N-heterocyclic carbenes as supporting ligands in homogeneous catalysis

by Marion, Nicolas

ABSTRACT In the last ten years, N-heterocyclic carbenes(NHCs) have gained tremendous popularity, notably as highly versatile ligands for transition metals. Their strong ?-donating properties, associated with high steric hindrance, often impart enhanced stability and activity to a given metallic center. Two main successes of the NHCs in homogeneous catalysis are arguably the ruthenium-mediated olefin metathesis and the palladium-promoted crosscoupling reactions.In this work, we have studied the effect of N-heterocyclic carbenes as supporting ligands in well-defined complexes of palladium(II), gallium(III), and gold(I) that we used in homogeneous catalysis.Notably, we have synthesized, in very straightforward manners, two families of palladium compounds of formulae [(NHC)Pd(L)Cl], where L, which is a R-allyl or R-acac moiety, acts as a protecting shell for the catalytically active [(NHC)Pd0] species. Hence, upon activation under the reaction conditions, these new Pd complexes were found extremely active in the Suzuki-Miyaura, the Buchwald-Hartwig, and the ?-ketone arylation cross-coupling reactions. More precisely, the more active R-allyl family allowed for reactions to be performed with as low as 10 ppm of palladium. A series of [(NHC)GaCl3] complexes wa synthesized via a simple one-step procedure. The resulting unprecedented NHCGaIII compounds were found extremely stable but showed only moderate activity in isomerization reactions.Demonstrating further the versatile application of NHCs in metal-based catalysis, we developed several new catalytic transformations using [(NHC)AuCl] complexes. Hence,these pre-catalysts, activated in situ with a silver salt, proved to be excellent activators of alkynes, allenes, and alkenes. This led to the development of efficient synthetically useful protocols, encompassing enyne cycloisomerization, indene cyclization,formation of conjugated enone, and allylic rearrangement. KEYWORDS gallium gold homogeneous catalysis N-heterocyclic carbene palladium 8