ON THE NATURE OF THE COPPER EFFECT IN THE STILLE CROSS-COUPLING

The effect of added CuI on the kinetics of a typical Stille coupling was studied. With triphenylphosphine as Pd ligand, cocatalytic Cu(I) salts can yield a > 100-fold rate increase over the traditional Stille conditions, but little effect was displayed when CuI was used in conjunction with a soft ligand, such as triphenylarsine. NMR studies suggest that CuI is an excellent scavenger for free ligand and, since strong ligands in solution are known to inhibit the rate-limiting transmetalation, one effect of copper salts is readily explained. In addition, however, when working in highly dipolar solvents like NMP and in the absence of strong ligands, unsaturated stannanes react with CuI to yield presumably an organocopper species, which then transmetalates to Pd(II). An example of altered group transfer selectivity due to cocatalytic copper is given, and it is suggested that this strategy may be of some general value.