The Suzuki And Sonogashira Reactions Through The Thermoregulated Phase-Transfer Catalysis In Water

Water is the most abundantly existing solvent in nature and widely used in organic reactions for reasons of nontoxicity, low cost, nonflammability and environmental concerns. However, the main difficulty in water-involved organic reactions is that most organic substrates are water-insoluble, resulting in a low reaction rate. To overcome this drawback has become one of the recent focuses in aqueous organic reactions.One of the key problems in the homogeneous catalysis is the separation and recycling of transition-metal catalysts. To solve this problem, a number of approaches termed as liquid/liquid biphasic catalysis have been reported in the literature, in which a catalyst is immobilized in one liquid phase and the substrate or the product is located in a separate immiscible phase at room temperature. So far, aqueous/organic biphasic system, fluorous biphasic system, thermoregulated liquid/liquid biphasic system, thermoregulated nanoparticle biphasic system and ionic liquid biphasic system have been developed.Based on the cloud point of the non-ionic phosphine ligands, a concept of the thermoregulated phase-transfer catalysis (TRPTC) has been developed by Jin and co-workers. The TRPTC system has been successfully applied in the aqueous/organic biphasic hydroformylation of higher olefins. In this paper, a new TRPTC system using water as sole medium has been developed to perform the palladium-catalyzed Suzuki and Sonogashira reactions smoothly, in which the organic solvent employed in the traditional TRPTC process was removed completely.Initially, a new protocol for the thermoregulated ligand Ph2P(CH2CH2O)nCH3(n=22)-Pd-catalyzed Suzuki reaction using water as sole medium was developed. This thermoregulated system demonstrated excellent catalytic activities and a wide range of functional group tolerance in the Suzuki reaction. This system has been successfully applied in the preparation of fluorinated biaryl derivatives, and six new fluorinated biaryl compounds were obtained (see page59, table3.3and page61, table3.4). In addition, the catalyst could be recycled three times with high activity.This new thermoregulated system was further extended to the palladium-catalyzed copper-free Sonogashira coupling between aryl halides and terminal alkynes. The results indicated that the catalytic system demonstrated excellent catalytic activity and the reusability of the catalyst. The reaction system was sensitive to the electronic nature of the substituents on the aryl bromides. Aryl bromides with an electron-withdrawing group showed higher reactivity than those with an electron-donating group. Moreover, this protocol has been applied to the synthesis of liquid crystals containing internal alkyne. Six new compounds were obtained (see page72, table4.2and page73, table4.3).In the last chapter, three PEG [poly(ethylene glycol)]-functionalized imidazolium salts (I, II, and III) have been synthesized. The catalytic performances of the catalyst generated in situ from palladium sources and imidazolium salts have been investigated in aqueous Suzuki reactions. The results indicated that the imidazolium salt â…¢ showed the highest catalytic activity towards the Suzuki reaction with excellent yields. The catalytic active species in the Suzuki reaction catalyzed by PEG-functionalized imidazolium salts-palladium was proved to be palladium nanoparticles via a mercury poisoning test.