(600s) 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