Palladium® complexes halogen ligands

In accord with calculations performed by Cavell et al. [110], the oxidative addition of C2-X functionalized azolium cations (X = halogen) to metal centers proceeds faster and with a more favorable reaction enthalpy than the oxidative addition of the C2-H substimted imidazolium cations [118, 119]. The former reaction was applied successfully for the preparation of nickel and palladium complexes bearing a variety of different ylidene ligands [119]. 

It is known that the halogen ligand on an organopalladium(II) halide is readily displaced by an alkoxy, hydroxy, or acetoxy anion to provide the reactive Pd-OR complexes 20 [58], which have been postulated as reaction intermediates [59] or isolated [55, 56] from the reaction of organopalladium(II) halides with sodium hydroxide or methoxide. It is not yet obvious, in many reactions, which process shown in Scheme 2-18 or 2-23 is predominant however, the formation of alkoxo-, hydroxo- or acetoxopalladium(II) intermediates should be considered to be one of the crucial transmetallation processes in base/palladium-induced cross-coupling reactions. 

The reaction of 1-alkenylborates with haloenones shows a characteristic feature of the (alkoxo)palladium mechanism (Scheme 2-24) [60]. The cross-coupling reaction with haloenones is accelerated by exceptionally weak bases such as NaOAc or even EtjN, when methanol is used as a solvent. The results cannot be explained by the ate complex mechanism shown in Scheme 2-18, and can be best understood by the formation of the (alkoxo)palladium(II) intermediate 27, since 26 readily exchanges the halogen ligand with methanol due to the strong trans effect of the electron-poor alkenyl group (Scheme 2-24). 

Fig. 3 Dimeric and monomeric palladium complexes for enantioselective halogenation. (5)-BINAP ligands and (5)-Segphos-ligand...

II.2.2 Palladium Complexes Containing Halogen and Oxygen Ligands... 

II.2.2 PALLADIUM COMPLEXES CONTAINING HALOGEN AND OXYGEN LIGANDS 45... 

It is known that the halogen or OTf ligand on Ar-Pd-X is readily displaced by an alkoxy, hydroxy, or acetoxy anion to provide a basic Pd-OR complex. ° Indeed, the (hydroxo)palladium complex reported by Alper gives a cross-coupled product (70%) together with biphenyl (15%) (Eq. 20). The electronic effects of substituted arylboronic acids... 

In addition to the examples described previously, a tremendous variety of compounds have been prepared using palladium-catalyzed cross-coupling reactions due to the broad functional group tolerance. The combination of copper and palladium complexes have been used to promote the formation of borane-protected arylphosphines [298]. Palladium acetate catalyzed the coupling of halogenated benzoic acid derivatives leading to the formation of water-soluble phosphine ligands [299]. Aryl triflates have also been used in the phosphination reaction [300]. Functionalized tyrosine derivatives were converted... 

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