Palladium complexes carbon-donors

Halides react with carbon monoxide, usually with palladium complex catalysts, in the presence of hydrogen donors, to give aldehydes (equation 147). 

In the C-0 bond cleavage reaction of vinyl epoxides by a palladium complex, formic acid acts as a good proton and hydride donor evolving CO2 (Eqs. 3.24 and 3.25). Zero-valent palladium complex favors the attack at an allylic carbon in an Sn2 manner to give i/ -allylpalladium(II) complex with inversion of configiuation, and formate anion coordinates to the palladium center (Scheme 3.50). Then, decarboxylation of the formate affords palladium hydride, which attacks the ry -allyl moiety from the endo side. Thus, 1,2-addition of hydrogen atoms takes place regioselectively with inversion at the allylic carbon [94]. 

A series of l-(pyridin-2-yl)benzimidazolium salts were synthesized by Chianese et al. and used as precursors to rigid, five-membered chelated biden-tate NHC-pyridine ligands [57]. The ligands were constructed with either very small (H, Me) or very bulky (2,6-diisopropylphenyl, 2,4,6-triisopropylphenyl) groups adjacent to the nitrogen and carbon donor atoms. Cationic palladium-allyl complexes were synthesized under mild conditions and used as catalysts for the allylic substitution reaction between allylic carbonates and either sodium dieth)f-2-methylmalonate or Af-methylbenzylamine. 

As well as phosphorus ligands, heterocyclic carbenes ligands 10 have proven to be interesting donor ligands for stabilization of transition metal complexes (especially palladium) in ionic liquids. The imidazolium cation is usually presumed to be a simple inert component of the solvent system. However, the proton on the carbon atom at position 2 in the imidazolium is acidic and this carbon atom can be depro-tonated by, for example, basic ligands of the metal complex, to form carbenes (Scheme 5.3-2). 

Electronic ligand effects are highly predictable in oxidative addition reactions a-donors strongly promote the formation of high-valence states and thus oxidative additions, e.g. alkylphosphines. Likewise, complexation of halides to palladium(O) increases the electron density and facilitates oxidative addition [11], Phosphites and carbon monoxide, on the other hand, reduce the electron density on the metal and thus the oxidative addition is slower or may not occur at all, because the equilibrium shifts from the high to the low oxidation state. In section 2.5 more details will be disclosed. 

The tetrahedral [160-163] and cis-planar [100,134,164-167] structures are characteristic for chelates of type 874 with coordination units NiN4 and MN2S2, respectively, as well as chelates 868 discussed above. Original polyhedral forms were discovered by x-ray diffraction for nickel and palladium ICC of the discussed type 874. It is accepted that, in case of a nickel complex, the compound with a carbon-carbon bond 875 is formed [165,166] formation of palladium chelates is accompanied by the cyclometallation reaction leading to tetranuclear clusters 876, where the tridentate ligand behaves as C,N,S-donor [168]. 

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