Bridge Nitrogen Heterocyclic Carbine-Palladium Catalyst Systems Applied In The Cross-Coupling Reactions

This dissertation deals with palladium-bridged imidazolium salts and palladium-bridged benzimidazolium salts catalyst systems and their applications in the Heck reaction and Suzuki cross-coupling reactions.Four kinds of bridged imidazolium salts and bridged benzimidazolium salts have been designed and prepared. Through formation of palladium catalytic species in situ, the structural influence of these bridged imidazolium and benzimidazolium salts on the catalystic activities of the corresponding palladium catalysts were investigated. The bridged N,N'-dibenzylbenzimidazolium salts was found to be less efficient than N.N'-dibutylbenzimidazolium analog in the Heck reaction under identical conditions. However, the length of the bridge was found not to affect the efficiency of these catalysts based on bridged N,N'-dibenzylbenzimidazolium salts. When palladium loading was lower than 0.03mol%, the catalyst system was found to be deactivated. An electron-donating group on benzimidazolium salts was found to depress the catalytic activity of the corresponding catalyst systems.The Pd-catalyzed Heck reaction of β-substituted acrylates in TBAB was also investigated. The stereoselectivity of the coupling products have been explored with respect to ligand, bases, and electronic effects of aryl moieties in substrates. No significant influence on the stereoselectivity of the reaction was observed from ligand. However, tiny differences of base can affect the stereoselectivity. After a systematic screening, the acetate zinic has been proved to be the best base in the Heck reactions of β-substituted acrylates, giving coupling products >90% E/Z selectivity. The electronic effects from olefins and aryl halides as found to show little effects on the stereoselectivity of the coupling products. However, the yields of the aryl halides bearing an acetyl group in the Heck coupling of β-substituted acrylates were low.The Suzuki cross-coupling of aryl halides with phenylboronic acid catalyzed by the above motioned bridged palladium-benzimidazolium catalyst systems was also explored biefly. The bridge length of the bridged benzimidazolium has been proved to show influence on the catalytic activity in the Suzuki cross-coupling. The catalyst system derived from (CH₂)₄ bridged benzimidazolium salt showed the highest activity and give coupling products in good yields.