Cyclopropenylidene carbene ligands in palladium catalysed coupling reactions: carbene ligand rotation and application to the Stille reaction

Reaction of [Pd(PPh3)(4)] with 1,1-dichloro-2,3-diarylcyclopropenes gives complexes of the type cis-[PdCl2(PPh3)(C-3(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(C-3(Ar)(2))](2) (Ar = Ph 7, p-(OMe)C6H4 8, p-(F)C6H4 9). Alternatively, when the reactions were performed in acetonitrile, the complexes trans-[PdCl2(NCMe)(C-3(Ar)(2))] (Ar = Ph 10, p-(OMe) C6H4 11 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)(C-3(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 6.3.25CHCl(3), 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.