Catalyst divalent palladium

Until recently, the hydroformylation using palladium had been scarcely explored as the activity of palladium stayed behind that of more active platinum complexes. The initiating reagents are often very similar to those of platinum, i.e., divalent palladium salts, which under the reaction conditions presumably form monohydrido complexes of palladium(II). A common precursor is (39). The mechanism for palladium catalysts is, therefore, thought to be the same as that for platinum. New cationic complexes of palladium that are highly active as hydroformylation catalysts were discovered by Drent and co-workers at Shell and commercial applications may be expected, involving replacement of cobalt catalysts. 

The Wacker-Hoechst process has been studied in great detail and in all textbooks it occurs as the example of a homogeneous catalyst system illustrating nucleophilic addition to alkenes. Divalent palladium is the oxidising agent and water is the oxygen donor according to the equation ... 

Whilst the catalysis begins with Pd°, the combination of Pd" with an alkene and nucleophile results in reduction to Pd°. Accordingly, the catalyst may also be introduced in the form of a divalent palladium complex (typically and conveniently more air stable). 

The carbonylation of chloroarenes has been described by Alper and Grushin [27] and Jenner and Bentaleb [28], While the former showed that square-planar complexes of divalent palladium, [ L2PdCl2], where L = tertiary phosphine, are active catalysts for the biphasic carbonylation of aromatic halides, including chloroarenes (when L = tricyclohexylphosphine), to the corresponding carboxylic acids, the latter demonstrated that chloroarenes can be converted into aromatic acids via catalytic reaction with aqueous methyl formate under biphasic conditions. [ PdCl2(PCy3)2] was the most efficient catalyst. The addition of [ Ru3(CO)12] and ammonium formate improved yield and selectivity of the carbonylation reaction. The mechanism should involve oxidative addition of the C—Cl bond to a zero-valent Pd species followed by CO insertion. However, the palladium catalyst may also directly activate methyl formate. Compared to other carbonylations of aryl-Hal compounds the procedure is quite convenient (no solvent, no initial pressurization) [27]. 

Most platinum metal catalysts (with the exception of Adams catalyst) are stable and can be kept for many years without appreciable loss of activity, but they can be deactivated by many substances, particularly by compounds of divalent sulfur. Catalytic activity is sometimes increased by addition of small amounts of platinum or palladium salts or mineral acid. The increase in the activity may simply be the result of neutralization of alkaline impurities in the catalyst. 

Olefin Dimerization and Polymerization The homogeneous catalytic dimerization of olefins, e.g., ethylene to butene, is another important reaction that can proceed via the addition, insertion, cleavage sequence. Among the catalysts that effect ethylene dimerization are RhCl3, PdCl2, and combinations of alkylaluminum halides with divalent iron, cobalt, and nickel compounds. The rhodium system has been thoroughly studied (177) and almost certainly proceeds by the sequence shown below (S = solvent). The nickel system probably functions similarly, but other mechanisms have been considered for the iron-, cobalt-, and palladium-catalyzed reactions. 

Ganguly, S. Roundhill, D.M. (1993) Catalytic hydration of diethyl maleate to diethyl malate using divalent complexes of palladium(ll) as catalysts, Organometallics, 12, 4825-32. 

Alkenes are reduced by addition of H2 in the presence of a catalyst such as platinum or palladium to yield alkanes, a process called catalytic hydrogenation. Alkenes are also oxidized by reaction with a peroxyacid to give epoxides, which can be converted into trans-l,2-diols by acid-catalyzed hydrolysis. The corresponding cis-l,2-diols can be made directly from alkenes by hydroxylation with OSO4. Alkenes can also be cleaved to produce carbonyl compounds by reaction with ozone, followed by reduction with zinc metal. In addition, alkenes react with divalent substances called carbenes, R2C , to give cyclopropanes. Nonhalo-genated cyclopropanes are best prepared by treatment of the alkene with CH2I2 and zinc-copper, a process called the Simmons-Smith reaction. 

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