Organopalladium compounds catalytic reactions

The vinyl substitution reaction often may be achieved with catalytic amounts of palladium. Catalytic reactions are carried out in different ways depending on how the organopalladium compound is generated. Usually copper(II) chloride or p-benzoquinone is employed to reoxidize palladium(0) to palla-dium(II) in catalytic reactions when methods (i) or (ii) are used for making the organopalladium derivative. The procedures developed for making these reactions catalytic are not completely satisfactory, however. The best catalytic reactions are achieved when the organopalladium intermediates are obtained by the oxidative addition procedures (method iii), where the halide is both the reoxidant and a reactant. Reviews of some aspects of these reactions have been published.u-le... 

Various methods of generation of organopalladium derivatives discussed in Sects. 1.2 and n.3.1 are also applicable to the syntheses of discrete organopalladium compounds that can be isolated and identified. In most of the catalytic reactions, then-formation is assumed, and their stoichiometric preparation discussed in this section lends strong support for such assumptions. So the stoichiometric preparation of... 

As far as activation of the C-H bond in aromatic compounds with palladium(ll) is concerned, the features of these catalytic reactions are the following (1) hydrogen atom is departed at the first step as a proton (2) electrons of the activated C-H bond are involved in the formation of organopalladium intermediate (3) palladium(ll) is reduced into palladium (O), which then undergoes oxidation with an external oxidant into palladium(ll) acetate, thus providing a cyclic catalytic process (Scheme 4). 

Halopalladation is a reaction in which a halide ion, as a nncleophile, adds to the palladium complexed unsaturated compounds. Chloropalladation of unsaturated organic substrates is an important reaction in organopalladium chemistry. Additions of Pd—Cl to allenes, " olefins,acetylenes, " and conjugated dienes have been reported. While a lot of works concerning the halopalladation reactions of unsaturated substrates were studied, only a few papers related to the catalytic reactions initiated by halopalladation of organic substrates were reported until recently. 

The addition of an organopalladium intermediate to the double bond with subsequent cleavage of palladium hydride and restoration of unsaturation is the basis of another important process involving organopalladium compounds, the Heck reaction. In a catalytic version, the organopalladium intermediate is formed by oxidative addition to carbon-halogen or other carbon-heteroatom bonds, and the hydridopalladium complex is believed to be recycled by base-promoted reductive elimination (Scheme 5.7). 

Olefinic compounds will often insert into carbon-transition metal bonds as CO does, and this reaction is an important step in many catalytic syntheses. When this step is combined with an oxidative addition of an organic halide to a palladium(O) complex in the presence of a base, a very useful, catalytic olefinic substitution reaction results (26-29). The oxidative addition produces an organopalladium(II) halide, which then adds 1,2 to the olefinic reactant (insertion reaction). The adduct is unstable if there are hydrogens beta to the palladium group and elimination of a hydridopalladium salt occurs, forming a substituted olefinic product. The hydridopalladium salt then reforms the... 

By use of aryl compounds prepared in this way, the arylation of alkenes and a variety of other organic unsaturated compounds can be achieved.52 A palladium compound, usually Li2PdCl4, is used as a transfer agent and organopalladium species are believed to be unstable intermediates by use of air and a Cu" salt the reaction can be made catalytic, since the Pd metal formed in the reaction is dissolved by Cu11 and the Cu1 so produced is oxidized by air (cf. the Wacker process, Section 24-B-5). A typical reaction is... 

Organopalladium(iv) intermediates probably occur in a range of (catalytic) synthetic reactions involving oxidative addition-reductive elimination sequences." In several cases, intermediates have been well characterized, for example, by NMR and with reference to authenticated structures of organopalladium(iv) compounds with similar spectroscopic data. 

To make C-glycosides, organopalladium(II) reagents produced from the corresponding organomercurials or -stannanes are used or iodoaromatics can be employed with catalytic proportions of palladium salts. Compound 19, made by treatment of the adduct 18 with triphenylphosphine, gives 20 (R = H) in near quantitative yield on reaction with aqueous sodium bicarbonate, but when it is simply heated briefly in toluene a different elimination occurs to give the ace-toxy compound 20 (R = OAc, Scheme 7) [30]. 

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