Palladium , reductive elimination from

The palladium(O) complex undergoes first an oxydative addition of the aryl halide. Then a substitution reaction of the halide anion by the amine occurs at the metal. The resulting amino-complex would lose the imine with simultaneous formation of an hydropalladium. A reductive elimination from this 18-electrons complex would give the aromatic hydrocarbon and regenerate at the same time the initial catalyst. 

In order to account for the high regioselectivities observed in the rhodium-catalyzed hydroboration of styrenes, Hayashi proposed a modified mechanism which proceeds through 73-benzyl-rhodium complex 22 as a key intermediate (Scheme 7). Reductive elimination from this 73-benzyl-rhodium complex 22 produces the secondary alkylborane regioselectively.12 A related 73-benzyl-palladium complex was recently isolated by Hartwig in studies of hydroamination.75... 

The Pd-PPh3 system (Scheme 3) is characterized by a two-electron reduction step of the cr-aryl-palladium intermediate [37], as also proposed previously for aryl-nickel complexes ligated to PPha [23, 38]. The formation of the biaryl proceeds by reductive elimination from the diarylpalladium and regeneration of Pd°. 

When reductive elimination from a late transition metal involves the formation of a carbon-carbon bond, the process is intramolecular and the groups have to be aligned cis to one another in the complex. In the formation of carbon-heteroatom bonds the reductive elimination from palladium might take place via competing pathways.16... 

Figure 3.9 Carbon-carbon bond formation via reductive elimination from a palladium complex.

Oxidative addition to a monophosphine palladium complex is unusual, but is a reasonable pathway if one bears in mind that reductive eliminations often occur from monophosphine palladium complexes [202,203]. These reductive eliminations from monophosphine Pdn species would form a monophosphine Pd° complex as the initial metal product, and these Pd° products are similar to the intermediate in the oxidative addition of aryl halide deduced from kinetic studies. 

Because the reductive elimination from DPPF-ligated palladium does not involve geometric rearrangements or changes in coordination number before the rate-determining step, the DPPF complexes allowed an assessment to be made of the electronic properties of the transition state in this reaction. The relative rates for elimination from amido groups were found... 

The insertion of CO into palladium carbon bonds is a common step in many palladium-catalyzed carbonylation reactions and polymerizations. This reaction takes place under moderate CO pressure (1-3 atm). From the range of compounds that can be carbonylated, it can be inferred that CO will insert into alkyl, aryl, and alkynic bonds (equation 13). One of the few types of Pd-C bonds inert to CO insertion is the Pd-acyl bond, thus only single carbonylations are normally observed. However, a few examples of double carbonylation have been reported. In the case of palladium-catalyzed formation of PhCOCONEt2 from Phi, CO, and NHEt2, reductive elimination from a bisacyl complex has been established as the mechanism, rather than CO insertion into a Pd-acyl bond. 

For the case of tri(o-tolyl)phosphine-ligated catalysts, the upper pathway appears to predominate. Oxidative addition occurs first via loss of a ligand from the bisphosphine precursor to form the oxidative adduct, which exists as a dimer bridged through the halogen atoms (equation 33). This dimer is broken up by amine, the coordination of which to palladium renders its proton acidic. Subsequent deprotonation by base leads to the amido complex, which can then reductively eliminate to form the product. When tert-butoxide is used as the base, the rate is limited by formation of and reductive elimination from the amido complex, while for the stronger hexamethyldisilazide, the rate-determining step appears to be oxidative addition. ... 

Palladium-catalyzed hydrostannation of isoprene was used for in-situ generation of allylstannane 9, which was trapped by an aldehyde to give alcohol 10 [Eq.(4)] [20]. It was suggested that an intermediate HPdSn(OAc)Cl2 is formed. The authors proposed two mechanisms for the hydrostannation, one according to Scheme 8-1 where Nu = Sn(OAc)Cl2 and another where insertion of the double bond into the palladium-tin bond is followed by reductive elimination from a (jr-allyl)palladium hydride. 

The use of nickel and rhodium in acylation has largely been supplanted by the growing use of palladium complexes. This is in large part due to the lethargic nature of reductive elimination from nickel(II)... 

Reductive elimination from the acylorganopalladium(II) complex is generally a facile process. The rate is influenced by the solvent polarity and added triphenylphosphine. Reductive >iiminnrif n has been shown to be faster than the elimination of palladium hydride from intermediate (111) in both chloroform and HMPA (Scheme 46). This makes the process useful even for alkyl group acylation. 

Loar, M. K., Stille, J. K. Mechanisms of 1,1-reductive elimination from palladium coupling of styrylmethylpalladium complexes. J. Am. 

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