Synthesis and Catalytic Application of Palladium Complexes with Picoline-Functionalized Benzimidazolin-2-ylidene Ligands

The picoline-functionalized benzimidazolium salts N-alkyl-N'-picolylbenzimidazolium bromide [alkyl = methyl (1), ethyl (2), n-propyl (3), n-butyl (4)] and the dipicoline-functionalized benzimidazolium bromide 5 have been synthesized. Reaction of the salts 1-4 with palladium acetate leads to the formation of the palladium complexes of the type [Pd(L)(2)]Br-2 (6-9; L = N-alkyl-N'-picolylbenzimidazolin-2-ylidene). Subsequent reaction of these complexes with AgBF4 gave the palladium complexes 10-13. The pincer-type palladium complex [PdBrL]Br (14, L = N,N'-dipicolylbenzimidazolin-2-ylidene) was prepared by deprotonation of the benzimidazolium. salt 5 with n-butyllithium and subsequent coordination of the unstable carbene intermediate to [PdBr(2)cod]. The molecular structures of the benzimidazolium. salts 3 and 5 and the palladium complexes 7-9 and 11 have been determined by X-ray diffraction. The bis(carbene) palladium complexes 6-9 as well as the monocarbene pincer-type complex 14 have been used as precatalysts in Heck-type coupling reactions of several aryl halides with styrene and n-butyl acrylate as olefinic substrates. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)