The synthesis of oligoether-substituted benzimidazolium bromides and their use as ligand precursors for the Pd-catalyzed Heck coupling in water

Abstract

The oligoether-substituted (CH 3(OCH 2CH 2) n –; n = 1, 2 or 3) benzimidazolium bromides ( 3– 7) and oligoether-linked (–CH 2(CH 2OCH 2) n CH 2–, n = 1, 2 or 3) bisbenzimidazolium dibromides ( 8– 13) were prepared by quarternization of N-substituted benzimidazoles ( 1 and 2) with the bulky benzyl bromides (ArCH 2Br: Ar = C 6H 2(CH 3) 3-2,4,6 and C 6(CH 3) 5). trans-Bis(carbene) palladium(II) complexes 14 and 15 derived from 4 and 6 were synthesized by using Ag complexes as carbene-transfer agents in dichloromethane at ambient temperature. In addition, the reactions of 4 and 6 with Pd(OAc) 2 and NaBr gave the Pd(II) dimers 16 and 17 which can readily be cleaved by triphenylphosphine to afford the benzannulated monocarbene (NHC) monophosphine Pd(II) complexes [PdBr 2(NHC)(PPh 3)] ( 18 and 19). All compounds have been fully characterized by using elemental analysis, 1H, 13C and 31P NMR spectroscopies. X-ray diffraction studies on single crystals of 19a and 19b confirm the cis square planar geometry. In situ formed complexes from Pd(OAc) 2 and benzimidazolium salts ( 3– 13) and preformed Pd(II) complexes 14, 15, 18 and 19 were tested as catalyst for the Heck coupling reaction in water. The influence of the oligoether and benzyl substituents on N atoms and CH 3-substituents on the 5,6-positions of benzimidazole frame were investigated under the same conditions in the Heck coupling reaction. In situ formed catalysts showed better conversions than the isolated Pd(II) complexes. The length of the oligoether spacer significantly increases the activity. The salts with two benzimidazole moieties connected by an oligoether as the spacer 8– 13 showed similar catalytic activities in the Heck coupling reaction with the mono salts 3– 7 bearing corresponding oligoethers on the N atom.