Cyclopropenylidenecarbeneligands in palladium catalysed coupling reactions: carbeneligand rotation and application to the Stille reaction

Abstract

Reaction of [Pd(PPh₃)₄] with 1,1-dichloro-2,3-diarylcyclopropenes gives complexes of the type cis-[PdCl₂(PPh₃)(C₃(Ar)₂)] (Ar = Ph 5, Mes 6). Reaction of [Pd(dba)₂] with 1,1-dichloro-2,3-diarylcyclopropenes in benzene gave the corresponding binuclear palladium complexes trans-[PdCl₂(C₃(Ar)₂)]₂ (Ar = Ph 7, p-(OMe)C₆H₄8, p-(F)C₆H₄9). Alternatively, when the reactions were performed in acetonitrile, the complexes trans-[PdCl₂(NCMe)(C₃(Ar)₂)] (Ar = Ph 10, p-(OMe)C₆H₄11 and p-(F)C₆H₄) 12) were isolated. Addition of phosphine ligands to the binuclear palladium complex 7 or acetonitrile adducts 11 and 12 gave complexes of the type cis-[PdCl₂(PR₃)(C₃(Ar)₂)] (Ar = Ph, R = Cy 13, Ar = p-(OMe)C₆H₄, R = Ph 14, Ar = p-(F)C₆H₄, R = Ph 15). Crystal structures of complexes 6·3.25CHCl₃, 10, 11·H₂O and 12–15 are reported. DFT calculations of complexes 10–12 indicate the barrier to rotation about the carbene-palladium bond is very low, suggesting limited double bond character in these species. Complexes 5–9 were tested for catalytic activity in C–C coupling (Mizoroki–Heck, Suzuki–Miyaura and, for the first time, Stille reactions) and C–N coupling (Buchwald–Hartwig amination) showing excellent conversion with moderate to high selectivity.