Palladium kinetic studies

The kinetics of hydrogenation of phenol has already been studied in the liquid phase on Raney nickel (18). Cyclohexanone was proved to be the reaction intermediate, and the kinetics of single reactions were determined, however, by a somewhat simplified method. The description of the kinetics of the hydrogenation of phenol in gaseous phase on a supported palladium catalyst (62) was obtained by simultaneously solving a set of rate equations for the complicated reaction schemes containing six to seven constants. The same catalyst was used for a kinetic study also in the liquid phase (62a). 

The formation and presence of both phases of the Pd-H system in the palladium catalyst samples investigated was confirmed by Brill and Watson by the values of the magnetic susceptibility of the samples investigated under the same conditions as in the kinetic studies. 

The transition-metal catalyzed decomposition of thiirene dioxides has been also investigated primarily via kinetic studies . Zerovalent platinum and palladium complexes and monovalent iridium and rhodium complexes were found to affect this process, whereas divalent platinum and palladium had no effect. The kinetic data suggested the mechanism in equation 7. 

Some of the details of the mechanism may differ for various catalytic systems. There have been kinetic studies on two of the amination systems discussed here. The results of a study of the kinetics of amination of bromobenzene using Pd2(dba)3, BINAP, and sodium r-amyloxide in toluene were consistent with the oxidative addition occurring after addition of the amine at Pd. The reductive elimination is associated with deprotonation of the animated palladium complex.166... 

The reactivity of a series of closely related substances can either be enhanced or inhibited depending on the type and extent of interaction with double-helical DNA. This was shown in a kinetic study of the substitution of ethylenediamine (en) or A-, A- -dimethylethylcncdiamine (Me2en) by thiourea in the palladium(II) complexes [Pd(4,4,-R2bpy)(en)](PF6)2 (R = H or Me), [Pd(en)2](PF6)2, and [Pd(Me2en)2](PF6)2, in water and in the presence of calf thymus DNA.183... 

It is tempting to associate directly the absence of ethylene oxide over catalysts with more than 40% Pd with the appearance of holes in the d-band. It could be assumed that ethylene is chemisorbed directly on Pd-rich alloys and rapidly decomposed, whereas on Ag-rich alloys ethylene is only adsorbed on top of an oxygen-covered surface leading to selective oxidation. However, the general conclusion from earlier kinetic studies (143) is that the rate-determining step over pure palladium also involves the latter mode of ethylene chemisorption. 

Alkoxycarbonylation has been known for a long time, but the rates and selectivities of the new catalysts are outstanding. The mechanism of the alkoxycarbonylation reaction catalysed by palladium has been the topic of research for many years [55], Stepwise reactions had shown the feasibility of two mechanistic pathways, shown in Figure 12.20, but kinetic studies and in situ observations on catalytic systems were lacking. 

The empirical observation that (—)-sparteine 55 is necessary for catalysis implicates a base-promoted pathway in the mechanism. In the first step, a palladium alk-oxide is formed after alcohol binding, followed by p-hydride elimination of the alkoxide to yield a ketone product. On the basis of a kinetic study of the enantio-selective oxidation of 1-phenylethanol, it was revealed that (—)-sparteine plays a dual role in the oxidative kinetic resolution of alcohols, as a ligand on palladium and an exogeneous base " ... 

A study of Pd(PPhs)4,4, reported recently by Roth and coworkers, reveals yet another mechanistic pathway for Pd° oxygenation [122]. Complete dissociation of one PPha ligand from Pd occurs in solution to produce a three-coordinate palladium(O) species, 26. Kinetic studies reveal that 26 reacts with dioxygen via parallel associative and dissociative pathways (Scheme 5). The latter dissociative pathway results in the formation of the two-coordinate complex 27, which undergoes very rapid reaction with dioxygen in a manner directly analogous to that of the well-defined two-coordinate NHC complexes 23 and 24. 

Reaction of siloxycyclopropane 1 with acid chlorides in the presence of a palladium catalyst also proceeds cleanly to give 4-ketoesters in high yields (Eq. 59, Table 14) [57]. Chloroform is a suitable solvent. Kinetic studies have revealed that the interaction between 1 and an acylpalladium chloride complex is the rate limiting step. 

