Palladium Compounds Tris -Triphenylphosphine

Methods (i) and (ii) require palladium(II) salts as reactants. Either palladium acetate, palladium chloride or lithium tetrachloropalladate(II) usually are used. These salts may also be used as catalysts in method (iii) but need to be reduced in situ to become active. The reduction usually occurs spontaneously in reactions carried out at 100 °C but may be slow or inefficient at lower temperatures. In these cases, zero valent complexes such as bis(dibenzylideneacetone)palladium(0) or tetrakis(triphenylphos-phine)palladium(O) may be used, or a reducing agent such as sodium borohydride, formic acid or hydrazine may be added to reaction mixtures containing palladium(II) salts to initiate the reactions. Triarylphosphines are usually added to the palladium catalysts in method (iii), but not in methods (i) or (ii). Normally, 2 equiv. of triphenylphosphine, or better, tri-o-tolylphosphine, are added per mol of the palladium compound. Larger amounts may be necessary in reactions where palladium metal tends to precipitate prematurely from the reaction mixtures. Large concentrations of phosphines are to be avoided, however, since they usually inhibit the reactions. 

Vinylic ethers (44) can be synthesized in high yields by the cross-coupling of aryl or benzyl halides with tris(2-ethoxyvinyl)borane (4S) in the presence of 1 molej% of a palladium compound such as tetrakis(triphenylphosphine)palladium and a base (Eq. 108) Since vinylic ethers (44) thus obtained can readily be hydrolyzed to aldehydes, this reaction provides a convenient procedure for converting aryl or benzylic halides into the corresponding aldehydes with two more carbon atoms. 

Reactions with palladium compounds share many common features with reactions involving other transition metals. During a reaction, palladium is coordinated to a variety of groups called ligands, which donate electron density to (or sometimes withdraw electron density from) the metal. A common electron-donating ligand is a phosphine, such as triphenylphosphine, tri(o-tolyl)phosphine, or tricyclohexylphosphine. 

Besides the use of copper and its compounds, palladium dichloride, rhodium trichloride, tris(triphenylphosphine)rhodium(i) chloride, and palladium diacetate have been reported to be effective in catalysing the reaction of ethyl diazoacetate with styrene at room temperature. Of these, palladium diacetate is the most effective. 

Under remarkably mild conditions aromatic cyanides can be prepared from halogen compounds with alkali metal cyanides in the presence of transition metal complexes. Complexes of palladium and nickel are particularly useful, for instance tetrakis(triphenylphosphine)palladium(0) (6), tris(triphe-nylphosphine)nickel(O) (7) or trun -dichlorobis(triphenylphosphine)nickel(II) (8 Scheme 7). 

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