Donor-Functionalized Heterocyclic Carbene Complexes of Palladium(II):  Efficient Catalysts for C−C Coupling Reactions

Abstract

PdII complexes of donor-functionalized heterocyclic carbene ligands have been synthesized through a route involving carbene transfer from AgI carbene precursors. The Ag complexes undergo facile reaction with PdCl2(MeCN)2 to yield Pd(C∧OPh)2Cl2 (3a), Pd(C∧OOMe)2Cl2 (3b), and Pd(C∧N)2Cl2 (3c) (C∧OPh = 3-methyl-1-phenacylimidazolin-2-ylidene, C∧OOMe = 3-methyl-1-(methylacetyl)imidazolin-2-ylidene, C∧N = 3-methyl-1-picolylimidazolin-2-ylidene), from which [Pd(C∧OOMe)2(MeCN)2][BF4]2 (4b) and [Pd(C∧N)2][BF4]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∧OOMe)Cl]2 (5b), PdMe(C∧N)Cl (5c), PdMe(C∧OOMe)2Cl (6b), and PdMe(C∧N)2Cl (6c) are obtained. The C∧OPh and C∧OOMe ligands are monodentate with a dangling functional moiety in all complexes, while the C∧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.