Organopalladium derivatives

Three methods are commonly employed for the in situ preparation of organopalladium derivatives (i) direct metallation of an arene or heterocyclic compound with a palladium(II) salt (ii) exchange of the organic group from a main group organometallic to a palladium(II) compound and (iii) oxidative addition of an organic halide, triflate or aryldiazonium salt to palladium(O) or a palladium(O) complex. 

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... 

Those interactions mentioned above lead to jr-complexation and oxidative addition, representing two of the several most widely employed routes to organopalladium derivatives along with transmetallation with Pd, hy-dropalladation, and heteropalladation defined as addition of Pd - X bonds to 7r-bonds, where X is any element other than C or H [2],... 

Organopalladium derivatives obtained by any of the methods indicated above can undergo carbometallation which may be defined as a process of addition, generally syn-addition, of a C - Pd bond to alkenes and alkynes (Schemes 1 and 2). The regiochemistry of carbometallation can be affected by mutually competing factors and is therefore often somewhat unpredictable. 

Since both starting organometals and products in carbopalladation are organopalladium derivatives (Schemes 1 and 2), the process of carbopalladation can, in principle, repeat itself as exemplified in the reaction shown in Scheme 4. Thus, unless intercepted by some C - Pd bond cleaving process, this living process will stay alive, and no catalytic process will result. In both Heck and Maitlis reactions, the carbopalladation steps are spontaneously followed by /1-dehydropalladation and dechloropalladation, respectively. In cases where no such process occurs spontaneously, some processes must be deliberately devised usually through addition of appropriate reagents. In addition to dehydropalladation and dechloropalladation shown in Schemes 3 and 4 as well as in Eqs. 1 and 2 in Scheme 5, several other reactions used for this purpose are exemplied with prototypical cases of catalytic cyclic carbopalladation shown in Scheme 5 [9-19]. 

Mechanistic consideration of the Wacker reaction, which is thought to involve nucleophilic attach of ethylene complexed with Pd by HjO, led to the discovery of a carbon-carbon bond-forming reaction of 1,5-cyclooctadiene-Pd ir-complex with ethyl malonate in the presence of Na2C03 by Tsuji et al. in 1965 ° (Scheme 3). Re-searchers admit that an analogy between the organopalladium derivatives in Scheme 3 and TT-allylpalladium complexes was drawn and exploited in the discovery of the reaction of TT-allylpalladium with malonate also in 1965 (Scheme 4). It is noteworthy that this reaction remained only stoichiometric in Pd for several years. Once its catalytic version f was developed, however, this reaction has been extensively developed by Tsuji,Trost, " and many others, as detailed in Sect. V.2. Today, it is widely referred to as the Tsuji-Trost reaction, and it represents one of the most widely investigated areas of the organopalladium chemistry (Scheme 4). 

Despite these interesting and promising possibilities, the current scope of the in situ generation of organopalladium derivatives via migratory deinsertion is still rather limited. 

Although no 1,2-migratory insertion is involved, aryldiazonium salts have been used to generate organopalladium derivatives via extrusion of a small molecule (i.e., (Scheme 23). 

Oxidative addition requires either the use or the in situ generation of a Pd(0) complex. Metal vapor techniques, for instance, have allowed the preparation of nonligated polymeric aryl-Pd complexes, (CgF5PdX) , where X = Cl or The higher thermal stability of the perfluoroorganylpalladium complexes as compared with their ordinary organopalladium derivatives is once again clearly indicated (vide infra). 

Other CO-containing organopalladium derivatives in which the CO group is not directly bonded to Pd are also known and implicated as transient intermediates (Part VI). Some such compounds have also been prepared as discrete species, as exemplified by... 

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... 

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