Palladium(II) Complexes with Benzimidazolin‐2‐ylidene and Phosphane Ligands and their Catalytic Activity in Mizoroki–Heck Coupling Reactions

Symmetrically and unsymmetrically substituted benzimidazolium bromides (1a–d = NHC·HBr) were synthesized, incorporating bulky benzyl and/or methoxyethyl substituents (a: R = pentamethylbenzyl, R′ = 2‐methoxyethyl; b: R = 2,3,5,6‐tetramethylbenzyl, R′ = methoxyethyl; c: R, R′ = pentamethylbenzyl; d: R, R′ = 2,3,5,6‐tetramethylbenzyl). The salts were used to prepare NHC palladium complexes. Reaction of 1a–d with Pd(OAc)2 and NaBr in a 1:1:3 ratio in dmso gave mono or dinuclear, bromide‐bridged complexes of type 2. Complexes of type 2 react with monodentate or bidentate phosphanes in CH2Cl2 to afford the mixed benzimidazolin‐2‐ylidene‐phosphane complexes 3–6. Reaction of salts 1a–d with Pd(OAc)2 in a 1:2 ratio, in the absence of NaBr, gave complexes trans‐[PdBr2(NHC)2] (7). All compounds were fully characterized by NMR spectroscopy, mass spectrometry (MALDI), and elemental analysis. In addition, the molecular structures of 2c, 3c, 3d, and 5b were determined by X‐ray diffraction. The catalytic activities of 2–7 in Mizoroki–Heck C–C coupling reactions of aryl halides with n‐butyl acrylate are described and compared. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)