Palladium acetate arene-alkene reaction

Functionalized benzenes preferentially induced ortho-para substitution with electron-donating groups and meta substitution with electron-withdrawing groups (see above). Additionally, the order of reactivity found with aromatics was similar to that of electrophilic aromatic substitution. These observations implicated an electrophihc metalation of the arene as the key step. Hence, Fujiwara et al. [4b] believed that a solvated arylpalladium species is formed from a homogeneous solution of an arene and a palladium(ll) salt in a polar solvent via an electrophilic aromatic substitution reaction (Figure 9.2). The alkene then coordinates to the unstable arylpalladium species, followed by an insertion into the aryl-palladium bond. The arylethyl-palladium intermediate then rapidly undergoes )8-hydride elimination to form the alkenylated arene and a palladium hydride species, which then presumably decomposes into an acid and free palladium metal. Later on, the formation of the arylpalladium species proposed in this mechanism was confirmed by the isolation of diphenyltripalladium(ll) complexes obtained by the C-H activation reaction of benzene with palladium acetate dialkylsulfide systems [19]. 

A very important series of reactions are those in which two hydrocarbons such as arenes and alkenes are homocoupled with loss of two hydrogens in the presence of a stoichiometric amount of palladium(II) compounds such as PdCl2 and Pd(OAc)2, or more usefully with a catalytic amount of palladium catalysts and a stoichiometric amount of oxidantsJ f The palladium(II)-promoted coupling reaction was first reported by Hiittel and Bechter in 1959, who found that a-substituted styrenes were coupled by PdCl2 in acetic acid in the presence of NaOAc (Scheme... 

During the development of palladium-catalyzed coupling reaction of arenes with alkenes, we found that a-arylpalladium complexes reacted with CO to give aromatic acids in AcOH, as shown in Scheme 1 [1], This carboxylation reaction of arenes with CO proceeds catalytically with regard to Pd at room temperature under atmospheric pressure of CO, when K2S208 is added as oxidant and trifluoro-acetic acid (TFA) is used as the solvent. 

The palladium-catalyzed arylation of alkenes and arenes offers one of the conceptually most intelligent solutions for the synthesis of PAHs from the appropriate aryl halides or triflates. The reaction is usually carried out in a polar solvent (AT,AT-dimethylformamide (DMF) or AT,AT-dimethylacetamide (DMA)) at relatively high temperatures (100-170°C) with a base to trap the acid formed, and a phase transfer catalyst. Most often Pd(II) acetate or a Pd(II) complex are used, that are converted to a catalytically active Pd(0) species in the course of the reaction. The mechanisms of these Heck reactions have been discussed widely throughout the pertinent literature [78,79]. 

A variety of arenes and heteroarenes react with alkenes in the presence of palladium(II) derivatives to produce alkenyl substitution products. Three methods are commonly employed for the in situ preparation of palladium derivatives (i) direct metallation of an arene or heteroarene with a Pd(II) salt (ii) exchange of the organic group from a main-group organometallic to a Pd(II) compound (iii) oxidative addition of an organic halide, an acetate, or triflate salt to Pd(0) or a Pd(0) complex. For catalytic reactions Cu(II) chloride or p-benzoquinone is usually used to reoxidize Pd(0) to Pd(II). 

---