Coupling Reactions In Microemulsion

A microemulsion is a thermodynamically stable, isotropic, optically transparent solutionconsisting of water, oil and a surfactant. Microemulsion could cosolubilize great amountsof hydrophilic and hydrophobic compounds, which would resolve the well-knownsolubilization problems. The interest of microemulsion in organic synthesis relates to theirextremely large interfacial areas, providing specific rate enhancement and overcomingreagent incompatibility. Microemulsion have nanometric size of droplets, which areparticularly suitable nano-reactors for organic reactions. In previous reports onmicroemulsion, most of studies concerned the use of microemulsion as reaction medium inorganic synthesis. Thus, the development of new routes on applying microemulsion inmodern organic synthesis is considered to be one of the most challenging fields.This dissertation focoused on coupling reactions in microemulsions, we found theYX10/heptane/butanol/water/ propylene glycol(PG) microemulsions were not only thereaction medium for ligand-free coupling reactions(Sonogashira, Heck and Ullmannreactions, et al.), but also reducing agents for the coupling reactions.TX10/heptane/butanol/water/PG microemulsions were prepared as insensitivemicroemulsion, which could be stable at 30～80℃'s temperature range and 0～0.3 M's baseconcentration range, and the insensitive microemulsion could be stable at the course of thecoupling reactions.Nanosized Pd⁰ for the coupling reactions could be in-situ formed in the microemulsionswithout other reducing agents. The microemulsions containing in-situ formed nano-Pd⁰were applied to the ligand-free coupling reactions. The effect of reaction parameters werediscussed. The results indicated that the aqueous phase concentration, base concentrationand temperature played key role on the reaction. Iodobenzene was convened to thecorresponding tolane quantitatively within 2 min. Therefore, TX 100/heptane/butanol/water/propylene glycol microemulsions containing in situ formed palladium nanoparticleswere very efficient catalyst systems for ligand-free coupling reactions.The heterogeneous catalysts (Pd/C,Pd(NH₃)₄²⁺/NaY) could catalyze ligand-freeSonogashira,Heck,Suzuki,Ullmann reactions. Excellent result of chlorobenzene in Pd/C-catalyzed Ullmann reaction could be achieved in microemulsions without other reducingagents. The effect of reaction parameters on the supported Pd-catalyzed coupling reactionsin the microemulsion were discussed. "Hot filtrations" were performed in the Pd/C- and Pd(NH₃)₄²⁺/NaY-catalyzedcoupling reactions in the microemulsion. The hot filtration indicated the "active catalyst"were the dissolved nano-Pd that leached from supports into the microemulsion.