Phosphine complexes of palladium

When the steric repulsion between a cis bidentate and two substituents R and CN increases in a series of bidentate phosphine complexes of palladium, the rate of reductive elimination of R-CN increases by several orders of magnitude [16], Actually, the steric effect and electronic effect are related and careful variation of only one property at a time is needed to distinguish between the two. 

The palladium-catalyzed coupling of aiyl and vinyl halides to organotin compounds, known as Stille coupling, is one of the most important catalytic methods of carbon-carbon bond formation. The reaction is generally conducted in polar organic solvents, such as dimethylformamide, with tertiary phosphine complexes of palladium, although phosphine-free complexes or simple Pd-salts are also frequently used as catalysts [8]. 

A series of substituted phosphine complexes of palladium have been synthesized by displacement of the allyl ligand from (2-methylallyl)PdCl2 with excess phosphine, and 13C n.m.r. studies have shown that the equilibrium (1) lies well to the left ... 

Complexes of other metals are used to sensitize emulsions, and some are used to sensitize even further (supersensitize) gold-sensitized emulsions. " Among these are (NH4)2[PtCl4], (NH4)2[PdCl4], (NH4)2[PdCy, K2[IrCl6] and similar compounds with ruthenium and the other halides. Sensitization by iridium salts and complex ions has been reviewed recently. Mechanisms of action of palladium(II) salts and complex ions on gold-sensitized emulsions have been studied. Several phosphine complexes of palladium and platinum, for example (2), are reported to be effective sensitizers, as are many macrocyclic polyamine compounds and their metal complexes,for example the Cu", Ni", Fe " and Rh " chelates of the cyclen ligand, 1,4,7,10-tetraazacyclododecane (3). 

On the other hand, true ligand acceleration (type 3 processes) shows preference for solvents of low polarity and lower Lewis basicity (toluene, dioxane and THF) with soluble tertiary amines as bases. In this respect, these Mizoroki-Heck reactions resemble cross-coupling processes, which also display strong preference for these solvents. Reactions in nonpolar solvents (toluene or xylene) have been known since Heck s seminal articles [8]. The halide remains a crucial subject of concern in reactions catalysed by phosphine complexes of palladium, aryl iodides prefer triarylphosphines and polar solvents, whereas reactions of aryl bromides and chlorides indeed prefer electron-rich trialkylphosphines and nonpolar solvents [63-65]. 

At least for aryl iodides, the application of phosphine complexes of palladium still continues to be practised in complex preparations where mild conditions are desirable to secure high selectivity. In these cases, halide scavenger additives are often used to enable the polar pathway, because phosphine ligands under mild conditions block the coordination places need for alkene binding. Owing to the low reactivity of those systems, high loadings of precatalyst are only tolerable in complex molecule synthesis. For example, this approach enabled clean vinylation of vinylic boronate 63 to afford complex diene 64 (62 64, Scheme 2.13) [62]. 

In contrast to unsubstituted 1-alkenes, the hydrosilylation of their derivatives with electron-acceptor substituents (e.g., acrylonitrile, styrene, or vinyl-trichlorosilane) in the presence of phosphine complexes of palladium leads to a selective formation of a-adducts (3,6,10,11,15). 

Two-Phase Catalysis by Hydrophilic Phosphine Complexes of Palladium. 

Zudin, V, Likholobov, V. and Yermakov, Y. (1979). Catalytic Synthesis of Hydrogen Peroxide from Oxygen and Water in the Presence of Carbon Monoxide and Phosphine Complexes of Palladium, Kinet. Rated. USSR) Engl. TransL), 20, pp. 1324—1325. 

It should be noted that cross-coupling in the presence of phosphine complexes of palladium usually requires high amounts of catalyst, with initial loadings of 25-30 mol% not being uncommon. An entirely new approach to the Suzuki reaction is phosphine-free palladium catalysis. The use of palladium catalysts without the addition of phosphine ligands for cross-coupling with organoboron compounds in aqueous media opened a new chapter in the story of this powerful synthetic method. This approach allows catalyst efficiency to be dramatically increased, and the reaction to be performed under milder conditions. 

Kiji, J. Okano, T. (1994) 2-Phase catalysis by water-soluble phosphine complexes of palladium, J. Synth. Org. Chem. Jpn., 52,276-84. 

The reaction is exoenergetic and is accompanied by the expected mass increase. Unlike these nickel(II) compounds, phosphine complexes of palladium(II) and platinum(II), [MX2(PR3)2l, do not undergo such a reaction. While phosphine oxidation might take place at high temperatures, it is obscured by decomposition of the phosphine ligand. Kukushkin and... 

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