Organopalladium compounds reaction with halides

Polystyrene-bound secondary aliphatic amines and /V-alkyl amino acids can be ally-lated by treatment with a diene and an aryl iodide or bromide in the presence of palla-dium(II) acetate (Entry 14, Table 10.3). As the diene, 1,3-, 1,4-, and 1,5-dienes can be used, and, besides aryl halides, heteroaryl bromides have also been successfully used [63], This remarkable reaction is likely to proceed via the formation of an aryl palladium complex, with subsequent insertion of an alkene into the C-Pd bond. The resulting organopalladium compound does not undergo ( -elimination (as in the Heck reaction), but isomerizes to an allyl palladium complex, which reacts with the amine to give the observed allyl amines. 

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... 

Acemoglu L, Williams JMJ (2002) Synthetic Scope of the Tsuji-Trost Reaction with Allylic Halides, Carboxylates, Ethers, and Related Oxygen Nucleophiles as Starting Compounds. In Negishi E, de Meijere A (eds) Handbook of Organopalladium Chemistry for Organic Synthesis. Wiley, New York, p 1689... 

Oxidative addition [1, 38] of 1-alkenyl, i-alkynyl, allyl, benzyl, and aiyl halides to a palladium(O) complex affords a stable rra .s-<7-palladium(II) complex (11). The reaction proceeds with complete retention of configuration for alkenyl halides and with inversion for allylic and benzylic halides. Alkyl halides having /3-hydrogens are rarely useful because the oxidative addition step is very slow and may compete with /3-hydride elimination from the a-organopalladium(II) species. However, it has been recently shown that iodoalkanes undergo the cross-coupling reaction with organoboron compounds (Section 2.4.5). 

Organopalladium compounds can be prepared by electrophilic paUadation, oxidative addition to aryl halides or reaction of Pd(II) with organometalhc reagents. These transformations are all vital for the palladium-catalysed reactions discussed later in this chapter. 

Palladium metal reacts with organic halides directly and organopalladium compounds are isolated by reaction with phosphine [44-48]. For example, organopalladium compounds are prepared by the reaction of organic halides with vaporized palladium or with metal slurry obtained by the reduction of palladium halide with an alkali metal as shown in eqs. (20.24)-(20.26). The last reaction shows higher yield. 

Nitrilepalladium or isonitrilepalladium is easily prepared in high yield by reaction with palladium halides or unsaturated palladium compounds as shown in eqs. (20.27)- 20.30) and these palladium compounds are used as the raw material for synthesis of other organopalladium compounds as shown in eqs. (20.3I)-(20.34) [11,12,49-57]. 

Fifth, although the relative inertness of carbonyl compounds excluding acyl halides was emphasized above, most everything in chemistry is relative, and organopalladium chemistry is no exception. Thus, in the absence of faster reaction paths, Pd and its complexes may react with aldehydes via C—H activation to give acylpalladium derivatives and subsequent decarbonylation (Sect. VI.5.1), while ketones may be reduced to alcohols and even to hydrocarbons, as discussed in Sect VII.2.3.1, although the presence of proximal 7T- or n-donors may be critical in such reactions. 

The Heck reaction consists in the Pd(0)-catalysed coupling of alkenes with an aryl or alkenyl halide or triflate in the presence of a base to form a substituted alkene (Scheme 7.1). The reaction is performed in the presence of an organopalladium catalyst. The halide or triflate is an aryl or a vinyl compound and the alkene contains at least one proton. 

Organopalladium intermediates are also involved in the synthesis of ketones and other carbonyl compounds. These reactions involve acylpalladium intermediates, which can be made from acyl halides or by reaction of an organopalladium species with carbon monoxide. A second organic group, usually arising from any organometallic reagent, can then form a ketone. Alternatively, the acylpalladium intermediate may react with nucleophilic solvents such as alcohols to form esters. 

The most common route to alkyl or aryl complexes of the type [AuRL] is by the treatment of a halide complex with an alkyl- or aryllithium reagent. The first reactions of this type were performed (15) in 1959 [Eq. (5)], and the methyl and phenyl compounds were found to have chemical and thermal stabilities intermediate between those of the previously known organopalladium and -platinum complexes. 

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... 

For Pd-catalyzed cross-coupling reactions the organopalladium complex is generated from an organic electrophile RX and a Pd(0) complex in the presence of a carbon nucleophile. Not only organic halides but also sulfonium salts [38], iodonium salts [39], diazonium salts [40], or thiol esters (to yield acylpalladium complexes) [41] can be used as electrophiles. With allylic electrophiles (allyl halides, esters, or carbonates, or strained allylic ethers and related compounds) Pd-i73-jt-allyl complexes are formed these react as soft, electrophilic allylating reagents. 

---