Palladium-amides

In addition to being a synthetically useful by-product, the isolation of 44 suggested that the moderate yields of the Heck coupling were due to fi-elimination of the butenyl side chain, perhaps via an intermediate palladium amide complex. While the reason for the readiness of this E-butenyl geometry to undergo elimination is unclear, it is clearly a facile process, having been observed several times previously (as noted). 

Mechanism of Palladium Amide Formation from Amines... 

In the original process using tin amides, transmetallation formed the amido intermediate. However, this synthetic method is outdated and the transfer of amides from tin to palladium will not be discussed. In the tin-free processes, reaction of palladium aryl halide complexes with amine and base generates palladium amide intermediates. One pathway for generation of the amido complex from amine and base would be reaction of the metal complex with the small concentration of amide that is present in the reaction mixtures. This pathway seems unlikely considering the two directly observed alternative pathways discussed below and the absence of benzyne and radical nucleophilic aromatic substitution products that would be generated from the reaction of alkali amide with aryl halides. 

The formation of alkoxo intermediates may be occurring when monophosphines are used, but the stability of the amine complexes favors the deprotonation of coordinated amine. Instead, the alkoxo complexes may be important in catalytic systems involving chelating ligands [51]. Indeed, the DPPF complex [Pd(DPPF)(p-Bu C6H4)(0-f-Bu) reacted with diphenyl amine, aniline, or piperidine, as shown in Eq. (48), to give the product of amine arylation in high yields in each case [51]. Since, no alkali metal is present in this stoichiometric reaction, the palladium amide is formed by a mechanism that cannot involve external deprotonation by alkali metal base. 

The cleavage of allcylamine N-H bonds by late transition metals to form metal amido complexes is also rare [69, 70]. When the transition metal is a low valent, late metal, the resulting amido complexes are highly reactive [71, 72]. It appears that the amination of aryl halides can involve an unusual N-H activation process by a palladium alkoxide to form a highly reactive palladium amide [65, 73]. 

In the coupling of more challenging substrates, reduction of the aryl halide is frequently observed [21]. Specifically, in the reaction of electron-rich aryl halides or sulfonates, reduced arene is a major by-product. Presumably, this side-product arises when the palladium amide can undergo /1-hydride elimination to generate an imine and a palladium (II) aryl hydride (Scheme 2). Subse-... 

Such reactions may proceed by carbonylation of an intermediate palladium amide. Giannoccaro et al. have carried out the stoichiometric reaction of amines with palla-... 

Recently, Oshima and co-workers reported Pd-catalyzed 2-pyridylmethyl transfer reactions by chelation-assisted cleavage of unstrained C—C bonds (Equation 11.45) [90]. A tentative mechanism has been proposed (Scheme 11.8). The key step is the coordination of nitrogen to the palladium center. With the aid of this coordination, the intermediate is likely to undergo cleavage of the C—C bond to yield a palladium amide [91]. This amide immediately isomerizes to aryl(2-pyr-idylmethyl)palladium to restore the aromaticity. Reductive elimination affords the final product. 

The reactions of diarylamines with aryl halides is more straightforward than the reaction of secondary alkylamines because these substrates possess no hydrogens on the a carbon and cannot, therefore, nndergo /3-hydrogen elimination as palladium amides. However, competitive formation of arene by an nnknown mechanism remains one side prodnct. For the most part, the synthesis of triarylamines has focused on the preparation of discrete molecules and polymers that are important for electronic materials applications,... 

Finally, several palladium-catalyzed processes appear to occur by insertion of alkenes into palladium amides. Cyclizations of aminoalkenes in the presence of aryl halides to form P5nrolidines appear to occur by intramolecular insertion of alkenes into palladium amides. One example of the cyclizations of aminoalkenes in the presence of aryl halides is shown in Equation 9.89a. The relative stereochemistry of the aryl group and the nitrogen in the ring structure of the product indicates that syn addition of palladium and nitrogen occurred across the alkene. 

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