Metal cation metathesis

Most of these metal-oxygen intermediates are expected to be reactive toward organic substrates and electrode surfaces. Hence, the presence of metal cations enhances the electron stoichiometry for the reduction of 02, but frequently passivates the electrode surface. Thus, the formation of (H20)4Znn(02) on the surface of a platinum electrode probably initiates a metathesis reaction ... 

Heterocyclization reactions with saturated moieties (alcohols, amines, thiols, etc.) or acids on unsaturated counterparts (alkenes, allenes, alkynes, etc.) are not covered in this chapter since they are addition, and not isomerization, reactions. Silver is also widely used as an activating agent for producing highly reactive metallic cations (anion metathesis), which, in turn, may catalyze cycloisomerization reactions. This aspect is covered only when the silver control experiments give substantial positive results. 

Ousters adsorbed on the outside surfaces of zeolites can often be easily extracted with neutral solvents or with salt solutions which remove the cluster ions by cation metathesis (ion exchange). Comparison of the infrared spectra of the extracted spedes and those of known spedes helps identify the encaged spedes. When treatment of a sample with such solutions fails to remove sorbed dusters, they are inferred to be trapped within the cages. The inference is supported when the same clusters adsorbed on the surface of a large pored material such as an amorphous metal oxide are removed by extraction. 

Bis(phosphonio)isophosphindolium cations were expected to act as attractive ambidentate ligands toward the transition-metal-containing moieties. These expectations did not materialize with respect to CpM(CO)3 (M = Mo, W), since the anionic metathesis occurred to yield the ionic products 178 (96PSS125). 

The mechanism for the reaction catalyzed by cationic palladium complexes (Scheme 24) differs from that proposed for early transition metal complexes, as well as from that suggested for the reaction shown in Eq. 17. For this catalyst system, the alkene substrate inserts into a Pd - Si bond a rather than a Pd-H bond [63]. Hydrosilylation of methylpalladium complex 100 then provides methane and palladium silyl species 112 (Scheme 24). Complex 112 coordinates to and inserts into the least substituted olefin regioselectively and irreversibly to provide 113 after coordination of the second alkene. Insertion into the second alkene through a boat-like transition state leads to trans cyclopentane 114, and o-bond metathesis (or oxidative addition/reductive elimination) leads to the observed trans stereochemistry of product 101a with regeneration of 112 [69]. 

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... 

In addition to the cations diseussed above, additional salts of the monoanion 2a have been prepared by substituting various pyridinium salts for the BU4NI utilized in Seheme 2. A signifieant number of salts of 2a have also been prepared via metathesis of [Bu4N][2a] with various metal PFe salts. These methods have thus produced salts of 2a with various organometallic cations ([M(Cp )2], were M = Fe, Mn, Co, Cr). 

Tetramethylammonium 1,1,2,3,3-pentacyanopropenide is useful for preparation of pentacyanopropenide salts of other metal and quaternary ammonium cations by metathesis.8 The free acid, which may be obtained by use of an ion-exchange resin,3 has an ionization constant comparable to that of a strong mineral acid (pK < —8.5 the anion is not detectably protonated in 12M sulfuric acid).4... 

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