Study On Transition-Metal Catalyzed Carbon-Carbon And Carbon-sulfur Bonds Formation Reactions In PEG

As environmentally benign organic synthesis is becoming more and moreimportant, it is desirable to avoid use of any easily volatile and toxic organic solvents.There are also significant economical and environmental reasons for developingrecyclable catalytic reactions from both academic and industrial perspectives. Tosatisfy both recyclability and environmental concerns, a more facile method is toimmobilize the catalyst in a liquid phase by dissolving it into a nonvolatile andnonmixing liquid, such as ionic liquids and poly(ethylene glycol)(PEG). PEG areknown to be nontoxic, nonvolatile, recoverable, and inexpensive liquid polymers andrepresent a very attractive medium for organic reactions. In continuing our efforts todevelop green synthetic pathways for organic transformations, we have investigatedcarbon-carbon and carbon-sulfur bonds formation reactions catalyzed bytransition-metals such as copper, ruthenium or gold in PEG-400.Firstly, we investigated the CuI-catalyzed cross-coupling of aryl iodides withthiophenols in PEG-400. It was found that the reaction proceeded smoothly in thepresence of5mol%CuI at80°C in PEG-400with KCO3as base yielding a variety ofunsymmetrical thioethers in good to excellent yields. A wide range of electron-donating or electron-withdrawing groups were well tolerated. The CuI/PEG-400system could be recycled and reused six times without significant loss of activity.Next, we studied the ruthenium-catalyzed1,4-addition reaction of arylboronic acidswith α,β-unsaturated ketones in PEG-400/H2O. The results showed that the reactionproceeded effectively in the presence of2mol%[RuCl2(p-cymene)]2at90°C inPEG-400/H2O (3/1) to give the corresponding β-arylketones in good to high yields.The [RuCl2(p-cymene)]2/PEG-400/H2O system could be recycled and reused fourtimes without significant loss of activity. Finally, we investigated gold-catalyzedtandem three-component reaction of aldehydes, terminal alkynes and amines inPEG-400/H2O (1:1). In the presence of1mol%AuBr3, the cascade reaction ofaldehydes, terminal alkynes and amines proceeded smoothly in PEG-400/H2O (1:1) at100°C to furnish a variety of propargyl amines in good to excellent yields. TheAuBr3/PEG-400/H2O catalytic system could be recycled and reused five timeswithout significant loss of activity.