Versatile Relay and Cooperative Palladium(0) N-Heterocyclic Carbene/Copper(I) N-Heterocyclic Carbene Catalysis for the Synthesis of Tri- and Tetrasubstituted Alkenes

Two new and efficient dual catalytic procedures for the synthesis of tri- and tetrasubstituted alkenes using aryl bromides and chlorides are reported. The formation of these vinyl arenes occurs independently in a one-pot relay and a one-pot cooperative procedure using well-defined Pd and Cu N-heterocyclic carbene (NHC) complexes. Mechanistic studies were performed to elucidate the key role of the solvent in each procedure as well as the formation of the key intermediate, [Cu(alkenylboron)(NHC)], which allows transmetalation with the Pd-NHC co-catalyst in the cooperative catalysis.