Palladium-amido complex, formation

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. ... 

The mechanism of this process has been studied in detail. The identity of the palladium(O) species that lies on the catalytic cycle/ the effect of anions on the oxidative addition step/ " the effect of amines in the dissociation of chelating ligands from the palladium(O) complex during the oxidative addition/ the mechanism of formation of the amido complex/ and the mechanism of reductive elimination of amine - - have all been studied. The oxidative addition of aryl chlorides and bromides is generally the turnover-limiting step of the catalytic cycle. 

Three general mechanisms can be envisioned for the formation of amido complexes from arylpalladium halides direct substitution of halide by alkali amide generated by simple deprotonation of free amine by free base, coordination of amine to the metal center followed by deprotonation of the more acidic coordinated amine, or formation of a palladium alkoxide... 

Intermolecular alkene insertion into the Pd-N bond was shown to be a syn-addition process as well (Scheme 12) [42,47]. Amido complexes of palladium were found to coordinate alkene, undergo migratory insertion and finally to form enamine product after (3-hydrogen elimination. Experimental evidence for the ethylene amido intermediate and for yw-addition process was obtained by NMR spectroscopy, including deuterium labeling study. The rate constant of the decay of observed intermediate complexes leading to the formation of the enamine corresponded to AG of 17 kcal/mol (8.7 x 10 " s -40 "C) [42]. 

The arylamines are generally formed in good yields (Table 1). Dehalogenation products are the only by-products observed, which pro-baly arise from base-induced )9-hydride elimination of the amido arylpalladium complex and subsequent reduction. Interestingly, the base employed has a decisive influence on the course of the reaction. In the amination of l-bromo-4-n-butylbenzene with free amines in the presence of silyl amides as base -in contrast to the coupling with tin amides -the rate-determining step in the catalytic cycle is the oxidative addition of bis(tri-o-tolylphosphine)palladium(O) to the aryl halide. However, when LiOrBu is used as base, the formation and reductive elimination of the amido arylpalladium complex is decisive for the rate of the reaction. In the presence of NaOtBu both reaction steps seem to take place at similar rates. [9]... 

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