Arylpalladium complexes bonds

Davidson and co-workers568-571 found that benzene reacts withPd(OAc)2 in acetic acid at 100°C to give roughly equal amounts of biphenyl and phenyl acetate. The reaction time was reduced from 16 hr to 5 min at 100°C in the presence of perchloric acid,568 and the yield of biphenyl was higher. Formation of phenyl acetate was also inhibited in the presence of oxygen.571 These authors concluded that oxidative coupling proceeds via an unstable o-bonded arylpalladium complex, which rapidly decomposes to biaryls and Pd(I), that is,... 

Addition of a cr-bonded organometallic compound across a carbon-carbon double bond, followed by elimination of a metal hydride species, results in net vinylic substitution. The arylpalladium complex (CCXXVIII)... 

The (n -allyl) arylpalladium complexes of the type [Pd(L)2bond cleavage on reaction with electrophiles (HCl Br-,NBS), whereas the corresponding (n -allyl)aryl palladium... 

Palladium-catalyzed C-H olefination is also a very useful protocol to synthesize oxygen-containing heterocycles, since it allows to functionalize the substrates with an unsaturated olefinic moiety which would constitute a Jt-conjugated structure or be elaborated further. Two pathways are often encountered in this approach (i) addition of the resulted arylpalladium complex to alkynes and subsequent protonation or transformation and (ii) addition of the arylpalladium complex to olefins and following palladium hydride elimination [7b, 20]. The current methods are mainly focused on the functionalization of aromatic C-H bonds to form benzo-oxacycles. 

Palladation of aromatic compounds with Pd(OAc)2 gives the arylpalladium acetate 25 as an unstable intermediate (see Chapter 3, Section 5). A similar complex 26 is formed by the transmetallation of PdX2 with arylmetal compounds of main group metals such as Hg Those intermediates which have the Pd—C cr-bonds react with nucleophiles or undergo alkene insertion to give oxidized products and Pd(0) as shown below. Hence, these reactions proceed by consuming stoichiometric amounts of Pd(II) compounds, which are reduced to the Pd(0) state. Sometimes, but not always, the reduced Pd(0) is reoxidized in situ to the Pd(II) state. In such a case, the whole oxidation process becomes a catalytic cycle with regard to the Pd(II) compounds. This catalytic reaction is different mechanistically, however, from the Pd(0)-catalyzed reactions described in the next section. These stoichiometric and catalytic reactions are treated in Chapter 3. 

The ij-arylpalladium bonds in these complexes are reactive and undergo insertion and substitution reactions, and the reactions offer useful methods for the regiospecific functionalization of the aromatic rings, although the reac-... 

During the cross-couplings to form C—N, C—O, C—S, and C—P bonds, the arylpalladium halide complexes are converted to arylpalladium amide, alkoxide, thiolate, and phosphide complexes. Examples of each type of complex have now been isolated, and the reductive elimination of the organic products has been studied. Although the reductive elimination to form carbon-hydrogen and carbon-carbon bonds is common, reductive elimination to form carbon-heteroatom bonds has been studied only recently. This reductive elimination chemistry has been reviewed.23... 

The reaction is initiated by oxidative addition of the halide to a palladium(O) species generated in situ from the Pd(II) catalyst. The arylpalladium(II) intermediate then forms a complex with the alkene, which rearranges to a a complex with carbon-carbon bond formation. The er-complex decomposes with regeneration of Pd(0) by /i-climination. 

Vinyl substitution of primary or secondary allylic alcohols with aryl halides usually produces 3-aryl aldehydes or ketones, respectively. The reaction is believed to involve an addition of the intermediate arylpalladium halide to die double bond, placing the aryl group mainly on the more distant carbon from the hydroxy group, followed by palladium hydride elimination, a reverse readdition and another elimination with a hydrogen atom on the carbon bearing the hydroxy group. The product is probably a ir-com-plex of the enol which ultimately either dissociates or collapses to a a-complex with palladium on the... 

The n-arylpalladium bonds in these complexes are reactive and undergo insertion and substitution reactions. The reactions offer useful methods for regiospecific functionalization of aromatic rings, although the reactions are difficult to make catalytic in most cases. Insertion of styrene to AA-dimcthylbcnzylaminc complex (180) to form the stilbene derivative 185 occurs smoothly at room temperature in AcOH [5]. The reaction has been extended to the functionalization of the dopamine analogue (A,A-dimethyl-2-arylethylamine) 187 via the six-membered o/Y/zo-palladated complex 186 [118]. 

Over 35 years ago, Richard F. Heck found that olefins can insert into the metal-carbon bond of arylpalladium species generated from organomercury compounds [1], The carbopalladation of olefins, stoichiometric at first, was made catalytic by Tsutomu Mizoroki, who coupled aryl iodides with ethylene under high pressure, in the presence of palladium chloride and sodium carbonate to neutralize the hydroiodic acid formed (Scheme 1) [2], Shortly thereafter, Heck disclosed a more general and practical procedure for this transformation, using palladium acetate as the catalyst and tri-w-butyl amine as the base [3], After investigations on stoichiometric reactions by Fitton et al. [4], it was also Heck who introduced palladium phosphine complexes as catalysts, enabling the decisive extension of the ole-fination reaction to inexpensive aryl bromides [5],... 

Abstraction of chloride from a vinylpalladium complex by silver acetate has been reported,45 as well as halide abstraction by silver triflate from arylpalladium halides.46 More interestingly in the context of creating C-C bonds, silver perchlorate was able to promote the reaction of (r -aryl)palladium bromide with dienes. Silver-promoted bromide abstraction led to the formation of (r 1-r 2-enyl)palladium complexes, which evolved further through the regioselective formation of a C-C bond between the aryl group and the former diene. Reactions with nonconjugated dienes suggested that the reaction proceeds via carbometallation of the less crowded double bond. Isomerization and (3 elimination led to a (ri3-allyl)palladium complex (Scheme 10.26).47... 

Scheme 2 shows the mechanism generally accepted for the catalytic arylation of olefins with aryl iodides in the presence of a tertiary phosphine-coordinated palladium catalyst and a base (4). Oxidative addition of aryl iodide (Arl) to a Pd(0) species (A), which is most commonly generated from palladium diacetate and a tertiary phosphine ligand, forms an arylpalladium iodide complex (B). Coordination of olefin on B followed by insertion of the coordinated olefin into the Pd-Ar bond forms a a-alkylpalladium species (C), which undergoes p-hydrogen elimination reaction to give the arylation... 

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