Palladium complexes arylpalladium

Besides rhodium catalysts, palladium complex also can catalyze the addition of aryltrialkoxysilanes to a,(3-unsaturated carbonyl compounds (ketones, aldehydes) and nitroalkenes (Scheme 60).146 The addition of equimolar amounts of SbCl3 and tetrabutylammonium fluoride (TBAF) was necessary for this reaction to proceed smoothly. The arylpalladium complex, generated by the transmetallation from a putative hypercoordinate silicon compound, was considered to be the catalytically active species. 

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... 

The presence of chelating groups in those complexes is necessary to stabilize the intermediate aryl-palladium complex for isolation but it does not seem necessary to cause palladation. The chelating group does, however, tremendously accelerate the palladation. Aromatic compounds reactive to electrophilic substitution apparently undergo palladation with palladium acetate in acetic acid solution fairly readily at 100 °C or above. Of course, the arylpalladium acetates presumably formed, are not stable under these conditions, and they decompose very rapidly into biaryls and palladium metal 34,35,36) ag do aryl palladium salts prepared by the exchange route 24>. If the direct palladation is carried out in the presence of suitable olefins, arylation can be achieved, so far, however, only in poor yields, arid with concurrent loss of stereospecificity and formation of isomers and other side products 37.38). 

In addition to the most important 1,2-difunctionalization assisted or catalyzed by palladium(II) complexes, a catalytic 1,1-arylamination process of alkenes, applied to the construction of nitrogen heterocycles from 4-pentenylamides, was realized29,30. The mechanism involves the formation of arylpalladium chloride from alkyl(aryl)stannanes, the addition to the alkene, the isomerization of the adduct to the more stable benzylic palladium complex, and the displacement of palladium by an internal nitrogen nucleophile. In the presence of a substituent, mixtures of diastereomers were generally obtained. 

The use of norbomene as a scaffold for aromatic C-H functionalization, a process we dubbed the Catellani Reaction, is a useful and mechanistically interesting method for the polyfunctionalization of aromatic molecules. Through the development and study of palladium complexes with norbomene, a powerful synthetic method has emerged which has been proven useful primarily through the research efforts of Catellani and Lautens. Future studies in this area should focus on expanding the already wide variety of products available, and to develop and/or utilize new reactions which can be performed on either the palladacycle intermediate or terminal arylpalladium(II) species. 

Palladium-catalyzed reactions have been widely investigated and have become an indispensable synthetic tool for constructing carbon-carbon and carbon-heteroatom bonds in organic synthesis. Especially, the Tsuji-Trost reaction and palladium(II)-catalyzed cyclization reaction are representative of palladium-catalyzed reactions. These reactions are based on the electrophilic nature of palladium intermediates, such as n-allylpalladium and (Ti-alkyne)palladium complexes. Recently, it has been revealed that certain palladium intermediates, such as bis-7i-allylpalladium, vinylpalladium, and arylpalladium, act as a nucleophile and react with electron-deficient carbon-heteroatom and carbon-carbon multiple bonds [1]. Palladium-catalyzed nucleophilic reactions are classified into three categories as shown in Scheme 1 (a) nucleophilic and amphiphilic reactions of bis-n-allylpalladium, (b) nucleophilic reactions of allylmetals, which are catalytically generated from n-allylpalladium, with carbon-heteroatom double bonds, and (c) nucleophilic reaction of vinyl- and arylpalladium with carbon-heteroatom multiple bonds. According to this classification, recent developments of palladium-catalyzed nucleophilic reactions are described in this chapter. 

The electronic effects on the reductive elimination of aryl ethers from BlNAP-ligated palladium complexes (Eq. 6) was studied by Widenhoefer and Buchwald. Because the reductive elimination of acyclic ethers required activated palladium-bound aiyl groups, the scope of the arylpalladium alkoxide complexes was limited. An example of elimination from an oxametallacycle that contained an unactivated aryl group demonstrated that the formation of cychc ethers may become more general. Nonetheless, studies on these systems showed that the factors that control elimination of ethers are similar to those that control elimination of amines. For example, alkoxide complexes that are more electron rich at the alkoxide oxygen underwent faster reductive elimination than those that are more electron poor. Elimination of diaryl ethers from arylpalladium phenoxides did not occur. 

There are two structurally characterized examples in which an arylpalladium(ll) coordinates to silver(l) in a type 11 fashion (Scheme 10). First, Kickham and Loeb [40] reported that an attempt to abstract the chloride ligand in a palladated thiacyclo-phane with two equivalents of silver(l) triflate unexpectedly resulted in the formation of a bimetallic complex in which the silver(l) ion is encapsulated by three oxygen atoms and an interaction with the Pd-C bond. Then, Braunstein et al. [41] synthesized a remarkable one-dimensional coordination polymer by reacting the phosphanyl immolate palladium complex depicted in Scheme 10 (bottom) with silver(l) triflate. 

The cleavage of substrates from a solid support using Pd-promoted or Pd-catalyzed reactions has some advantages over other cleavage methods. Since most protecting groups and functionalities are resistant toward palladium complexes, a selective surgical cutoff is frequently possible. In addition, intermediate 7r-allyl- and cr-arylpalladium complexes can in principle be used for further derivatization with the use of appropriately versatile linker types. 

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