Arylation palladium complexes

Bromophenyl)ethanol undergoes a palladium-catalyzed carbonylation which results in the formation of isochroman-l-one (79H(12)92l). It is considered that the reaction involves formation of an aryl-palladium complex (624) through insertion of the zerovalent palladium complex (623) into the aryl halide. Insertion of carbon monoxide followed by reductive elimination of the metal as a complex species leads to the isochromanone (Scheme 242). 

Polystyrene-bound secondary aliphatic amines and /V-alkyl amino acids can be ally-lated by treatment with a diene and an aryl iodide or bromide in the presence of palla-dium(II) acetate (Entry 14, Table 10.3). As the diene, 1,3-, 1,4-, and 1,5-dienes can be used, and, besides aryl halides, heteroaryl bromides have also been successfully used [63], This remarkable reaction is likely to proceed via the formation of an aryl palladium complex, with subsequent insertion of an alkene into the C-Pd bond. The resulting organopalladium compound does not undergo ( -elimination (as in the Heck reaction), but isomerizes to an allyl palladium complex, which reacts with the amine to give the observed allyl amines. 

The catalytic cycle consists of the following basic reactions. In the first step (A) of the cycle, Pd species 29 undergoes oxidative addition of an alkenyl or aryl halide 30. The result is a a-alkonyl or a-aryl palladium complex 31. In the second step, alkene 32 coordinates with Pd(II) compound 31 This is followed by syn insertion (B) of the double bond into the alkenyl or aryl palladium bond... 

In particular, cleavage of substrates from a solid support by use of palladium-promoted or -catalyzed reactions has some advantages over other cleavage methods. Because most protecting groups and functionalities are resistant toward palladium complexes, selective surgical removal is frequently possible. In addition, intermediate 7z>allyl- and er-aryl-palladium complexes can be used in principle for further derivatization with the use of appropriate linker types. 

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). 

The aryl palladium complexes that are formed via oxidative coupling of aryl halides with palladium(O) can undergo the Heck reaction.238,239 -p e Heck reaction is the coupling of aryl halides or aryl sulfonate esters with alkenes.340 The reaction proceeds with formation of a palladium species such as 401, with elimination of palladium to give arylated alkene derivatives.241 Aryl halides differ greatly in their reactivity, Arl being the most reactive, followed by aryl bromides. In general, aryl chlorides are very unreactive in the Heck... 

The C-C bond formation from 7t-allyl(aryl)palladium complexes follows the cis elimination process, as confirmed by the following experiments using two geometrical isomers (Eqs. 9.8 and 9.9) [43e]. 

The mechanism of coupling between aryl iodides and aryl palladium complexes has been investigated. The formation of a T-shaped intermediate [LPd(Ar)-(Ar )] is necessary to achieve a mutual cis arrangement of the aryl groups before reductive elimination can take place. Crossover experiments show that bimetallic... 

Subsequent studies on the reactivities of neutral and cationic alkyl- and aryl- palladium complexes revealed that the creation of a vacant site adjacent to the alkyl or aryl ligand causes marked enhancement in reactivity toward j8-hydrogen migration. The implications of these results on the fundamental processes of the transition metal alkyls and aryls with the mechanisms of Pd-catalyzed organic synthesis, such as arylation of olefins and carbonylation of aryl halides, have been discussed. 

Scheme 1.49 Ionic mechanism for Mizoroki-Heck reactions catalysed by a Pd(0) coordinated to one or two C—C saturated or C=C unsaturated fi-heterocyclic monocarbenes (only one way for the coordination of the alkene is presented). The reactive species is PdP(Cb) for a bulky carbene and Prf(Cb)2 for a nonbulky carbene. The aryl-palladium complex formed in the oxidative addition is always ligated by two Cb ligands delivered by the Pd(0) or PdfII) precursor even if Pcf(Cb) is the reactive species.

Amatore, C., Godin, B., Jutand, A. and Lemaitre, F. (2007) Rate and mechanism of the reaction of alkenes with aryl palladium complexes ligated by a bidentate P,P ligand in Heck reactions. Chem. Eur. J., 13, 2002-11. 

Shortly thereafter, a more general palladium-catalyzed carbonylative Heck reaction of aryl halides was able to be developed by our group [34]. For the first time, various aromatic and aliphatic alkenes were used successfully in this system, and good yields of the corresponding a,jS-unsaturated ketones were obtained (41-90 %). Starting from easily available aryl iodides and bromides, interesting building blocks were obtained under mild conditions (Scheme 7.15). With respect to the reaction mechanism, the aryl palladium complex and acyl palladium complex were characterized by X-ray, and the mechanism was studied step by step. The results fit well with DFT calculations. 

Most recently, the synthesis of chalcones from aryl bromides in the presence of PPha as a ligand was achieved (Scheme 7.16) [35]. Later on, this group extended our methodology to vinyl ethers [36]. Based on both experimental results and DFT calculations, a proposed mechanism for this reaction is shown in Scheme 7.17. It begins with the oxidative addition of ArX to the Pd° center to form the corresponding aryl palladium complex. Followed by the coordination and insertion of CO, the respective acyl palladium complex is produced. After coordination, addition, and elimination processes, the desired chalcone is produced. Under the assistant of the base, Pd is regenerated and starts the next reaction cycle. 

