Organopalladium compounds coupling

Isolable aliphatic organopalladium compounds can also be coupled with alkynes in the presence of copper(I). One example of such a process is outlined in Figure 5.19. 

Direct alkoxylation of organopalladium compounds based on a new type of C-O coupling... 

In addition, steps 3-5 of Figure 16.27 so far offer the most precise model of how the trans-metalation of the other stoichiometrically employed organometallic compounds proceeds in a Pd-catalyzed C,C coupling to give the organopalladium compound ... 

If the organopalladium compound has a j8-hydrogen, it will rapidly undergo an elimination reaction and the coupling reaction will not occur. This explains why vinylic and aryl halides are the reactants in these reactions—they cannot undergo an elimination reaction under the conditions used to carry out the coupling 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. 

In the presence of appropriate organometallic compounds, the trapping of organopalladium intermediate can be followed by the carbonylative cross-coupling pathway (Scheme 15)/" The cascade can be applied to the construction of heterocycles (Scheme 16). 

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. 

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). 

Negishi, E.-i. Palladium-catalyzed cross-coupling involving 3-hetero-substituted compounds. Palladium-catalyzed a-substitution reactions of enolates and related derivatives other than the Tsuji-Trost allylation reaction. Handbook of Organopalladium Chemistry for Organic Synthesis 2002, 1, 693-719. 

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