Metal groups palladium complexes

The ease of formation of the carbene depends on the nucleophilicity of the anion associated with the imidazolium. For example, when Pd(OAc)2 is heated in the presence of [BMIM][Br], the formation of a mixture of Pd imidazolylidene complexes occurs. Palladium complexes have been shown to be active and stable catalysts for Heck and other C-C coupling reactions [34]. The highest activity and stability of palladium is observed in the ionic liquid [BMIM][Brj. Carbene complexes can be formed not only by deprotonation of the imidazolium cation but also by direct oxidative addition to metal(O) (Scheme 5.3-3). These heterocyclic carbene ligands can be functionalized with polar groups in order to increase their affinity for ionic liquids. While their donor properties can be compared to those of donor phosphines, they have the advantage over phosphines of being stable toward oxidation. 

Octaethylbilindione (H3OEB) is a convenient model for the bile pigment biliverdin IXa. Key redox states of this ligand as observed in its complexes are shown in Figure 14. The redox behavior of the palladium complex of octaethylbilindione was examined in order to determine the generality of the redox behavior of this group of transition metal complexes.202 A preliminary report on the novel tetrameric Pd4(OEB)2, which contains palladium) ) ions 7r-bonded to C=C bonds of the tetrapyrrole ligand, and of Pdn(OEB ) has appeared.203... 

However, the practical, direct synthesis of functionalized linear polyolefins via coordination copolymerization olefins with polar monomers (CH2 = CHX) remains a challenging and industrially important goal. In the mid-1990s Brookhart et al. [25, 27] reported that cationic (a-diimine)palladium complexes with weakly coordinating anions catalyze the copolymerization of ethylene with alkylacrylates to afford hyperbranched copolymers with the acrylate functions located almost exclusively at the chain ends, via a chain-walking mechanism that has been meticulously studied and elucidated by Brookhart and his collaborators at DuPont [25, 27], Indeed, this seminal work demonstrated for the first time that the insertion of acrylate monomers into certain late transition metal alkyl species is a surprisingly facile process. It spawned almost a decade of intense research by several groups to understand and advance this new science and to attempt to exploit it commercially [30-33, 61]. 

Recently, another type of catalytic cycle for the hydrosilylation has been reported, which does not involve the oxidative addition of a hydrosilane to a low-valent metal. Instead, it involves bond metathesis step to release the hydrosilylation product from the catalyst (Scheme 2). In the cycle C, alkylmetal intermediate generated by hydrometallation of alkene undergoes the metathesis with hydrosilane to give the hydrosilylation product and to regenerate the metal hydride. This catalytic cycle is proposed for the reaction catalyzed by lanthanide or a group 3 metal.20 In the hydrosilylation with a trialkylsilane and a cationic palladium complex, the catalytic cycle involves silylmetallation of an alkene and metathesis between the resulting /3-silylalkyl intermediate and hydrosilane (cycle D).21... 

The principal developments in hydrostannation during the past decade are in the use of hydrides R2SnXH, where X is an electronegative group such as halide, carboxylate, or sulfonate, and in the use of the hydrides complexed to a transition metal, particularly palladium. 

More recently a variation of this mechanism was reported by Novak [37], The mechanism involves nucleophilic attack at co-ordinated phosphines and it explains the exchange of aryl groups at the phosphine centres with the intermediacy of metal aryl moieties. After the nucleophilic attack the phosphine may dissociate from the metal as a phosphonium salt. To obtain a catalytic cycle the phosphonium salt adds oxidatively to the zerovalent palladium complex (Figure 2.38). Note where the electrons go . 

Metallic groups as in case (c) lead to electrophilic or even carbocation-like carbene complexes. Typical examples are Fischer-type carbene complexes [e.g. (CO)5Cr=C(Ph)OMe] and the highly reactive carbene complexes resulting from the reaction of rhodium(II) and palladium(II) carboxylates with diazoalkanes. Also platinum ylides [1,2], resulting from the reaction of diazoalkanes with platinum(Il) complexes, have a strong Pt-C o bond but only a weak Pt-C 7t bond. In situation (d) the interaction between the metal and the carbene is very weak, and highly reactive complexes showing carbene-like behavior result. Similar to uncomplexed carbenes. 

Recently, the oxidative addition of C2-S bond to Pd has been described. Methyl levamisolium triflate reacts with [Pd(dba)2] to give the cationic palladium complex 35 bearing a chiral bidentate imidazolidin-2-ylidene ligand [120]. The oxidative addition of the levamisolium cation to triruthenium or triosmium carbonyl compounds proceeds also readily to yield the carbene complexes [121], The oxidative addition of imidazolium salts is not limited to or d transition metals but has also been observed in main group chemistry. The reaction of a 1,3-dimesitylimidazolium salt with an anionic gallium(I) heterocycle proceeds under formation of the gaUium(III) hydrido complex 36 (Fig. 12) [122]. 

The third-generation dendrimer palladium complex (31, containing 24 PdC groups) was applied in the Stille coupling of methyl-2-iodobenzoate with 2-(tributyl-stannyl)thiophene in DMF (Scheme 10 lmol% catalyst). In contrast to what was observed with the monomer (PPh3)2PdCl2, no palladium metal formation was... 

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