Catalysts, Ligands and Reagents

Heck reactions are often carried out using palladium(O) complexes. Tetrakis(triphenylphosphine)palladium(0) is frequently used, but does not allow the chemist to vary either the identity of the ligand, or the lig-andipalladium ratio. A more convenient mixture is to use the air-stable palladium(O) dibenzylideneacetone complex with the added ligands of choice. It is, however, not necessary to use a palladium(O) pre-catalyst. Palladium(II) salts, especially the more soluble palladium(II) acetate, are often used, with added phosphines. The palladium(II) salts are reduced to palladium(O) in situ. 

The most commonly used ligand is PPhs. The ratio of ligand to palladium is of great importance. Too much ligand will inhibit the reaction (due to Le Chatelier s principle). If too little ligand is present, the catalyst may decompose to give a palladium mirror. As iodide (if an aryl iodide is employed) and coordinating solvents such as acetonitrile, are used, then the amount of phosphine can be reduced even to zero. 

For the base, triethylamine or inorganic carbonates are most commonly used. Other bases, such as sodium acetate and pempidine (1,2,2,6,6-pentamethylpiperidine) have also been used. When silver salts are added, the counter ion is sometimes used as a base, such as in silver carbonate and silver phosphate. 

An interesting set of conditions are the Jeffrey conditions, using a polar solvent and a quaternary ammonium salt (Table 5.1). Detailed investigations have shown that the correct choice of ammonium salt and addition of a small amount of water can be critical. Under the right conditions, many Heck reactions can mn at or near room temperature. The main effect is due to the cation of the phase-transfer catalyst, not the anion, and the effect was most marked when inorganic bases were used. The phase-transfer catalyst may facilitate the final step of the Heck mechanism. 

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