Arylpalladium intermediates

The diazonium salts 145 are another source of arylpalladium com-plexes[114]. They are the most reactive source of arylpalladium species and the reaction can be carried out at room temperature. In addition, they can be used for alkene insertion in the absence of a phosphine ligand using Pd2(dba)3 as a catalyst. This reaction consists of the indirect substitution reaction of an aromatic nitro group with an alkene. The use of diazonium salts is more convenient and synthetically useful than the use of aryl halides, because many aryl halides are prepared from diazonium salts. Diazotization of the aniline derivative 146 in aqueous solution and subsequent insertion of acrylate catalyzed by Pd(OAc)2 by the addition of MeOH are carried out as a one-pot reaction, affording the cinnamate 147 in good yield[115]. The A-nitroso-jV-arylacetamide 148 is prepared from acetanilides and used as another precursor of arylpalladium intermediate. It is more reactive than aryl iodides and bromides and reacts with alkenes at 40 °C without addition of a phosphine ligandfl 16]. 

The poor regioselectivity of alkyne insertion in our polycychc aromatic hydrocarbon synthesis (Scheme 17) suggested to us that perhaps the palladium intermediate in that process was actually undergoing migration from one aromatic ring to the other, perhaps by a Pd(IV) hydride intermediate, to establish an equilibrium mixture of two regioisomeric arylpalladium intermediates under our reaction conditions (Scheme 18). This, indeed, appears to be true as... 

Arylpalladium intermediates bearing neighboring imine functionality have... 

Zhao and Larock have described the synthesis of carbazoles, indoles, and dibenzofurans 118 via a Ic type cyclization that follows a sequence of Pd-catalyzed cross-coupling of alkynes and aryl iodides 116, then nitrogen-directed palladium migration to an arylpalladium intermediate 117 that undergoes an intramolecular Mizoroki-Heck ring closure <06JOC5340>. 

Arylation of cycloalkenes.1 Aryl halides undergo Heck coupling with cy-cloalkenes in the presence of a palladium catalyst. The reaction involves addition of an arylpalladium intermediate to the double bond followed by elimination of a palladium hydride. 

Several routes have been proposed starting from the cr-arylnickel and the ff-arylpalladium intermediate. 

Scheme 3.1 Formation of arylpalladium intermediate by oxidative addition...

Eq. 1) [5]. On the other hand, Kikukawa and his co-workers found that the reaction of trimethyl(a- or -styryl)silane with arenediazonium tetrafluoroborates gave regioisomeric mixtures of coupling products [6-8]. The catalytic cycle of the reaction is considered to involve carbopalladation toward the C-C double bond of an alkenylsilane by an arylpalladium intermediate followed by tetraflu-oroborate-assisted elimination of the silyl group and regeneration of a palladi-um(0) species (Scheme 1). 

The reaction is considered to involve two mechanistic patterns i.e.,the reactions of arylpalladium intermediates with (a) phenolates at the ortho-positions, this being similar to the a-arylation of ketones (see Sect. 2.2 and Scheme 4), and with (b) thus formed biphenyl-2-ols as in Eq. (56). While the latter proceeds in both DME and xylene, the use of the less polar solvent is essential for the former to occur effectively. However, the intramolecular cyclization of halophenyl-linked phenols is known to occur in DMA [ 122]. It is worth noting that 0-arylation of phenols to give diaryl ethers occurs when bulky phosphine ligands are used (Eq. 60) [26-28]. This may imply that in the aryl(aryloxy)palladium intermediates, reductive elimination to give the ethers is enhanced by the ligands (Scheme 4). 

In this reaction, Pd(OAc)2 is first reduced to Pd(0) and then chloride ion is coordinated to form a chloride-ligated Pd(0) species. Oxidative addition of the 2-iodoaniline occurs to Pd(0), which becomes coordinated by the alkyne, leading to the arylpalladium intermediate... 

Formation of dihydroquinol-4-ones from 3-(o-iodoarylamino)propanoate esters involving arylpalladium intermediates which may exist in the palladacycle form and thereby derive special activity for intramolecular attack on the ester group.Under similar conditions, five-membered ring oxime derivatives are prepared. ... 

The reaction proceeds via a-arylation of the ketone followed by aromatic ort/zo-arylation. The latter may occur via coordination of the enolate oxygen to an arylpalladium intermediate as in Scheme 3 and Eq. 44. 

Analogous chemistry utilizing stoichiometric arylpalladium intermediates has been employed to generate 2ff-l-benzopyrans and 1,2-dihydroquinolines (Eq. 66) [127, 128]. When intermediates bearing chiral diamine ligands are utilized, high enantiomeric excesses can be obtained [128]. 

The key to the success of the reaction is the presence of pyridine. The reaction appears to proceed by generation of an arylpalladium intermediate and sequential insertion of the alkyne and CO, followed by lactone formation. This sequence is a bit unusual, since the literature suggests that CO should insert more readily than the alkyne. It may well be that CO insertion does indeed occur first, but that it is reversible and that alkyne insertion is not reversible. 

