Cationic palladium catalysts

Fig. 3 Cationic palladium catalyst showing coordination of last inserted acrylate unit...

Hydrosilylation of 1,6-dienes accompanied by cyclization giving a five-membered ring system is emerging as a potential route to the synthesis of functionalized carbocycles.81,81a,81b 82 As its asymmetric version, diallylmalonates 86 were treated with trialkylsilane in the presence of a cationic palladium catalyst 88, which is coordinated with a chiral pyridine-oxazoline ligand. As the cyclization-hydrosilylation products, //ww-disubstituted cyclopentanes 87 were obtained with high diastereoselectivity (>95%), whose enantioselectivity ranged between 87% and 90% (Scheme 25).83 83a... 

The catalytic cycle proposed for the cyclization-hydrosilylation with the cationic palladium catalyst is classified into the type D in Scheme 2. The reaction consists of an olefin insertion into palladium-silicon bond and the metathesis between palladium-carbon and hydrogen-silicon bond, regenerating the silylpalladium intermediate and releasing the product where migratory insertion of the pendant olefin into the alkylpalladium is involved before the metathesis (Scheme 26).83a... 

A new class of cationic palladium catalysts for the carbonylation of alkynes is described which, under mild conditions, shows unprecedented high activity and selectivity for the carbonylation of (higher) alkynes. As a particularly interesting application, the catalysts allow the development of a commercially attractive and environmentally friendly process for the carbonylation of propyne to methyl methacrylate. 

The nickel catalysts described in the preceding section are neutral in character, in our hands cationic nickel catalysts such as those described by Brookhart et al. [20] are ineffective in the copolymerization of norbornene and ethylene. Indeed norbornene has been described as a strong catalyst poison (for both nickel and palladium) by the same workers. Surprisingly we found that cationic palladium catalysts, with a wide variety of chelate ligands, are very effective in the copolymerization of ethylene and norbornenes [89]. These catalysts possess the same ability as the above-described neutral nickel catalysts to incorporate norbornenes bearing functional groups, but overcome two of the major limitations ... 

The cationic palladium catalysts are typically prepared by reacting (cyclooctadi-enejpaUadium methyl chloride with a stoichiometric amount of the bidentate ligand to afford the (ligand) Pd(Me)Cl adduct The adduct is then reacted with an activator such as silver hexafluoroantimonate, to afford the final cationic catalyst, (hgand) l l(Me) SbF[]. The polymerization reactions were typically run at ambient temperature in toluene to afford a viscous solution of the desired copolymer. Other activators used include tris(pentafluorophenyl)borane/triethylaluminum mixtures, Nal (C,H 5(fT j)2)4 and mefhaluminoxane. 

The introduction of methoxy substituents at the ortho position of the aryl rings of diphosphine-modified cationic palladium catalysts results in markedly increased catalyst performance [49]. With the fully o-OMe-substituted analogue of ligand 1, ethylene-CO copolymers with molecular weights of up to l,2xl0" g mob and narrow polydispersitities ca. 2) were obtained in polymeriza-... 

The discovery that group IV metallocenes can be activated by methylaluminox-ane (MAO) for olefin polymerization has stimulated a renaissance in Ziegler-Natta catalysis [63]. The subsequent synthesis of well-defined metallocene catalysts has provided the opportunity to study the mechanism of the initiation, propagation, and termination steps of Ziegler-Natta polymerization reactions. Along with the advent of cationic palladium catalysts for the copolymerization of olefins and carbon monoxide [64, 65], these well-defined systems have provided extraordinary opportunities in the field of enantioselective polymerization. 

In the early 1980s. it was discovered that cationic palladium catalysts with bidentate tertiary phosphines exhibited remarkable reaction rates for olefin/... 

Goodall, B. L. McIntosh, L. F. HI. Method for the preparation of copolymers of ethylene and norbornene-type monomers with cationic palladium catalysts. PCT Int. Pat. Appl. W098/56839 A1 (B.F. Goodrich), December 17,... 

Mitsudo K, Shiraga T, Tanaka H (2008) Electrooxidative homo coupling of arylboronic acids catalyzed by electrogenerated cationic palladium catalysts. Tetrahedron Lett 49 6593-6595... 

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