Palladium metal exchange reaction

The a- and y-position activation has a remarkable impact on the regiochemical outcome for the Pd-catalyzed reaction of heterocycles. For example, the palladium-catalyzed reaction of 2,5-dibromopyridine takes place regioselectively at the C(2) position, although the halogen-metal exchange reaction takes place predominantly at C(5). Pd-catalyzed reactions of chloropyrimidines take place at C(4) and C(6) more readily than at C(2). 

Metal complexes are often used in hydrogenation reactions. These metal catalysts can be supported on a resin in the form of nanoparticles [8]. Palladium metal nanoparticles are useful as hydrogenation catalysts. They are produced by the metal exchange reaction of Pd(OAc)2 onto an appropriate acid containing resin. Once the Pd(II) is in place, the resultant polymer supported Pd(II) is reduced by NaBH4 [9]. 

In recent years, some interesting reactions that involve metal-exchange reactions of arylmercury compounds with Pd(II) salts have been developed. The resulting organopalladium species serve as a source of reactive intermediates that lead to products. Some of the types of compounds that can be formed via the unstable palladium intermediates are illustrated below ... 

The mechanism of 1 1 complex formation between palladium(II) and catechol and 4-methylcatechol has been studied in acidic media, and the rate of 1 1 (and 1 2) complex formation between silver(II) and several diols is an order of magnitude higher in basic solution than in acidic. The kinetics of formation and dissociation of the complex between cop-per(II) and cryptand (2,2,1) in aqueous DMSO have been measured and the dissociation rate constant, in particular, found to be strongly dependent upon water concentration. The kinetics of the formation of the zinc(II) and mercury(II) complexes of 2-methyl-2-(2-pyridyl)thiazolidine have been measured, as they have for the metal exchange reaction between Cu " and the nitrilotriacetate complexes of cobalt(II) and lead(II). Two pathways are observed for ligand transfer between Ni(II), Cu(II), Zn(II), Cd(II), Pb(II) and Hg(II) and their dithiocarbamate complexes in DMSO the first involves dissociation of the ligand from the complex followed by substitution at the metal ion, while the second involves direct electrophilic attack by the metal ion on the dithiocarbamate complex. As expected, the relative importance of the pathways depends on the stability of the complex and the lability and electrophilic character of the metal ion. 

Although a metal exchange reaction is not often thought of as a preparative method, such reactions can be useful since they generally proceed in good yield. Palladium is replaced by iron (2-8) by conversion of tetraphenylcyclo-butadiene palladium dibromide to tetraphenylcyclobutadiene iron tricarbonyl. The driving force involves the formation of the more stable tetraphenylcyclobutadiene iron tricarbonyl complex. 

Excellent examples of metal exchange reactions are provided by complexes of tetraphenylcyclobutadiene. When either iron pentacarbonyl or nickel carbonyl is reacted with tetraphenylcyclobutadiene palladium bromide, the corresponding metal is exchanged with the release of palladium(O) metal. The reaction proceeds only in aromatic solvents and seems to be generally applicable to a variety of transition metal carbonyls. As illustrated in Fig. 6-8, the first step is proposed to involve the formation of uncomplexed tetraphenylcyclobutadiene, a mixed metal halide carbonyl, carbon monoxide. 

Thus, evidence has accumulated in support of hydrogen exchange in benzene by a mechanism involving associatively chemisorbed benzene, and without the necessity to postulate the participation of chemisorbed C Hs. One attractive test of these ideas which, so far as we know, has not been made, would be to repeat, for example, the reaction of para-xylene with deuterium using as catalyst a palladium thimble. This system would allow the exchange reaction to proceed either in the presence of molecular deuterium (both reactants on same side of the thimble) or in the presence of atomic deuterium only (xylene and molecular deuterium on opposite sides of the thimble, so that the hydrocarbon reacts only with chemisorbed atomic deuterium that arrives at the surface after diffusion through the metal). 

A small amount of MIBK is made.from a new European-originated process. A complex catalyst system involving palladium metal and a cation exchange resin is used. The reaction permits going directly from acetone to MIBK. 

The exchange reaction of methane with deuterium has been found to show a decrease in specific activity upon alloying of palladium with gold (82), reflecting the decrease in number of reactive surface metal atoms upon alloying. 

Fig. 5. Compensation behavior in reactions on palladium metal. The lines (Table 11) were calculated (Appendix II) for cracking ( ) and for exchange (x) and oxidation (o) processes (data references given in text).

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