Pyridin-, quinolin- and acridinylidene palladium carbene complexes as highly efficient C-C coupling catalysts

A series of four new complexes bearing N-heterocyclic carbene ligands (NHCs) as well as four compounds bearing N-heterocyclic carbene ligands with remote heteroatoms (rNHCs) of the general types [(NHC)(PPh₃) ₂PdCl]⁺BF₄^- and [(rNHC)-(PPh ₃)₂PdCl]⁺BF₄^-, respectively, have been prepared in high yields. Crystal and molecular structures have been determined for four representative examples. These compounds proved to be efficient catalysts for aryl coupling reactions of the Heck and Suzuki types (reaching TONs of as high as 6,200,000). Both aryl bromides and aryl chlorides can be used as substrates. Like the well known mixed, standard (NHC)(phosphine) compounds, the new six-numbered, one-N-heterocyclic carbene complexes (and in particular certain rNHC-containing ones) also combine the advantageous stability of bis(carbene) and the high activity of bis(phosphine) complexes. Furthermore, their good catalytic performance and, especially, their easy synthesis based on cheap and commercially available starting materials, make them by far superior when compared to the mixed (NHC) (phosphine) catalysts known thus far.