Palladium hydride, decomposition

Studies in deuterated water have shown that the hydroxyl proton does not end up in the ethanal formed. The decomposition of the 2-hydroxyethyl is not a simple P-elimination to palladium hydride and vinyl alcohol, which then isomerises to ethanal. Instead, the four protons stemming from ethene are all present in the initial ethanal product [6] (measured at 5 °C in order to suppress deuterium/hydrogen exchange in the product) and most authors have therefore accepted an intramolecular hydride shift as the key-step of the mechanism (see Figure 15.2). There remains some doubt as to how the hydride shift takes place. 

The reaction of compound Vc afforded a comparatively stable complex considering the fact that it contains a hydrogen /3 to the palladium and therefore can decompose by a ft elimination of palladium hydride. Evidently, the triphenylphosphine ligands in this complex provide some added degree of stability toward this decomposition pathway. 

Bimetallic catalysts can be prepared by a direct redox method when a cationic complex of a metal of higher electrochemical potential is reduced by another metal of lower electrochemical potential that has been deposited and reduced first187 (see Table 4.10) PdAu/SiO 88 and PdAu/C189 have been made in this way, gold being deposited after the palladium. Small amounts of the metals were found in filtrates, and XRD and temperature-programmed decomposition of palladium hydride indicated a substantial... 

The existence of a free carbonium ion such as VII in a strongly solvating medium is highly improbable. Only if VII could exist in association with the palladium could decomposition to vinyl acetate be expected to occur with a reasonable degree of frequency, in competition with the reaction with acetate to form ethylidene diacetate. Similar results have been reported in the Wacker acetaldehyde synthesis when D2O is used as the solvent (25). Stern (54) has reported results in which 2-deuteropropylene was used as substrate in the reaction. Based on assumed /J-acetoxyalkylpalladium intermediates, on the absence of an appreciable isotope effect in the proton-loss step, and on the product distribution observed, excellent agreement between calculated (71%) and observed (75%) deuterium retention was obtained. Several problems inherent in this study (54) have been discussed in a recent review (I). Hence, considerable additional effort must be expended before a clear-cut decision can be made between a simple / -hydrogen elimination and a palladium-assisted hydride shift in this reaction. 

Allyl alkyl carbonates, prepared from various alcohols except simple primary ones, are converted into aldehydes or ketones in the presence of a phosphine-free palladium catalyst. Acetonitrile as coordinating solvent is necessary for the success of this reaction. A mechanism via palladium alkoxides was proposed (Scheme 8). Ruthenium hydride complexes work similarly. A similar mechanism operates for the palladium-catalyzed decomposition of allylic carbonates. The reaction can be utilized for the mild deprotection of amines, e.g., for peptide synthesis shown in equation (20). 

Reduction of 2a by LiBEtjH in the presence of PPhj afforded q -(Si-H)Pd(0) complex 43 bearing a Si-H a-bond coordinated to Pd(0) although the same reaction without PPhj resulted in some decomposition reactions probably via palladium hydride (Scheme 9.11) [21, 22]. X-ray and NMR analyses disclosed that the q -(Si-H)Pd(0) structure is retained both in solid and solution (Figure 9.4). The Si-H, Pd-H, and Pd-Si distances are 1.60(3), 1.67(3), and 2.4283(8) A, respectively. 

As early as 1923 Hinshelwood and Topley (27) noted the exceptionally erratic behavior of palladium foil catalyst in the formic acid decomposition reaction within 140-200°C. The initially very high catalytic activity decreased 102 times during the exposure of palladium to hydrogen, which is a product of the reaction. Though the interpretation does not concern the /3-hydride formation, the authors observation deserves mentioning. 

Fig. 8. Arrhenius plots for the formic acid decomposition on palladium foil (1) and small pieces of this foil (2) at a higher temperature range, when hydrogen evolving as a product of the reaction was absorbed by Pd and transformed into the /3-Pd-H hydride phase. At the lower temperature range the reaction proceeds on the a-Pd-H phase, with a higher activation energy when the foil was hydrogen pretreated (2a), and a lower activation energy for a degassed Pd foil (3a). After Brill and Watson (57).

In a study of thermal stability and hydrogen sorption characteristics of a series of sorbent tablets composed of hydride-forming metals dispersed in polymers under a 50% hydrogen in argon atmosphere, it was found that tablets of 80% palladium in PTFE, and 80% of 1 5 atom lanthanum-nickel alloy in PTFE could not be used above 247° C because of explosive decomposition of the PTFE. 

Much work has gone into the optimization of results with functionalized alkenes. In the reaction of cyclic alkenes, ds-decomposition of organopalladium halide-alkene complex gives -palladium complex 41. This subsequently undergoes syn-p-hydride elimination since only one such hydrogen is available, deconjugation to 3-substituted alkenes should... 

The reaction of tt -allyl palladium complexes with hydrogen causes decomposition of the compounds, and produces either alkenes or alkanes, depending on the degree of substitution of the TT-aUyl complex and the reaction conditions. Hydride reagents, such as NaBUi and LiAUTi, also reduce the aUylic group to alkanes or alkenes. 

The next step in the reaction scheme—decomposition of the a-bonded alkylpalladium (XIV or XV)—has caused some controversy. To account for the results of several deuterium-labelling studies (15, 36, 54), a. palladium-assisted hydride transfer reaction (Reaction 4) has been proposed (36, 54). A number of inconsistencies in the studies using 2-deuteropro-pylene as substrate (54) have been discussed (i). In addition, the formation of a free carbonium ion such as VII [as proposed by Moiseev (36)], while accounting well for the formation of ethylidene diacetate, is much less satisfactory in accounting for the production of the unsaturated esters in an acetate-acetic acid medium. A simple elimination of -hydrogen (Reactions 13a and b) could also account for the products formed. While not necessary for the reaction, chloride assistance for proton removal is a possibility and has been postulated previously for a similar reaction (i, 37). 

Fortunately, we can postulate a plausible decomposition route from the known chemistry of Pd (II) alkyls. Pd (II) with a stabilizing ligand, such as phosphine, forms methylpalladium(II), but alkyls with a -hydrogen are unstable (4). The reason for this instability is not solvolytic decomposition but probably )3-hydrogen elimination to give olefin and apparently an unstable palladium (II) hydride, which decomposes to Pd(0) and H". 

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