Alkenes vinyl substitution with palladium complexes

When coordinated to palladium, the rr-indenyl ligand tends to slip from the if - to the 77 -coordination mode, and most of the complexes synthesized show severe distortions or clear -indenyl coordination. Thus, although being a cyclopentadienyl analog, it is rare to find a true Tj -indenyl coordination to palladium, however common for other transition metals. Metathesis with Li[indenide] and ligand-substitution reactions are common preparative routes for indenyl derivatives. The insertion of an alkyne into Pd-G bonds of vinyl substituted aryls, followed by intramolecular alkene insertion, also leads to highly substituted indenyl palladium complexes. Equation (66) shows one of these examples. ... 

Particularly interesting is the reaction of enynes with catalytic amounts of carbene complexes (Figure 3.50). If the chain-length between olefin and alkyne enables the formation of a five-membered or larger ring, then RCM can lead to the formation of vinyl-substituted cycloalkenes [866] or heterocycles. Examples of such reactions are given in Tables 3.18-3.20. It should, though, be taken into account that this reaction can also proceed by non-carbene-mediated pathways. Also Fischer-type carbene complexes and other complexes [867] can catalyze enyne cyclizations [267]. Trost [868] proposed that palladium-catalyzed enyne cyclizations proceed via metallacyclopentenes, which upon reductive elimination yield an intermediate cyclobutene. Also a Lewis acid-catalyzed, intramolecular [2 + 2] cycloaddition of, e.g., acceptor-substituted alkynes to an alkene to yield a cyclobutene can be considered as a possible mechanism of enyne cyclization. 

Normally, the most practical vinyl substitutions are achieved by use of the oxidative additions of organic bromides, iodides, diazonium salts or triflates to palladium(0)-phosphine complexes in situ. The organic halide, diazonium salt or triflate, an alkene, a base to neutralize the acid formed and a catalytic amount of a palladium(II) salt, usually in conjunction with a triarylphosphine, are the usual reactants at about 25-100 C. This method is useful for reactions of aryl, heterocyclic and vinyl derviatives. Acid chlorides also react, usually yielding decarbonylated products, although there are a few exceptions. Likewise, arylsulfonyl chlorides lose sulfur dioxide and form arylated alkenes. Aryl chlorides have been reacted successfully in a few instances but only with the most reactive alkenes and usually under more vigorous conditions. Benzyl iodide, bromide and chloride will benzylate alkenes but other alkyl halides generally do not alkylate alkenes by this procedure. 

Pd(II) compounds coordinate to alkenes to form rr-complexes. Roughly, a decrease in the electron density of alkenes by coordination to electrophilic Pd(II) permits attack by various nucleophiles on the coordinated alkenes. In contrast, electrophilic attack is commonly observed with uncomplexed alkenes. The attack of nucleophiles with concomitant formation of a carbon-palladium r-bond 1 is called the palladation of alkenes. This reaction is similar to the mercuration reaction. However, unlike the mercuration products, which are stable and isolable, the product 1 of the palladation is usually unstable and undergoes rapid decomposition. The palladation reaction is followed by two reactions. The elimination of H—Pd—Cl from 1 to form vinyl compounds 2 is one reaction path, resulting in nucleophilic substitution of the olefinic proton. When the displacement of the Pd in 1 with another nucleophile takes place, the nucleophilic addition of alkenes occurs to give 3. Depending on the reactants and conditions, either nucleophilic substitution of alkenes or nucleophilic addition to alkenes takes place. 

This approach has been used to efficiently assemble 3-vinyl indoles 13 with alkenes (Scheme 6.20) [28]. It was observed in this case that the nature of the nitrogen substituent influences the ability to trap the 3-palladated indole, with carbamates providing the highest yields. The elimination of HX from the palladium after P-hydride elimination creates a Pd(0) complex that is unable to mediate subsequent cyclizations. As such, co-oxidants, such as stoichiometric copper(II) salts, are used in this reaction to regenerate the palladium(II) catalyst. However, by modifying reaction conditions, Lu has found that the addition of excess LiBr can inhibit P-hydride elimination, and instead allow the formation of the reduced product (Scheme 6.21) [29]. This not only allows access to 3-alkyl substituted indoles, but also eliminates the need for stoichiometric oxidants. 

Another of the standard palladium-catalyzed C—C bond formations is the Mizoroki-Heck reaction. This reaction is based on the palladium-catalyzed coupling of olefins with aryl or vinyl halides under basic conditions [21]. The catalytic cycle that is typically proposed for this reaction is outlined in Scheme 7.7. The first step of the reaction postulated in the mechanism is an oxidative addition of R -X to a Pd(0) complex. The next step is the insertion of the alkene to the Pd complex II. In order for this to be possible, an uncharged ligand has to break away giving a neutral Pd(II) complex that will be coordinated by the olefin. The insertion of the alkene into the Pd— bond results in the C C bond-forming step to give BO. Rotation around the C—C bond and 3-hydride elimination yields the new substituted olefin and intermediate 131. Regeneration of the active catalytic species occurs by the addition of a base. 

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