260254 Heck Coupling with Sol-Gel Immobilized Palladium Catalysts
The palladium-catalyzed C-C coupling between aryl/vinyl halides and activated alkenes (e.g. acrylic acid and derivates, styrene's) in the presence of a base is referred to as "Heck Reaction". This reaction is one of the important steps in different pharmaceutical syntheses for example of naproxen, prosulfuron, singulair and sunscreen agents and also for the production of fine chemicals. The reaction rate depends not only on temperature, choice of base and amounts of catalyst and substrate but also on the substrate selection. The donor or acceptor properties of these reactants influence the kinetics of the reaction, which is catalyzed by an in-situ activated palladium-(0) species. Different palladium salts and ligands, for example Pd(OAc)2 or PdBr2 with/without TPPTS or Xantphos, can be used as catalysts. The addition of reducing agents, surfactants, ionic liquids or ethylene glycol allows the ligandfree, cheep and not air sensitive catalysis. The replacement of traditional organic solvents by the solutions of different surfactants in water and by use of o/w-microemulsions stabilized allows not only the more environmentally friendly procedure but also increases the solubility of hydrophobic aromatic substrates in water and also facilitates the product separation through phase separation. But Pd(OAc)2 is expensive therefore it is preferred to use it as heterogeneous catalysts in the synthesis. This catalyst could be entrapped into hydrophobically modified sol-gel matrix containing triethoxyphenylsilane or trimethoxy (octyl)silane as gel building agent and applied in the reaction. The immobilized catalyst could be reused more than 4 times without decrease in activity. After about 6 hours the full conversion was observed for the reaction of bromobenzene with styrene to trans-stilbene in aqueous microemulsion.
Figure 1: TEM images of sol-gel immobilized Pd(OAc)2 catalyst
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