Donor-functionalized heterocyclic carbene complexes of palladium(II): Efficient catalysts for C-C coupling reactions

Pd-II complexes of donor-functionalized heterocyclic carbene ligands have been synthesized through a route involving carbene transfer from Ag-I carbene precursors. The Ag complexes undergo facile reaction with PdCl2(MeCN)(2) to yield Pd(C boolean AND OPh)(2)Cl-2 (3a), Pd(C boolean AND OOMe)(2)Cl-2 (3b), and Pd(C boolean AND N)(2)Cl-2 (3c) (C boolean AND OPh = 3-methyl-1-phenacylimidazolin-2-ylidene, C boolean AND OOMe = 3-methyl-1-(methylacetyl)imidazolin-2-ylidene, C boolean AND N = 3-methyl-1-picolylimidazolin-2-ylidene), from which [Pd(C boolean AND OOMe)(2)(MeCN)(2)][BF4](2) (4b) and CPd(C boolean AND N)(2)]CBF4](2) (4c) can be prepared by halide abstraction. When PdMeCl(cod) (cod = cyclooctadiene) is treated with the Ag complexes 2b and 2c, the Me-Pd carbene complexes [PdMe(C boolean AND OOMe)Cl](2) (5b), PdMe-(C boolean AND N)Cl (5c), PdMe(C boolean AND OOMe)(2)Cl (6b), and PdMe(C boolean AND N)(2)Cl (6c) are obtained. The C boolean AND OPh and C boolean AND OOMe ligands are monodentate with, a dangling functional moiety in all complexes, while the C boolean AND N ligand exhibits both monodentate and chelating behavior. Several complexes give rise to highly active and very stable catalysts for Heck, Suzuki, and Sonogashira coupling reactions, with turnover numbers of up to 1 700 000 (Heck) and 127 500 (Suzuki) being obtained.