The mechanistic propositions in Scheme 23 are further supported by a number of experimental findings, namely, the isolation and structural characterization by X-ray methods of both organic and organometallic spirocyclic derivatives in a number of reactions, such as IV and 93, the trapping of an analog of D to yield 92 by an intramolecular cyclization reaction in a triphenylphosphine-induced depalladation process," and the structural characterization of some rare q -bound aryl palladium complexes, such as 94 found in Chart 1." ... 

As described in Sect. Vin.2.2, the Pd-catalyzed coupling reaction of arenes and olefins proceeds via o--aryl-Palladium complex intermediates to give aryl-substituted olefins (Scheme 1). 

During the development of this arylation reaction, it was found that the rr-aryl-Palladium complexes reacted with CO to give aromatic acids, when AcOH was used as a solvent (Scheme... 

To access useful arylamines derivatives, the palladium catalyst is usually prepared as a cationic aqua or diaqua palladium(ll) complex or other organopalladium complex. In the Scheme 6.23, a putative catalytic cycle for the Pd-catalyzed asymmetric addition reaction is shown. The hydroxyl-Pd species (A) is formed in the presence of a base or MS that undergo transmettJlation with an organometallic reagent such as ArM to produce the corresponding aryl palladium complex (B). Insertion of an imine substrate into the Ar-Pd bond, followed by hydrolysis, affords the desired product [29b]. 

It has been shown earlier that the presence of electron-withdrawing ligands favors the reductive elimination in palladium 77 -allyl complexes. Benzoquinone has been used to perform the clean reductive elimination in -allyl aryl palladium complexes, that otherwise decompose by other competing routes such as /3-H elimination. The intermediate complex with a coordinated benzoquinone molecule has been characterized in solution (Scheme 72). Benzoquinones have also been used to promote the reductive elimination of chloro and an -allyl Cl as a nucleophile toward Pd 77 -allyls usually attacks in a trans- (or exo-) fashion. A different stereochemistry (cis- or /rrfo-attack) has been achieved in this work. It was found that the more electron withdrawing the quinone the more favored the m-attack, as supported by theoretical calculations. Allyltin trichloride can be obtained by a reductive elimination reaction from Pd(SnCl3)(77 -allyl)L. The reaction is accelerated by addition of an electron-withdrawing olefin such as allyl chloride. Tin dichloride also increases the reaction rate. ... 

The Pd(OAc)2"Catalyzed addition of arylboronic acids to per-acetylated glycals has been investigated. The reaction proceeds via. -addition of the relevant aryl-palladium complex to the gly-cal double bond and this is followed by an anri-elimination process that then delivers the illustrated Sn 1-type product (eq 134). 

This synthetic strategy brings into account interesting aspects of the Heck-Matsuda arylations. As it does not rely on external ligands, the substrates behave as such, not only stabilizing the cationic 8-aryl-palladium complex but also providing structural bias that controls the regio- and stereoselectivity... 

The most commonly accepted mechanism for this coupling was initially proposed by Hiyama and Hatanaka, which involves three steps.The first step is the oxidative addition of the aryl halide to the palladium(O) catalyst to give arylpalladium complex 1. The second step involves the transmetallation of the arylpalladium complex 1 with the anionic arylsilicate 2 to give bis(aryl)palladium complex 3. Finally, the cross-coupled product 4 is produced and the palladium(O) catalyst is regenerated through reductive elimination of the bis(aryl)palladium(II) complex 3. The key intermediate to this process is the requirement for the pentacoordinate arylsilicate anion 2, typically formed by treatment of the tetracoordinate silane with the activating anion, such as tetrabutylammonium fluoride (TBAF). 

Benzene substituents, although not affecting the overall rate of coupling, do, however, have orientation effects. This has been interpreted in terms of a or-bonded aryl-palladium complex which is not formed in a rate-determining step of the reaction. However, phenylation of styrene with benzene and... 

Cyclic aryl palladium complexes (palladacycles) are readily available by palladation reactions starting from palladium(Il) salts and an arene having a directing group (Scheme 5-1). 

The reaction of lithium organyls with stoichiometric amounts of aryl palladium complexes has been discovered by Murahashi. However, the catalytic variant... 

The aminocarbonylation of phenols with carbon monoxide and ammonia has been achieved using palladium catalysis. After in situ conversion of the phenols to the corresponding nonaflates, oxidative addition to the palladium gives an aryl palladium complex which undergoes carbon monoxide insertion. Nucleophilic attack of ammonia results in the formation of the benzamide. ... 

The palladium-catalysed reaction of the pyrazolo-pyrimidine derivative (141) with 3-bromotoluene may result in arylation at the 3-position in the pyrazole ring or at an sp hybridized site in the 7-methyl side-chain depending on the base and ligands used. After initial insertion of the palladium catalyst into the aryl halide bond, palladation of (141) occurs by a concerted metalation-deprotonation pathway and is followed by reductive elimination. Concerted metalation-deprotonation is also likely in the palladium-acetate-catalysed reaction of imidazo[l,2-a]pyridines with aryl bromides to give 3-substituted derivatives such as (142). A careful mechanistic study of the arylation of pyridine A-oxide by bromotoluene, catalysed by palladium acetate and t-butylphosphine, has shown that direct reactions of an aryl palladium complex with... 

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