Experimental evidence supporting a Cu-III 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 coppe r salt, has been studied. These reactions yield a mixture of homo- and cros s-coupled 1,5-diene products. The product ratios obtained are close to thes e expected for a reaction proceeding via a triallylcopper(III) intermediate consisting of three equivalent allyl groups bound to copper. When the reac tion is performed with a stoichiometric amount of a preformed diallylcuprat e, a homo-coupling/cross-coupling ratio larger than that predicted for a Cu -III intermediate is observed. However, on dilution this ratio decreases an d becomes close to the predicted ratio. The deviation from the predicted ho mo-coupling/cross-coupling ratio was accounted for by an olefin-induced hom e-coupling, as demonstrated in control experiments. The possibility of the allylic ligands to coordinate to the metal center in a eta (3) or eta (1) f ashion provides an opportunity for the stabilization of the intermediate Cu -III species.