Pd arylation

The alkylphenylacetyi chloride 843 and benzoyl chloride undergo decarbo-nylative cross-condensation to give the enone 845 in the presence of EtiNf723]. The reaction is e.xplained by the insertion of the ketene 844 into the Pd-aryl bond and, 3-elimination. To support this mechanism, o, d-unsaturuted ketones are obtained by the reaction of ketenes with aroyl chlorides[724]. 

Palladium(0)-catalyzed cross-coupling of aryl halides and alkenes (i.e., the Heck reaction) is widely used in organic chemistry. Oxidative Heck reactions can be achieved by forming the Pd -aryl intermediate via direct palladation of an arene C - H bond. Intramolecular reactions of this type were described in Sect. 4.1.2, but considerable effort has also been directed toward the development of intermolecular reactions. Early examples by Fu-jiwara and others used organic peroxides and related oxidants to promote catalytic turnover [182-184]. This section will highlight several recent examples that use BQ or dioxygen as the stoichiometric oxidant. 

Sequential displacement reactions of Pd(CF3COCHCOCF3)2 with triarylformazans provides the highly colored chelates Pd(CF3COCHCOCF3)[(aryl)3CN4] and Pd[(aryl)3CN4]2.250... 

Four oxidation states of palladium are encountered in organometallic chemistry see Palladium Inorganic Coordination Chemistry) In order of importance, they are Pd , Pd , Pd, and Pd . With the reduction of palladium from Pd to Pd , the metal changes its reactivity from electrophile to nucleophile. However, unlike main group nucleophiles such as thiolates or cyanide, Pd complexes react with both alkyl halides and aryl or vinyl halides. Reactions of Pd complexes with these latter sp halides generate new Pd aryl or vinyl bonds through the process of oxidative addition. 

Reaction of tra i -[PdAr2(PEt2Ph)2] with Mel gives good yields of ArMe. Mechanistic stndies show that most of the elimination prodnct arose not from a Pd intermediate, but from a cis Pd -aryl methyl complex (11), which comes about from exchange in a complex such as (10), as shown in Scheme 9. 

The reaction of palladium reagents with amines, phosphines, and other organic ligands to produce chelated complexes with Pd-C bonds is the Cyclometalation reaction (equation 7). It has been used to synthesize thousands of complexes with Pd-alkyl and Pd-aryl bonds. These complexes are beginning to be used as very stable, high turnover number catalysts and as intermediates in the synthesis of complex natural products. The scope and limitations of this reaction are detailed in Section 8. 

Experimentally it has been found, generally through X-ray analysis of the product complexes, that nucleophilic addition is almost always exo leading to trans products (see Table 1). An interesting case of endo addition involves the phenylation of norbornadiene-PdCl2. The phenylation reaction was postulated to proceed via a Pd-aryl intermediate, I, which could not be isolated or detected ... 

Thus, the reaction between [NBu4 [M(C6F5)2(acac)] and [Ag(0C103)(PPh3)] affords the expected dinuclear compound 6 when M = Pt, (Eq. (9)) but when M is Pd, arylation of the silver center takes place and the dinuclear compound cannot be obtained (Eq. (10)). ... 

Palladium complexes bearing NHC ligands are generally very robust systems with impressive thermal and atmospheric stability. From this point of view, they seem to be ideal candidates for catalytic processes. However, the palladium-carbenic carbon bond can be visualized as a single bond and the usual chemistry of Pd-aryl or Pd-alkyl, such as elimination of C-C, C-H, or C-X, has been both observed and investigated from a theoretical perspective. 

The 1,2-insertion of alkenes into transition metal-carbon o-bond leads to C-C bond formation under mild conditions, as described in Chapter 6. This reaction is considered to be a crucial step in the coordination polymerization and carbometalation of alkenes catalyzed by transition metal complexes. A common and important carbometalation is the Heck-type arylation or vinylation of alkene catalyzed by Pd complexes [118], The arylation of alkene, most typically, involves the formation of arylpalladium species and insertion of alkene into the Pd-aryl bond as shown in Scheme 5.20. The arylpalladium species is formed by the oxidative addition of aryl halides to Pd(0) complexes or the transmetalation of aryl compounds of main group metals with Pd(II) complexes. Insertion of alkene into the Pd-aryl bond produces 2-arylalkylpalladium species whose y6-hydrogen elimination leads to the arylalkene. Oxidative chlorination of the 2-arylalkylpalladium intermediate forms chloroalkanes as the product. 

In contrast to a facile intermolecular insertion of alkenes and alkynes to Pd-aryl and Pd-alkenyl bonds as observed in Heck reaction, curiously intermolecular insertion of alkenes and alkynes to Pd-allyl bond in 7r-allylpalladium is very rare. On the other hand, intramolecular insertion of alkenes and alkynes to r-allylpalladium occurs smoothly to form five- and six-membered rings. Oppolzer and co-workers have developed a useful synthetic method of cyclic compounds by intramolecular insertions of alkenes and alkynes, and they explained the reaction as Pd-ene reaction as shown by 404. The method has been applied to the syntheses of several natural products and complex cyclic compounds [155,156]. 

Mycophenolic acid was synthesized by a three-component coupling between the lactone 26, isoprene and dimethyl malonate (Eq. (23)) [47]. The reaction proceeds by the usual mechanism, with oxidative addition of the aryl halide to Pd(0) and subsequent insertion of isoprene into the Pd-aryl bond to give a t-allyl complex followed by nudeophilic attack by the malonate carbanion. Compound 27 was subsequently transformed to mycophenolic add. 

A general mechanistic manifold for Pd -catalyzed direct arylations involves (i) Pd"-mediated C—H activation to afford 1, (ii) oxidation of 1 to Pd -aryl complex 3, and (iii) C—C bond formation via reductive elimination to release the product and regenerate the Pd" catalyst (Scheme 24.3). Aryl iodides (Scheme 24.3a) or aryl iodonium salts (Scheme 24.3b) are the most commonly employed... 

Three-membered thiopalladacycles 146 with X = Br, G1 were synthesized via insertion of alkenyl sulfides into a Pd-aryl bond. Subsequent reaction with Br2 and PPhs were studied as well as the decomposition of these complexes at elevated temperatures. 

Some palladium(ii) complexes with Pd-arene interactions studied and characterized are catalytic intermediates involved in the systems arising from nbd or nb insertion into Pd-aryl bonds, represented in Scheme 28. These have been extensively studied by the groups of Catellani and Milstein (for nb), " and Cheng (for nbd). ... 

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