This hitherto unknown reaction has attracted attention as a potentially important method for carbon-carbon bond formation. Now all reactions which proceed via insertion of alkenes and also alkynes to the arylpalladium intermediates 1 (Scheme 3.1) and the alkenylpalladimn 14 (Scheme 3.5) are called Mizoroki-Heck reactions or Heck reactions (abbreviated to HR in this chapter). A number of reviews have already been published [6]. The reaction is certainly the most useful and versatile method of carbon-carbon bond formation involving sp carbons. 

Larock found that the reaetion of 2-iodo -methylbiphenyl (34) with acrylate provided two products 35 and 36 in equal amounts using cesium pivalate as a base. Of course, 35 is an expected produet [27]. Also the reaction of 37 afforded the mixture of 35 and 36. Gallagher also diseovered a similar migration using 3-bromo-4-phenylpyridine (38) and acrylate to afford 39 and 40 [28]. Although the mechanism of the migration process to form the crossover produets is not clear, certainly a reversible 1,4-Pd shift of arylpalladium intermediates 41 and 43 via the palladacycle 42 is occurring. 

Meyers et al. reported that arylpalladium intermediates are generated from electron-rich aromatic acids 88 by decarboxylation and undergo HR in 5 % DMSO-DMF when palladium trifluoroacetate was used. However, addition of 3 equivalents of Ag2C03, which may accelerate the decarboxylation, is necessary [58]. 

Interestingly, when the aryl bromide homolog 23 was used, two regioisomeric isochroman products 24 and 25 were obtained in 1.7 1 ratio (Scheme 3.5). The desired product 24 is presumably formed via an alkylpalladium intermediate 24. On the other hand, an arylpalladium intermediate 25, generated by 1,4-palladium alkyl to aryl migration, would give product 25. This type of palladium migration has also been observed by Larock and coworkers [9] in the synthesis of fused polycycles. 

Since the early reports of the Fujiwara-Moritani reaction [2], catalytic alkenylation procedures for a broad range of aromatic substrates with various alkenes have been developed. A proposed mechanism for these Fujiwara-Moritani-type reactions is illustrated in Scheme 18.4 [4]. The reaction is initiated by electrophilic attack on an arene by a cationic palladium species [PdOAc]+, generated in situ from Pd(OAc)2, to form an arylpalladium intermediate. Subsequent alkene insertion and fi-hydrogen elimination may occur to produce an alkenylarene derivative and HPdOAc. The latter may be reoxidized by an oxidant to regenerate Pd(OAc)2. 

On the left hand of Scheme 2 is shown the catalytic cycle to produce a-keto amide (Cycle 1), whereas the right-hand catalytic cycle shows the route to amide (Cycle II). The process common to both processes is oxidative addition of aryl halide to give arylpalladium halide. Further CO coordination to the arylpalladium intermediate gives a CO-coordinated complex. If CO insertion into the aryl-palladium bond takes place, an acylpalladium complex is produced to drive the double carbonylation cycle. Further coordination of CO followed by attack of amine on the carbonyl ligand produces the aroyl(carbamoyl)palladium species as the bis-acyl-type intermediate. Reductive elimination of the a-keto amide by combination of the benzoyl ligand with the carbamoyl ligand regenerates the Pd(0) species that carries the catalytic cycle. 

In a remarkable biomimetic synthesis of narwedine shown in Scheme a doubly chelated arylpalladium intermediate must undergo a Tl-induced C—C bond formation. Even thongh it is only stoichiometric in Pd, the high efficiency and selectivity associated with this synthesis might well justify the stoichiometric use of Pd at least on a recycling basis. 

The aryne C—C triple bond is highly reactive toward palladation processes, and the initially formed arylpalladium intermediate can undergo a variety of transformations with an appropriate reagent in an intermolecular, giving rise to 1,2-functionalized benzene derivatives, or in an intramolecular manner, leading to benzo-fused (hetero)carbocyclic systems. 

Carbopalladation of Arynes by n-Allylpalladium and Related Complexes. Pioneering work from Yamamoto established that ji-allylpalladium species are particularly effective for the carbopalladation of arynes leading to an arylpalladium intermediate 109, which can be employed in three-component couplings with a second aryne to give phenanthrenes such as 110, or with alkynes to produce naphthalenes such as 111. In the first case, 109 evolves through insertion of another benzyne (1) molecule (carbopalladation of 1 by an arylpalladium) and further carbopalladation of the pendant alkene to afford a tricyclic species that undergoes p-hydride elimination (Scheme 12.55) [96],... 

No reaction was observed with stoichiometric amounts of Pd(OAc)j under argon, suggesting that Oj is likely involved in the product formation step rather than reoxidation of Pd(0). Labeling studies using and supported a direct oxygenation of the arylpalladium intermediates with Oj instead of an acetoxylation/hydrolysis sequence. Pyridyl group also enabled direct Cu-catalyzed orf/io-selective acetoxylation of aryl C—H bonds with in AcOH/ACjO [39]. 

Tanaka, D., Myers, A. G. (2004). Heck-type arylation of 2-cycloalken-l-ones with arylpalladium intermediates formed by decarboxylative palladation and by aryl iodide insertion. Organic Letters, 6,433-436. 

Unsymmetrical biaryls can be piepared by reaction of aryl Grignard reagents or aryllithiums with aryl halides in the presence of an arylpalladium intermediate which serves as a catalyst. The arylpalladium catalyst is generated from the halide by oxidative addition. 

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