Fundamental Reactions of Pd Compounds

The following six fundamental reactions of Pd complexes are briefly explained in order to understand how reactions either promoted or catalyzed by Pd proceed [32a], In schemes used for explanation, spectator or iimocent phosphine ligands are omitted for simplicity. First, a brief explanation of the chemical terms specific 

Although Pd is cheaper than Rh and Pt, it is still expensive. In Pd(0)- or Pd(ll)-catalyzed reactions, particularly in commercial processes, repeated use of Pd catalysts is required. When the products are low-boiling, they can be separated from the catalyst by distillation. The Wacker process for the production of acetaldehyde is an example. For less volatile products, there are several approaches to the economical uses of Pd catalysts. As one method, an alkyldi-phenylphosphine 9, in which the alkyl group is a polyethylene chain, is prepared as shown. The Pd complex of this phosphine has low solubility in some organic solvents such as toluene at room temperature, and is soluble at higher temperature[28]. Pd(0)-catalyzed reactions such as an allylation reaction of nucleophiles using this complex as a catalyst proceed smoothly at higher temperatures. After the reaction, the Pd complex precipitates and is recovered when the reaction mixture is cooled. 

Pd can also be recovered as insoluble complexes such as the dimethylglyox-ime complex, or PdCUiPhiP) by treatment with HCl and PI13P. When water-soluble phosphines are used, the catalyst always remains in the aqueous phase and can be separated from a product in the organic phase, and is used repeatedly. 

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Migratory insertion and its microscopic reversal, that is, migratory deinsertion, are two of the 20 or so fundamental processes discussed in Sect. 1.2 in which Pd participates. In principle, they should be observed with a wide variety of substrates. In reality, however, the current scope of these processes observable with organopalladiums is almost totally limited to those that involve CO and related compounds, such as isonitriles. Conse-qnently, the migratory insertion-deinsertion chemistry of organopalladium compounds at present is essentially synonymous with their carbonylation-decarbonylation reactions. 

Compounds with Unsupported Pd(l)-Pd(l) Bond - Corner-Sharing Geometry This type of Pd(I)-Pd(I) compound has a long history and is fundamentally important in understanding the nature of the Pd(I)-Pd(I) bond, in homogeneous catalytic reactions, and as model complex for cluster and for metallic surfaces [57]. The two four-coordinate square planar Pd(l) centers are connected by an unsupported Pd(l)-Pd(I) bond, with the bond length usually less than 2.6 A,... 

The general chemistry of aromatic compounds and particularly their substitution reactions have changed fundamentally with the advent of Pd- and other metal-complex catalysts. 

A starting material that is suitable for the direct construction of a heterocycle by an intramolecular Heck-type reaction has to fulfil some simple but fundamental requiranents there has to be the halide function or a triflate for the oxidative addition onto the Pd catalyst, a side chain with an unsaturated functionality such as an alkene or an alkyne in an appropriate distance, and of course the heteroatom in this side chain. Figure 1 presents a substructure typical for very many starting materials, which were transformed to heterocycles by intramolecular Heck-type reactions (X = halide, Het = heteroatom). This type of substructure with an allylic side chain is easily accessible by derivatization of 2-bromo- and 2-iodo anilines, phenols, and thiophenols and leads to interesting heterocycles such as indoles and benzofurans, which are related to many natural products and other biological active compounds. 

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