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 eta3 or eta1 fashion provides an opportunity for the stabilization of the intermediate CuIII species.