Several mechanisms were proposed to interpret bond shift isomerization, each associated with some unique feature of the reacting alkane or the metal. Palladium, for example, is unreactive in the isomerization of neopentane, whereas neopentane readily undergoes isomerization on platinum and iridium. Kinetic studies also revealed that the activation energy for chain branching and the reverse process is higher than that of methyl shift and isomerization of neopentane. 

The metal-catalysed hydrogenation of cyclopropane has been extensively studied. Although the reaction was first reported in 1907 [242], it was not until some 50 years later that the first kinetic studies were reported by Bond et al. [26,243—245] who used pumice-supported nickel, rhodium, palladium, iridium and platinum, by Hayes and Taylor [246] who used K20-promoted iron catalysts, and by Benson and Kwan [247] who used nickel on silica—alumina. From these studies, it was concluded that the behaviour of cyclopropane was intermediate between that of alkenes and alkanes. With iron and nickel catalysts, the initial rate law is... 

T[[dotb]he nature of the initial attack by the water (eq. 10) is a matter of some controversy (205,206). Stereochemical and kinetic studies of model systems have been reported that support trans addition of external water (207,208) or internal addition of cis-coordinated water (209), depending on the particular model system under study. Other palladium-catalyzed oxidations of olefins in various oxygen donor solvents produce a variety of products including aldehydes (qv), ketones (qv), vinyl acetate, acetals, and vinyl ethers (204). However the product mixtures are complex and very sensitive to conditions. 

Finally a kinetic study of the oxidation of ethylene by palladium (II) acetate gave a rate-[NaOAc] profile similar to Figure 1 which could also be interpreted as conversion of less reactive trimer to more reactive dimer. However at [NaOAc] > 0.2M the decrease in rate with increase in [NaOAc] is much greater than that shown in Figure 1 and corresponds to a 1/[NaOAc] term in the rate expression for reaction of dimer. This difference in rate expression between exchange and olefin oxidation could have very interesting mechanistic implications. For instance, the added acetate inhibition term could result from the need for a vacant coordination site on the Pd (II) before hydride elimination can occur. The scheme is shown in Equations 31 and 32. 

Kinetic Studies Provide Only Limited Mechanistic Information. While such studies are invaluable and frequently indicate the nature of pre-rate-determining steps, they provide almost no information concerning such vital fast steps as electron transfers and rearrangements. For example, despite extensive studies of the kinetics of acetaldehyde and vinyl acetate syntheses, it is clear only that olefin, nucleophile, and palladium combine in a complex. The nature of the rate-determining step as well as the details of post-rate determining product forming steps remains uncertain (7,94). In some cases—e.g., the metal-catalyzed autoxi-dation of thiols to disulfides—re-oxidation of metal to its catalytically... 

Another useful technique in kinetic studies is the measurement of the total pressure in an isothermal constant volume system. This method is employed to follow the course of homogeneous gas phase reactions that involve a change in the total number of gaseous molecules present in the reaction system. An example is the hydrogenation of an alkene over a catalyst (e.g., platinum, palladium, or nickel catalyst) to yield an alkane ... 

Oxidative addition to a monophosphine palladium complex is unusual, but is a reasonable pathway if one bears in mind that reductive eliminations often occur from monophosphine palladium complexes [202,203]. These reductive eliminations from monophosphine Pdn species would form a monophosphine Pd° complex as the initial metal product, and these Pd° products are similar to the intermediate in the oxidative addition of aryl halide deduced from kinetic studies. 

In particular, more structural data, more kinetic studies, a more thorough investigation of the platinum and palladium chemistry, the use of new techniques, such as NMR spectroscopy, and the determination of bond energies could increase our understanding of the bond nature and reactivity and make the interpretation of important catalytic mechanisms of these compounds easier. 

Palladium(II) acetate has been found to be a catalyst for the dimerization of benzene to biphenyl in perchloric acid-acetic acid solutions 80). Kinetic studies suggested the formation of an intermediate complex between a benzene molecule and a Pd(II) ion, and the following reaction sequence was proposed ... 

The next step, however is discussed contradictorily. In the first experiments it was already found that chloride and hydrogen ions inhibit the reaction between ethylene and palladium chloride [1, 10, 13] and it was concluded that an OH ion would attack the complexed olefin. This was confirmed by kinetic studies carried out by Moiseev et al. [14-16] and Henry [17, 18], leading to eq. (9) where K is the equilibrium constant for eq. (8) and k an overall rate constant. 

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