close to those expected for a reaction proceeding via a triallylcopper(III) intermediate consisting of three equivalent allyl groups bound to copper. When the reaction is performed with a stoichiometric amount of a preformed diallylcuprate, a homo‐coupling/cross‐coupling ratio larger than that predicted for a CuIII intermediate is observed. However, on dilution this ratio decreases and becomes close to the predicted ratio. The deviation from the predicted homo‐coupling/cross‐coupling ratio was accounted for by an olefin‐induced homo‐coupling, as demonstrated in control experiments. The possibility of the allylic ligands to coordinate to the metal center in a η3 or η1 fashion provides an opportunity for the stabilization of the intermediate CuIII species. The allylic substitution reactions between allylic esters and allylic cuprates have been studied with the aim to achieve a better understanding of the reaction mechanism. The mixtures of homo‐ and cross‐coupled 1,5‐diene products formed through reductive elimination in these reactions provide strong support for the earlier proposed mechanism via a CuIII intermediate (see scheme).